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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!
Satellite imagery indicates a still quiet tropics this evening. You can note in water vapor loop imagery (2nd satellite loop), the lack of moisture over a majority of the Atlantic wide basin.
I know folks have been wondering what has been going on, as to why the quiet spell. For one, the SAL (Saharan Air Layer) has been a factor. July is the month that contains the most and strongest SAL outbreaks. We saw some early tropical activity in June, as the SAL was below what the average. For July, the SAL soared to above average. The latest SAL and water vapor images seem to indicate the SAL has lessened, and could be making it’s way toward average.
Another culprit, which plays a role in the SAL, is the sub-tropical ridge having shown strong MSLP values, which or the month, have averaged around 1028 – 1030 mb (30.37 in – 30.42 in). The average pressure for the ridge rests at around 1024 mb (30.24 in). When the ridge is as strong as it has been, the winds around the periphery increase. Given the winds come around from the N to ENE on the eastern periphery (over western Africa), dust gets picked up and transported into the Atlantic. The stronger ridge also provides stronger subsidence (sinking air). As air sinks from the mid and upper layers of the atmosphere, it compresses. This compression allows the surrounding atmosphere to warm, which tends to provide for a drier atmosphere. Now, even though wind shear has been running at or below climatology thus far, Vertical Instability over the Tropical Atlantic has been running below climatology. This is caused by the SAL blocking out sunlight from the ocean, and along with the warm SAL layer, it creates an inversion (700-600 mb level), hence, warm air and moisture cease to rise above that point (stable atmosphere). Subsidence also creates a stable environment. Either one of these will create a stable environment. So far, we’ve seen these two phenomenon combined for most of July.
TROPICAL ATLANTIC VERTICAL INSTABILITY
Based on the current NAO forecast from various models, the ridge should begin to weaken, which should allow for conditions to improve over the Atlantic.
Based on analysis of current forecast conditions for the MJO, from Dr. Michael Ventrice, the MJO seems to continue to slowly move toward our side of the world.
REAL-TIME MJO FORECAST
Current forecast 200 CHI anomalies (200 mb Vertical Velocity Potential Anomalies), we may be looking at improved conditions as we near the end of the month, with the forecast indicating upward motion over the Atlantic Basin (indicated by blue/purple contours). Generally, we really don’t have to rely on the MJO as much during Aug. and Sep., as by that time in the season, tropical waves should be strong enough to support themselves within the ITCZ. However, it doesn’t hurt to have a favorable MJO signal over any waves that may exit into the Atlantic.
200 MB VERTICAL VELOCITY POTENTIAL ANOMALIES
Based on my analysis of current satellite imagery from METEOSAT (AFRICA VIEW), there is some activity over the continent. A wave is noted near the African west coast, however forecast conditions are not appearing entirely favorable for the development of this one. Based on global model analysis this afternoon, I believe the wave located much further east may stand a better chance for some development after it enters the Atlantic. You’ll notice in the water vapor image from the SAL site from CIMSS, the wave in question has some fairly high water vapor content at mid levels, and also in the upper level of the atmosphere.
WATER VAPOR IMAGES
In fact, I believe this may be the area that the ECMWF and CMC (the only two models indicating a developing low) are hinting at. The ECMWF for 2 runs now, indicates the possibility of a low trying to develop in about 8 – 10 days from now. The CMC is a little quicker and indicates something in about 5 – 6 days. The UKMET is also onboard with a very weak low. I’d like to see further runs, and continued consistency before jumping onboard, and would prefer to see more model support. However, I am not ready to make fun of the CMC as of yet, as surprisingly, it was the ONLY model to initialized and develop Tropical Storm DON. It is noted by day 5, the AEJ becomes active and shows a fairly strong signature.
ECMWF 500 MB NORMALIZED ANOMALIES
AEJ 120 HOUR FORECAST
Based on the wind shear forecast from the GFS, and should the MJO/200 mb Vertical Velocity Potential anomalies forecast be correct, we could see some slow develop in about 1 week or so. The GFS indicates upper level winds may be somewhat conducive for slow develop by that time in the period
I wanted to throw these graphics in, regarding the MJO. You’ve heard me mention different “phases” or “octants” of the MJO. These graphics should help clarify the octants. You’ll notice, the favorable areas for enhanced convective activity or development for the Atlantic Basin and Caribbean is octants 8 and 1.
MJO ASSOCIATED GRAPHICS (EXAMPLES)
I will be monitoring this situation throughout this week until Wed., and then next week again beginning on Sunday.
Elsewhere…I am not expecting tropical storm development during the next 5-7days.
Have a blessed day!
T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST /SEVERE WEATHER SPECIALIST
MEMBER WEST CENTRAL FLORIDA AMS