KoritheMan's WunderBlog

Tropical weather analysis - May 30, 2013

By: KoritheMan, 5:05 AM GMT on May 30, 2013

Barbara

After briefly becoming a hurricane this morning, Barbara is well inland and weakening over southern Mexico. As of the latest NHC advisory, the following information was available on the decaying cyclone:

Wind: 50 mph, with higher gusts
Location: 17.1°N 93.8°W
Movement: NNE at 9 mph
Pressure: 995 mb

The satellite presentation is not very impressive; the central convection is rapidly decreasing, with only one loose curved band still extant to the south of the center.



Figure 1. Latest infrared satellite image of Tropical Storm Barbara. Image credit: NOAA's Satellite Services Division (SSD).

Unfortunately, the cyclone is out of range of the Puerto Angel radar, and the radar station in Alvarado, located along the coast of the Bay of Campeche, is currently not operational. In addition, I have not been able to acquire any helpful microwave passes, but it is likely that the low- and mid-level circulations are significantly displaced.

Now that Barbara is well inland, a continued rapid decay rate is expected. There have been no surface reports of tropical storm force winds, with no indication of those conditions lingering over the coastal waters, either. It is interesting to note that the SHIPS does not weaken Barbara appreciably, even taking into the inland decay rate algorithm incorporated into that model. In all reality, this will not be the situation, as Barbara is demonstrably weaken via satellite photographs.

With the cyclone no longer a hurricane, the primary threat shifts to continued heavy rains due to the fairly slow motion of the tropical cyclone; as much as 5 to 10 inches of rain, with localized amounts of 15 inches, are possible across much of southern Mexico over the next couple of days, which will likely result in flash flooding and mudslides.

All of the model guidance -- statistical and all -- brings Barbara into the southern of Bay of Campeche on Thursday. In all likelihood, Barbara will enter the Gulf of Mexico as a decaying remnant low, and it is not expected to persist beyond 24 hours as a viable surface entity. While the cyclone will be monitored for signs of possible regeneration once it enters the Bay of Campeche proper, water vapor imagery shows strong southwesterly upper-level flow blowing across that area, which would be an inhibitor to redevelopment; the surrounding air in that part of the Gulf is also fairly dry. None of the guidance indicates regeneration, although the steering will be weak in that region, and water vapor and model analyses suggests that the strongest vertical shear is located a little to the north of where Barbara will enter -- and likely meander.

Given the weak nature of the steering pattern forecast in that region, and the associated model discrepancy, my best estimate is that Barbara or its remnants will move slowly and erratically northwestward to northward before gradually moving back into Mexico. An alternate scenario is that mid-level troughing over the southern plains will be strong enough to turn Barbara -- primarily its mid-level remnants -- northeastward toward the northern Gulf Coast. This would be fairly aclimatological, however, as it generally takes a lot of deep southerly flow to pull systems out of the Bay Campeche in September when troughs are stronger and more frequent, much less late May/early June.

It is possible that Barbara's remnants will contribute to decreasing surface pressures across the southern Gulf of Mexico and western Caribbean over the next few days as the GFS and CMC ensembles continue to indicate lower pressures across that region, albeit no tropical cyclone.

5-day intensity forecast

Initial 05/30 0300Z 45 KT 50 MPH...INLAND
12 hour 05/30 1200Z 25 KT 30 MPH...NEAR BAY OF CAMPECHE AS A REMNANT LOW
24 hour 05/31 0000Z 20 KT 25 MPH...REMNANT LOW IN BAY OF CAMPECHE
36 hour 05/31 1200Z...DISSIPATED

5-day track forecast



Figure 2. My 5-day forecast track for Barbara.

NHC storm information

NHC storm information

ZCZC MIATCPEP2 ALL
TTAA00 KNHC DDHHMM

BULLETIN
TROPICAL STORM BARBARA ADVISORY NUMBER 7
NWS NATIONAL HURRICANE CENTER MIAMI FL EP022013
800 PM PDT WED MAY 29 2013

...BARBARA DUMPING HEAVY RAINFALL ACROSS PORTIONS OF SOUTHEASTERN
MEXICO...

SUMMARY OF 800 PM PDT...0300 UTC...INFORMATION
----------------------------------------------
LOCATION...17.1N 93.8W
ABOUT 50 MI...85 KM WNW OF TUXTLA GUTIERREZ MEXICO
ABOUT 85 MI...135 KM SSE OF COATZACOALCOS MEXICO
MAXIMUM SUSTAINED WINDS...50 MPH...85 KM/H
PRESENT MOVEMENT...NNE OR 20 DEGREES AT 9 MPH...15 KM/H
MINIMUM CENTRAL PRESSURE...995 MB...29.38 INCHES

WATCHES AND WARNINGS
--------------------
CHANGES WITH THIS ADVISORY...

NONE.

SUMMARY OF WATCHES AND WARNINGS IN EFFECT...

A TROPICAL STORM WARNING IS IN EFFECT FROM...
* SALINA CRUZ TO BOCA DE PIJIJIAPAN MEXICO.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.

DISCUSSION AND 48-HOUR OUTLOOK
------------------------------
AT 800 PM PDT...0300 UTC...THE CENTER OF TROPICAL STORM BARBARA WAS
LOCATED INLAND OVER SOUTHEASTERN MEXICO NEAR LATITUDE 17.1 NORTH...
LONGITUDE 93.8 WEST. BARBARA IS MOVING TOWARD THE NORTH-NORTHEAST
NEAR 9 MPH...15 KM/H. A TURN TOWARD THE NORTH AND NORTH-NORTHWEST
AT A SLOWER FORWARD SPEED IS EXPECTED TONIGHT AND ON THURSDAY. ON
THE FORECAST TRACK...THE CENTER OF BARBARA WILL CONTINUE TO MOVE
OVER SOUTHEASTERN MEXICO OVERNIGHT...AND EMERGE OVER THE EXTREME
SOUTHWESTERN GULF OF MEXICO AS A REMNANT LOW ON THURSDAY.

MAXIMUM SUSTAINED WINDS HAVE DECREASED TO NEAR 50 MPH...85 KM/H...
WITH HIGHER GUSTS. WEAKENING WILL CONTINUE OVERNIGHT...AND BARBARA
IS EXPECTED TO DEGENERATE TO A REMNANT LOW ON THURSDAY.

TROPICAL STORM FORCE WINDS EXTEND OUTWARD UP TO 45 MILES...75 KM
FROM THE CENTER.

THE ESTIMATED MINIMUM CENTRAL PRESSURE IS 995 MB...29.38 INCHES.

