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fxus62 kkey 211931 
afdkey 


Area forecast discussion for the Florida Keys 
National Weather Service Key West Florida 
230 PM EST Sat Nov 21 2009 


Discussion...in the middle and upper levels...700-200 mb...latest 
water vapor available satellite imagery overlaid with streamline 
analysis as of 2 PM continue to depict a split flow pattern across 
the Continental U.S.. a 75 to 100 knot southern jet at 250 mb traverses 
sections of southern North America from the Baja California...northern old 
Mexico...extreme south Texas and the Gulf and then streams 
northeastward towards the middle Atlantic coast. A disorganized middle and 
upper low continue to slide northeastward near the Ozarks. The 
Florida Keys are beneath the southern periphery of the wide band of 
these jet stream winds...with spotty cirrus moving well overhead of 
the Keys. 


In the lower to middle levels...surface to 700 mb...latest infrared 
satellite imagery overlaid with available surface and marine 
observations detail as of 200 PM a 1008 mb low over the northwestern 
Gulf of Mexico. Showers and storms continue in the central Gulf well 
in advance of this surface low. A warm front is analyzed passing 
near the Gulf Coast to the east of the low as far as the Florida 
Panhandle...while a cold front swings southwest of the system into 
central Mexico. Closer to the Keys...visible imagery suggests just a few 
stubborn but weak lower level confluent boundaries over The Straits. 


Currently...as of 200 PM...skies are mostly sunny across the 
islands and adjoining waters. Just a few cumulus clouds are left 
over from the late morning confluent boundary that produced a few 
light showers over the Middle Keys. Temperatures have reached the 
lower 80s with dewpoints near 70 degrees. Winds along the Florida 
reef are now east to southeast near 10 knots...and east near 10 miles per hour 
over the Keys island chain. 


Short term (rest of today)...in the middle and upper 
levels...(700-200 mb)...tonight through Monday night...the 12z 
European model (ecmwf)...NAM...and GFS are consistent with indications that the 
weakening upper low now near the Ozarks will eject northeastward as 
it dampens out...with the leftover energy or impulses reaching the 
middle Atlantic coast on Monday. Meanwhile the Keys will remain beneath 
a somewhat stronger than normal southern stream of the jet 
stream...or also on the northwestern periphery of a deep and warm 
core middle and upper near 590 decameters at 500 mb anticyclone 
centered near Hispaniola through Monday night. The result of this 
will be continued middle and upper drying...with only some cirrus 
moving across the Keys from time to time. 


Monday night through Saturday. The next piece of energy dives out of 
Canada into the northern plains Monday night. 12z European model (ecmwf)...NAM...and 
European model (ecmwf) all agree that this system will migrate eastward and amplify 
into a full latitudinal East Coast gyre by Thanksgiving day. As this 
system is carved out on Tuesday and Wednesday...deep middle level 
moisture will be transported northward from the tropics ahead of the 
main trough axis on Tuesday and Wednesday...with latest time height 
forecasts suggesting ample middle level Omega in place over the Keys 
during Wednesday. All of this will help to allow for above and much 
above normal rain chances for the period Tuesday through 
Thanksgiving day. Given the weak upper dynamics...may need to 
include a slight chance for thunder on Wednesday and Wednesday 
night. 


For Thursday through Saturday...much drier and cooler will be 
transported southward to the Keys in the middle levels within large 
and deep cyclonic flow. The main impact of this is that large scale 
rain chances will be in either the very low or nonexistent 
categories. 


In the lower to middle levels...surface to 700 mb...tonight through 
Monday night. All large scale models indicate that the 1008 mb Gulf 
low appears to weaken a bit as it moves northeast during the next 
several days and slides to off of the middle Atlantic coast by Monday 
night. Nevertheless...the latest available 12z NAM and GFS forecast 
soundings illustrate a fairly moist lower troposphere...with 
pwat(precipitable water) remaining between 1.25 and 1.50 inches 
through Monday night. Thus a slight chance of showers will be kept 
across the islands through the short term period...with the best 
chance for showers late at night and early in the morning given the 
lower level confluence flow is always greatest across the Gulf 
Stream. But the bottom line is that sunny...warm and humid weather 
is expected each afternoon with temperatures rising to a few degrees 
above normal maxima...and overnight low temperatures a few degrees 
above minima. 


Tuesday through Thanksgiving day...on Tuesday and Tuesday night 
another low pressure system developing over the Mississippi Valley 
will drag a strong cold front southwest into the western Gulf of 
Mexico. Like the present frontal structure over the Gulf...another 
low pressure area is prognosticated to develop over the western Gulf on 
Tuesday along this frontal system. However...in this case...a 
much deeper lower to middle level moisture profile is indicated in 
advance of this front...with NAM and GFS forecast soundings 
indicating precipitable water (precipitable water) remaining near 2.00 inches 
through Thanksgiving day. By Thursday evening...12z European model (ecmwf) and GFS 
models consistent with indications that weak lower pressure near the 
Bahamas will aid in bringing a strong cold front through the Keys. 


Given that we are still several days away...have only raised rain 
chances up 10 percent on Wednesday and Wednesday night...when the 
deepest moisture and Omega is prognosticated to be over the area. So for 
now will advertise 40 percent for those two periods...which is well 
below the avail mex numerical model guidance for 12 hour probability of precipitation for 
those two periods. After this front pulls through the Keys Thursday 
night...decent 1000-850 mb cold thickness advection may result in 
some light showers...but expect breezy to windy conditions to 
develop across the islands thereafter. 


Extended...Friday and Saturday...much cooler...dry and breezy to 
windy conditions are expected as strong high pressure builds in from 
the northeast. 


&& 


Marine...tonight through Monday night...a gentle east flow will be 
accompanied by just isolated showers later at night and early in the 
morning. 


Tuesday through Thanksgiving day...another area of lower pressure in 
the Gulf will move eastward dragging a cold front southward towards 
central and South Florida. Expect gentle to to moderate winds across 
the waters. Winds and seas will be higher on Tuesday and Wednesday 
within scattered showers. Isolated thunderstorms will be possible on 
Wednesday. Winds will increase and veer to a westerly direction by 
late in the afternoon on Thanksgiving day. There is a good 
possibility regarding the posting of for marine cautionary headlines 
or small craft advisories beginning on Thursday night. 


&& 


Aviation...through 22/18z...prevailing VFR conditions are expected 
at eyw and mth. 


&& 
Preliminary point temps/pops... 
Key West 75 83 74 83 73 / 10 10 20 20 20 
Marathon 75 84 74 84 72 / 10 10 20 20 20 
&& 


Key watches/warnings/advisories... 
Florida...none. 
GM...none. 
&& 


$$ 


Public/marine/grids............futterman 
aviation/nowcasts..............kasper 
data acquisition/upper air.....Ross/fuentes 




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