147 
fxus63 karx 200812 
afdarx 


Area forecast discussion 
National Weather Service La Crosse WI 
210 am CST Thursday Nov 20 2008 


Short term...today through Saturday night 


Forecast concerns focus on how quickly the clouds will clear 
tonight...and the resulting low temperatures...along with the light snow 
potential Friday night. 


500 mb trough axis and surface low continue to exit east into the New 
England states early this morning...but the cyclonic flow will still 
influence the region through the daytime hours. Current fog/stratus 
satellite imagery indicating a expansive cloud shield across the 
upper Mississippi River valley...but with breaks here and there. 
Enough low level moisture...per relative humidity fields/BUFKIT soundings...and 
hefty 1000-850 mb lapse rates around 9 c/km...for cumulus/SC formation in 
these breaks. Toss in some isentropic upglide along the 280-290 k 
sfcs...and a few flurries should also be possible. Some hints of 
shortwave energy aloft...but Don/T feel the cloud depth will be deep 
enough for snow showers...as an inversion above 800 mb should put a 
lid on things. 


Tonight...an area of high pressure will build in from northern 
plains...with the high moving southeast into the Ohio River valley by 
Friday night. This should lead to a clearing trend from west to east 
tonight...although how quickly these clouds clear is uncertain...and 
have a large bearing on how cold it will get. Looking again at the 
latest fog/stratus imagery point to a broken cloud mass west into the 
high pressure...with more clearing from the high center westward. 
West to east running time/height x-section across the forecast area 
hangs onto a thin layer of moisture sub 850 mb through 
12z...although it also shows considerable subsidence. Also a factor 
for clearing tonight is the amount of diurnally driven cloud 
elements that develop today...as they will quickly diminish this 
evening...leading to large breaks in any lingering cloud cover. An 
additional factor for the overnight lows are the winds. The models 
have slowed down the eastward progression of the ridge a bit...which 
hangs a stronger pressure gradient over at least the eastern 1/2 of 
the forecast area through much of tonight. There should be some 
decoupling of the wind field...although winds are still nearly 
Uni-directional sub 900 mb...with 925 mb winds prognosticated at 15-20 kts 
through 12z Friday. So...what was looking like an ideal radiational 
cooling night looks less so at the moment. Best radiational 
conditions should be west of the Mississippi River...and this is 
where the coldest Friday morning lows //for the most part// should be 
found. 


Both the 20.00z GFS and NAM drive a 500 mb shortwave trough out of 
the Pacific northwest...across the northern plains...and into Iowa by 06z Sat. 
Good qg convergence through the layers with this shortwave 
energy...along with 850 mb warm air advection and isentropic upglide 
on the 280-295 k sfcs. The brunt of the forcing is focused across Iowa 
at this moment...and between 06-12z Sat. The system will bring its 
own saturation with it...per the relative humidity fields...so would seem to have 
plenty going for it for some precipitation chances. However...this system is 
trying to work into the high pressure that moves in on Friday...and 
both the GFS and NAM weaken the shortwave the farther 
east/southeast it moves. Looking at the resulting quantitative precipitation forecast...trends are 
to diminish the precipitation as it moves out of the northern plains...with 
the GFS and European model (ecmwf) hanging on to some light quantitative precipitation forecast for Iowa overnight 
Friday...while the NAM and sref diminish it completely. Overall...the 
biggest impact locally may be in the form of clouds...but still 
enough forcing to continue small probability of precipitation for northeast Iowa overnight 
Friday. 


Long term...Sunday through Wednesday 


Decent agreement between the European model (ecmwf) and GFS with the overall upper 
level pattern...deepening a 500 mb trough as it drops across the 
Great Lake states on Monday...meanwhile amplifying a ridge to the 
west. At the surface...an associated cold front will slide across the 
region during the day on sun...with neither the GFS nor ec 
developing much precipitation along this boundary until its well to the east. 
They both generate some quantitative precipitation forecast to the north...near and north of the surface 
low. Chances increase as the surface low departs...and areas of light 
snow/snow showers develop in the wrap around area of the low. A surface 
high will then build in for tues/Wed. However...one difference with 
the models is that the 19.12z ec drives a rather vigorous shortwave 
from the West Coast and into the ridge Mon/Tue...then spinning it 
across the upper Mississippi River valley on Wednesday. The GFS has a much 
weaker wave...if at all. This is a new feature for the ec...as it 
has been similar to the consistent GFS over the past few runs. The 
ensembles have a mix of both solutions...lending to decreased 
forecast confidence. That said...the ec does take the shortwave 
through the surface high...and produces little if any precipitation...so it could 
be a non-issue. The 20.00z also now points to a much weaker 
shortwave...more like the GFS. Will stay dry for now. 


&& 


Aviation... 


A weak shortwave trough will drop southeast across the area this 
morning...while cyclonic flow remains in place throughout the day. 
Rather uniform ceilings expected through much of the period with 
generally VFR conditions...although some MVFR ceilings likely 
through about 16z today at krst. 


High pressure begins to build westward tonight from the plains...but 
ridge axis remains west of the area. Question will be if any 
clearing can occur. Models showing strong subsidence...but some 
moisture remaining in the lower layers. Did go with a scattered deck 
by late afternoon/evening...but confidence low on clearing this 
quickly. 


Raberding 


&& 


Arx watches/warnings/advisories... 
WI...none. 
Minnesota...none. 
Iowa...none. 
&& 


$$ 


Rieck