HAZARDS AFFECTING LAND
----------------------
RAINFALL...BARBARA IS EXPECTED TO PRODUCE TOTAL RAIN ACCUMULATIONS
OF 6 TO 10 INCHES...WITH ISOLATED MAXIMUM AMOUNTS OF 20 INCHES
POSSIBLE...OVER PORTIONS OF SOUTHEASTERN MEXICO. THESE RAINS COULD
CAUSE LIFE-THREATENING FLASH FLOODS AND MUD SLIDES.

WIND...TROPICAL STORM FORCE WINDS...ESPECIALLY IN GUSTS...WILL
AFFECT PORTIONS OF SOUTHEASTERN MEXICO OVERNIGHT.

NEXT ADVISORY
-------------
NEXT INTERMEDIATE ADVISORY...1100 PM PDT.
NEXT COMPLETE ADVISORY...200 AM PDT.

$$
FORECASTER BERG/BEVEN

NNNN



Invest 91E

The persistent area of low pressure located about 650 miles south-southwest of the southern tip of Baja California continues to tenaciously generate thunderstorms that, once again, can never gain any real organization due to continued easterly shear.



Figure 3. Latest infrared satellite image of Invest 91E. Image credit: NOAA's Satellite Services Division (SSD).

Following a brief burst of cold-topped convection that developed -- for the first time -- close to the low-level center, southeasterly shear has again removed the convection from the partially exposed center. I have not been able to identify the state of the surface circulation with this system due to lack of conclusive data, but last light visible satellite images earlier in the day suggested a small but fairly robust low-level wind circulation that could be closed. A couple of microwave passes from the last several hours suggested that the mid-level circulation is very well-defined, but it was harder to get a good grasp on the surface center.

While this system theoretically has a chance to become a tropical cyclone over the next day or so, the models have thus far done an unsavory job at forecasting the upper flow pattern around the low; the shear has continued despite continuous forecasts to the contrary, and there is also ample mid-level dry air in the vicinity of the disturbance. Having said all that, environmental conditions are not favorable for development, but, at the risk of sounding like a broken record, they should become a little more favorable over the next 12-24 hours as the system continues gaining latitude. This isn't just speculation based on faulty model predictions, either; this can legitimately be corroborated by water vapor imagery, which show somewhat lighter upper tropospheric flow as the system moves northeastward.

Regardless of development, this system is not expected to threaten land. By Friday afternoon, the system is forecast to encounter cooler water temperatures and a much more stable airmass, which should kill off any potential tropical cyclone that develops by then.

Probability of development in 48 hours: 50%

I am not going to touch on the potential Atlantic development in this post pending the evolution of Barbara as its energy reaches the Bay of Campeche and southern Gulf of Mexico, because there really isn't anything new to report in the model fields.

2013 Pacific hurricane season 2013 Atlantic hurricane season Tropical Storm Barbara Invest 91E

Updated: 5:09 AM GMT on May 30, 2013

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Tropical weather analysis - May 29, 2013

By: KoritheMan, 5:57 AM GMT on May 29, 2013

Barbara

Tropical Storm Barbara developed Tuesday evening from what was previously "Invest 92E". As of the just released intermediate advisory from the National Hurricane Center, the following information was posted on Barbara:

Wind: 50 mph, with higher gusts
Location: 14.5°N 95.7°W
Movement: NE at 3 mph
Pressure: 1000 mb

Satellite images show a remarkable central dense overcast pattern, with some evidence of a periodic warm spot -- perhaps Barbara's first attempts at an eye -- on infrared and microwave satellite data.



Figure 1. Latest infrared satellite image of Tropical Storm Barbara. Image credit: NOAA's Satellite Services Division (SSD).

Since Barbara appears to be developing an inner core, rapid intensification near the coast is a distinct possibility given the highly favorable environment encapsulating the cyclone. The majority of the guidance is, interestingly, not strengthening Barbara much, with the highest I've seen being 45 kt from the GFDL. Given the extremely favorable outflow pattern associated with the tropical storm, along with the general 'funneling effect' off the Sierra Madre mountain range which helps to assist convection in this area, I am going to go more bullish than virtually all of the guidance, and bring Barbara to the coast at just under hurricane strength. However, if the inner core quickly becomes solidified during the diurnal convective maximum this morning, Barbara could reach the coast as a hurricane. Rapid weakening is expected after landfall as the small circulation traverses the mountains of southern Mexico.

Synoptic data indicates that Barbara is situated between a well-established subtropical ridge over the western Atlantic extending into the southeastern United States, with an amplifying shortwave trough over the southern plains. While this pattern is likely to force a north-northeast motion toward the coast, recent satellite and microwave pictures during the night suggested that Barbara is moving rather slowly for now. Primarily due to the observed slow motion, and considering there is still zonal flow over the western Gulf of Mexico (implying that the trough hasn't reached that location), I am giving Barbara a little more time over water, with landfall indicated just prior to 0z Thursday. However, given the capricious nature of the Sierra Madre mountain range and its associated effect on tropical cyclones, and inner core fluctuations as the storm becomes better established, unpredictable trochoidal wobbles are likely, and with them, a change in the forward speed. In summary, it is impossible to predict exactly how many hours Barbara has left over water, but 12-18 hours seems like a good guess.

It should be emphasized that the primary threat accompanying Barbara remains the potential for very heavy rains capable of causing life-threatening flash floods and mudslides. These rains are likely to be especially prodigious in mountainous areas.

In addition to the flooding, sustained tropical storm force winds are anticipated within about 50 miles of where the center makes landfall and spreading inland; winds could gust to hurricane force, especially over areas of higher elevation, and particularly if Barbara strengthens as forecast. There is a small chance of sustained hurricane force winds lashing the immediate coast.

5-day intensity forecast

Initial 05/29 0300Z 45 KT 50 MPH
12 hour 05/29 1200Z 50 KT 60 MPH
24 hour 05/30 0000Z 60 KT 70 MPH...INLAND
36 hour 05/30 1200Z 30 KT 35 MPH...INLAND
48 hour 05/31 0000Z...DISSIPATED

5-day track forecast



Figure 2. My 5-day forecast track for Barbara.

NHC storm information


000
WTPZ32 KNHC 290550
TCPEP2

BULLETIN
TROPICAL STORM BARBARA INTERMEDIATE ADVISORY NUMBER 2A
NWS NATIONAL HURRICANE CENTER MIAMI FL EP022013
1100 PM PDT TUE MAY 28 2013

...BARBARA STRENGTHENING IN THE GULF OF TEHUANTEPEC...


