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Next 12 Hours

Forecast data from the National Digital Forecast Database.

5 Day Forecast

• Tuesday
• 
• High: 86 °
• Low: 61 °
• Partly Cloudy

• Wednesday
• 
• High: 84 °
• Low: 63 °
• Chance of T-Storms

• Thursday
• 
• High: 82 °
• Low: 55 °
• Chance of T-Storms

• Friday
• 
• High: 75 °
• Low: 57 °
• Mostly Cloudy

• Saturday
• 
• High: 84 °
• Low: 64 °
• Chance of T-Storms

Area forecast discussion 
National Weather Service La Crosse WI 
605 PM CDT Tuesday may 22 2012 


Short term...tonight through Friday 
303 PM CDT Tuesday may 22 2012 


Main forecast concerns are when precipitation chances move into the 
area and severe weather potential for Thursday. 


Currently as of 20z...water vapor loop and rap 500mb height analysis 
showed troughing over southwest Canada and the northern rockies... 
with ridging building downstream over the upper Mississippi River 
valley region. A shortwave ejecting out of the troughing and 
downsloping upper flow off The Rockies has led to low pressure 
development over the Dakotas. Increasing southerly flow downstream 
of this surface low on the order of 20-40kt 850mb winds per profiler 
data is spreading warmer air northward. Modifying through advection 
and daytime heating processes from 12z radiosonde observations...the rap suggests 
850mb temperatures have risen to 10-14c over the forecast area. With plenty 
of sun and the increasing winds...surface temperatures have climbed 
into the 70s to low 80s. Farther west...the 850mb temperatures increase to 
18-25c or higher over much of the plains. Moisture is also 
returning...more over the plains...where surface dewpoints are 
mostly in the 50s. 


Tonight and Wednesday...troughing over the northern rockies is 
forecast to deepen across the western u... in 
strengthening ridging downstream into the Great Lakes. A couple of 
shortwaves ejecting out of this troughing will help lift the low in 
the Dakotas into far northwest Ontario...dragging a cold front into 
western Minnesota by 00z. Increasing south to southwest flow aloft 
ahead of the front will help bring warmer air into the forecast area 
with 850mb temperatures climbing to at least 16-18c by 18z Wednesday. We 
should also see an increase in altostratus and cirrostratus... 
already evident over eastern Montana and Wyoming. These clouds may 
temper highs somewhat...but readings should easily reach into the 
80s given the strength of the warm advection/winds and sun. Lows 
tonight also expected to be warmer with the increasing winds... 
especially across southeast Minnesota. Regarding precipitation 
chances...any convection should be strongly tied to the front. Ahead 
of the front...models suggest quite a bit of cin...even with their 
higher surface dewpoints like the NAM suggests. There is also mix 
down potential of the dewpoints with plentiful dry air below 700mb. 
Therefore...thinking the entire forecast area should be dry through 
Wednesday. Last item of note is winds could get close to advisory 
criteria in southeast Minnesota late in the morning into middle afternoon... 
just based on the speeds themselves. Something to watch. 


Wednesday night and Thursday...at the base of the deepening western 
trough...a new surface low should have developed over far southeast 
Colorado by 00z Thursday. This surface low is expected to lift 
northeast...along the cold front that moved into western Minnesota 
late Wednesday...due to another collection of shortwaves ejecting 
out of the western trough. Therefore...the front Wednesday night is 
likely to not make much if any progress eastward. In fact...we need 
to wait until the low moves north of the area to drive the cold 
front eastward. This drive eastward too will be enhanced by a fairly 
potent shortwave coming up into Minnesota. For Wednesday night... 
again convection should remain tied around the cold front in 
Minnesota. Various things going for convection include a right 
entrance region of a jet lifting out of the Dakotas...increasing low 
to middle-level frontogenesis on the cold front...and a plume of 
500-1000 j/kg of MUCAPE being transported into the frontogenesis 
zone by a strengthening low level jet. As it stands right now...most 
models suggest the front and bulk of convection will end up west of 
the forecast area. Have maintained some chances over the northwest 
ends...though...as convective outflows may try to push the front 
eastward. Thursday is definitely more concerning as the potent 
shortwave approaches in the afternoon. Some slight timing 
differences exist on the cold frontal passage and approach of the 
shortwave...but there is good agreement that the dynamical and 
thermodynamical forcing will likely lead to a squall line along the 
front early to middle afternoon...along with Post-frontal 
precipitation. This precipitation then pushes east into the evening. 
Raised precipitation chances to 50-70 for the afternoon. Plentiful 
0-6 km and 0-3km shear and MLCAPE of 500-1000 j/kg should be enough 
to produce some severe weather...with damaging winds main hazard. 
Temperatures likely to stay warm Wednesday night and Thursday due to 
breezy winds...with a possible fall in temperatures in the afternoon 
as the front/precipitation move through. Winds could get close to 
advisory criteria on Thursday. 


