TROPICAL CYCLONE FORMATION: INVEST 99L (40%) / T.S. FIONA / EATL DIST (90%)
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(T. F. “Storm” Walsh III)
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CURRENT 2016 ATLANTIC TROPICAL CYCLONE TOTALS:
TOTAL STORMS: 6
INTENSE HURRICANES: 0
StormW’s Seasonal Hurricane Forecast:
TOTAL STORMS: 13-15
INTENSE HURRICANES: 3-4
Good evening everyone!
This evenings forecast will focus on Tropical Disturbance INVEST 99L, as Tropical Storm Fiona continues to move to the WNW. One thing about Fiona however is, the forecast remnant low may stall in 4 days and meander for a couple days, as steering currents become weak. Based on analysis of the updated steering forecast maps, Fiona should slowly re-curve toward the north by days 6-7 as steering around the western periphery of a ridge center located near 40N;45W strengthens by then. The EATL disturbance (now designated INVEST 90L) that exited the African coast, has been given a HIGH 90% probability of cyclone formation by the NHC in the 2:00 p.m. 5 day Graphical Tropical Weather Outlook. Based on analysis of forecast steering, I expect this system to track toward the WNW during the next 36-48 hours, with a turn more toward the NW thereafter. It is suggested by the forecast steering currents maps, that this may not re-curve right away as thought, as by day 6, forecast steering indicates a small ridge to build back in over the system, giving it a brief jog toward the WNW to W. Global models at this time, indicate the possibility of this system making it close to 60W before moving due north. It is suggested that 90L will become a hurricane in 4 days, as conditions are forecast to be conducive for development.
NHC 5 DAY GRAPHICAL TROPICAL WEATHER OUTLOOK
Satellite loop images indicate INVEST 99L is looking very ragged. The center has been difficult to located, however the latest ATCF BTK information contained the following on INVEST 99L:
2:00 PM AST Sun Aug 21
Location: 13.9°N 44.7°W
Moving: WNW at 19mph
Min pressure: 1010 mb / 29.83 in
Max sustained: 25 mph
Placing information in the LAT/LONG distance calculator, 99L has been averaging a 19 mph forward speed during the past 12 hours from 06Z to 18Z.
The disturbance has not been fairing well since late yesterday evening, and the NHC has lowered the 5 day probability of cyclone development to MEDIUM (50%). Think back to me always mentioning, how quick conditions in the tropics can change. This would be one of these times. Is it that my forecast was incorrect? I tend to believe not really, as the majority of indicators such as warm SST’s, improving upper pattern, and high TPW pointed toward slow development of this (remember, I believe it was the day before yesterday, and yesterday that the NHC had this at a 60% probability of development. If they had believed development would not occur, they would not have placed the probability that high, which means to me, that I was analyzing the same parameters as they were). So, evidently, it was unforeseen that some of the SAL would be ingested along with some drier air, which has occurred last night and today. 99L did knock out some drier air yesterday, however water vapor imagery shows the moisture shield has been eroded somewhat. It does appear though, 99L is trying hard to push the dry air ahead of itself.
There is one other item I looked at, which was evident in earlier satellite loop imagery, early this morning. Given the proximity of Fiona in relation to the disturbance, it was noted that with Fiona also taking in dry air, her thunderstorms were collapsing. When this occurs, cooler and drier downdrafts (outflow boundaries) are created from the collapse of the convection, and in analysis today out flow boundaries were noted moving right over 99L. So, this may be adding to the struggle of 99L.
At the moment, I am a little perplexed as to what the intensity guidance models are looking at. Based on forecast conditions, I am cautious as to not write this system off yet, as if you remember pre Hurricane Earl, he began pretty much the same way, not really being discernible on satellite, also having a brief problem with dry air and then he hit a more favorable environment. If I remember correctly, when Earl was an INVEST, I believe the models killed him off as well, and then the ECMWF was the only model to initialize him, as a very small area in the 500 mb field. The 18Z intensity guidance has backed off slightly, however the most widely used intensity guidance models still indicate 99L to become a Hurricane (CAT1) in 4-5 days, and the LGEM a low to mid end Tropical Storm. Having tossed this around in my mind, I don’t know if this is because there are 3 areas models are looking at or there is the lack of an LLC to track? However, given the consistency of the intensity guidance indicating development, the case may be, given the current and forecast wind shear products, which indicated an upper level anticyclone becoming better established near the reported center, and the premise of this upper level feature remaining with the INVEST, the models may be looking at this, the presence of still high TPW, and the fact the current dynamic model guidance bringing 99L into higher Ocean Heat Content (OHC). These conditions, should they prevail, should be enough for 99L to eventually mix out any dry air, as the higher OHC would help with an increase of moisture. Based on these factors, I cannot say definitely at the moment, that 99L will develop, however I cannot write this off if the forecast conditions are correct. In fact, the global models do indicate MSLPA falls over the Bahamas in about 7 days.
GFS AND ECMWF MSLPA (Mean Sea Level Pressure Anomalies)
The 18Z dynamic guidance still indicates 99L to enter the Caribbean based on the TVCN/TVCC consensus models. I prefer this forecast, however again, both intensity and track guidance are to be considered preliminary, as there is still no closed LLC (Low Level Circulation) for the models to initialize. The global models and other dynamic models move this north of the Caribbean. Based on analysis of the MSLPA, it appears whatever happens to this system, could visit the Bahamas.
Right now, the best solution on this, is to continue to monitor the situation for any significant changes, as we have just witnessed how fast changes can occur.
Elsewhere, I still believe a decent increase in activity may be in order, as the NASA GEOS model still calls for moisture surges in TPW to continue to come off Africa in days 3, 5, and 8. In addition to this, the forecast appears to call for the MJO to be over a portion of the EATL and over Africa beginning very soon, through Sep. 04. Analysis of the 200 mb velocity potential anomalies also indicates we may see favorable conditions by the 26th of this month, with upward motion (divergence) at the 200 mb level, noted by the blue contours.
Have a blessed evening!
T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST /SEVERE WEATHER SPECIALIST
MEMBER WEST CENTRAL FLORIDA AMS