SUMMARY OF 1100 PM PDT...0600 UTC...INFORMATION
-----------------------------------------------
LOCATION...14.5N 95.7W
ABOUT 120 MI...195 KM SSW OF SALINA CRUZ MEXICO
MAXIMUM SUSTAINED WINDS...50 MPH...80 KM/H
PRESENT MOVEMENT...NE OR 40 DEGREES AT 3 MPH...6 KM/H
MINIMUM CENTRAL PRESSURE...1000 MB...29.53 INCHES


WATCHES AND WARNINGS
--------------------
CHANGES WITH THIS ADVISORY...

NONE.

SUMMARY OF WATCHES AND WARNINGS IN EFFECT...

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* LAGUNAS DE CHACAHUA TO BOCA DE PIJIJIAPAN MEXICO.

A TROPICAL STORM WARNING MEANS THAT TROPICAL STORM CONDITIONS ARE
EXPECTED SOMEWHERE WITHIN THE WARNING AREA...IN THIS CASE WITHIN THE
NEXT 12 TO 24 HOURS.

FOR STORM INFORMATION SPECIFIC TO YOUR AREA...PLEASE MONITOR
PRODUCTS ISSUED BY YOUR NATIONAL METEOROLOGICAL SERVICE.


DISCUSSION AND 48-HOUR OUTLOOK
------------------------------
AT 1100 PM PDT...0600 UTC...RADAR DATA FROM PUERTO ANGEL MEXICO
INDICATE THAT THE CENTER OF BARBARA WAS LOCATED NEAR LATITUDE 14.5
NORTH...LONGITUDE 95.7 WEST. BARBARA IS MOVING TOWARD THE
NORTHEAST NEAR 3 MPH...6 KM/H...AND AN ACCELERATION TOWARD THE
NORTHEAST OR NORTH-NORTHEAST IS EXPECTED ON WEDNESDAY. ON THE
FORECAST TRACK...THE CENTER OF BARBARA SHOULD REACH THE COAST OF
SOUTHERN MEXICO ON WEDNESDAY AFTERNOON OR EVENING.

MAXIMUM SUSTAINED WINDS HAVE INCREASED TO NEAR 50 MPH...80 KM/H...
WITH HIGHER GUSTS. ADDITIONAL STRENGTHENING IS FORECAST OVERNIGHT
AND ON WEDNESDAY BEFORE THE CENTER OF BARBARA REACHES THE COAST.
WEAKENING IS FORECAST AFTER LANDFALL...AND BARBARA SHOULD DISSIPATE
OVER SOUTHERN MEXICO BY THURSDAY.

TROPICAL-STORM-FORCE WINDS EXTEND OUTWARD UP TO 60 MILES...95 KM
FROM THE CENTER.

THE ESTIMATED MINIMUM CENTRAL PRESSURE IS 1000 MB...29.53 INCHES.


HAZARDS AFFECTING LAND
----------------------
RAINFALL...BARBARA IS EXPECTED TO PRODUCE TOTAL RAIN ACCUMULATIONS
OF 3 TO 6 INCHES OVER SOUTHERN OAXACA MEXICO...WITH ISOLATED MAXIMUM
AMOUNTS OF 10 INCHES POSSIBLE IN SOUTHEASTERN OAXACA.

WIND...TROPICAL STORM CONDITIONS ARE EXPECTED TO FIRST REACH THE
COAST WITHIN THE WARNING AREA EARLY WEDNESDAY...MAKING OUTSIDE
PREPARATIONS DIFFICULT OR DANGEROUS.


NEXT ADVISORY
-------------
NEXT COMPLETE ADVISORY...200 AM PDT.

$$
FORECASTER KIMBERLAIN/BROWN



Invest 91E

The persistent area of low pressure located about 850 miles west-southwest of the southern tip of Baja California continues to produce shower activity. This activity has been persistent, but unable to gain any real organization due to continued easterly shear, in part due to the outflow mechanism from Tropical Storm Barbara. Central convection has decreased a little over the past couple of hours as some dry air appears to have worked its way into the circulation center; however, the center appears to be closer to the convection than it was yesterday, although it is still along the eastern side of the convection, implying that the threat of easterly shear arresting development still exists.



Figure 3. Latest infrared satellite image of Invest 91E. Image credit: NOAA's Satellite Services Division (SSD).

The subtropical jet is enhancing outflow to the north and west, and the upper-level wind pattern is forecast to ease up to more conducive levels, although the models have done a poor job at handling any areal decreases in shear with this system. Since the shear does appear to be slowly lessening, dry air appears to be the bigger inhibitor at the present time. By late Friday, the system is forecast to encounter cooler waters, which should cause weakening. Most of the models move this system northward (confirmed by recent satellite pictures as well) as it comes under the influence of the southern United States trough, and it no longer appears likely that it will get absorbed into the circulation associated with Barbara.

Probability of development in 48 hours: 30%



Atlantic development still possible next week

The global model ensembles continue to suggest lowering of surface pressure across the western Caribbean and southern Gulf of Mexico as the upward MJO moves into the area and a strong upper high develops over the western Atlantic. It also appears that some of Barbara's mid-level vorticity and moisture will move into this area and serve as the partial catalyst to the initiation of an area of disturbed weather within that region. The timing and placement of the low remains off though, especially among the operational runs. For this type of situation, we generally want to see agreement amongst the ensembles in regards to the future pattern, and for the most part, we have.

It should be noted that the last couple runs of the GFS ensembles have showed the area of lowest pressure originating the Bay of Campeche, which could happen given the close proximity of Barbara to that region.

The eventual trajectory of such a system is still speculative for now, but one thing that hasn't changed is that most of the models still really like the west coast of Florida.

2013 Pacific hurricane season 2013 Atlantic hurricane season Tropical Storm Barbara Invest 91E

Updated: 6:04 AM GMT on May 29, 2013

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Tropical weather analysis - May 28, 2013

By: KoritheMan, 6:21 AM GMT on May 28, 2013

Invest 91E

An area of low pressure located about 825 miles southwest of Manzanillo, Mexico has become a little better organized this evening. The system is accompanied by a large burst of convection that is ongoing, although it is displaced some 50 miles west of the low-level center. A couple of SSMIS microwave passes from around 0340Z showed that the low- and mid-tropospheric circulations are much more aligned than they have been the last several days, suggesting a significant decrease in easterly shear; this trend was confirmed by analyzing water vapor images as well.



Figure 1. Latest infrared satellite image of Invest 91L. Image credit: NOAA's Satellite Services Division (SSD).

