Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 240322

Area Forecast Discussion
National Weather Service La Crosse WI
922 PM CST Mon Nov 23 2020

Issued at 921 PM CST Mon Nov 23 2020

The latest CAMS have slowed the northward progress of the snowband
overnight. As a result, there was an adjustment made to the
arrival of the snow in northeast Iowa and southwest Wisconsin. At
this time, it looks like it will not make it into the I-90
corridor until 6-8 AM. This is 3 to 6 hours slower than it looked
like this afternoon. Some of this is a result of the dry conveyor

The 24.00z NAM 4km greatly increased its snow totals near the
Interstate 90 corridor on Tuesday. There appears to some
convective feedback. One indicator is the very strong 700 mb
moisture convergence with 60 knots converging with 10 knots over a
3 county area. While this seems unrealistic, it may be an
indication of a potential for convective snow bands development.


.SHORT TERM...(This evening through Wednesday)
Issued at 225 PM CST Mon Nov 23 2020

Very tricky forecast over the next couple days as a wintry system
brings a mix of snow, rain, and perhaps a little bit of ice to
the area tonight through Wednesday. Early afternoon analysis
showed an area of snow moving through southern Iowa and northern
Missouri, tied so some shortwave energy sliding through. The
general idea is that this area of snow will expand northward
tonight as additional shortwave energy moves in and moisture
transport, warm air advection, and isentropic lift all increase
east of surface low pressure in the lee of the Rockies. Looking
like a fairly impressive moisture transport signal, with
precipitable water values getting into the 0.75" to 1" range,
which is above the 90th percentile of climatology for this time of
year. Good agreement in short term guidance of a decent band of
snow forming on the nose of said moisture transport, with some
frontogenesis showing up in the lower to mid levels. Latest timing
suggests this band would move into the forecast area from the
south just before midnight and reach the I-90 corridor or perhaps
just north by 8 a.m. or so. Therefore, would expect impacts to the
morning commute for areas near and south of I- 90.

While that sounds straightforward, this is one of those cases
where temperatures will play a large role in precipitation types
and snow amounts, as temperatures look to hover right around or
just above the freezing mark through the morning. Therefore, it`s
looking like ridge tops could see a bit more snow than valleys.
Competing elements will be going into the temperature forecast, as
southerly flow will be bringing in warm air through the morning,
but some pockets/bands of heavier snow will keep temperatures from
increasing too fast. So again, a very tricky forecast here. As
far as amounts go, HREF guidance suggests a band of of 4"+
somewhere in the area with more widespread amounts in the 1-4"
range. Consensus would place the heaviest band somewhere along and
near a line from roughly Charles City, IA, to Adams, WI, again
with highest amounts at higher elevations. While rates don`t look
overly impressive, generally remaining below 1" per hour according
to the HREF, they should still be high enough for snow to
accumulate on some potentially warmer roads, especially where some
slightly more convective elements form. It should be noted that
some short term guidance suggests some 6" amounts are not out of
the question, but hesitate to go that high given the low
confidence in the temperature forecast. For now, going with a
Winter Weather Advisory for all of northeast Iowa, parts of
southeast Minnesota, and into southwest and central Wisconsin.
Will need to keep an eye on temperature trends for potential
expansion this evening.

Snow should gradually switch to rain through the day with nearly
the entire area being rain by mid-afternoon (other than
Taylor/Clark counties). Temperatures should generally stay high
enough for rain to be the main precip type Tuesday night into
Wednesday, though with loss of in-cloud ice, could see some light
freezing rain where temperatures get closer to freezing (again,
more Taylor/Clark counties). That said, ice accumulations look to
be pretty minimal as the heavier precipitation looks to remain
south of there. For Wednesday, the surface low and 500 mb trough
will slide south of the forecast area, with rain continuing to
fall through at least the morning hours, mainly along and south of
I-90. Temperatures once again look to remain above freezing
during this time period, so would not anticipate much, if any,
wintry precipitation.

.LONG TERM...(Wednesday night through Monday)
Issued at 225 PM CST Mon Nov 23 2020

The shortwave and associate surface low pressure system continues
eastward, away from the region into the northeast US. The flow
aloft becomes southwesterly as the upper-level jet swings around a
deeper longwave trough stretched from northern Saskatchewan,
Canada into the far southwest coastal region of the US. An
associated shortwave trough moves across the region Friday, but
not expecting precipitation with its passage. A dry forecast is
expected to hold through the end of the week into the weekend.
Afternoon temperatures reach into the 30s and 40s with overnight
lows dipping into the 20s, which would be around the seasonable
normals to slightly above.

The upper-level flow becomes disjointed forming a mid to upper-
level cutoff low that slowly moves through the desert southwest of
the US over the weekend. Another shortwave swinging eastward
along the Canadian and US border moves through the region
returning precipitation chances to the area on Sunday. Decided to
stick with the blend as there are differences between the models
on the progression of this wave.


.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 529 PM CST Mon Nov 23 2020

A low pressure area currently over eastern Colorado will move east
into eastern Kansas by 25.00z. As this occurs, a snow/rain band
will move northeast across the forecast area. This band will
produce IFR/MVFR ceilings and visibilities.

This band will move into KRST around 24.08z. It will be initially
be snow and the transition to rain and snow by 24.16z and rain by
24.19z. Snow amounts will be in the 1 to 2 inch range.

This band will move into KLSE around 24.09z. It will be initially
be rain and snow and transition to rain by 24.18z. Snow amounts
will be up to 1 inch.


WI...Winter Weather Advisory until noon CST Tuesday for WIZ041>044-

MN...Winter Weather Advisory until noon CST Tuesday for MNZ088-095-

IA...Winter Weather Advisory until noon CST Tuesday for IAZ008>011-



LONG TERM...Peters
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