Small town USA guy. Politics nerd. Soccer fan. Interested in eyewalls, deformation zones, and hook echos.
By: TropicalAnalystwx13 , 10:13 PM GMT on May 15, 2013
Not dissimilar to the 2012 Pacific hurricane season, which featured both Tropical Storm Aletta and Hurricane Bud in May, we are once again starting the hurricane season right on cue. What was once Invest 90E intensified into Tropical Depression One-E early this morning and further to Tropical Storm Alvin as of the latest update. The most recent information from the National Hurricane Center states maximum sustained winds have increased to 40 mph, and the minimum barometric pressure has fallen to 1005 millibars. Alvin is tracking west-northwest at 13 mph. Visible satellite loops reveal a well-developed and intensifying tropical cyclone, with increasing spiral banding in all semicircles and bursting convection near the low-level center. Despite a bit of dry air around the tropical cyclone, Alvin remains further within a well-developed moisture bubble and is tracking within a moisture belt which should help to aid development and prevent many dry air intrusions. The latest satellite intensity estimates from SAB and UW-CIMSS-ADT remain at T2.0/30 knots and T2.5/35 knots, respectively. A recent 1612 UTC ASCAT pass revealed a closed circulation with 35 mph winds in the northern semicircle. Using a blend of the above, I would not have upgraded the system quite yet, but it makes little difference when considering the cyclone likely would have met the criteria of a tropical storm by the next update time.
Figure 1. MODIS image of Alvin taken earlier this afternoon. At the time, it was a tropical depression with winds of 35 mph.
Before getting into specific model forecasts for Alvin, I would like to take the time to commend the CMC model for nailing the development of the tropical cyclone well in advance; over a week out, specifically. The GFS model only caught on about four days ago, and the ECMWF model does not yet acknowledge Alvin as a system. One of the main fall-backs of the European model is its tendency to "miss" tropical cyclogenesis, however.
There continues to be huge discrepancy in the forecast track and intensity forecast of Tropical Storm Alvin. The 12z run of the GFS had a fairly decent initialization, with a pressure of 1007 millibars, two closed isobars, and winds of roughly 30 knots. By 24 hours out, the system is at, or near, hurricane intensity with a minimum barometric pressure of 988 millibars. By late Friday evening, Alvin attains its peak intensity with a minimum barometric pressure of 983 millibars. Its track solution has not changed since yesterday evening, with the tropical cyclone headed west-northwest for 3-4 days. After that point, the cyclone turns to the north and eventually north-northeast while slowly weakening over cooler waters and a more stable environment.
The CMC remains the outlier. Not only is the model the weakest, with Alvin only peaking as a mid-grade tropical storm, but it is also one of the only models to show a west-northwest track and eventually a westward turn as the cyclone gets "stuck" under the southeastern periphery of the large ridge of high pressure to its northwest, over the northeastern Pacific. Unlike the GFS, the CMC does not amplify an upcoming trough over the West Coast; therefore, the eastern side of the ridge is not eroded, and no weakness for the cyclone to escape is made. This should be discounted for the time being given the strong model support towards the other solution.
On a brief note again, the HWRF and GFDL remain very bullish with the intensity of Alvin. The 12z HWRF showed intensification into a minimal hurricane by 24 hours, a Category 2 hurricane by 36 hours, and a peak intensity of 93.9 knots/965.1 millibars by 54 hours. The 12z GFDL showed intensification into a minimal hurricane by 30 hours, a high-end Category 2 hurricane by 54 hours, and a peak intensity equivalent to a major hurricane by 66 hours, at 97.1 knots/959.1 millibars. Both models show the northward turn of the cyclone, both they are too quick and too strong with the weakness in all likelihood. It should be stated that the GFDL and HWRF have a long history of being extremely inaccurate.
Forecast for Alvin
The intensity forecast for Alvin needs to be adjusted higher than the one from yesterday. The idea that Alvin will track through the Pacific moisture belt and has developed a tight moisture bubble means that dry air intrusions may not be as big of an issue as was thought yesterday. The SHIPS indicates lower than 15 knots of wind shear through 96 hours, and Alvin is forecast to track over sea surface temperatures of at least 28C through that time. Relative humidity values are forecast to continue in the low to mid-70s, sufficient for at least steady intensification of the cyclone. Rapid intensification would be a possible if it were not for low ocean heat content values, at 45 kJcm^-2 and decreasing. My forecast is, as aforementioned, higher than yesterday's and shows Alvin attaining hurricane intensity in 36 hours, with a peak of 80 knots thereafter. By 96 hours, increased wind shear from the trough forecast to be over the West Coast, decreasing sea surface temperatures, and lowering relative humidity values should all lead to steady weakening of the cyclone. Dissipation should occur shortly after the end of the forecast period.
Despite the CMC solution, a majority of the dynamical, statistical, and global models show a continued west-northwest track across the open eastern Pacific through 96 hours, a result of a small subtropical ridge over western Mexico. After 96 hours, a deepening trough—aided by a jet streak forecast to round the base of it—should erode both the eastern periphery of the large ridge over the northern Pacific and the western periphery of the subtropical ridge, creating a weakness over Baja California. For this reason, a turn towards the north is expected at that time; thereafter, the system is forecast to track north-northeastward as the eastern periphery of the ridge builds west of the cyclone and the trough moves through the Colorado Rockies.
INIT 15/2200Z 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 70 KT 80 MPH
72H 18/1800Z 80 KT 90 MPH
96H 19/1800Z 60 KT 70 MPH
120H 20/1800Z 40 KT 45 MPH
I'll have a new blog tomorrow,
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.