The environment ahead of the system looks a little more favorable than it has been the last few days, although there is still some shear. While by no means do I anticipate the environment to become anticyclonic, it is at least not as hostile as it was yesterday. Significant development will likely be inhibited in a couple of days as easterly shear increased, denoted by nearly all of the model guidance.

The low appears to be moving west-southwest, although the accompanying motion is rather slow, making it difficult to gauge the true nature of the forward motion. The guidance is spread, with some of the statistical models showing a motion toward the northwest, while the global models generally indicate a northeasterly motion. The global models are generally superior performs, and their solution is thus preferable. However, the system appears isolated from any significant steering influences at the moment, hence the slow motion observed in satellite images. Looking at the 500 mb vorticity fields in the global model fields, it appears that the trough moving into the southern plains will have not enough influence to recurve the system; thus, it appears that the low-level cyclonic flow associated with a developing disturbance south of the Gulf of Tehuantepec will be the primary steering mechanism for the purported poleward motion advertised by the models. This system will likely ultimately become absorbed into the larger circulation associated with Invest 92E.

Probability of development in 48 hours: 20%



Invest 92E

A secondary area of disturbed weather, this one located about 190 miles south of Salina Cruz, Mexico, is much better organized than the former low, and actually appears destined to become the season's next tropical cyclone. The associated convection nearly vanished during the culmination of the diurnal convective minimum yesterday, but recent satellite trends show a system that is continuing to organize; deep convection has erupted over the center, although there is little curvature in the associated thunderstorms.



Figure 2. Latest infrared satellite image of Invest 92E. Image credit: NOAA's Satellite Services Division (SSD).

The large-scale environment continues to appear rather favorable for development of this low, with little vertical shear, very warm sea surface temperatures, and high atmospheric moisture content. The low also has a well-developed outflow pattern, with dual channels -- one to the north and one to the south. While no recent scatterometer or microwave data were available, low cloud lines on shortwave infrared satellite images indicate that the low-level circulation is well-defined, and continues to consolidate. There seems to be little reason why this system will not become a tropical depression or tropical storm later this morning, and interests along the coast of southern Mexico from Puerto Angel to Salina Cruz should monitor its progress.

92E is currently situated between a mid-level ridge over the western Atlantic and a trough over the southern plains, a pattern which is forcing a slow northward motion toward the coast. The majority of the guidance agrees with a landfall along the coast on Wednesday afternoon. While it is still too early to pinpoint the precise location and timing of landfall, right now I favor a track just to the east of Puerto Angel based on the forecast steering pattern and observed forward motion.

Although this system has limited time over water, Tropical Storm Alma in 2008 formed in a similar location (albeit a little farther east), and was able to strengthen fairly quickly despite only having 24 hours over water. Thus, it is not advisable to discourage or underestimate this system's potential for rapid growth near the coast, particularly given the very favorable upper-level environment. Right now I would anticipate at least a 45 kt tropical storm, and that could be conservative if the system takes full advantage of its environment and doesn't sputter during the diurnal minimum.

Tropical storm watches and warnings could be issued by the National Hurricane Center on short notice this morning, as the system does appear to be developing.

Regardless of development, torrential rains and associated flash flooding/mudslides will accompany this system as it impacts southern Mexico over the next several days.

The global models generally show some residual mid-level energy getting left behind in the western Caribbean from this system, which would help to initiate the development of a disturbance across that region during the first week of June.

Probability of development in 48 hours: Near 100%



Western Caribbean development still possible during the first week of June

After several days of repeating myself, there's not much else to say. The global model ensembles -- the GFS, CMC, and ECMWF, continue to indicate the development of lower surface pressures in the western Caribbean through next week as the upward MJO moves into the area. As previously noted, the mid-level energy associated with Invest 92E is likely to move into the northwestern Caribbean Sea over the next few days and enhance convective potential there.

While the long-range track of this system is still in question, in general the models have come into better agreement on a 500 mb pattern favoring a strike along the western coast of Florida. However, there is enough margin for error that the system could still pose a threat to panhandle, particularly if it originates in the Bay of Campeche like the 0z GFS suggested; it is of course, also possible that this forecast tropical cyclone will not materialize at all. But the models appear to be gaining stronger agreement, and this will soon be out of the long-range domain.

2013 Pacific hurricane season 2013 Atlantic hurricane season Invest 91E Invest 92E

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Tropical weather analysis - May 26, 2013

By: KoritheMan, 2:41 AM GMT on May 27, 2013

Invest 91E

The persistent area of low pressure we have been tracking well to the southwest of Manzanillo, Mexico for the last several days has become better defined today. Last light visible satellite images of the day suggested a small but well-defined low-level circulation, which is in direct contrast to the state of affairs just 24 hours ago. The system is accompanied by an area of deep convection about 50 miles west of the exposed low-level center due to strong easterly shear associated with a mid-tropospheric ridge over northern Mexico.



Figure 1. Latest infrared satellite image of Invest 91E. Image credit: NOAA's Satellite Services Division (SSD).

There are mixed signals amongst the global model forecast fields as to how favorable the large-scale environment will be. There is some easterly shear over the system, and the GFS doesn't recognize it. The 12z CMC seemed to have a better depiction of the reality of the upper air pattern over the area, so that forecast was relied more heavily upon than that of the GFS. The former model shows the shear lessening in about 24 hours, which would allow for some slow development beyond that time. Despite this, the odds of 91E eventually becoming a tropical cyclone appear low, as the SHIPS/GFS/CMC forecast an abrupt increase in easterly shear in about 72 hours, presumably in response to a developing vortex to the east -- modeled tropical cyclone in the global model forecast fields. In addition, the 850 mb vorticity fields within the models show 91E gradually getting absorbed into the secondary low during that time while the 500 mb center -- the mid-level center -- meanders aimlessly in weak steering between a mid-level ridge to the north and broad-scale cyclonic flow to the east.

This low is expected to move slowly westward under the influence of a weak low-level ridge before moving eastward beyond day three as the secondary system begins to dominate the cyclonic flow in the region.

Probability of development in 48 hours: 20%



Invest 92E

A second area of low pressure is located about 250 miles southeast of Salina Cruz, Mexico, is showing some signs of organization, with a small burst of cold-topped convection ongoing over the center.



Figure 2. Latest infrared satellite image of Invest 92E. Image credit: NOAA's Satellite Services Division (SSD).

