777 
fxus63 kmkx 220932 
afdmkx 


Area forecast discussion 
National Weather Service Milwaukee/Sullivan WI 
332 am CST Sat Nov 22 2008 


Forecast focus on precipitation chances today...and again Sunday 
night through Monday night. 


Latest water vapor imagery indicated a potent shortwave trough 
over northeastern South Dakota...moving southeast. At the 
surface...a low pressure system was located beneath the shortwave 
trough. A band of light snow and flurries was located from around 
the Twin Cities to Rochester...and into east central Iowa. 


Today...NAM and GFS have the shortwave trough and the core of the 
vorticity maximum passing to the southwest of the forecast area. 
Current light snow band associated with 850mb low level jet nose 
should pivot around shortwave trough this morning. This should 
bring some of the reflectivity into the southwestern counties this 
morning. 


Any light snow falling from this feature will have to contend with 
a very dry lowest 5000 feet of the atmosphere per area forecast 
soundings. Not too confident in ability of the light snow to 
penetrate this dry layer beyond having scattered flurries. 
Also...850mb low level jet nose weakens considerably. 


Thus...will pull the slight chance probability of precipitation in the far southwestern 
counties this morning...and go with scattered flurries from mainly 
a Wisconsin Dells...to Madison...to Beloit line. 


Tonight and Sunday...warm air advection kicks in tonight and 
lasts through Sunday...with 1000/500mb thicknesses rising from 
around 530 dkm to the lower to middle 540s dkm range. This should 
allow for lows tonight to climb to around 20...with highs rising 
into the lower 40s for Sunday. Middle and high clouds will increase 
later on Sunday...in advance of next weather system. 


Sunday night through Monday night...NAM/GFS/ECMWF all have a cold 
front with a lead vorticity maximum following the front for mainly 
later Sunday night. NAM and GFS show a fair amount of quantitative precipitation forecast ending 
at 12z Monday...with up to 0.08 inches on the GFS...and up to 0.20 
inches on the NAM. This mainly due to differential cyclonic 
vorticity advection and positive low to middle level frontogenesis 
response fields. 


May see a period of a rain and snow shower mixture Sunday 
evening...as area soundings indicated a chance for precipitation 
beginning as liquid before quickly changing to snow. Have 
increased probability of precipitation for late Sunday night. This may be the first 
widespread light snowfall of the winter season. Using more 
conservative GFS quantitative precipitation forecast amounts and a 10 to 1 ratio...snowfall 
accumulations up to an inch are possible...mainly in the eastern 
areas. 


Secondary vorticity maximum associated with the main shortwave 
trough to affect the area Monday into Monday evening. Chances for 
light snow showers should continue during this time. 


Tuesday through Friday...GFS/ECMWF blend per HPC blended fields 
product shows a large surface high pressure system moving through 
the region. This was despite quite a bit of disagreement among 
these models with various upper air and surface features across 
the West Coast and southeast. So...this would mean quiet weather 
would be possible over the area. Temperatures would appear to be 
near seasonal normals. 


&& 


Aviation... 


Upper disturbance producing two areas of precipitation. First is 
in isentropic lift on nose of 850 mb low level jet. Second more 
associated with vorticity maximum. The first area should push into 
south central Wisconsin this morning but weaken as low level jet 
weakens. Dry air initially in place so expect mainly virga...but 
some snow may reach the ground mainly west and southwest of 
Madison. Mainly VFR ceilings...but some MVFR visibilities and possibly some 
MVFR ceilings briefly in light snow again mainly west and southwest of 
Madison. 


Second area of precipitation with upper disturbance should move 
southeast of the forecast area. 


&& 


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


$$ 


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