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Area forecast discussion 
National Weather Service La Crosse WI
505 PM CST Tuesday Jan 27 2015

Short term...(this evening through Thursday night)
issued at 228 PM CST Tuesday Jan 27 2015

The focus of the forecast in the near term is on the incoming
system for Wednesday night into Thursday morning.

The 27.12z models in good agreement in bringing two short wave
troughs across the region in this time frame. The first wave is
the system currently over Nevada while the second is coming onto
the British Columbia coast. Both of these waves will continue to
work east...topping the ridge Wednesday and then sliding across
the region Wednesday night into Thursday. As of now...the models
do not phase these two short waves and bring them through
separately. Two rounds of pv advection in the 500-300 mb layer
should occur and both be moderate in strength. However...the bulk
of the qg convergence will be tied to the first wave and should
be moderate in strength and extend through a deep layer from
1000-300 mb Wednesday evening. The qg convergence then becomes
weak in strength and limited to the 500-300 mb layer overnight
Wednesday into Thursday morning as the second wave slides across
the region. A band of weak frontogenesis in the 850-500 mb layer
should work across the area late Wednesday night into Thursday
morning behind the weak surface low that is expected to track from
southern Iowa toward Southern Lake Michigan. The best isentropic
up Glide will occur Wednesday afternoon into the evening ahead of
the surface low with the GFS showing between 3 and 5 ubar/S
occurring on the 285k surface. There is also a decent moisture
transport signal on this surface into the area. This moisture
transport and isentropic up Glide will go into saturating the
atmosphere with the subsequent forcing expected to be enough to
squeeze out some light precipitation. Looking at the NAM and
BUFKIT soundings...neither model shows any ice will be in the
clouds during this event which should lead to freezing drizzle and
drizzle being the primary precipitation types. Both models show
the saturation layer extending up to about five thousand feet with
some Omega through the layer to produce the drizzle but this layer
likely not deep enough to produce any rain drops. Have raised the
precipitation chances up to 60 percent Wednesday night across the
northeast where the best chances of saturation occurring will be.
Also bumped up the chances a little across the west...but
concerned that these chances will have to go higher across the
entire area. Also think that there will be some light ice
accumulations with this event and that a Winter Weather Advisory
will likely be needed with later forecasts. If an advisory becomes will come down to surface temperatures for
Wednesday night. Parts of northeast Iowa into southeast Minnesota
may not dip below freezing until late in the night resulting in
more drizzle than freezing drizzle while east of the Mississippi
River...the temperatures could be below freezing much longer.

Long term...(friday through tuesday)
issued at 228 PM CST Tuesday Jan 27 2015

A large cut off low is expected to form over the Desert Southwest
late this week. This system should weaken heading into the weekend
and actually get pushed to the south as the northern stream
increases and becomes the dominant flow across the country. As the
northern stream strengthens...a short wave trough is expected to
track across the area...but there are some differences between the
27.12z European model (ecmwf)...GFS and Gem on the timing and strength of this
wave. All these models also agree that moisture will return north
ahead of this wave...but with the differences in the timing of the
wave...the models differ on whether the moisture will arrive and
produce precipitation over or east of the local area. The European model (ecmwf)
looks to be the slowest with the wave and thus shows the most
precipitation over the area with the GFS the fastest and has the
least while the Gem is in the middle. The model consensus grids
does a decent job of averaging out these differences and gives
about a 20 percent chance for some snow Saturday and 30 to 40
percent Saturday night and Sunday.


Aviation...(for the 00z tafs through 00z Wednesday evening)
issued at 505 PM CST Tuesday Jan 27 2015

IFR/MVFR ceilings have setup across the region...and look like they will
stick around through the better part of Thursday. Low level moisture will
be stuck under an inversion...with no impetus for drying/clearing
until Thursday night/Friday when high pressure is slated to move in.

Meanwhile...a couple upper level shortwaves and a surface boundary will
sweep west to east across the upper Mississippi River valley Wednesday
night/Thu. There could be enough thermodynamic/frontogenetic lift to
produce fzdz/dz Wednesday night into Thursday morning. The bulk of the low
level warming will likely go into deepening the saturation
initially. Models currently suggest 03-09z Thursday as the most favorable
time for precipitation.


Arx watches/warnings/advisories...


Short term...04
long term...04

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