Analysis of the model fields and water vapor imagery this evening shows a very favorable environment for development of this low; the aforementioned high over northern Mexico is gradually retograding eastward as weak upscale troughing develops over the four corners region. This is causing the flow over the eastern Gulf to back and become more diffluent, a pattern which would theoretically enhance the anticyclonic outflow in the eastern semicircle; this is supported by the GFS as well, which shows the subtropical jet in a very favorable position to furnish a poleward outflow channel with this disturbance. Interestingly, the CMC shows an environment that is only marginally conducive for development, and accordingly barely acknowledges the existence of a future tropical cyclone in the 12z run. Since this model does not appear to have a good handle on the upper pattern in this area of the basin, its solution has been discounted. Underlying sea surface temperatures are very warm, and the system will be passing over progressively higher heat content over the next couple of days.

With low to nonexistent shear, a very moist atmospheric column, a well-established outflow pattern, and warm sea surface temperatures, there appears to be little reason why this low will not gradually strengthen. The only inhibitor I can detect right now is the fact that the low is still embedded in the monsoon trough. Since the system currently lacks much in the way of a reflection in the surface wind field (as denoted by satellite, microwave, and scatterometer data), I do not anticipate rapid development of this system, despite the SHIPS giving a whopping 78% chance of an increase of 25 kt or more during the next 24 hours. Once the system detaches from the monsoon trough, however, then rapid development is a distinct possibility given the favorable environment and the small size of the system.

This system is expected to move west-northwest to northwest and make landfall along the coast of Mexico in about four or five days. Interests from Acapulco to Puerto Angel should monitor its progress carefully through midweek.

Probability of development in 48 hours: 50%




Western Caribbean development remains viable during the first week of June

While the operational runs of the global models have remained inconsistent, the GFS/CMC/ECMWF ensembles continue to indicate lowering pressures over the western Caribbean region during the first week of June. This would coincide with the passage of the upward MJO in this area of the world, a pattern which can be seen enhancing convection over the tropical Pacific (91 and 92E).

It is still too early to definitively state whether or not a tropical cyclone will develop from this pattern, but regardless, very wet weather appears to be on tap for this region over the next 7 - 10 days as the monsoon trough lifts northward.

2013 Pacific hurricane season 2013 Atlantic hurricane season Invest 91E Invest 92E

Updated: 2:49 AM GMT on May 27, 2013

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Tropical weather analysis - May 24, 2013

By: KoritheMan, 12:47 AM GMT on May 25, 2013

Invest 91E

An area of low pressure over the tropical Pacific several hundred miles southwest of Acapulco, Mexico remains highly disorganized, and the potential for significant development appears to be diminishing.



Figure 1. Latest infrared satellite image of Invest 91E. Image credit: NOAA's Satellite Services Division (SSD).

Visible satellite images show several mid-level circulations competing for dominance within a convectively active monsoon trough. None of these circulations are showing signs of organization, which again, is to be expected with such strong competition for inflow. There also appears to be some easterly shear over the area, along with lingering dry air. In addition, the gyre as a whole appears to have moved very little over the last few days; while there is a mid-level high over the Gulf of Mexico that would ordinarily furnish a definitive steering mechanism for this disturbance, it is straddling the 10N latitude line, and this has isolated it from any large-scale steering influences. In addition, there is a lot of discrepancy amongst the models on which way this system is going to move over the next few days; such a spread usually heralds little movement. The lack of solid movement is not going to enable this system to gain much, if any, latitude throughout the next several days, and that will also bar development potential by keeping it embedded within the monsoon trough and its associated cyclonic circulation. It is possible that whatever is left of this system will contribute some vorticity and energy to a system well to the east near the Pacific coast of Costa Rica, which the models show becoming the next tropical cyclone of the basin in a few days.

Probability of development in 48 hours: Near 0%




Costa Rica disturbance

The aforementioned disturbance about 150 miles southwest of the Pacific coast of Costa Rica is a much bigger threat for development over the next several days. The associated shower activity is currently disorganized, and minimal near the center, but that is to be expected of a tropical disturbance at this stage. I couldn't find a decent ASCAT pass, but extrapolation of what little coverage I did get showed that the low-level center is extremely ill-defined, and the cyclonic rotation seen with the system on satellite images appears to be primarily mid-level.



Figure 2. Latest infrared satellite image of the Costa Rica disturbance (far right). Image credit: NOAA's Satellite Services Division (SSD).

Examining the upper tropospheric fields within the GFS and CMC, it appears that the large-scale environment will be diffluent and of light shear, a forecast trend which appears reasonable based on analysis of water vapor images this evening. The GFS hints that the environment could become more solidly anticyclonic as the system approaches the southern coast of Mexico in about 5 - 6 days, which would be a potentially dangerous situation if we were to get an intensifying tropical cyclone making landfall.

The large-scale pattern continues to favor a gradual motion to the west-northwest at about 5 to 10 mph over the next 72 hours as the low moves along the southern periphery of the subtropical ridge over the Gulf of Mexico. After that time, the global models are in agreement that a piece of shortwave energy will amplify the jet stream over the four corners region. The associated trough is etched as moving eastward through the Rockies by the end of the period, which would allow the system to turn northwest and north toward the southern coast of Mexico. Interests along that coast from Acapulco to Puerto Angel should monitor the progress of this system over the next 5 to 7 days, as virtually all of the models show this system making landfall. There are some timing differences, naturally, but these will be resolved with time as the models come into better agreement on the synoptic patten and the amplitude of the purported western United States trough.

Probability of development in 48 hours: 20%



Early June western Caribbean development still possible

Although during the last several cycles the operational GFS dropped the tropical cyclone it had in the western Caribbean during the June 3 - 4 timeframe, its respective ensembles, as well as the Canadian ensembles, continue to forecast a large-scale pattern that is conducive for lowering pressures in the western Caribbean during the first week of June. The forecast of this possible development may be inconsistent (in fact, I would actually expect it to be, since again, the global models tend to have notorious difficulty and struggle in correctly resolving the climatological increase in convection across this region this time of year), but the timing appears to be steadily improving, and, more importantly, the ensembles have remained remarkably consistent on a synoptic situation conducive for lower pressures across this region during that timeframe.

At the very least, it appears that an increase in moisture is on tap for the region over the next 7 - 10 days, but what, if anything, will come of that remains to be seen.

2013 Pacific hurricane season 2013 Atlantic hurricane season Invest 91E

Updated: 12:57 AM GMT on May 25, 2013

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Tropical weather analysis - May 23, 2013

By: KoritheMan, 4:01 AM GMT on May 24, 2013

Invest 91E

The area of low pressure located about 550 miles south-southwest of Acapulco, Mexico remains poorly-organized. A fortuitous microwave overpass captured the system just after 00z, and showed that the low-level center is broad and ill-defined; an ASCAT overpass confirmed a similar signature. There is a pronounced area of concentrated rotation within the convective ball from 8 to 9N, but analysis of satellite imagery suggests that this circulation is a mid-level vortex, and the actual low-level center is closer to 10N.



