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Area forecast discussion 
National Weather Service La Crosse WI 
1038 am CST Sat Nov 21 2009 


Update... 


Fog lifting and dissipating quickly after the 10 am hour this 
morning. Some dense fog was still being observed across the 
northern parts of the forecast area yet late this morning...but 
with this being more of a localized threat and that the majority 
of the area has seen improvement...let the dense fog advisory 
expire. A different caveat of the forecast is a stratus deck which 
has pushed north into the forecast area from eastern Iowa this 
morning. The majority of the models are not picking this moisture 
up very well...with the latest RUC being the only one to have any 
indication of it being there. Although through the visible 
imagery portions of it may be thinning...do think these clouds 
will affect the southeast parts of the forecast area through at 
least middle afternoon. 




&& 


Previous discussion... /issued 342 am CST Sat Nov 21 2009/ 


Short term...today through Monday 


At 3 am...high pressure...centered over the Ohio Valley...was in 
control of the weather in the upper Mississippi River valley. The 
combination of light winds and a moist boundary layer has resulted 
in the development of dense fog early this morning. This fog 
should persist across the forecast area through 10 am this 
morning. As a result...no changes will be needed to dense fog 
advisory that was issued on Friday evening. 


The BUFKIT soundings continue to show a very dry air mass located 
above the initially moist boundary layer today. As diurnal heating 
occurs...we should easily mix up to 900 mb. This in turn will 
bring this dry air to the surface. As typical in these 
situations...the MOS dew point guidance are too high and the gfe 
dew point too was used to lower these dew points to around 
30 degrees this afternoon. 


From tonight into Sunday morning...the models continue to show that 
the moisture below 800 mb dramatically increases ahead of an 
approaching long wave trough in the northern and Central Plains. 
This moisture is currently producing LIFR/IFR ceilings and 
visibilities across Oklahoma this morning. There was enough 
confidence to include areas of dense fog in the grids into the 
higher elevations of northeast Iowa and southeast Minnesota after 
midnight tonight. This fog will likely persist through middle to late 
morning. The drizzle is still in question for late tonight... 
because there is very little lift /if any/ in the deep moisture 
layer. 


In addition to the increasing moisture...the warm air advection 
should result in steady to slowly rising temperatures across the 
forecast area tonight. As a result...a non diurnal temperature 
curve was used account for this from late tonight into Sunday 
morning. 


From Sunday night into Monday...it was the consensus that the 
slower operational NAM/WRF...Gem...and European model (ecmwf) solutions were 
preferred over the faster operational GFS. Due to this...the 
temperatures were raised some and the precipitation chances were 
lowered. 


Long term...Monday night through Friday 


Much of the time this morning was spent working on the forecast 
grids from Monday night into Wednesday night. 


The 21.00z models continue to struggle with the evolution and 
timing of a low pressure system moving through the upper 
Mississippi River valley somewhere from Monday night through 
Thanksgiving. Much of this has to deal with how this system 
interacts with another system dropping into the mean trough behind 
the initial strong wave. While the operational GFS has slowed its 
movement of this system from 24 hours ago...it continues to be 
much faster than the European model (ecmwf) and Gem. About two thirds /8 out of 12/ 
of its family members support this faster solution. Meanwhile the 
remaining GFS members are clustered with the slower European model (ecmwf) which 
moves the system through our area between Tuesday through 
Thanksgiving. This adds to quite a bit of uncertainty to this time 
period...but the overall consensus of our office and the 
surrounding weather forecast offices was to continue to trend our forecast grids 
toward the slower European model (ecmwf) and Gem solutions. This ultimately 
resulted in some changes to temperatures /raised temperatures from 
Monday night into Tuesday night/...precipitation chances /lowered 
chances on Monday night and then increased them some from Tuesday 
into Wednesday night/...and weather /delayed change over to snow/ 
grids from Monday night through Wednesday night. 


The European model (ecmwf) actually generates quite a bit of snow /4 to 8 inches/ 
across the northern Iowa and southern Minnesota...and then 
northeast into Upper Michigan from late Wednesday into 
Thanksgiving. The operational GFS actually show a similar 
totals...but it is further northwest and slightly earlier. While 
these models are showing this...the Cobb data only shows minor 
snow accumulations in the GFS. With such differences...we decided 
not to highlight this system in our hazardous weather outlook. 
Hopefully this will become a bit clearer in the upcoming days. 


Aviation...today and tonight 


Primary concerns were focused on IFR conditions in low clouds and 
fog during the valid taf period. 


Radiational cooling and a moist boundary layer has allowed for 
widespread IFR conditions in fog early this morning. As south winds 
increase today and mixing commences...improvement is expected. 
However...haze should linger through the day with persistent 
southerly flow. This same southerly flow continues drawing higher 
relative humidity northward tonight. Various data sets were strongly 
suggestive of IFR and/or MVFR in low clouds/fog again tonight. 
Poorest conditions anticipated across higher elevation airports 
mainly west of the Mississippi River...such as krst taf site...where 
confidence was high enough to forecast conditions at instrument 
approach minimums. 


&& 


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


$$ 


Update...Brooks 
short/long term...boyne 
aviation...Thompson/aj 

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