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Area forecast discussion 
National Weather Service La Crosse WI 
310 PM CST Sat Nov 21 2009 


Short term...tonight through Monday night 
main forecast concern is on chances for light precipitation chances 
through the period. 


Currently as of 21z...water vapor loop and RUC 500mb height analysis 
showed ridging across the upper Mississippi Valley and Great 
Lakes...troughing from Alberta south into Utah...and a more potent 
trough just south of the Gulf of Alaska. Subsidence under the upper 
ridge has kept much of the area east of The Rockies and say north of 
I-40 precipitation free today. However...numerous issues exist the 
cloud department...where a zone of two different stratus layers can 
be seen over the Central Plains and over the forecast area. Both 
areas have mostly evolved from a combination of mixing out morning 
fog...and then that moisture staying trapped under the subsidence 
inversion caused by the upper ridge. The clouds over the forecast 
area have held temperatures down...with readings mostly in the middle 
to upper 40s. Return low level flow...between high pressure over the 
Ohio Valley and a Lee trough is bringing 40s dewpoints northward 
through the plains. 


Tonight...upper trough over the intermountain west is prognosticated to 
push east into the Western Plains...causing upper level flow to turn 
southwesterly over the area. In response in the low levels... 
southerly flow should increase...as the Lee trough/cold front moves 
into the eastern Dakotas. This increasing southerly flow will help 
bring the clouds/moisture over the Central Plains into the forecast 
area. Past and current GFS/European model (ecmwf) runs continue to show development 
of light quantitative precipitation forecast across the upper Mississippi Valley region...especially 
after 06z. However...past and current NAM runs remain dry. Looking 
at GFS soundings...the light quantitative precipitation forecast looks to be a result of drizzle... 
since saturation and lift is confined to an area at or below 5000 
feet. Previous forecast was dry...and will continue that idea but 
maintain a mention of patchy drizzle. Only area that may not see 
drizzle is from Juneau/Adams counties southwest into Grant County... 
where enough drier air influence from central Illinois coming 
through the return flow will prevent precipitation. Another issue to 
deal with tonight is the moisture coming up from the south...running 
into a cooler airmass over the forecast area. This may result in 
some fog development...especially over the ridges of southeast 
Minnesota and northeast Iowa. Have maintained the going areas of 
dense fog...but did not issue an advisory at this time due to 
uncertainty of the fog being widespread enough to warrant the 
advisory. Regarding low temperatures...with the higher dewpoints 
moving into the area from the south...lows will likely be this 
evening with rising temperatures afterwords. 


One caveat to watch for tonight...with the winds increasing... 
developing low level saturation may be tricky as temperatures may 
not want to fall. If this occurs...then all we could end up with is 
some passing stratus clouds with bases at 3000 feet or so...as seen 
over eastern Nebraska and northeast Kansas. Future shifts will need 
to monitor cloud trends and dewpoint depressions. 


Sunday...upper trough in the Western Plains at 12z will continue its 
March eastward...aided by the potent trough currently south of the 
Gulf of Alaska digging into Idaho and northern Utah. Although the 
upper trough is approaching the area...the associated cold front 
ahead of it is prognosticated to fall apart over western Minnesota... 
possibly due to new low pressure forming in Wyoming. In any 
event...southerly flow is prognosticated to continue over the area... 
bringing more moisture up from the south. Same scenario exists again 
with the GFS/European model (ecmwf) runs producing precipitation while the NAM is 
dry. Soundings from the GFS or NAM are not that impressive...with 
little lift and have ceilings climbing through the day to help 
prevent drizzle. Isentropic lift forecasts are fairly benign too with 
little if any lift. Tempted to dry out the forecast...but with the 
consistent signal by the GFS/ECMWF...have maintained the low 
probabilities of precipitation. Any fog from tonight should 
dissipate by noon with heating and warm advection. Regarding that 
warm advection...temperatures should be able to climb into the 50s 
in most locations. 


Sunday night and Monday....upper trough that was marching eastward 
towards the forecast area on Sunday should move through Monday 
morning...while the potent trough digs down into Nebraska and 
Kansas. Although the upper trough is moving through the area...the 
trough is much weaker than what it looks like on water vapor...due 
to heights trying to rise ahead of the potent trough. Therefore... 
the overall DPVA forcing is quite weak over the forecast area...as 
well as the isentropic lift. If anywhere could see precipitation 
during this period...it looks like northeast Iowa...southeast 
Minnesota and into central Wisconsin where most of the weak forcing 
is located. Elsewhere...this period looks mainly cloudy...though 
would not be surprised to see some clearing of the clouds from Grant 
County northeast into Adams. This clearing is in response to a wedge 
of dry air in the low to middle levels that advects in on southeast 
winds. With plenty of clouds and relatively high dewpoints for late 
November...temperatures should remain on the mild side at night and 
not rise a whole during the day Monday. Previous forecast lows 
seemed reasonable. 


