242 
fxus62 kmfl 231754 aaa 
afdmfl 


Area forecast discussion...updated 
National Weather Service Miami Florida 
1254 PM EST Monday Nov 23 2009 


Aviation...generally VFR conds to continue under fast moving 
cirrus canopy around 25 thousand feet. Weak nearly stationary front 
generally across central Florida but is diffuse with a very light 
surface wind pattern over Mainland South Florida. Sea breezes 
still expected at airfields on both Atlantic and Gulf coasts but 
winds will be light and variable once again this evening through 
tonight. Isolated rain showers expected especially around Lake Okeechobee 
possibly affecting kpbi and kapf this afternoon but briefly. 


&& 


Previous discussion... /issued 1000 am EST Monday Nov 23 2009/ 


Update...will make slight changes to the current forecast package and 
mainly to increase cloud cover to the northern portions of South 
Florida where mostly middle/high level clouds will keep overcast over 
much of the period today and this evening. This may contribute to 
keep maximum afternoon temperatures to rise above l/M 80s this afternoon. Will 
expect isolated thunderstorms over lake Okee/Palm Beach region due to proximity 
of the frontal boundary, however will keep only showers southern 
sections. 


Previous discussion... /issued 427 am EST Monday Nov 23 2009/ 


Discussion... 
generally quiet weather conditions persist across South Florida 
this morning...with recent infrared satellite imagery indicating a 
continuation of the thick cirrostratus deck in place over the 
region. Despite the high-level cloud cover...will continue to 
monitor potential fog development toward morning...especially 
interior zones. Short term models are in fairly good agreement on 
depicting a continuation of strong southwesterly flow aloft over 
South Florida for the next couple of days...with a broad 590 dm 
middle-level anticyclone forecast to remain centered north of the 
Greater Antilles...and a strong upper trough expected to dig 
southeastward across the northern rockies...and develop into 
cutoff upper low before shifting eastward across the Central 
Plains later today and tonight. Overall...will follow a 
persistence forecast for today/tonight...with highest probability of precipitation (but 
only low chance) in vicinity of a weak/stalled frontal boundary 
across central Florida. Although most of the precipitation should 
come in the form of showers...would not be entirely surprised if 
an isolated thunderstorm developed. Will also stick to a 
persistence forecast for high temperatures...and lower MOS numbers 
by a 1-3 degrees...based on expected amount of high-level cloud 
cover. 


The forecast becomes a bit more complicated on Tuesday...as all 
available guidance suggests an active subtropical branch of the 
upper level jet will evolve over the area...with several low 
amplitude perturbations noted in middle-level vorticity fields 
extending from the eastern Pacific across central Mexico. Although 
the various global models offer different solutions regarding 
timing and strength of the individual waves as they approach South 
Florida from the west...the general trend of broad/very weak 
surface cyclogenesis somewhere across the Gulf by the middle of 
the week is becoming more clear. This...in combination with strong 
surface low pressure -- associated with Central Plains upper low 
-- moving across the upper Midwest should result in the stalled 
frontal boundary beginning to lift northward late on Tuesday. A 
strengthening low-level warm advection regime will likely be in 
place by Tuesday night...and when coupled with the onset of 
moderate easterly flow at the surface and precipitable waters  rising into the 
1.75-2 inch range...should lead to a fairly widespread coverage of 
shower activity along at least the southeast coast by Wednesday 
morning. 


A trend toward wetter conditions is now being advertised by not 
only the GFS and NAM...but also the more reliable UKMET and 
European model (ecmwf)...which leads to a higher level of confidence that Wednesday 
and Thanksgiving day will feature a fair amount of shower activity 
across South Florida. Rich/deep moisture will be in place during 
this period...with precipitable waters  on forecast soundings expected to be near 
or in excess of 1.75 inches -- well above normal for south 
Florida's dry season. Rain chances may be enhanced by the presence 
of a weak inverted low-level trough...and locally heavy rainfall 
may occur at times...but there is too much uncertainty to address 
this as a substantial hazard at this time. A wave of low pressure 
moving northeastward along this inverted trough on Thanksgiving 
day still appears as if will bring an end to rain chances and open 
the flood gates for a much cooler/drier Canadian air mass to 
spread over the area beginning on Thursday night. Moderate cold 
advection and potential for lingering high clouds will likely keep 
high temperatures in the upper 60s/lower 70s on Friday. Saturday morning 
should be the coolest of the event...with low-level thickness 
values northwest interior ranging from 1350 M (ecmwf) to 1335-1340 
M (gfs). With dew points possibly as low as the middle/upper 30s and 
light winds...this may allow temperatures to fall into the lower 40s near 
the lake on Saturday morning...and perhaps even a few degrees 
cooler in wind sheltered locations. A continuation of dry weather 
and a slow warming trend is expected through end of the weekend. 


Marine... 
generally benign weather conditions are anticipated on the local 
waters for the next couple of days...with a more pronounced 
easterly component to low-level winds becoming established at 5-10 
knots by mid-week. A strong cold front still appears as if it will 
pass across the local area on Thanksgiving day...with increasing 
northwest winds following frontal passage and scec conditions 
likely on Thursday night. Small Craft Advisory for all waters appears very likely 
from Friday into Friday night...with north/northwest winds around 
20 knots resulting in increasingly hazardous marine conditions... 
especially in the Gulf Stream. Winds will veer to northeast and 
slowly subside through the remainder of next weekend...with high 
seas expected to linger in the Gulf Stream. 


Fire weather... 
there are no fire weather concerns through Friday. However... 
depending on amount of diurnal heating on Friday/Saturday...red 
flag conditions will be possible for at least a portion of the 
forecast area on both afternoons...as a dry Canadian air mass 
spreads over the region. A slow modification of air mass and 
gradual increase in low-level moisture is expected through the end 
of next weekend. 


&& 


Preliminary point temps/pops... 
West Palm Beach 83 70 83 71 / 30 20 20 50 
Fort Lauderdale 84 75 84 72 / 20 10 20 60 
Miami 84 74 84 72 / 10 10 20 60 
Naples 84 68 84 69 / 30 10 30 40 


&& 


Mfl watches/warnings/advisories... 
Florida...none. 
Am...none. 
GM...none. 
&& 


$$ 


Long term...47/rhg 
short term/aviation...50/rlp 

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