908 
fxus63 klmk 210152 
afdlmk 


Area forecast discussion 
National Weather Service Louisville Kentucky 
952 PM EDT Sat Mar 20 2010 


..updated aviation discussion... 


..Forecast update... 


Low pressure developing across central Arkansas at this time as 
upper level low...noted on latest water vapor imagery...spins over 
the Red River valley. Will see increasing middle to high clouds 
overnight. Eastern zones starting out with less clouds than forecast 
this evening...so have made a switch in wording from mostly cloudy 
to increasing cloudiness as those middle and high clouds build in. 
Farther west...expect just enough deep moisture to see some 
sprinkles develop around daybreak west of I-65. Only other change 
were some small tweaks to temperature forecast...which were mostly 
transparent in the updated zones already issued. 


Short term (tonight and sunday)... 


Afternoon satellite imagery showing partly to mostly cloudy skies 
across the region on this first day of Spring. Afternoon 
temperatures have continued to warm with readings in the middle to 
upper 60s in southern Indiana and central Kentucky. So far..only 
Evansville (kevv) has made it past 70. While initial low stratus 
field broke up a bit earlier...extensive cumulus field has redeveloped 
and put a damper on solar insolation. So feel that maximum highs today 
will likely be the current temperatures or just a degree or two more. 


Latest suite of numerical model simulations continue to support a 
much slower eastward progression of the upper level trough over the 
plains and associated surface low and cold front. Currently...little if 
any of the model data support precipitation chances at all during 
the overnight hours today. Therefore...have gone ahead and trended 
the forecast to one that is dry for the overnight with mostly cloudy 
skies. Deep south to southwest planetary boundary layer flow will result in mild 
overnight low readings ranging from the middle 40s in the northern 
sections of our southern Indiana counties with upper 40s to around 
50 in our central Kentucky regions. 


Model solutions all agree that upper level trough will close off with 
a 500mb low cutting off across the Red River valley of OK/TX. This 
feature is then forecast to slowly roll to the east while surface 
cyclogenesis occurs over central Arkansas during the day on Sunday. 
As the surface low heads off to the northeast...we should see an 
increasing and widespread rain shield push up from the southwest by 
middle afternoon and overspread the entire forecast area by evening. 
With that said...will be allowing probability of precipitation to increase throughout the 
day culminating to 100 percent by evening. At this time...best 
instability and surface heating will primarily be limited to areas near 
the Gulf Coast as return flow moisture this far north seems very 
unlikely. However...model solutions continue to show some very weak 
elevated instability and with increasing colder air coming in 
aloft...a few rumbles of thunder are very possible Sunday afternoon 
and evening. Thus...we will continue to mention isolated 
thunderstorms in this forecast package. Temperatures for Sunday 
will be cooler than today due to cloud cover and expected rainfall. 
Highs are expected to warm into the upper 50s to the lower 60s. 


Long term (sunday night - saturday)... 


For the most part will use a blend of the GFS/gefs/Gem ensemble 
mean...which are in fair agreement at least through midweek. 


Cold upper low over northwest Mississippi at the beginning of the 
period will advance to the Carolinas by Monday evening and then head 
off the East Coast Tuesday morning. At the surface low pressure will 
move from the Land Between The Lakes to eastern Kentucky Monday 
morning to the Mason Dixon line Tuesday morning. The operational GFS 
aggressively dry slots US Sunday night...but this seems suspect at 
this time. Will go ahead and continue high probability of precipitation for Sunday night. 
Rain will then continue Monday before tapering off from west to east 
Monday night into early Tuesday morning. 


Despite it being Spring...there are some questions regarding 
precipitation type with this system. 1000-500hpa thicknesses do drop 
below the old 540 dam benchmark over most of the County Warning Area Sunday night 
through Monday. However looking at soundings it seems that the very 
great majority of the precipitation should be rain. The best chance 
of seeing any wet snowflakes mix in with the rain will be in the 
Lake Cumberland region around dawn Monday morning. Even there the 
chances are small and surface temperatures at sunrise should be in 
the middle 30s. Thanks to jkl for coordinate. 


Weak ridging at the surface and aloft will work in to give US dry 
weather Tuesday afternoon and night. A very weak cold front will 
make a run at US...slipping out of the Midwest Tuesday and into the 
Ohio and Tennessee River valleys Wednesday. Frontolysis will occur 
with the already weak front as it crosses the lmk County warning forecast area. An afternoon 
shower could potentially pop up somewhere...especially in southern 
Kentucky...but coverage should be less than 15% so will keep dry 
weather in the forecast. 


On Thursday it appears a storm system will take shape over the 
Southern Plains and head east...moving off the southeast U.S. Coast 
Saturday morning. However there is precious little agreement among 
models and even among perturbations of the ensembles...leading to 
rather washed out ensemble means. Mref plumes show a large 
spread...though there is a general signal that the best chances for 
rain will be Friday/Friday night. With such low overall confidence 
though probability of precipitation will need to be kept low. 


Temperatures this week won't stray too far from normal...other than 
on Monday as that cold upper low moves overhead and keeps temperatures 
colder than normal. 


&& 


Aviation (00z taf issuance)... 


VFR conditions expected for most of the taf period as a scattered to 
broken middle deck continues through Sunday morning. Low level moisture 
will increase as a surface low approaches the area. Light rains will 
start ahead of this low for the taf sites around noon...with 
worsening conditions through the afternoon. 


&& 


Lmk watches/warnings/advisories... 
Kentucky...none. 
In...none. 
&& 


$$ 


Update.........Rjs 
short term.....Mj 
long term......13 
aviation.......rjs