16-yr-old weather aficionado, with primary focus on tropical cyclones. High school and college student, working towards the National Hurricane Center.
By: TropicalAnalystwx13 , 10:17 PM GMT on July 04, 2013
It's been a few days since my last blog. Since that time, Dalila has become a hurricane and since succumbed to easterly wind shear and dry air as initially forecast. What did not pan out was my intensity forecast of a 90 mph Category 1 hurricane at peak intensity; it reached 75 mph and leveled off instead. My higher forecast...compared to the statistical, dynamical, and global model guidance...was under the expectation of a brief period of rapid intensification from Dalila. This may have been the case had it not been for the unusually dry air environment around the tropical system. I noted in my first forecast that it may end up being the most detrimental to Dalila, and indeed it had been. Quite a bit of southeast to easterly wind shear affected the system in its formative stages as well, and still it actually. As of the latest National Hurricane Center, Tropical Storm Dalila was located within 20 nautical miles of 17.3N 110.4W, or about 390 miles south of the southern tip of Baja California. Maximum sustained winds were steady at 40 mph, the pressure was up one millibar to 1004 millibars, and the storm was moving towards the west at 7 mph. Visible satellite imagery indicates Dalila hardly meets the criteria of a tropical cyclone, with a recent burst of thunderstorm activity atop an otherwise exposed low-level swirl today. This disheveled appearance is the result of the aforementioned mid-level dry air and brisk wind shear; ocean heat content has fallen off to 0 units as well, which is unfavorable for sustainability or intensification. The latest intensity estimates from SAB, TAFB, and UW-CIMSS-ADT were T1.5/25 mph...T2.0/30 mph...and T2.1/36 mph, respectively. Dalila is likely to lose tropical cyclone classification over the next day.
Figure 1. Visible satellite imagery of Tropical Storm Dalila.
Forecast for Dalila
There's not much more to be said about the forecast for Tropical Storm Dalila. Continued easterly wind shear and mid-level dry air entrainment should combine with increasingly cool sea surface temperatures and a lack of ocean heat content to cause steady weakening of the system. Dalila is expected to become a tropical depression within 12 hours, and degenerate into a non-convective remnant area of low pressure & post-tropical cyclone in 24 hours, if not sooner. The system is currently moving westward underneath the southeast periphery of the ridge of high pressure over the West United States. This motion is expected to continue for the next day or so, until the ridge over the Northwest shifts southeastward and provides southeasterly flow. This should prompt a turn towards the northwest. Regardless, Dalila and/or its remnants is not expected to be a threat to land.
...FORECAST POSITIONS/MAX WINDS...
INIT 04/2100Z 17.3N 110.4W 35 KT 40 MPH
12H 05/0600Z 17.3N 111.2W 30 KT 35 MPH
24H 05/1800Z 17.5N 112.6W 30 KT 35 MPH...POST-TROP/REMNT LOW
36H 06/0600Z 17.7N 113.4W 25 KT 30 MPH...POST-TROP/REMNT LOW
48H 06/1800Z 17.9N 114.2W 20 KT 25 MPH...POST-TROP/REMNT LOW
72H 07/1800Z 18.2N 116.7W 15 KT 20 MPH...POST-TROP/REMNT LOW
Tropical Depression Five-E
Since my last blog, we've seen yet another spin-up in the East Pacific. The formation of Tropical Depression Five-E comes as a result of a broad area of low pressure that developed and organized just south of the Gulf of Tehuantepec. As of the latest National Hurricane Center advisory, the system was located within 30 nautical miles of 14.2N 99.2W, or about 190 miles south-southeast of Acapulco, Mexico. Maximum sustained winds and the minimum barometric pressure remained consistent at 30 mph and 1006 millibars, respectively, and the storm was moving towards the northwest at 10 mph. Visible satellite loops reveal a tropical cyclone that is slow organizing despite unfavorable upper-level wind shear. While previous OSCAT and ASCAT passes reveal a relatively wind-free center of circulation, winds of tropical depression intensity are occurring well away from the low. Latest satellite intensity estimates from SAB, TAFB, and UW-CIMSS-ADT were T1.5/25 mph...T2.0/30 mph...and T2.7/45 mph, respectively. Shower and thunderstorm activity has been increasing in coverage and intensity near the center of the past few hours, and spiral banding is becoming increasingly noticeable on satellite loops, so it is likely Five-E will attain tropical storm status...at which point it would be named Erick...over the next few hours.
Figure 2. Visible satellite imagery of Tropical Depression Five-E.
Forecast for Five-E
Five-E is currently located within a marginally favorable environment, at best, for intensification. An upper-level anticyclone is located over Mexico, just south of the Bay of Campeche; this is offset from the center of the tropical cyclone, and providing 10-15 knots of easterly wind shear accordingly. This upper-level high is expected to become better aligned from Five-E over the next 12 to 24 hours, which should provide a more favorable shear environment. Sea surface temperatures are currently near 30°C and should remain favorable for intensification up until 72 hours or so; after that point, Five-E...which should be Erick at the time...is expected to cross the 26°C isotherm and gradually weaken. Mid-level relative humidity values were analyzed in the lower 80s (%) by the SHIPS, which is more than favorable for intensification. However, these values are expected to steadily drop to the lower 70s and upper 60s by 72 hours; these values are equivalent to those Dalila experienced when it began weakening. Thus, steady weakening after 72 hours seems like a good bet. Ocean heat content remains low after 36 hours or so. My forecast is in agreement with the statistical intensity consensus and ever so slightly above the National Hurricane Center's.
