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fxus63 karx 191930 
afdarx 


Area forecast discussion 
National Weather Service La Crosse WI 
230 PM CDT Tuesday Aug 19 2008 


Short term...through Friday night 


Primary forecast concerns were with cloud trends tonight...then 
rain chances during the latter part of the week. 


Early afternoon surface analysis had high pressure over northern 
Great Lakes...while subtle front extended from northern Illinois into 
southeast Minnesota. Surface dew points were a good indicator of relative 
humidity gradient across the forecast area...with values ranging 
from middle 50s in parts of central WI to upper 60s in northeast Iowa 
and southeast Minnesota. 


Visible satellite imagery indicated stratus that affected part of 
forecast area this morning had dissipated. However...stratus still 
lingered across parts of eastern WI. Water vapor imagery and 19.12z 
upper air data indicated ridge aloft over upper Mississippi Valley. 
Meanwhile...closed 500mb low was situated near Texas/OK border...with 
middle level trough carving out across the Pacific northwest. 


19.12z NCEP and European deterministic models were in overall good 
agreement with regard to upper level and surface features through 
23... evening. Therefore...confidence has been increasing 
that the forecast area should receive rain during the latter part of 
the week. This was also corroborated by probabilities of measurable 
quantitative precipitation forecast in the 19.09z sref data increasing to 60-65 percent by Friday. 


Tonight...low level southeasterly flow suggests stratus could develop 
and expand across portions of the forecast area. In fact...boundary 
layer relative humidity in latest RUC...NAM and arxmesowrf indicated 
stratus should emanate from eastern WI...where low clouds currently 
reside. Probabilities for low clouds appear to be highest especially 
for locations along and east of the Mississippi River. Please 
reference aviation discussion below for additional details. 


Thursday and Thursday night...NCEP data suite has trended toward a 
sooner arrival of rain chances on Thursday associated with ejection 
of Southern Plains middle level wave...which European models had 
indicated previously. As a result...confidence was high enough to 
expand rain chance over a bit more of the forecast area on Thursday. 


Ample Gulf moisture expected to be brought northward...as evidenced 
by precipitable water values potentially reaching 1.5-2.0 inches. 
Even so...forcing appears to be weak...which was indicated by weak 
upward vertical motion signal in time/height cross sections. Thus... 
rain chances were maintained in the low-end chance category at this 
point...which seems reasonable. 


Friday and Friday night...rain probabilities increase in this time 
frame. In fact...sufficient forcing along approaching cold front 
suggests rain chances will probably warrant an increase Friday night. 
At this time...maintained rain chances as in current data base... 
although consistency of forcing signals should lend higher confidence 
for increasing rain chances Friday and/or Friday night with 
subsequent forecasts. 


Long term...Saturday through Tuesday 


Primary concerns were with rain chances lingering into Saturday 
associated with passage of cold front...then dry weather potential 
through rest of the weekend and into first part of next week. 


19.12z deterministic GFS and European model (ecmwf) continued to have good consensus 
concerning overall details of upper level and surface wave advancing 
across southern Canada on Saturday. As a result...surface cold front 
attendant to this system appears to advance through the forecast 
area Saturday morning. 


Deterministic models have suggested front should scour clouds and 
rain out of forecast area Saturday afternoon. On the other hand... 
19.00z gefs supported high probabilities of quantitative precipitation forecast lingering through the 
day. Confidence was not high enough to end rain chances over any one 
part of the forecast area this far ahead of time. However...opted to 
trend rain chances downward Saturday afternoon. 


Based on consistency in deterministic GFS and European model (ecmwf) data suite...as 
well as 19.00z gefs...signal of building ridge at surface and aloft 
lends fairly high confidence for a period of dry weather Sunday into 
Tuesday. Caveat to this would be eventual track of remains of 
Tropical Storm Fay...which at this time...remains highly uncertain. 


&& 


Aviation...tonight and Wednesday 


Visible satellite imagery shows that another area of lake stratus is 
moving into southeast Wisconsin this afternoon. Both the 19.12z RUC 
and NAM/WRF have a fairly good handle on the clouds currently. They 
show that IFR/MVFR clouds will spread into southwest...central...and 
west central Wisconsin this evening...and then they move north into 
northwest and north central Wisconsin during the overnight as the 
winds become more southerly aloft. Klse looks to be on the western 
edge...so I went with a scattered to broken 1500 foot deck during 
this time period. The remainder of the area will see the diurnal 
clouds quickly dissipating between 19/23z and 20.01z. 


Both the GFS and NAM/WRF MOS suggest that dense fog will develop at 
klse in the 10z to 13z time frame on Wednesday. With the MOS winds 
around 5 to 7 knots and temperature/dew point spreads at 3f or 
greater...I am unclear whether this will actually occur...so I have 
just kept 4sm br in the klse between 20.10z and 20.13z. 


&& 


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


$$ 


Short/long term...Thompson 
aviation..........boyne