594 
fxus63 kgrb 220909 
afdgrb 


Area forecast discussion 
National Weather Service Green Bay WI 
309 am CST Sat Nov 22 2008 


Synopsis...large scale pattern will become increasingly split during 
the forecast period. That will allow temperatures to moderate...with readings 
ending up near or maybe even a little above normal for the period. There 
will be a chance of precipitation late in the weekend and early in the weak. Could 
see another round of lake effect in that wake of that system. 
&& 


Short term...tda/tngt/sun. Weakening upper system will slide by to the 
SW of the forecast area today. Main impact will be to push cirrus across the 
area...with some middle clouds probably working into the SW. After a 
cold start and with some clouds overhead...guidance forecast maximum temperatures seemed 
awfully optimistic. All the bias-corrected guidance was lower than 
the actual forecast numbers...and that seemed the way to go. 


Remnants of the moisture from the system to the west should work northward over 
the forecast area tonight. Previous forecast was pretty aggressive with bringing 
patchy fzdz across the entire area after midnight. But forecast soundings 
off both the NAM and GFS indicated atmosphere barely gets saturated by late 
tonight. So removed the fzdz from all but the far northwest...where GFS and 
NAM had the most moisture late tonight. Also confined fzdz to after 09z. 
Used a blend of guidance for min temperatures. 


Similar reasoning supported removing fzdz from the forecast for Sun 
morning over all but the far northwest. By late in the day...band of 
middle-level moisture working eastward ahead of the next system should 
reach the area. Areal coverage of precipitation chance again looked too big...so 
pulled from all by the northwest. A blend of temperatures again supported edging maximum 
temperatures back a bit. 


Long term (sun night through thu)...main forecast concerns revolve 
around precipitation chances early in the period when northern and southern 
stream systems phase over the central Great Lakes. Although models 
are in reasonable agreement...uncertainties remain revolving around 
timing of the phasing...and the amplitude of the resulting upper 
low. The 00z GFS has the most support from the NCEP ensemble...but 
also tried to blend in some 12z European model (ecmwf) ideas. 


Sunday night into Monday...northern stream wave will dig south 
towards the western Great Lakes...while the southern energy will 
gradually draw Gulf of Mexico moisture north into the Ohio Valley. 
Better moisture and forcing (-divq and isentropic ascent) will 
reside with the southern shortwave across the Ohio Valley. Some of 
this moisture will try to sneak NE along a weak cold front...which 
will move southeast across the area mostly during the overnight hours. 
Although precipitation chances seem fairly low during the evening 
hours...if any light precipitation does form...the southwest forecast area 
will have a chance of seeing a rain/snow mix before the bl cools 
overnight. The Fox River valley northeast to areas adjacent to 
Green Bay will continue to see the best chances for accumulating 
snow overnight...perhaps reaching up to an inch. The northern 
stream upper low moves overhead on Monday with an additional lobe of 
middle-level moisture...mostly across the western forecast area. With 
inversion heights increasing through the day as the cold pool aloft 
moves in and arrival of deeper moisture...seems like a day for 
afternoon diurnal snow showers. Will also have a lake enhanced case 
to contend with as low level winds veer around to the northwest or north-northwest. 
850mb temperatures arent especially cold behind the front (-10c)...but will 
still have sufficient instability (delta T/S 15c)...cyclonic 
flow...upper support...and moisture in the dendritic growth zone. 
Better accumulations will arrive come afternoon as wind trajectories 
become more favorable. Would not be surprised to see 1-2 inches 
over Vilas County...although will temper expectations at this point 
in the game. 


Monday night and Tuesday...upper low consolidates over the eastern 
Great Lakes and increasing subsidence along with a surface high will 
make its presence known...especially on Tuesday. Diurnal snow 
showers will dissipate early Monday evening across the area. 
Assuming wind trajectories cooperate...any kind of show will reside 
over northern WI where lake effect snow will continue Monday night 
with thermal troughing overhead. Lake effect will gradually wind 
down on Tuesday because of the aforementioned subsidence and drier 
850-700mb air moving in. Will beef up cloud cover a bit with the 
thermal troughing lingering...but still have more sun in the 
afternoon. 


Rest of the forecast...very little changes made here. Upper low 
moves further east while high pressure and subsidence keeps NE 
Wisconsin dry. 850mb temperatures moderate a degree or two each day 
with highs returning to near normal in the middle to upper 30s. 
&& 


Aviation...VFR conditions expected...with some MVFR ceilings possible 
across the northwest late tonight. Some wind shear expected tonight...but did not 
seem strong enough to warrant inclusion in the tafs. 
&& 


Grb watches/warnings/advisories... 
none. 
&& 


$$ 
Skowronski/mpc