Weather

231 
fxus63 kgrb 052036 
afdgrb 


Area forecast discussion 
National Weather Service Green Bay WI 
236 PM CST Monday Jan 5 2009 


Synopsis...upper ridge will remain along/just off the West Coast 
throughout the forecast period. Mean trough posn over the Continental U.S. Expected 
to migrate into the eastern states during the weak. Then western ridge will 
being to amplify starting late this weak. Still some concerns about 
actual sharpness of the ridge and depth of downstream troffing...but 
trend of the past few guidance suites has been for sharper ridge and 
lower heights downstream. 


Temperatures will start out cold...then rebound closer to normal as upper trough 
shifts across the region. Amplified pattern with a ridge near the West 
Coast and a trough over central noam will set the stage for a cold 
period of weather across the region for the latter part of the forecast period and 
beyond. Light precipitation event expected middle-weak with the passage of the upper 
trough...with another chance late in the weak and during the start of the 
weekend. 
&& 


Short term...tonight and Tuesday...latest RUC analysis and 
visible/IR/WV imagery show high pressure centered over the central Ohio 
Valley with a weaker ridge axis extending NE across the western 
Great Lakes. Most of the area is cloud free except for a thin 
stripe across far NE Wisconsin in a region of weak low level 
confluence. Looking upstream...high clouds diving southeast overtop a weak 
shortwave ridge and already moving into northern Minnesota...while low 
pressure is just moving to the Lee of The Rockies. Main forecast 
concerns include temperatures and cloud cover tonight...and small precipitation 
chances tomorrow. 


Tonight...high clouds will pour into far northern Wisconsin this 
evening and gradually thicken into the overnight. Have a hard time 
imagining they will have a big impact on temperatures though. 
Rather baggy pressure gradient will keep a light south flow as high 
pressure shifts to the east. SW winds are beefier aloft and think 
that the bl inversion will allow winds to decouple. Looking 
upstream to South Dakota last night...which was relatively in the 
same location in relation to the surface high as we will be 
tonight...there was a wide range in low temperatures depending on 
the terrain (ie. Cold spots). With this in mind...tried to show a 
wide range in temperatures from the negative middle single digits across 
the higher terrain of north-c Wisconsin (from Eagle River to Merrill and 
marshfield)...to near 10 above by the Lake Michigan shoreline. 


Tomorrow...shortwave over Alberta/Montana will drive the broad area 
of low pressure from the northern Great Plains into the upper 
Mississippi Valley...while stronger energy shifts into the Southern 
Plains. This southern system will rob much of the moisture from 
reaching the northern Great Lakes...although some moisture will try 
to sneak into our neck of the Woods...but will get shunted to the 
east. 12z GFS is most bullish with this moisture...pushing the 
deeper moisture into the far southeast part of the forecast area. This 
looks too aggressive given the other model solutions and modest 
strength of the southwest flow. So have backed off on precipitation 
chances tomorrow afternoon. Just kept a chance of flurries across 
the northwest...where somewhat deeper moisture will reside (still 
fairly dry below 750mb)...and closer to the low level trough. Partly 
to mostly sunny skies should still remain for much of the morning 
before clouds increase in the afternoon. Highs in the middle 20s to 
low 30s. 


Long term...Tuesday night through next Monday. System crossing the region middle- 
weak looks like a high pop-low precipitation amount type event. Took probability of precipitation up to 
likely over most of the forecast area for Wednesday. At this point expect up 
to an inch of snow over the southwestern part of the forecast area...with an 
inch or two in the NE. Could need to bump those numbers up a bit if 
things get going a little more quickly or linger a little 
longer...but compared to the numerous sig snow events we/ve had 
thus far this winter this looks fairly tame. 


Some lake-effect off Lake Superior will probably follow for Wednesday night 
into Thursday. Trajectory looks somewhat marginal and wonder just how dry 
the incoming air will be...so kept probability of precipitation in chance category for now. 


Better snow maker then will cross the region late in the weak. Canadian 
and GFS still a bit faster than European model (ecmwf)...but all indicate snow 
sometime from late Thursday night/Fri/Fri night. Took probability of precipitation up to high chance 
category...and might have gone likely if there was more of a 
consensus on timing. System looks like it will produce a swath of 3-5 
inches of snow across or somewhere near the forecast area. Introduced 
system into severe weather potential statement yesterday...and will continue to mention it today. 


The ugliest weather of the forecast period will probably follow right behind... 
as its now looking like a strong surge of acrtic air will blast across the 
area in the wake of the system. Blowing and drifting snow combined 
with falling temperatures could make Sat a downright nasty day. Once cold 
air is in place...it/S tough to see it getting dislodged as long as 
upper flow stays northwesterly. Pulled temperatures down in all later periods of the 
forecast...in some cases substantially. Still a little nervous as models 
have been waffling around on the strength of the western ridge/depth of 
downstream troffing. But large scale pattern strongly suggests 
cold...so was comfortable enough to make the changes. In fact...if what 
the models have now actually ends up verifying...we may need to pull 
back more on temperatures with later forecasts. 


That could just be the percursor before the main show. The latest 
runs of the European model (ecmwf)...GFS...and Canadian model all suggest a major 
Arctic outbreak with even colder air surging into the forecast area 
during the middle of next weak. Seems like medium range models end up 
forecasting more of these cold outbreaks than what actually occur. The 
outbreak next weak would still be a day or two beyond the end of the 
forecast...so we/ve got some time to see how this plays out before 
dealing with it in the forecast. 
&& 


Aviation...VFR conditions with only some invading highs clouds 
tonight. Clouds eventually thicken and lower...but that looks to be 
more tomorrow afternoon. May be able to squeak out a flurry...but 
not much more. 
&& 


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


$$ 
Mpc/skowronski 

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