Figure 1. Latest infrared satellite image of Invest 91E. Image credit: NOAA's Satellite Services Division (SSD).

There are some southwesterlies ahead of the system as per water vapor images, but these winds appear to be gradually moving westward ahead of 91E. The GFS, CMC, and SHIPS continue to maintain a fairly light upper tropospheric shear regime over the system, this flow in fact appears lighter in today's model runs. Nevertheless, the system is close enough to upper-level westerlies that I am going to be a little cautious. Conditions appear favorable for slow development, although the close proximity of the system to the monsoon trough could be an inhibiting factor to rapid development. There is also some dry air around.

The system remains south of a low- to mid-level ridge over the Gulf of Mexico. Normally this would favor a path toward the west-northwest given the amplitude of the ridge. However, 91E is far enough to the south that I anticipate a more uniform westward movement over the next several days; while the system might gain a degree or so of latitude during the next 72 hours, any significant poleward component of motion will likely have to wait until days four and five, when a larger-scale trough begins to amplify over the western United States as per the global models. This system is not forecast to threaten land, although the 12z ECMWF has an interesting binary interaction toward the coast between this feature and a tropical wave behind it that is expected to become a tropical cyclone in the global model sea level pressure fields. This should be taken as an interesting triviality, and not an accurate portrayal of reality.

Probability of development in 48 hours 30%



Elsewhere

The NHC has officially removed the tropical wave we've been tracking over the last several days from the 1800/0000 UTC surface charts, portraying instead a feature that has gotten absorbed into the cyclonic gyre associated with the monsoon trough. I can't find any easy evidence of a tropical wave either, but regardless of the nominal status of the system, this is the one the global models have been consistently transforming into a tropical cyclone during the next several days. Extrapolation of the tropical wave's position relative to recent days would place it in the vicinity of the far eastern Pacific right now, which would be consistent with a broad area of cyclonic turning noted near 8 to 9N near 90W.

Conditions appear quite conducive for development for this wave as it moves off to the west-northwest. Interests along the southern coast of Mexico should be aware of a possible tropical cyclone in the next five to seven days, as the large-scale pattern continues to favor a landfall along the south coast of Mexico from Acapulco to Puerto Angel.

On the Atlantic side of things, the large-scale 7-10 day pattern continues to favor the possibility of some sort of tropical development -- or at least the formation of a broad area of low pressure -- in the western Caribbean. The GFS and Canadian ensembles -- as well as the operational -- show increasing heights over the southern United States, which would tend to lower pressures farther south in the Caribbean, and consequently cause an increase in surface convergence, which is beneficial for the development of thunderstorms. This would be coincident with the entrance of the upward MJO into that area during that time, and it would also coincide with climatology. It is possible that whatever is left of the eastern Pacific tropical wave/purported tropical cyclone will help to initiate the development of this disturbance, similar to how the mid-level remnants of Hurricane Carlotta assisted in the formation of Atlantic Tropical Storm Debby almost one year ago. This is a complex meteorological situation that, while climatological, also tends to cause enhanced convective feedback issues amongst the global models as they react to all this increased moisture, which introduces a lot of uncertainty into the forecast.

Nevertheless, the GFS has slowly come into better agreement on the timing of this potential development, showing a tropical cyclone in the western Caribbean east of the Yucatan Peninsula around the June 3 to 4 timeframe over the last several model cycles. Again, we may not get a cyclone out of this, but the large-scale pattern is definitely shaping up to be a wet one in this region. The potential track of such a system is still up in the air, but the operational GFS shows it being a threat somewhere along the United States Gulf Coast; climatology would favor a smooth shot into peninsular Florida, but it could just as easily go westward into Central America or Mexico under such a fickle and complex pattern.

2013 Pacific hurricane season 2013 Atlantic hurricane season Invest 91E

Updated: 4:01 AM GMT on May 24, 2013

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Tropical weather analysis - May 22, 2013

By: KoritheMan, 1:38 AM GMT on May 23, 2013

Invest 91E

I'm usually pretty good at catching things, but I missed this one. An area of disturbed weather has developed well to the south of the Mexican coast. The National Hurricane Center has officially designated this system as an "invest" (91E).

Following the typical progression of a disturbance in its early stages, deep convection has practically vanished, likely due to the diurnal minimum suppressing the development of deep convection. It could also have to do with the fact that the system still appears to be embedded in the ITCZ. The synoptic situation looks somewhat similar to Alvin, in that the large-scale environment is not the most conducive; the evolution of the upper-level winds will depend on how far south weak upper low south of Cabo San Lucas dives. Currently the models showing it moving northeast and decaying, which would favor a more conducive upper air pattern. Water vapor imagery shows fast upper-level winds a couple hundred miles to the west of the system, and there is some dry air in the vicinity of the disturbance. The GFS has changed its tune compared to a week ago and shows an environment that is more accurately diffluent than falsely anticyclonic. The CMC also shows a little bit of southwesterly shear during the early period before the upper flow transitions to a more uniform easterly regime. The SHIPS, which is based on the GFS, doesn't show much shear. Under the assumption that the westerlies may inflict more harm to 91E than the models are currently seeing, along with the disheveled nature of the system at present, I am going to withhold on mentioning the potential for a tropical cyclone for now. Interestingly, the normally aggressive GFS is not showing cyclogenesis from this disturbance, while the typically conservative ECMWF is.

The disturbance is currently south of a mid-level ridge. A small weakness is forecast to develop within the ridge to a trough over the southern United States, which may allow for the system to move more to the west-northwest over the next day or so. There are no apparent signs of this motion at the moment, however, and I feel the models may be a bit too far north with its trajectory. It should be noted that the upper-level flow looks a little more favorable the farther south the system stays, according to analysis of water vapor imagery and forecasts from the global models. The system is expected to turn back to the west in about three days, never threatening land.

Probability of development in 48 hours: 20%

Elsewhere

Elsewhere in the tropics, a tropical wave is currently in the southwestern Caribbean according to the 1800 UTC NHC surface map. This wave is enhancing convection across extreme northwestern South America and adjacent Central America. This wave has been consistently shown by the GFS and CMC to develop into a tropical cyclone that threatens southern Mexico in about six days. Given the consistency of this development in the GFS, which usually does extremely well with genesis forecasts (recall that it successfully foresaw the development of Alvin well before any of the other models), this wave will need to be carefully monitored for tropical development when it enters the eastern Pacific during the next day or two.

