TROPICAL CYCLONE FORMATION: MEDIUM (50%)
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(T. F. “Storm” Walsh III)
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The area of disturbed weather I have been monitoring just north of the Bahamas, was designated INVEST 91L at 12Z this morning. The NHC has now designated a MEDIUM (50%) probability of formation during the next 5 days.
Based on information contained in the 18Z ATCF BTK product indicated the following on INVEST 91L :
Based on the current steering layer mean, and extrapolating the previous position data from ATCF, a mean motion of NNE 3-5 mph was noted. Based on the current steering product however, it does appear that steering is beginning to collapse. I would have to agree on this, as analysis of the current steering layers forecast maps from PSU e-WALL, valid for 00Z this evening, indicate the steering pattern to change to a east-west flow in approximately 18-24 hours. This would allow INVEST 91L to slow down tonight, and eventually begin more of a track toward the west. By late evening on Thursday, or early on Fri, forecast steering indicates by the setup (orientation of the ridging), a more WNW track may be in order. This should be considered preliminary, as the global models do not have a well defined surface circulation to initialize as of yet.
As far as the global models, each has their own solution with the GFS leaving this an open trof, passing over FL. in about 5-6 days, the ECMWF bring a sub-tropical or tropical depression close to the SC/GA coast, then dissipating it, and the CMC GGEM bring a 1016 mb low over SC. Again, based on the lack of a well defined surface circulation at the moment, my decision is to wait and see what type of development may occur, and if the upper level pattern changes to a more favorable scenario during the next 3-4 days, based on the shear forecast.
The following track guidance is not to be taken as gospel. This will most likely change, once 91L becomes better organized (if and when). Most of these are STATISTICAL MODELS. We go by the DYNAMIC model guidance, which hasn’t materialized as of yet.
Analysis of the recent wind shear product indicates at the moment, 91L is under 35-40 knots of SWLY wind shear.
The current wind shear forecast from the GFS zonal shear map does indicate zero zonal shear IVO the forecast position of 91L in about 72-84 hours. Information from the NOAA HFIP model page, with the GFS solution, indicates wind shear to fall to 17 knots in 48 hours over 91L. So, this may be when we begin to see better organization.
The only negating factors I see at the moment having analyzed various forecast tools is, although the TPW in the area is moderate to high, there is dry air in front of 91L. In fact, in the visible and RGB satellite loop imagery, some very small outflow boundaries were detected, which may indicate some very slight dry air intrusion. The current 200 mb streamline forecast, even though wind shear is forecast to relax to favorable levels briefly, is not optimal for evacuation of the system in the upper levels of the atmosphere. It’s not impossible for development with that setup, it’s just very, very difficult. The last item is seas surface temperature. Based on the current path projection and SST’s, 91L will be traversing “minimal” supporting SST’s of 26.0-26.3C as it moves further WNW. Based on analysis of everything, this “could” possibly have the potential to become a minimal sub-tropical or tropical storm, however at the moment, the forecast conditions only support what the models indicate right now, a sub-tropical or tropical depression. If and when development occurs, I’ll have better data to work with, and will be able to determine if the system is warm core or not.
The following is the initial forecast intensity guidance. The three most widely looked at models are the SHIP, DSHIP, and LGEM
Elsewhere, Tropical Storm formation is not expected during the next 5-7 days. I won’t be available tomorrow through Saturday due to work. My next update will be Sunday, if needed.
Have a blessed day!
T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST /SEVERE WEATHER SPECIALIST
MEMBER WEST CENTRAL FLORIDA AMS