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Good day everyone!
I am still monitoring an area of disturbed weather over the Bahamas, which is currently associated with a mid-upper level low.
Analysis of lower level winds, and vorticity maps tend to indicate this area of disturbed weather may be slowly trying to develop a surface reflection. There has been an increase in vorticity at the 925mb level, up through 850 mb. If you notice the yellow arrows I’ve drawn in on the low level steering flow map, you can detect a slight shift in the wind field over the area, with a weak westerly component beginning to take shape. Current surface analysis indicates however, surface pressures are still high over the area.
The current wind shear product from CIMSS indicates wind shear values over the area are only on the order of 5-10 knots, with a decreasing shear tendency over the past 24 hours. However, the flow aloft is counter-clockwise at the moment, which is the only hindrance at the moment.
This area of disturbed weather is located in weak steering currents at the moment. With the current flow, I expect this to pretty much drift toward the NW or NNW during the next 48 hours, with the probability of being shunted back toward the west as a small ridge builds north of the area, as per the current steering layers forecast maps valid for 12Z this morning.
I will continue to monitor this area for any significant changes, and monitor for development of a surface low over the next 72 hours.
Elsewhere, Tropical disturbance INVEST 91L is east of the Cape Verde islands, and is currently moving to the WNW near 15 mph. Information from the ATCF tracking product from 12Z provided the following on INVEST 91L:
Location: 12.5°N… 26.8°W
Moving: WNW at 15 mph
Min pressure: 1009 mb/29.80 in
Max sustained: 30 mph
Based on current and forecast steering maps, I expect this motion to continue pretty much for most of the day, before the system takes a slight bend toward the west, possibly nosing just slightly south, before resuming a WNW motion. In approximately 72 hours from 12Z this morning, 91L should take more of a NW turn, and by 108 hours out in the period, could shift back toward the west briefly, as the subtropical ridge briefly builds in to the north of the system. Thereafter, if everything follows the forecast pattern, the disturbance should begin to feel a weakness in the ridge which is forecast to be well north of the Bahamas, and east of the SC area, well offshore. This SHOULD begin to recurve the disturbance, however this will depend on how long the trof hangs off the U.S. east coast which will induce the weakness. Based on this, I prefer the track of the TVCN/AVNI models in the 12Z dynamic guidance forecast
ATCF 12Z DYNAMIC GUIDANCE
INVEST 91L is currently underneath an upper level anticyclone, which will continue to ventilate the disturbance. The current run of the GFS zonal wind shear forecast indicates upper level winds are forecast to remain conducive for development for at least the next 6 days. (Area near 15N…40W)
The only negating factor I can see at the moment, is dry air ahead of the disturbance. In fact, early RGB and Visible satellite loop imagery indicated some small outflow boundaries, which is indicative of dry air intrusion. RGB and Visible imagery also indicate a stable environment, indicated by the Stratocumulus cloud surrounding the large circulation. Water vapor imagery also shows dry air to the west of the system, in which a dry slot appears as if it could wrap its way in from the west.
Given the disorganized appearance of 91L, and the possibility of dry air continuing to affect it, there are really only 2 scenarios for this…It will either succumb to the dry air and remain weak, traveling further west, with the good probability of dissipating, or fight off the dry air, eventually being able to attain depression status, and continue as shown in the dynamic model guidance / steering forecast maps. The NHC has designated a HIGH (70%) probability of tropical cyclone development over the next 5 days
I will continue to monitor this system, and if time allows, may have another short update on both areas this afternoon. If not, I should have another update in the a.m.
Tropical wave activity has increased slightly this morning, and there are at least 2 areas that will have to be monitored as they exit the African continent.
Albeit we are pretty much in the “climatological” peak of the season, I still feel we may peak just a little later, as I have suggested before. From experience, the season does not always peak at the climatological mark. I have seen it peak 1 week earlier, to 1 week later…which I feel may be the case this season. Some of my reasoning for this is, the current increase of the wave activity over Africa, along with a good probability of the MJO upward motion phase moving into phase 1, and the premise of upper level winds being favorable within the next 10-14 days with very low wind shear values forecast over the MDR, and a possible dive in the NAO to a moderate to strong negative.
ZONAL SHEAR 360 HOURS
Have a blessed day!
T. F. “STORM” WALSH III
GMCS, USCG (ret)
METEOROLOGIST / HURRICANE SPECIALIST / SEVERE WEATHER SPECIALIST
MEMBER WEST CENTRAL FLORIDA AMS
OFFICIAL SKYWARN SPOTTER (ADVANCED)