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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
Good day all!
I guess by now, you’re probably tired of hearing the same old talk. Not much change in the picture over the past 2-3 days. Satellite imagery pretty much looks the same as it did yesterday. Really nothing worth while to speak of regarding current disturbed areas and potential for development, albeit I will be vigilant, as we have seen how quickly conditions can change.
The NHC has included a LOW (20%) probability area in the far eastern Atlantic, off the coast of Africa, for cyclone development over the next 5 days. A tropical wave over the continent is supposed to enter the Atlantic by tomorrow.
Based on the location of the probability area, this wave appears to be at the lower latitude of near 8.0N;7.0W, which is SSE of the current wave sitting on the coast near 14.0N;15.0W. I have an arrow on the NRL Monterey satellite map pointing to the wave the NHC is speaking of.
NRL MONTEREY SATELLITE IMAGE
Based on analysis of the current wind shear forecast from the GFS, this MAY have a probability of acquiring some decent, slow organization by day 5, as wind shear is forecast to relax over the area indicated. Wind shear, or upper level winds could remain somewhat favorable as it moves westward, however based on the wind shear forecast at the moment, will most likely run into increasing shear as it nears 60W. Even though shear may relax for that brief period, one negating factor is positive 200 mb vertical velocities over the area. Maps are posted in a discussion at the end of the synopsis.
Based on analysis of the global models this morning, strong development is not expected, and models right now on the 00Z run (ECMWF) and 06Z run (GFS) dissipate this wave. I will be monitoring this feature when I can during the remainder of the week, and intend to have an update on late Sunday. I will not be available to issue updates for the remainder of the week due to work.
For the time being, as I’ve stated, it looks as if we may have somewhat of a quiet period until near the middle portion of July. I am basing this on analysis of the current MJO forecast, which the trend is indicating a continued eastward movement into phase 2 by mid Jul., wind shear forecast, RH values at 850 and 700 mb, and an increase in the AEJ (African Easterly Jet). The one item which may be a negating factor once again, is the 200 mb vertical velocities which still indicate sinking air over the area at that time. However, we’ll have to see if that changes, as the vertical velocities forecast doesn’t correlate with the MJO forecast. I’m going to try and explain some these maps, pretty much in layperson terms, as I know there are some folks that read these synopses whom don’t fully understand the maps. I DO understand that some of it can be confusing, so hopefully this will sort of help you out.
MJO (Madden Julian Oscillation):
In simple terms, the MJO forecast map shows where the MJO upward motion or rising air (BLUE SHADING) and downward motion or sinking air (YELLOWS AND REDS) is forecast to be within the specified forecast periods posted above each graphic, to the right. In general, the way it works is, the blue areas indicate where conditions are deemed more favorable for an increase in tropical rainfall (enhanced) and the yellows and reds indicating conditions are less favorable for an increase in tropical rainfall (suppressed). The blue areas indicate upward motion or rising air, which is needed for the formation of rain clouds. Click on the image for a good explanation on this feature:
You almost ALWAYS here me mention current and forecast wind shear. Wind shear is a difference in wind speed and/or direction with height over a relatively short distance in the atmosphere. IRT to tropical systems, westerly wind shear (coming from the west) will shear thunderstorm tops from the system. It also spreads the latent heat energy over a much larger area, which zaps the energy from a tropical storm or hurricane. Shear which is coming from the east (easterly shear) which is common off the coast of Africa, isn’t too detrimental to a developing system, however it still becomes a challenge for a storm to develop. You often hear me speak of an upper level anticyclone. In simple terms, this is a high pressure area, or clockwise flow above the storm at around 250 – 200 mb level. This evacuates the warm air and moisture that rises to the top of the storm, which allows for more convergence at the surface, thereby lowering the pressure. Since this is somewhat a complicated process, if you look at the wind shear maps I post, a simple analogy will help you in whether a wave or disturbance may organize and develop. Blues to very light green, just like a traffic signal, pretty much indicate GO! Yellows, reds, and pinks pretty much indicate STOP! Click on the following wind shear forecast map to access the looping properties.
GFS SHEAR FORECAST DAYS 3, 5, AND 10
AEJ (African Easterly Jet)
The AEJ plays a big role in the formation of Tropical Waves over the African Continent. As a general rule of thumb, African waves become more prevalent when there is a strengthening of the AEJ. The AEJ is located in the atmosphere between 700 – 600 mb level in the atmosphere. Since we don’t commonly utilize the 600 mb map in models, the AEJ can be found in the 700 mb forecast maps. Click the first graphic for a more detailed definition regarding the AEJ
The last map I’ll touch on is, the 200 mb vertical velocity forecast map. This pretty much ties in hand in hand with the MJO. The blue colors represent upward vertical motion in the 200 mb level of the atmosphere, and divergence aloft NEGATIVE VERTICAL VELOCITIES). You need divergence aloft as it helps to create convergence at the surface, which is needed for a tropical system to organize and maintain itself. This is how a convective system works…convergence at the surface, which allows warm air and moisture to rise up to the upper levels of the atmosphere, where divergence, or a spreading out of this air occurs. This is what we term as a convective current. The red and brown colors are indicative of downward vertical motion, or “sinking” air (convergence aloft…POSITIVE VERTICAL VELOCITIES). This tends not to be very conducive for tropical development, as it has the tendency to create a more stable environment in the mid to lower atmosphere (which can promote drier air). The dry air is caused, as when air sinks from the upper levels, it compresses, which warms the surrounding atmosphere, which in turn makes the surrounding air drier. Hence, no convection. This is a small abstract from an article written by former NHC forecaster Stacy Stewart:
TC-genesis is most favorable in areas of negative 200 mb Velocity Potential Anomalies (VPA) where westerly winds have shifted or begun to shift to an easterly direction, thus lessening the vertical shear.
GFS 200 MB VELOCITY POTENTIAL ANOMALY FORECAST DAYS 5, 7, 10 AND 14
Elsewhere, Tropical Storm formation is not expected during the next 7-10 days.
Have a blessed day!
T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST /SEVERE WEATHER SPECIALIST
MEMBER WEST CENTRAL FLORIDA AMS