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Area forecast discussion 
National Weather Service La Crosse WI 
227 PM CST Wednesday Nov 19 2008 


Short term...tonight through Saturday 


Forecast concerns continue to be focused on colder air filtering 
into the area tonight through Friday...and precipitation chances. 


Data analysis this afternoon had an area of low pressure over Eastern 
Lake Superior with a cold front extending southwest through eastern 
WI into northern Illinois. Visible satellite imagery showing expansive 
stratus/stratocumulus filling in behind the front in cold air 
advection. Northwest winds were gusting into the 25 to 30 miles per hour range 
across the higher terrain of southeast Minnesota. 


19.12z GFS and NAM in good agreement through Friday...then diverge a 
bit with disturbance riding southeast through the northern 
plains/Midwest Friday night/Saturday. Went with the 19.09z sref for 
this time frame which was a good compromise of the NAM/GFS. 


The area of low pressure over Eastern Lake Superior moves into the 
eastern Great Lakes tonight as a strong/cold high noses in from the 
central Canadian provinces into the northern plains. Brisk 
conditions will prevail across the area as a result of the stiff 
pressure gradient and cold air advection between the two systems. 
Also noted was increasing northerly fetch of 925mb winds across Lake 
Superior... tapping the lake effect snow machine across the u.P. And 
far northern WI. Concerned that a few of the resulting snow shower 
streets/stringers may reach into Taylor/Clark counties. The local 
rucwrf composite reflectivity was hinting at this. Otherwise...cold 
air advection/broad cyclonic flow will likely produce a few flurries 
across the rest of the area overnight. 


Broad cold cyclonic flow aloft continues across the area tomorrow 
with steep 1000-850mb lapse rates in the 9-10c/km range. This will 
continue the brisk conditions and scattered flurry activity across the 
area as temperatures struggle into the middle/upper 20s. Northwest 
winds in the 15 to 25 miles per hour range with a few higher gusts will 
definitely add to the chill. Kept flurry mention into Thursday 
evening with cyclonic continuing. Heights begin to build Thursday 
night/after midnight as ridge builds in from the plains. Subsidence 
will break up stratocu from west to east. Winds will also slacken 
with the ridge. This will set up good radiational cooling with lows 
slipping into the upper single digits to middle teens. 


High pressure will be overhead on Friday. Despite ample sunshine... 
the cold start and lack of mixing will produce chilly daytime highs 
in the middle and upper 20s. 


As mentioned above in the model discussion...GFS/NAM continue to show a 
middle-level shortwave trough dropping from the Montana rockies into 
the northern plains/Midwest Friday night/Saturday. The GFS is about 
3-6 hours faster with this feature and a bit further north. With 
this in mind...decided to go more with the sref which seems to be a 
good compromise between the two. Bottom line is there will be an 
increase in cloud cover from west to east Friday evening through 
Saturday. Believe airmass will be too dry across much of the area 
for any precipitation chances...which the sref precipitation probability 
indicates. Will maintain a very small chance of -sn across far 
southwest portion of the forecast area in the 06-12z Sat time frame. 


Long term...Saturday night through Wednesday 


19.12z GFS and 19.00z European model (ecmwf) showing better agreement through the 
extended period. Low pressure with origins on the leeside of the 
Alberta rockies will move southeast into the northern plains/upper 
Midwest Saturday night. This will push a warm front through northern 
MN/WI. Best forcing/saturation associated with the front appears 
like it will stay north of the area...so kept Saturday night dry for 
now. On Sunday...the low will occlude/close off as it continues to 
move into Minnesota with the cold front pushing east through WI. It appears 
there may be enough moisture convergence/lift along the front to 
produce a mix of rain or snow showers...mainly east of the 
Mississippi River. The stacked/closed low will continue to drift 
through WI and the western Great Lakes region Sunday night through 
Monday. This will keep cloud cover widespread across the area with 
small chances of snow showers. Both models showing a strong ridge of 
high pressure building from the plains into the upper 
Midwest/forecast area Tuesday through Wednesday for a dry period 
with temperatures warming back to seasonably normal levels. 


&& 


Aviation... 


Cold front passage and associated cold air advection has swept in 
stratus field creating MVFR conditions that are expected to last 
well into Thursday. Fairly uniform cloud and wind field expected 
next 24 hours as large high pressure area slowly builds in. Short 
wave trough swings through quickly Thursday morning but only impact 
may be to dry low levels out so expect stratus field to move out or 
dissipate across the area Thursday PM. 


&& 


Arx watches/warnings/advisories... 
WI...none. 
Minnesota...none. 
Iowa...none. 
&& 


$$ 


Short/long term...das 
aviation...Shea