Tropical weather analysis - September 11, 2013

By: KoritheMan , 7:32 AM GMT on September 11, 2013

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Gabrielle

Tropical Storm Gabrielle continues to move just west of Bermuda. As of the 0600Z NHC intermediate advisory, the following information was available on Gabrielle:

Wind: 60 mph, with higher gusts
Location: 32.2°N 65.3°W
Movement: NNW at 8 mph
Pressure: 1009 mb



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

The satellite signature is not impressive, with all of the remaining shower activity displaced at least 75 miles to the east of the low-level center due to about 25 kt of southwesterly shear as diagnosed by the University of Wisconsin CIMSS laboratory. If convection does not return soon, Gabrielle could degenerate into a remnant low later today, as noted in the latest NHC forecast discussion. Considering the diurnal convective maximum period is fast approaching and the cyclone has nothing to show for it, this may occur sooner rather than later.

My forecast generously allows Gabrielle to live for another 36 hours as a tropical cyclone, but I would not be surprised in the slightest if this were to occur much, much sooner.

Bermuda has been reporting sustained tropical storm force winds for the last several hours. These winds could continue for the next 12 hours or so even without the assistance of convection, as the low-level circulation is quite vigorous. Interests there should continue to monitor Gabrielle until it has safely passed.

In about 96 hours, the global models show Gabrielle's low-level vortex elongating and becoming absorbed by a frontal trough forecast to move offshore the eastern United States in a couple of days.

Gabrielle is temporarily being steered by a low- to mid-level ridge near Bermuda, as the large scale flow to the north of the island has become zonal as an upper low over the Bahamas retrogrades to the west. In a couple of days, the global models show another trough, now over the northern United States, moving offshore the east coast, which is expected to accelerate Gabrielle northeastward to north-northeastward. The guidance is in good agreement, although there has been another westward shift during the first 24-36 hours while the tropical storm gets steered by the ridge. Gabrielle is forecast to lose tropical characteristics in about 72 hours, if it doesn't completely lose its identity before then.

Intensity forecast and positions

INITIAL 09/11 0600Z 32.2°N 65.3°W 50 KT 60 MPH
12 hour 09/11 1800Z 33.1°N 66.1°W 45 KT 50 MPH
24 hour 09/12 0600Z 34.3°N 66.8°W 40 KT 45 MPH
36 hour 09/12 1800Z 37.0°N 66.0°W 35 KT 40 MPH
48 hour 09/13 0600Z 41.9°N 63.8°W 30 KT 35 MPH...POST-TROPICAL/REMNANT LOW
72 hour 09/14 0600Z 48.4°N 56.5°W 30 KT 35 MPH...POST-TROPICAL/EXTRATROPICAL
96 hour 09/15 0600Z...ABSORBED BY FRONTAL ZONE

Track forecast



Figure 2. My forecast track for Gabrielle.

NHC storm information

NHC storm information


000
WTNT32 KNHC 110551
TCPAT2

BULLETIN
TROPICAL STORM GABRIELLE INTERMEDIATE ADVISORY NUMBER 11A
NWS NATIONAL HURRICANE CENTER MIAMI FL AL072013
200 AM AST WED SEP 11 2013

...CENTER OF GABRIELLE PASSING WEST-SOUTHWEST OF BERMUDA...
...TROPICAL STORM CONDITIONS TO CONTINUE THROUGH WEDNESDAY
MORNING...


SUMMARY OF 200 AM AST...0600 UTC...INFORMATION
----------------------------------------------
LOCATION...32.2N 65.3W
ABOUT 40 MI...65 KM WSW OF BERMUDA
MAXIMUM SUSTAINED WINDS...60 MPH...95 KM/H
PRESENT MOVEMENT...NNW OR 345 DEGREES AT 8 MPH...13 KM/H
MINIMUM CENTRAL PRESSURE...1009 MB...29.80 INCHES


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

NONE

SUMMARY OF WATCHES AND WARNINGS IN EFFECT...

A TROPICAL STORM WARNING IS IN EFFECT FOR...
* BERMUDA

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


DISCUSSION AND 48-HOUR OUTLOOK
------------------------------
AT 200 AM AST...0600 UTC...THE CENTER OF TROPICAL STORM GABRIELLE
WAS LOCATED NEAR LATITUDE 32.2 NORTH...LONGITUDE 65.3 WEST.
GABRIELLE IS MOVING TOWARD THE NORTH-NORTHWEST NEAR 8 MPH...13 KM/H.
THIS GENERAL MOTION IS EXPECTED TO CONTINUE WITH A DECREASE IN
FORWARD SPEED TODAY. ON THE FORECAST TRACK...THE CENTER OF
GABRIELLE IS EXPECTED TO MOVE AWAY FROM BERMUDA LATER TODAY.

MAXIMUM SUSTAINED WINDS ARE NEAR 60 MPH...95 KM/H...WITH HIGHER
GUSTS. LITTLE CHANGE IN STRENGTH IS FORECAST DURING THE NEXT 48
HOURS.

TROPICAL STORM FORCE WINDS EXTEND OUTWARD UP TO 115 MILES...185 KM
FROM THE CENTER. AN ELEVATED STATION AT COMMISSIONERS POINT ON
BERMUDA RECENTLY REPORTED A WIND GUST OF 56 MPH...90 KM/H.

THE MINIMUM CENTRAL PRESSURE JUST REPORTED BY AN AIR FORCE RESERVE
HURRICANE HUNTER AIRCRAFT IS 1009 MB...29.80 INCHES.


HAZARDS AFFECTING LAND
----------------------
WIND...TROPICAL STORM CONDITIONS WILL CONTINUE ON BERMUDA THROUGH
THIS MORNING.