Thursday night and Friday...after the squall line and cold front 
move through in the evening...strong subsidence is indicated to move 
into the forecast area. Raised precipitation chances east of the 
Mississippi River to 50-70 to handle the squall line. Conditions 
should then dry out after midnight thanks to the subsidence... 
produced mostly by that potent shortwave which lifts into Upper 
Michigan. The cold front looks to stall over northern Missouri 
Friday morning. New deeper troughing then forms in the western 
u... ridging to re-build downstream into the 
middle-Mississippi Valley. Depending on this timing of the ridging 
building up...the front may start moving back north in the 
afternoon...as indicated in the 22.12z NAM/Gem. Moisture transport 
over this front could lead to a few showers and thunderstorms over 
northeast Iowa and far southwest Wisconsin...though definitely 
better chances would be over southern Iowa closer to the front. 
Cooler air advecting in behind the front should help send 
temperatures back down to seasonable levels. 


Long term...Friday night through Tuesday 
303 PM CDT Tuesday may 22 2012 


Models and GFS ensembles are increasingly becoming in good agreement 
for the weekend...indicating troughing to persist over the western 
U.S. While strong subtropical ridging builds over the eastern two 
thirds of the country. With this building ridging...the warm front 
on Friday situated either over southern Iowa or northern Missouri 
will gradually lift north across the forecast area. The speed of 
this may be modulated by periods of convection forming on the warm 
frontal boundary...in response to frontogenesis and especially 
increasing low level jet/moisture transport at night. Best time 
period for convection across the area would probably be Friday night 
into Saturday morning. Saturday night into Sunday morning could 
occur too over far northern portions of the area...depending on how 
far north the warm front reaches. Either round of convection could 
be severe...given 1-6 km shear values of 40kt or more and likely 
plenty of instability being advected over the frontal zone. Once the 
warm front passes...850mb temperatures jump to 20c or higher. Sunday looks 
to be the warm day with temperatures likely well into the 
80s...perhaps even 90s. 


Things get a little more murky for early next week...in particular 
with the handling of the western trough. The 22.00z/22.12z European model (ecmwf)/Gem 
and some of the 22.12z GFS ensembles suggest the western trough will 
be slower to eject northeastward than the 22.12z GFS and the rest of 
its ensembles. This seems reasonable given the strength of the 
ridging ahead of the trough. Therefore...likely the cold front 
associated with the trough will be slower to move through the 
forecast area. This results in a warmer day on Monday than what 
the GFS would suggest. Tuesday would be cooler after the frontal 
passage. Did maintain the consensus approach for precipitation 
chances Monday through Tuesday...given the differences. 


&& 


Aviation... 
605 PM CDT Tuesday may 22 2012 


Winds will be the main aviation concern tonight into Wednesday 
afternoon. Tightening pressure gradient ahead of a cold front will 
keep southerly winds brisk tonight. BUFKIT rap/nam12 soundings point 
to 50 kts by 2 kft at krst...enough to warrant a low level wind shear mention for 
that location. A bit less at klse...so will not add mention there. 


Winds go from south to southwest aloft...and not overly 
Uni-direction. Still...with only a few high clouds spreading in from 
the west...there should be enough mixing to lead to higher 
gusts...already by early morning...continuing into the afternoon. 
This could be border-line Wind Advisory for parts of southeast 
Minnesota...which would include krst. 


Trends have been to move the front in a bit later...and therefore 
the front will likely be more of a factor on Thursday. Convection will be 
ongoing along and behind it as it moves in...likely impacting 
krst/klse during the afternoon. 


&& 


Arx watches/warnings/advisories... 
303 PM CDT Tuesday may 22 2012 


WI...none. 
Minnesota...none. 
Iowa...none. 


&& 


$$ 


Short term...aj 
long term....aj 
aviation.....Rieck