Monday night...trough over Nebraska and Kansas is prognosticated to slowly 
shift east...reaching a position near Kansas City by 12z Tuesday. 
Height falls...isentropic lift and an open Gulf of Mexico feed of 
moisture ahead of it will allow for the formation of both warm and 
cold conveyor belts with the upper low. Models forecast that the warm 
conveyor belt may get into northeast Iowa and southeast 
Minnesota...and have raised chances of precipitation up in this 
area. Farther north and east...have maintained going low chances of 
precipitation...mostly due to concerns that the upper low could 
progress a little bit faster as the 21.12z GFS shows. Imagine over 
time the chances of precipitation can be refined such that less area 
is affected...though. Along with the incoming warm conveyor belt 
should be a mass of at least high and middle clouds...if not low clouds 
too. Therefore...another cloudy time period is expected...which 
should help keep low temperatures well above normal. 


Long term...Tuesday through Saturday 


Focus of this period remains on the system forecast in the central 
U.S. Tuesday morning and its impacts into the Thanksgiving Holiday. 
Problem is...models are yet to come to full agreement on the timing 
of the system. The 21.06z GFS and 21.00z Canadian...along with their 
respective ensembles...all moving the system along fairly quickly. 
By 12z Wednesday...they show the system in lower Michigan. Of note 
is that the Canadian...which a few days ago was one of the 
slowest...has now turned to the fastest scenario. The 21.00z European model (ecmwf) 
as well as the 20.12z European model (ecmwf) ensembles...are the slowest...reaching 
only the Chicago area around 00z Thursday. The timing of this system 
is critical too...since all models show a fairly potent shortwave 
trough in the northern stream flow dropping down into the upper 
Mississippi Valley late Wednesday. This trough will help bring 
colder air in...and if the system is still lingering around like the 
21.00z European model (ecmwf) shows...a better chance exists for an accumulating 
snow. For now...the preferred option is to stay closer to the 21.00z 
European model (ecmwf)...as per HPC forecasts. Interesting that the 21.12z European model (ecmwf) has come 
in faster. Therefore...with the wide model variety...the Tuesday 
through Thursday time frame is definitely subject to change. Enough 
confidence exists to raise chances of precipitation to 60-70 on 
Tuesday...despite the timing differences...as majority of models 
show the area affected by a warm conveyor belt. After Tuesday...have 
kept the precipitation chances no higher than 50 until certainty 
increases. Will start changing the precipitation type over to snow 
towards Wednesday night...though...as that northern stream trough 
drops in. Have extended chances of precipitation into Thanksgiving 
as well...but only low probabilities at this point. 


For Thursday night through Saturday...models and ensembles depict 
ridging over the western U.S. Pushing east into the upper 
Mississippi Valley...albeit flattening out. This ridge is being 
pushed east by a trough moving into the western U.S.. looks of this 
trough appears to be one of a more progressive nature...especially 
given the flattening ridge ahead of it. However...it appears any 
precipitation associated with it should not arrive until after 
Saturday. Therefore...the forecast for this time frame will stay 
dry. Looking for a cool Thursday night as a surface ridge builds 
in...and could be even cooler if snow cover exists. Gradual warming 
is then planned for Friday into Saturday as return flow sets up in 
the wake of the ridge. It should be noted that a cold frontal 
boundary may be entering the picture for Saturday...ahead of the 
western trough pushing eastward. 


&& 


Aviation...tonight and Sunday 


Not a high confidence forecast for the next 24 hours which begins 
with poorly modeled stratus across the area this afternoon. RUC has 
been the only model even close to indicate this low level 
moisture...however it also has not had a real good handle on it. 
Visibility restrictions continue across much of the area 
today...however finally seeing improvements early this afternoon. 
Winds have also begun to pick up a bit with tightening pressure 
gradient this afternoon which should continue through tonight. 


Another area of fog/stratus currently over eastern Nebraska/Kansas 
and Missouri will push northeast towards the area for tonight and 
Sunday morning. This signal has been pretty consistent over the past 
few days with bringing increased moisture into locations mainly west 
of the Mississippi River. With clouds increasing and winds staying 
up...rather than radiation fog development...this looks to be an 
advection fog episode affecting more of the higher terrain areas 
west of the Mississippi River...with stratus elsewhere. Stratus/fog 
looks to push into the krst/klse around 22.03-04z. Held off on 
significantly lowering visibilities over krst until after midnight 
when T/dew point spread is lowest. Did lower visibilities at klse...however 
kept ceilings/visibilities in MVFR category. 


&& 


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


&& 


$$ 


Short/long term...aj 
aviation...jlb 

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