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fxus63 karx 190801 
afdarx 


Area forecast discussion 
National Weather Service La Crosse WI 
200 am CST Wednesday Nov 19 2008 


Short term...today through Friday night 


Forecast concerns continue to focus on the freezing drizzle 
possibilities this morning...then any linger precipitation chances followed 
by cold air for the next couple days. 


The potential for freezing drizzle/drizzle continues this 
morning...and whether or not it manifests depends on the amount of 
low level saturation. Isentropic upglide continues to be hefty along 
the 280-295 k sfcs though 18z today...as does the 925-850 mb warm 
air advection...so ample low level thermodynamics to fuel precipitation. This 
is all ahead of a cold front...associated with a surface low to the 
north...and a 500 mb shortwave trough currently dropping into 
northern Minnesota. Again though...saturation is the question. A 
north-south running time/height x-section shows an increase in the 
saturation from north to south during the day today...from 850 mb to 
the surface. The saturated layer is rather shallow however...generally 
only around 2 k feet thick. Meanwhile...BUFKIT soundings between 
12-18z this morning...when the thermodynamics are at their 
peak...still point to a near surface dry layer. Current surface dewpoint 
depressions well over 10 degrees for much of the forecast area...so 
considerable moistening of this dry layer will need to be done 
before precipitation can reach the surface. RUC dew point depressions in the 1000-950 mb 
layer show a decrease this morning...but it looks to be too 
little...too late. Feel this near surface/surface dry layer will not be 
overcome by the time the broadscale thermodynamics move east...nor 
is the cloud depth likely to be deep enough across the local area to 
get a drizzle process going. Farther north will have a better 
chance...but feel the probabilities are too low at the moment to 
include in the local forecast. Obviously this will have to be 
monitored closely through the morning hours. 


For the afternoon...the cold front will have slid east into eastern 
WI...with a good amount of cold air advection sub 850 mb Post the 
front...with a strong inversion above that. Better saturation/cold 
air should result in some cold air cumulus/SC flat-topped development. 
Could squeeze out a few flurries where the saturation is 
deepest...in the north...but Don/T feel there will be a snow shower 
threat at this time. 


Tonight trough Thursday...the upper Mississippi River valley will 
stay under the influence of cyclonic flow aloft...as the 500 mb 
trough exits across the New England. Shortwave energy will spin 
across the region on the backside of this trough...while an 
associated surface trough also sinks across the area. The 850 mb to surface 
layer remains relatively moist tonight...but drying in the surface layer 
occurs on Thursday...per the BUFKIT soundings. 1000-850mb lapse rates 
Jump Up to 9.5 c/km for Thursday. So...expect clouds to hang around Wednesday 
night/Thu...with cumulus/SC development in any breaks on Thursday. Some 
flurries will also be possible through this period. 


A surface high looks to build across the upper Mississippi River valley 
on Friday...while a 500 mb shortwave trough drives southeast across 
the northern plains...into Iowa. 19.00z NAM and GFS models runs paint 
some quantitative precipitation forecast with this shortwave across the plains...but diminish it as 
it works into the high. The sref supports this. So...some clouds to 
the west and south with this shortwave...but will keep the local 
area dry for now. 


For temperatures...seasonable readings can be expected today...with 
highs likely in the morning. With the frontal passage by noon for 
much of the forecast area...cold air will flow in behind it... 
leading to more steady...even falling temperatures. The flux of cold air 
will continue for Thursday...with highs struggling to reach freezing. In 
addition...winds will be on the increase tonight and stay up for 
Thursday. Wind chills in the upper teens to 20s should result. Skies will 
start to clear from west to east Thursday night...with decreasing winds. 
This could lead to some chilly Friday morning lows...with single digits 
in some locations. 


Long term...Saturday through Tuesday 


Medium range models showing much better agreement this go 
around...converging on a more joint solution...as opposed to 
variances that were almost 180 degrees out of phase with each other 
previously. Clipper type shortwave trough prognosticated to move east out 
of Canada...strengthening as it spins into the upper Mississippi 
River valley by Sunday night. Better precipitation chances currently look to 
be north of the local area as this system moves through. Inverted 
surface trough Post the associated surface low...along with cyclonic flow 
and some shortwave energy aloft...will continue precipitation chances on the 
backside of the system for Monday...although...better chances look 
to be north and east of the local area. High pressure will then 
build in for tues/Wed. Seasonable temperatures should return for new 
work week. 


&& 


Aviation... 


A surface low was moving into western Minnesota this morning with a 
trailing cold front back into Nebraska. Ahead of this 
system...southerly winds have advected warmer air into the region. 
Biggest question will be the onset of MVFR ceilings at the taf 
sites. Models suggesting that this onset will be Post frontal...with 
frontal passage expected at around 15z at krst and 16z at klse. Thus 
have kept VFR ceilings through middle morning with MVFR ceilings of 
2k-3k feet from late morning through the afternoon. Ceilings are 
expected to lower after 00z Thursday but remain in the MVFR 
category. Rather gusty northwest winds are expected after frontal 
passage with gusts of 24 to 30 knots at both taf sites. 


Raberding 


&& 


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


$$ 


Rieck