RAINFALL...GABRIELLE IS EXPECTED TO PRODUCE RAINFALL AMOUNTS OF
2 TO 4 INCHES OVER BERMUDA...WITH ISOLATED MAXIMUM TOTALS OF 6
INCHES POSSIBLE.

STORM SURGE...A STORM SURGE OF 2 TO 3 FEET ABOVE NORMAL TIDE LEVELS
IS EXPECTED ON BERMUDA. IN ADDITION...ROUGH SURF CONDITIONS WILL
CONTINUE TO AFFECT BERMUDA TODAY.


NEXT ADVISORY
-------------
NEXT COMPLETE ADVISORY...500 AM AST.

$$
FORECASTER BEVEN


Humberto

Humberto remains just under hurricane status as of the 0300Z NHC advisory:

Wind: 70 mph, with higher gusts
Location: 15.4°N 28.4°W
Movement: NW at 7 mph
Pressure: 995 mb

After looking a little ragged earlier on Tuesday, Humberto is making a bit of a comeback this morning, likely in response to instability enhancement caused by the diurnal convective maximum. Convection has increased over the center, and microwave data suggests a better defined inner core region; on the other hand, conventional satellite images still do not suggest the presence of an eye. Satellite estimates are hovering between 3.5 and 4.0 depending on the agency you pick, and Humberto may tie or surpass the record for the latest-forming first hurricane since Hurricane Gustav of 2002, which became a hurricane near 1200 UTC 11 September. Keep in mind that this formation is the latest after the advent of aerial reconnaissance in 1944, and a major hurricane in October of 1905 finally became the first hurricane of the year on October 8; also, 1907 and 1914 did not have hurricanes at all.



Figure 3. Latest infrared satellite image of Tropical Storm Humberto. Image credit: NOAA's Satellite Services Division (SSD).

Humberto still remains over reasonably warm water and in an environment of seemingly low shear. As long as dry air does not interfere again, the cyclone should become a hurricane very soon. It no longer appears that a period of rapid intensification into a major hurricane is unlikely, and Humberto is forecast to cross the 26C isotherm in about 24 hours, after which time intensification is expected to be halted. Simultaneously, the GFS shows an abrupt increase in southwesterly shear in response to a large upper-level trough over the central Atlantic, an evolution which is forecast to cause Humberto to steadily weaken, and possibly even rapidly weaken.

Humberto turned northwestward on Tuesday, and appears to still be moving along that general heading as it rounds the western periphery of a deep-layer ridge centered near the Azores. The guidance remains in good agreement on the synoptic evolution, with Humberto forecast to turn sharply back to the west-northwest or west near the end of the period as it escapes the influence of any large-scale cyclonic flow capable of creating an amplified weakness in the subtropical ridge. It is unlikely Humberto makes it to the United States coast even with the projected westward turn, as it will be moving across a region of ocean where few storms historically make the crossing successfully.

My forecast track is similar to the one from the National Hurricane Center.

Intensity forecast and positions

INITIAL 09/11 0300Z 15.4°N 28.4°W 60 KT 70 MPH
12 hour 09/11 1200Z 16.3°N 29.1°W 65 KT 75 MPH
24 hour 09/12 0000Z 17.9°N 29.5°W 75 KT 85 MPH
36 hour 09/12 1200Z 20.0°N 29.7°W 75 KT 85 MPH
48 hour 09/13 0000Z 21.6°N 30.1°W 70 KT 80 MPH
72 hour 09/14 0000Z 23.3°N 32.5°W 55 KT 65 MPH
96 hour 09/15 0000Z 24.7°N 35.3°W 45 KT 50 MPH
120 hour 09/16 0000Z 25.3°N 39.6°W 35 KT 40 MPH

Track forecast



Figure 4. My forecast track for Humberto.


Invest 93L

An area of low pressure is moving across northern Belize. Although shower activity is currently not well organized, the GFS shows a very favorable upper-level environment over the Bay of Campeche, which would favor development of a tropical cyclone in that region as the low moves over water in the next 24 hours.



Figure 5. Latest infrared satellite image of Invest 93L. Image credit: NOAA's Satellite Services Division (SSD).

The models made a northward shift in Tuesday's cycles, with the 18z run of the GFS showing a landfall in south Texas, while the 12z NAVGEM showed a hurricane heading toward the central Gulf Coast. The NAVGEM is historically unreliable, but a model never postulates possibilities without reason. The most likely scenario is for a landfall in eastern or northeastern Mexico, but there is enough fragility in the pattern over the United States that this system could still pull northward to Texas or extreme western Louisiana (more likely Texas if the more northerly projections ensued). If the ECMWF starts trending more northward as well, the more northern solution will have to be considered more than a remote possibility. The northward turn is possible if a trough amplifying over the western United States moves eastward quicker than the models are showing. The modeled trough is not forecast to arrive in the central United States until around four or five days, and a lot change in that time. It also depends on where a definite center forms in the Bay of Campeche, and also how slow the system moves while in the Bay of Campeche.

Regardless of development, heavy rains and flooding are possible over portions of the Yucatan Peninsula, Belize, Guatemala, and eastern Mexico over the next several days, especially in mountainous areas.

Probability of development in 48 hours: 30%

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2. WunderAlertBot (Admin)
7:15 AM GMT on September 13, 2013
KoritheMan has created a new entry.
1. BahaHurican
8:23 AM GMT on September 11, 2013
Nice, Kori. Thorough and well written as usual.
Member Since: October 25, 2005 Posts: 19 Comments: 22322

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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.

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