The 7-10 day pattern in the Atlantic continues to favor the possibility of tropical development in the western Caribbean. The upward MJO is slowly edging back into our area of the world, which of course favors enhanced precipitation. Simultaneously, the pattern during this period favors low-level convergence manifesting farther south, into the western Caribbean. Based on the 500 mb height anomaly pattern seen by the GFS during the 8 day forecast period, I see what I feel are three decent analogs for any potential tropical cyclone that develops in this region:

Abby (1968)
Arlene (2005)
Arthur (2008)



Figure 1. 500 mb height analogs for the 8 day forecast period as depicted by the 0z GFS. The analogs are given on the right.

I have no reason to favor one over the other at this time.

2013 Pacific hurricane season Invest 91E

Updated: 2:02 AM GMT on May 23, 2013

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Tropical weather analysis - May 17, 2013

By: KoritheMan, 5:41 AM GMT on May 17, 2013

Alvin

Perhaps Simon will do better in 2014. In a bit of a surprise, westerly shear, associated with a large upper-level trough several hundred miles northwest of the tropical cyclone, began to increase over Alvin this morning. This shear has exposed the center to view, with Alvin generating only disorganized convection in broken bands southeast of the low-level center. Props to the ECMWF and CMC which anticipated this development several days ago.

As of the most recent NHC advisory, the following information was contained on the cyclone:

Wind: 40 mph, with higher gusts
Location: 9.8°N 110.8°W
Movement: WNW at 14 mph
Pressure: 1006 mb

Alvin is extremely disorganized, and I am honestly having trouble distinguishing it from another ITCZ disturbance to the west.



Figure 1. Latest infrared satellite image of Tropical Storm Alvin. Image credit: NOAA's Satellite Services Division (SSD).

An earlier ASCAT pass suggested that the circulation had decayed into a sharp surface trough. Low cloud lines on shortwave infrared imagery are certainly not suggestive of an existing closed surface circulation, and evening microwave data reveals that even the mid-level center is beginning to lose definition. Westerly shear continues to affect Alvin, and the global models forecast this to continue throughout the forecast period. The GFS still stubbornly assumes that the upper tropospheric flow will improve and become mildly diffluent, but even that model is starting to recognize the presence of westerly shear within its 850-200 mb forecast fields. Given the hostile environment, Alvin is expected to degenerate into a remnant low within the next 48 hours, although it could occur much sooner if a closed circulation definitely does not exist. I will generously keep a distinct vorticity maximum throughout the five day forecast period, but I am not expecting Alvin to last that long after dissipation.

Alvin remains south of a weak mid-level ridge over southern Mexico. This is the primary steering mechanism for now, but the global models forecast a gradual turn to the west in about two or three days as Alvin becomes a shallower vortex and responds to the lower-tropospheric flow. Compared to yesterday, the model consensus is actually a consensus, and this increases confidence in my track forecast. I am well to the south of the National Hurricane Center forecast track as well as the model consensus given the likelihood that the cyclone no longer has a closed circulation. I could have gone even farther south, but at least some continuity with the models is desirable. Alvin -- or more likely its post-tropical remnants -- are anticipated to turn southwestward near the end of the forecast period while the mid-level center moves northward to northeastward ahead of the trough near Baja.

5-day intensity forecast

INIT 17/0300Z 35 KT 40 MPH
12H 17/1200Z 35 KT 40 MPH
24H 18/0000Z 30 KT 35 MPH
36H 18/1200Z 30 KT 35 MPH
48H 19/0000Z 25 KT 30 MPH...POST-TROP/REMNT LOW
72H 20/0000Z 25 KT 30 MPH...POST-TROP/REMNT LOW
96H 21/0000Z 20 KT 25 MPH...POST-TROP/REMNT LOW
120H 22/0000Z 20 KT 25 MPH...POST-TROP/REMNT LOW

5-day track forecast



My 5-day forecast track for Alvin.

Elsewhere

There is the potential for long-range development in the Caribbean or western Atlantic if the long-range pattern in the GFS verifies, with a high over the southern United States, which will tend to focus convergence and upward motion farther south in tandem with the arrival of the upward MJO into this area of the world. I will probably not touch on this much, however, until the timeframe and pattern become more certain.

There is also a tropical wave moving westward across the central Atlantic south of 10N approaching 45W. While upper-level winds are not favorable for development in the Atlantic, this wave will need to be monitored for potential development when it enters the eastern Pacific in about a week.

2013 Pacific hurricane season Tropical Storm Alvin

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Tropical weather analysis - May 15, 2013

By: KoritheMan, 11:59 PM GMT on May 15, 2013

Alvin

Right on cue with the calendar, Tropical Storm Alvin formed today. As of the latest NHC advisory, the following information was available on the cyclone:

Wind: 40 mph, with higher gusts
Location: 8.7°N 105.1°W
Movement: WNW at 13 mph
Pressure: 1005 mb

Alvin looks well organized on satellite images, especially for a 35 kt tropical storm. Satellite estimates do not currently support an intensity higher than 35 kt, but the 18z Dvorak prognoses were in lieu of the ongoing convective burst near the center. In addition, an ASCAT pass from several hours ago showed believable 30 kt winds within the deepest convection orbiting around the north side of the center. Accepting the well-documented low bias of that instrument, and the subsequent organization of Alvin on satellite pictures since then, it is probable that the winds have come up a little, and I expect the 0z Dvorak classifications will find a stronger system. There is a prominent curved band to the south of the center, and a more loose one to the north. Upper-level outflow is well-established over the tropical cyclone, and the environment may be becoming more anticyclonic than diffluent.



Figure 1. Latest infrared satellite image of Tropical Storm Alvin. Image credit: NOAA's Satellite Services Division (SSD).

For this reason, I expect a short-term rapid intensification trend, and Alvin could be near hurricane strength tomorrow night. I should note that the GFDL and HWRF bring Alvin to the threshold of major hurricane strength in three days, and while these models notoriously have a difficult time furnishing accurate intensity forecasts, intensification to a category two hurricane is not out of the question, particularly if the upper wind environment in the GFS materializes. By day four, cooler waters and increasing southerly to southwesterly shear is forecast to bring about weakening. It should be noted that I don't have a lot of confidence in the intensity forecast, especially during the strengthening phase.