Tropical Depression Five-E is moving gradually northwest. This motion, as depicted by many of the statistical and dynamical models, is expected to continue for the next 36 hours in response to a weak ridge over inland Mexico. After this time, the ridge over the Pacific Northwest is expected to weaken and shift southeastward, deflecting the storm on a more west-northwest track. How quick this high builds southeast is crucial, as the current model track average brings Five-E dangerously close to the coastline of Mexico. In fact, the NHC track forecast from this morning now lies below the southernmost model member; they have since adjusted more north. Because of this shift, the agency has issued a Tropical Storm Watch stretching from Acapulco to La Fortuna. A Tropical Storm Warning will be needed for the same stretch of coastline if any further shifts towards the east in the model guidance occurs. A Tropical Storm Watch means tropical storm conditions, with winds of 40 mph, are possible within 36 hours; a Tropical Storm Warning means tropical storm conditions are expected within 36 hours. Regardless of how close the center gets to the coastline, heavy rainfall and gusty winds from the storm's spiral bands should work inland and lead to mudslides, landslides, and sporadic flooding. Increased swells and rip currents are expected along the coastline of southwestern Mexico. By 96 hours, Five-E is expected to turn westward without further consequence.
...FORECAST POSITIONS/MAX WINDS...
INIT 04/2100Z 14.2N 99.2W 30 KT 35 MPH
12H 05/0600Z 15.1N 100.5W 35 KT 40 MPH
24H 05/1800Z 16.2N 101.7W 40 KT 45 MPH
36H 06/0600Z 16.9N 103.4W 45 KT 50 MPH
48H 06/1800Z 17.8N 105.1W 50 KT 60 MPH
72H 07/1800Z 19.3N 108.2W 55 KT 65 MPH
96H 08/1800Z 20.8N 111.4W 45 KT 50 MPH
120H 09/1800Z 21.6N 115.7W 35 KT 40 MPH
Figure 3. From right to left — a disorganized Gulf disturbance, a newly-formed Tropical Depression Five-E, a dying Tropical Storm Dalila, and a potent extratropical low intertwined with the remnants of what was once Hurricane Cosme.
Several Atlantic disturbances
The Atlantic is heating up, though all of the disturbances below should dissipate without much consequence to the United States or elsewhere.
In the eastern Gulf of Mexico and Southeast United States lies a disorganized area of low pressure and an associated mass of deep shower and thunderstorm activity. Due to strong divergence aloft, a weak circulation was able to develop prior to moving into Florida and Alabama this morning. As expected, however, no tropical cyclogenesis occurred, and it is no longer viable with the storm overland. Regardless, heavy rainfall is expected to cause flooding across much of the East United States, a region that has already been plagued with rain over the past month.
A surface trough is located over the Yucatan Peninsula. Though visible satellite loops reveal the feature is disorganized, and sheared as a result of the unusually intense upper-level trough to its northwest, some slow development is possible as it moves west and then due north. The trough currently shearing it is expected to break off into two cut-off lows, with one moving northeast into the Northeast U.S. and the second drifting southwest into inland Mexico. The backing upper-level low should allow an upper-level high ridge, an anticyclone, to balloon over the disturbance and the western/central Gulf of Mexico over the next day or two, providing favorable ventilation to whatever tries to develop. With a lack of focused convergence, however, a bonified area of low pressure may be hard to come by. None of the reliable computer models show tropical cyclone development, though the NAM is indicating the formation of a tropical depression and all of the GFS ensembles indicate the presence of a low pressure center. This feature is expected to make "landfall" along the south Texas coastline on Thursday or Friday. Much like the first disturbance, heavy rainfall and gusty winds will be the main concerns, though it will be very beneficial in this area.
There has been no change in the thinking of the upper-level low setup north of Puerto Rico in the western Atlantic in a few days. Water vapor loops indicate the presence of an elongated upper-level trough wedged between two highs - one off the North Carolina coastline, the other south of the Azores Islands - over the central Atlantic. This trough is expected to weaken and lift northeast, but not before leaving a cut-off mid- to upper-level low behind. As a result of bridging high pressure, this upper-level low is expected to retrograde southwestward and eventually westward. At the same time, a tropical wave over the central Atlantic is tracking west-northwest and should combine with the low in a few days. Such a setup has led to tropical cyclone formation in the past, and with sea surface temperatures between 27-29°C, this should be watched. However, above-average trade winds will likely make it difficult for a definitive low-level circulation to close off. Regardless of development, at least spotty rainfall is likely to affect the Bahamas and Florida this weekend into early next week.
I'll have a new blog tomorrow,
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