Area Forecast Discussion
Issued by NWS La Crosse, WI

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839
FXUS63 KARX 171737
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1237 PM CDT Tue Apr 17 2018


.SHORT TERM...(Today and tonight)
Issued at 351 AM CDT Tue Apr 17 2018

Main short-term fcst concerns are clouds/temps today then incoming
wintry system late tonight.

06z data analysis had the broad sfc-mid level low over NE NOAM, with
its deep cyclonic flow still over WI. Clouds have been slow to clear
across WI in this flow. A narrow ridge of high pressure extended
from the MN/Dakotas border to the mid MS valley, with a decrease of
clouds over MN/IA. Further west, pressure were falling as the next
low was taking shape over WY/CO.

17.00z model runs initialized well. Solutions similar as shortwave
ridging builds over the region today and the next strong shortwave
trough swings thru the central/southern Rockies. Some timing/track
differences remain as the Rockies trough moves across the central
plains toward the mid/upper MS valley tonight. Consensus has the mid
level low near KOMA/KSUX at 12z Wed with the main shortwave swinging
into IA. Short-term fcst confidence is average to good this cycle.

In the short-term, clouds problematic today as 925-850mb flow turns
NE this afternoon, ahead of the sfc-850mb developing in the plains.
Plenty of lower level moisture/clouds laying NE of the fcst area
early this morning, to be advected SW today. Model soundings showing
there should be quite a bit of diurnal cumulus under the
strengthening inversion this afternoon. At the same time, the lower
level flow this afternoon is anticyclonic and there is lower level
cold advection/subsidence thru the 850mb layer.  For now, will stay
with the sky grid trend of more mostly cloudy along/NE of I-94 to
mostly sunny west of the MS river. Blend of guidance highs in the
mid 30s to lower 40s for today quite reasonable.

Subsidence/loss of diurnal heating looks to clear out the lower
clouds this evening. Any period of mostly clear skies looks brief as
mid/high level moisture/clouds ahead of the system coming across the
plains already spreads into the area overnight. Initial column
over the area at 06z Wed is dry thru most of its depth, and will
take some time to saturate. Initial stronger/deep layered lift
arriving later tonight works to saturate the column before any
precip would occur. The evaporative cooling during column
saturation along with deep layered lifting of the column still on
track to rather quickly erode the 850-700mb warm layer aloft.
Given lift/divergence aloft, ice in the cloud tops should not be a
problem once the deep lift arrives. Trend is for column to be
cold enough with ice in the cloud tops by the time bulk of precip
would arrive in the fcst area. With the initial dry column, model
consensus is slower to spread precip into the SW side of the fcst
area later tonight. Did continue with mainly a chance for a
wintry mix spreading into the SW side of the fcst area after 09z,
but moved the winter weather headlines to start at 12z Wed.

.LONG TERM...(Wednesday through Monday)
Issued at 351 AM CDT Tue Apr 17 2018

For Wednesday thru Thursday night: main fcst concerns this period
are track of the Wed system and where and how much snow may fall
Wed into Wed evening.

17.00z models in agreement to bring a closed 500mb low east across
the region Wed, continue to differ on its timing and track. GFS the
fast outlier this cycle. Differences in this and the timing/track of
the associated sfc-850mb low have impacts on the sensible weather
across the area Wed into Wed evening. Trough to pass quickly Wed
night, with decent agreement for rising hgts/ridging aloft to build
in Thu/Thu night. Given the detail differences Wed/Wed night having
larger impacts on the fcst, fcst confidence Wed/Wed night is
average, then on the good side for Thu/Thu night.

Pretty much a given the dynamic sfc-mid level low/shortwave trough
will spread a round of wintry precip (mainly snow) across much of
the area Wed into Wed evening. Big question with the differences is
where will the heaviest snow fall and how much. Shortwave/trough was
just coming on the west coast at 17.00z and may not have been fully
captured by the RA-OB network/ Thus the timing/track differences
among the models as it moves across the region Wed/Wed evening.
Consensus has been a slight southward shift in the heavier precip
band. Consensus remains for strong dynamic lifting aloft to quickly
cool the column and erode the warm layer aloft, with cloud tops cold
enough for ice. Precip as snow until it gets to the boundary layer.
Given the load of frozen precip particles falling into the
marginally warm BL, any BL temps above 35-36 degrees expected to
quickly cool to 32-34 degrees, with any mixed precip quickly
changing to snow. With bulk of precip expected to fall as snow Wed
into Wed evening, a swath of 4" to 8" of snow is expected across the
central and south parts of the fcst area. Will issue winter storm
warning for roughly the western 1/3 of the fcst area from 12z Wed to
06z Thu where confidence in 5-8" is higher and blends with
neighboring grids. Will expand the winter storm watch east across
the remainder of the central/south parts of the fcst area from 12z
Wed to 06z Thu. Confidence in snow amounts these areas is not carved
in stone just yet and system could still shift a bit north or south.
Trended most highs Wed down into the lower/mid 30s to account for
falling snow much of the day.

System exits quickly Wed night, with Thu continuing to trend as the
start of string of dry days with gradually warming temps. Thu should
see plenty of sunshine with mixed 925mb temps supporting highs in
the 40s.

For Friday thru Monday (days 4 to 7): main fcst concerns this period
are warming temperatures.

Medium range model runs of 17.00z in reasonable agreement for
ridging aloft to build across the Upper Midwest Fri, a NW flow
shortwave to drop across the great lakes Sat, then more ridging
aloft to build in for Sun/Mon. Fcst confidence in the day 4-7 period
is average to good this cycle.

With ridging aloft, high pressure is the dominate weather feature
across the Upper Midwest Fri-Mon. With dry sfc high pressure over
the region, little more than clouds expected with the passing
shortwave on Sat, with Fri-Mon continuing to trend dry. With the
center of the sfc-850mb high nearby most of this period, little in
the way of warm advection progged, with the airmass having to modify
in place over a melting snow cover. Temps expected to add a few
degrees each day, with highs by Monday looking to be close to
normal. La Crosse may see its first 60 degree high of 2018 by Sunday
or Monday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1236 PM CDT Tue Apr 17 2018

VFR conditions expected through tonight as high pressure remains
in control across the area. Low pressure will then move in
Wednesday morning, spreading snow/IFR conditions into KRST by 14z
and snow/MVFR conditions into KLSE by 16z. Northwest winds will be
less than 10 kt through this afternoon, becoming northeast at 5 to
10 kt overnight.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Storm Watch from Wednesday morning through late Wednesday
     night for WIZ032>034-041>044-053>055-061.

MN...Winter Storm Warning from 7 AM Wednesday to 1 AM CDT Thursday
     for MNZ086-087-094-095.

     Winter Storm Watch from Wednesday morning through late Wednesday
     night for MNZ079-088-096.

IA...Winter Storm Warning from 7 AM Wednesday to 1 AM CDT Thursday
     for IAZ008>010-018-019.

     Winter Storm Watch from Wednesday morning through late Wednesday
     night for IAZ011-029-030.

&&

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION...DAS



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