426 
fxus63 kmkx 230946 
afdmkx 


Area forecast discussion 
National Weather Service Milwaukee/Sullivan WI 
346 am CST sun Nov 23 2008 


Forecast focus on snow for tonight and Monday. 


Latest water vapor imagery indicated a shortwave trough to the 
south of the area...with a weak upstream ridge over the western 
Great Lakes region. Another shortwave trough was located upstream 
of the ridge over eastern Idaho and western Wyoming. At the 
surface...high pressure was located over the Ohio River 
valley...with low pressure developing over the Central High 
plains. 


Today...warm air advection takes place today over the area...with 
1000/500mb thicknesses rising into the lower 540s dkm range. 
Amount of cloud cover will have final say on highs for today. 
Current area of clouds should mix out or move out of the area 
early. Middle to high level clouds...with some embedded lower 
clouds...trying to move east into the area. 


However...drier air may inhibit movement of some of the clouds 
into the area. Will go with partly sunny skies for today. Source 
region of airmass for today from east central Missouri...where 
highs yesterday were in the lower to middle 40s. Will go near these 
values for highs today. 


Tonight and Monday...main issue for forecast will be amount of 
snow with approaching system. Area forecast soundings indicated 
some rain may occur before mixing with and changing to snow from 
west to east across the area...as low levels take a few hours to 
cool below freezing during air column saturation. Will time this 
from the west Sunday evening...and to the east from 06z to 09z 
Monday. 


Prefer the GFS handling of this system over the NAM. Lead vorticity 
maximum from Idaho/Wyoming shortwave trough will move eastward 
today and tonight...affecting southern Wisconsin between 06z and 
12z Monday. During this time...upward vertical motion will be the 
strongest across the area. Strong q vector convergence of 20 to 30 
units seen moving across the area during this time in both the low 
and middle levels. Low and middle level frontogenesis and isentropic 
lift modest at best...but is there. Ingredients method does show 
some middle level qpv corresponding with the vorticity maximum. 
So...some potential for convective banding of the snow in the middle 
levels. Given all of these parameters...will maintain the likely 
wording...and bump up the probability of precipitation some late Sunday night. 


Best upward vertical motion shifts east of the area on Monday. 
Still...vorticity maxima moving through the area with another 
shortwave trough from the northwest should keep snow showers 
chances going for most of the day. 


GFS had mainly between 0.10 and 0.20 inches of liquid quantitative precipitation forecast for late 
Sunday night and Monday. Using a snow to liquid ratio of around 11 
to 1...snow accumulations of 1 to 2 inches appear likely. The 
highest amounts would be in the eastern counties...where the best 
upward vertical motion lingers the longest. Will continue to 
mention the snow in the hazardous weather outlook for now...as it 
would be the first widespread snowfall of the winter season. 
Monday morning rush hour traffic looks to be affected by the 
snow...but expected accumulations do not warrant an advisory 
headline at this time. 


Monday night...vorticity maxima move southeast of the area during 
the evening. Snow showers should diminish during the evening 
hours...with scattered flurries after midnight in the central and 
eastern counties. Strengthening surface low to the east of the 
area should cause a tightening pressure gradient and subsequent 
brisk northwest winds. Cold air advection Monday into Monday night 
should bring cold temperatures back to the area. 


Tuesday through Saturday...European model (ecmwf) and GFS in fair agreement with 
bringing an upper level ridge across the northern plains Tuesday 
and Wednesday...before flattening it out on Thanksgiving day. 
Surface high pressure moves through the western Great Lakes region 
during this time...so generally quiet weather is expected. Cool 
temperatures should remain across the area. Models then diverge 
greatly with features Friday and Saturday. The European model (ecmwf) and European model (ecmwf) 
ensembles are the preferred models per HPC blended fields. 
Still...with the uncertainty seen lately in this period...will not 
change the going forecast. 


&& 


Aviation... 


Mainly VFR conditions to continue today. South winds ahead of 
advancing shortwave will increase moisture a bit this 
afternoon...but not enough for widespread stratus/stratocumulus 
development. As result mainly cirrus then altostratus ahead of 
advancing shortwave this afternoon. By later tonight leading 
shortwave reaches southern Wisconsin. Increased moisture and 
upward motion should result in lowering ceilings and precipitation. 
MVFR with some IFR ceilings expected early Monday morning. Mix of 
precipitation during the early morning...mainly south central WI 
west of Madison should turn to mainly snow behind cold front. Main 
cyclogenesis to occur Monday over Great Lakes region. 


&& 


Marine... 


Increasing south flow ahead of cold front expected to increase to 
10 to 20 knots. Waves should increase but remain below small craft 
criteria. As low deepens over Michigan Monday...increasing 
northwest winds should exceed 20 knots with gusts of 25 to 30 
knots Monday afternoon and continuing Tuesday and Tuesday night. 


Small Craft Advisory will likely be needed for Monday afternoon 
through Tuesday night...possibly into Wednesday for brisk winds 
and waves. 


&& 


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


$$ 


Short/long term...08/wood 
aviation/marine...06/hentz