871 
fxus63 kmkx 212117 
afdmkx 


Area forecast discussion 
National Weather Service Milwaukee/Sullivan WI 
315 PM CST Friday Nov 21 2008 


Short term...tonight through Monday. 
Immediate forecast concern focuses on threat of light snow Saturday 
morning. GFS showing better run to run consistency in regards to 
compact short wave trough currently over the Dakotas...when compared 
to latest NAM guidance...however differences rather small. All 
short term guidance including mesoscale models weaken Dakota wave as 
it progresses southeast across Iowa/Minnesota later tonight. It continues 
southeast track into Illinois on Saturday with further weakening into 
upper level confluence zone. Time height cross section shows low 
levels remain dry with a failure to saturate over southern WI. Yet brief 
period of impressive layer q-vector convergence skims my southwest. 
Q-vectors at the upper levels as strong as 30 units. Even more 
impressive is the southwest also gets brushed by an area of 
impressive isentropic Omega depicted on the 285 Theta surface Sat 
morning. Condensation pressure deficits lower to less than 10mb on 
this level with brief period of cross isobar flow of 20 to 30kts as 
it skims the southwest. -Sn could work its way to the surface 
during stronger forcing Sat morning...so added low probability of precipitation in my southwest 
per coordination with dvn/arx. WRF-4g...hi res European model (ecmwf) as well as cras 
paint out light quantitative precipitation forecast in my far southwest. 


Leaning toward NAM and GFS overdoing amount of low level moisture 
lingering over upper Midwest in wake of weakening short wave 
Saturday afternoon and night...however some clouds may get caught in 
strengthening inversion due to warm air advection Sat night. At 
this point...will go with p/c wording. Milder day anticipated on 
Sunday with 850 mb temperatures warming above zero. Deepening moisture and 
short wave trough looking more likely to bring areas of rain and 
snow to southern Wisconsin Sun night with lingering -shsn into Monday/Monday 
night as short waves rotate around amplifying upper low over gtlakes. 
Low level thicknesses favor potential for rain and snow sun evening 
before cooling later in the night. 


Long term...Monday night through Friday. 
Considerable agreement in extended guidance including hi res 
European model (ecmwf)...GFS...Canadian and ukmt at the beginning of the period in 
deepening upper low becoming cutoff over eastern gtlakes...and then 
meandering northeast or remaining nearly stationary for the later 
periods. Compared to previous model runs...appears long wave 
troffing and upper low slower moving and more persistent Wednesday and 
Thursday. This scenario would keep upstream ridging and mainly dry 
weather across western gtlakes through Thanksgiving. Latest medium 
range ensemble percent of measureable precipitation based on GFS 
showing less than 10 percent chance through Thursday. So after 
small threat of -shsn Monday night on backside of amplifying low pressure 
will continue dry forecast through Thursday. Southern stream of split flow 
more active later next week and not impossible system passing to the 
south on Friday could take a more northerly track considering 
upstream developing short wave in south central Canada and potential 
for some phasing. For now will go with dry wording on Friday but 
uncertainty quite low as ensemble members showing alot of 
variability for this system. Will need to watch this closely. 


Day 11 of 5 day 500mb means from GFS valid 00z/02 shows upstream 
positive height anomoly weakening with more unsettled west to 
northwest flow over WI by first days of December. 


&& 


Aviation...brisk west winds on north side of polar high should 
diminish quickly late this afternoon with sunset. Potent upper 
disturbance producing heavy snow in western Dakotas expected to turn 
east-southeast and brush southern Wisconsin tomorrow. Bone dry air 
in low levels...but strong ascent shown around 10-15k feet above 
ground. Will likely be virga with the snow struggling to make it 
through the dry lower levels. At this time expect the snow reaching 
the ground to remain south of the taf sites...with VFR broken to 
overcast middle layer clouds. 


&& 
Mkx watches/warnings/advisories... 
WI...none. 
Lm...none. 
&& 


$$ 


Short/long term...mbk 
aviation/marine...jpc