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(T. F. “Storm” Walsh III)
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STORM W’s SEASONAL HURRICANE FORECAST
TOTAL STORMS: 14-16
MAJ. HURRICANES: 3-4
MAJ. HURRICANES: 0
U.S. LANDFALLS: 1
Good day all!
At 8:00 a.m. this morning INVEST 95L was located approximately 750 miles east of the Windward Islands. The NHC has designated a MEDIUM (50%) probability of cyclone development over the next 5 days. Another strong Tropical Wave was located approximately 800 miles WSW of the Cabo Verde Islands. The NHC has designated a LOW (30%) probability of cyclone development during the next 5 days.
As of the 1200Z ATCF BTK product, the following was available on INVEST 95L:
8:00 AM AST Mon Jul 17
MOVEMENT: W 17 mph
PRESSURE: 1011 mb / 29.85 in
MAX SUSTAINED WIND: 30 mph
Based on my analysis of the latest available current wind shear product from CIMSS, INVEST 95L is in an area where wind shear values are relatively weak. Albeit the pattern is currently lacking an upper level anti-cyclone, conditions are marginal to somewhat favorable, as an upper level low positioned N of the wave, is aiding in evacuating air in the upper levels. A somewhat diffluent pattern is noted.
CIMSS WIND SHEAR MAP FOR 95L
Based on the current wind shear forecast from the GFS, upper level winds may become a little more conducive during the next 24 hours or so, developing somewhat of a small, upper level anticyclone over the wave as it approaches the Windward Islands. Based on this, there may be a small window of opportunity for some slow development over the next 36-42 hours, as the upper level winds remain somewhat favorable. Past 42 hours, INVEST 95L should begin to experience increasing westerly wind shear as it enters the Caribbean Sea. One item which could act as a negative is, should this develop and become stronger, it stands there is a high probability it could entrain the dry air to its north.
GFS WIND SHEAR FORECAST (BOTH AREAS OF INTEREST)
CIMSS SAL MAP
The GFS and ECMWF do not develop this wave, with the CMC still indicating a very small area of tropical storm force winds as it enters the southern Caribbean. Model Intensity guidance (SHIP, DSHP and LGEM) bring INVEST 95L to Tropical Storm status in 24-36 hours. Based on my analysis, I am having the tendency to believe this system may struggle to even make it to depression status, but we’ll see how future conditions pan out. Both track and intensity guidance at this time, should be considered preliminary at best, until models have a better defined system to work with.
12Z ATCF INTENSITY GUIDANCE
INVEST 95L is currently south of the subtropical ridge which is moderately strong at the moment, and is in a strong westerly flow, which is allowing the westerly motion. I expect this motion to continue during the next 24-36 hours, with a possibility of the INVEST gaining somewhat of a small northward component due to a current weakness in the ridge located around the western periphery of the ridge. Soon thereafter, the ridge axis shifts more toward the west, and the pattern takes on a more negative NAO type pattern, with a ridge building in north of Puerto Rico, and another center more toward the Eastern Atlantic. The resultant flow SHOULD continue to send the system westward, and is most likely what current model guidance is picking up on. As I stated, I believe the wave could lift slightly more northward over the next 24-36 hours, but then resume a more westerly component. The ECMWF EPS, GEFS, and CANADIAN ENSEMBLE members all indicate this, which is also reflected in the dynamic model guidance.
CURRENT LAYER MEAN STEERING
DYNAMIC MODEL TRACK GUIDANCE
Elsewhere,the strong tropical wave east of INVEST 95L appears to be somewhat better organized.
This wave may have a somewhat better chance at slow development, as an upper level anti-cyclone is forecast to be near or over the wave in about 24 hours, and upper level conditions could be favorable for about 72 hours. However, based on the forecast steering pattern, as the ridging and trof pattern change over the course of the next 4-5 days, this wave will most likely head for a weakness that develops in the sub-tropical ridge. In fact, given its location, relative to the current weakness, it may begin to take on a WNW motion. As the forecast ridge/trof pattern evolves, a weakness develops between the ridge axis south of Bermuda, and the ridge axis closer to 35N;40W, and the system should move toward this weakness. Once again, the same fate may lay in wait for this wave, in that should it strengthen enough, it could draw in the drier air north of it, as it heads toward the drier air.
In any case, I will continue to monitor the two areas for any significant changes.
My overall assessment at the moment regarding the Tropical Atlantic, is even though wind shear is not a big factor at the moment, the Sub-tropical ridge is fairly strong, or stronger than the average. One way to assess this is by looking at the NAO. During a fairly strong positive NAO, the Azores-Bermuda ridge becomes stronger, as does the Icelandic low. The NAO pretty much graphs this in that the pressure difference, or strength of both features is what defines the NAO. In a positive NAO, the ridge and low are both stronger, while the opposite occurs in a negative NAO situation. So, right now, the easterly trades are stronger than normal, which is why we are seeing the increase in SAL. Second, the strong high is creating subsidence (sinking motion) over the central and eastern Atlantic. This allows for the surrounding atmosphere to warm and dry out the air. Based on the current NAO forecast, we could see improving conditions beginning around the 21st of the month.
NAO (North Atlantic Oscillation)
Here is a shot of what is happening over Africa:
Elsewhere, Tropical Storm formation is not expected during the next 5 – 7 days.
Have a blessed day!
T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST /SEVERE WEATHER SPECIALIST
MEMBER WEST CENTRAL FLORIDA AMS