Alvin is currently south of a weakening mid-level ridge. The global models suggest that large-scale troughing will dominate over the southwestern United States over the next few days, which is expected to cause cyclone to continue gaining latitude. There is considerable dichotomy between the global models in how they respond to the amplifying upper flow pattern. The GFS, GFDL, and HWRF show Alvin turning northwestward and northward between the 72-120 hour period, presumably because the cyclone vortex is deeper in these models. The CMC, NAVGEM, and ECMWF show Alvin eluding the trough and getting caught up in a building mid-tropospheric ridge at longer ranges. Since these models do not appear to have an accurate portrayal of Alvin in the sea level pressure fields, and considering the upper flow pattern over the western United States would tend to favor a more poleward-moving cyclone, I am apt to lean closer to the GFS/GFDL/HWRF camp, albeit not fully committing to a complete northward turn, under the presumption that Alvin will begin to shear off. A slowing of the forward speed is indicated late in the period to be cognizant of the shear stripping the vortex of convection, and also in deference to the mid-level trough.

5-day intensity forecast

INIT 15/2100Z 35 KT 40 MPH
12H 16/0600Z 40 KT 45 MPH
24H 16/1800Z 55 KT 65 MPH
36H 17/0600Z 65 KT 75 MPH
48H 17/1800Z 75 KT 85 MPH
72H 18/1800Z 80 KT 90 MPH
96H 19/1800Z 65 KT 75 MPH
120H 20/1800Z 45 KT 50 MPH

5-day track forecast



Figure 2. My 5-day forecast track for Alvin.

2013 Pacific hurricane season Tropical Storm Alvin

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Tropical weather analysis - May 14, 2013

By: KoritheMan, 2:07 AM GMT on May 15, 2013

Invest 90E

A broad area of low pressure centered over the eastern Pacific about 650 miles south of Acapulco, Mexico continues to be a threat to eventually become a tropical cyclone.

The center is a little easier to locate this evening, possibly due to a somewhat better definition in the surface wind field compared to yesterday (as per low-cloud lines on last light visible satellite imagery). There is still not much evidence of vertical shear over the system, and both the SHIPS and GFS keep a light shear environment over the system throughout the forecast period (five days). I should note that the upper flow in the GFS has been inconsistently anticyclonic and diffluent; either scenario would favor intensification, but the former would be more beneficial in breeding a hurricane. There is a well-established equatorial outflow channel in the western semicircle, and a well-defined poleward outflow channel in the northern semicircle due to a narrow mid- to upper-level trough trailing from an upper low over northwest Mexico. There is some indication that southerly to southwesterly shear could increase a little in about five days, but there is not enough evidence to settle on this yet. Either way, the system is likely to become a tropical cyclone in a few days, and it seems likely to become the first tropical storm of the season. However, the circulation is still not well-defined as denoted by some recent ASCAT data and SSMIS data. There is, however, a formative curved band in the eastern semicircle, and overall the cloud pattern looks less disheveled than yesterday.




Figure 1. Latest infrared satellite image of Invest 90E. Image credit: NOAA's Satellite Services Division (SSD).

A weak mid-level ridge over the Gulf of Mexico is being maintained to the north of the system for now, which is imparting a mostly westward motion. The global models forecast a turn to the west-northwest over the next 24-48 hours as broad upscale troughing amplifies over the western United States. A southeastward-moving upper low off the Pacific northwest should provide some additional amplification to the trough over northwest Mexico over the next couple of days, which should allow for a more northwestward motion in about 72-96 hours. There is some disagreement amongst the models as to how far north the system goes as it comes under the influence of the trough, with the CMC and NAVGEM (old NOGAPS) actually forecasting the trough to bypass 90E, which causes the system to slide westward underneath a building mid-level ridge near the end of the forecast period. The GFS shows more amplification of the trough, a pattern that would favor a long-range threat to southern Baja. Since the guidance has a tendency to flip back and forth with systems in their formative stage, I will go with the trend of the previous forecast and maintain a poleward motion out to day five, but not nearly enough to pull the system toward the coast.

Probability of development in 48 hours: 40%

The CMC continues to show the development of a secondary system behind 90E, which goes on to become a tropical cyclone. I still don't see solid evidence of a tropical wave, but ASCAT wind barbs show what appears to be a cyclonic wind shift around 6N 91W; visible satellite and CIMSS 850 to 700 mb vorticity data confirmed a broad cyclonic rotation signature at this general latitude all the way east to about 87W as well, so there is at least something to monitor as possible catalyst for another area of low pressure to form in a few days.

2013 Pacific hurricane season Invest 90E

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Tropical weather analysis - May 13, 2013

By: KoritheMan, 12:43 AM GMT on May 14, 2013

For the second year in a row, an offseason tropical disturbance has manifest in the eastern north Pacific basin. The National Hurricane Center (NHC) tagged this disturbance as an "invest" (90E) earlier today. This disturbance has the potential to develop into a tropical cyclone in the next few days.

Invest 90E

Satellite images suggest a fairly decent cloud pattern. I do not see any evidence of vertical shear over the system; in fact, a weak mid-level trough well to the north over central Mexico may be slightly assisting outflow in the northern semicircle for the moment.



Figure 1. Latest infrared satellite image of Invest 90E. Image credit: NOAA's Satellite Services Division (SSD). I neglected to use the floater because it's currently positioned too far to the south (center is at the top of the image on the floater, whereas the view is plainer here).

Upper-level winds are forecast to remain conducive for steady development, and most of the global models, with the exception of the ECMWF, show this system becoming a tropical storm in a few days. Since the latter model tends to perform very poorly with tropical cyclogenesis, its solution has been discounted. Looking at the large scale, I am not expecting any surprises when it comes to a sudden increase in vertical shear, although the expected pattern in the global models may favor a slight increase in southerly shear very late in the period. All of that being said, there seems to be little reason not to expect gradual development with this system as it moves off to the west and then west-northwest in a couple of days. As of this time, despite the amplification of a broad trough west of Baja in four or five days, there is does not appear to be sufficient poleward forcing to turn the system toward the coast, which is in agreement with the model guidance. The trough might, however, be strong enough to slow the system's forward speed a little bit on days four and five.

The 12z CMC shows a second system following this one and developing into a tropical cyclone. None of the other global models show this, and for now I see very little evidence of an approaching tropical wave that could give birth to the modeled cyclone.

Probability of development in 48 hours: 20%

2013 Pacific hurricane season Invest 90E

Updated: 12:52 AM GMT on May 14, 2013

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About KoritheMan

I'm just a 23 year old with an ardent passion for weather. I first became aware of this interest after Tropical Storm Isidore struck my area in 2002.