Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 221047

Area Forecast Discussion
National Weather Service La Crosse WI
547 AM CDT Thu Mar 22 2018

.SHORT TERM...(Today and tonight)
Issued at 235 AM CDT Thu Mar 22 2018

Main short-term fcst concerns are cloud and temperature trends.

06z data analysis had high pressure ridging north-south across the
Upper Midwest. Another night of light winds across the region, with
clear skies under/east of the ridge axis and strong radiational
cooling over much of WI/NE MN. West of the sfc ridge axis, some sct-
bkn-ovc lower clouds remain over much of MN/western IA, with early
morning near/above freezing.

Model runs of 22.00z initialized well. Models offer a rather tight
consensus of solutions today/tonight as ridging aloft builds east
across the central CONUS and sfc high pressure ridging remains over
the western great lakes. Short term fcst confidence is good this

In the short term, a continuation of the quiet weather today/tonight
under the influence of rising hgts aloft and sfc-700mb high
pressure. High pressure to the north of Lk Superior strengthens a
bit later today/tonight, with 925-850mb flow to swing from NW this
morning back to the NE and east later today/tonight. This lower
level flow with an increasing easterly component to spread drier low
level air westward across the area. Clouds on the west edge of the
fcst area may linger for a time this morning, but are expected to be
pushed west this afternoon/tonight. Will have to watch for some
lower relative humidities across the WI portion of the fcst area
again this afternoon as it pertains to fire weather. This drier low
level flow and some weak subsidence under/east of the sfc-805mb
ridge axis to keep the clouds at bay, with sunny/mostly sunny skies
expected today and mostly clear skies tonight. Mixed 925mb temps
today supporting highs mainly in the 40s. Light winds, mostly clear
skies and a dry low level airmass favor strong radiational cooling
again tonight. Leaned toward cooler of guidance lows tonight,
especially near/NE of I-94.

.LONG TERM...(Friday through Wednesday)
Issued at 235 AM CDT Thu Mar 22 2018

For Friday through Saturday night: Main fcst concerns this period
are the snow chances/accumulation potential Fri night/Sat morning.

22.00z model runs in reasonable agreement that the shortwave energy
well off the southern CA coast this morning will move into the
central plains by 00z Sat then across southern IA/northern MO/
western IL by 12z Sat. This track favors more southerly of the
earlier runs. Good agreement for hgts to quickly rise behind the
departing shortwave trough with ridging aloft by later Sat/Sat night.
Fcst confidence this period is average to good this cycle.

Fri to start out dry with increasing mid/high clouds as lead
moisture ahead of the shortwave trough and 250-300mb ageostrophic
flow/divergence arrive. 925-700mb moisture eventually arrives across
the SW side of the fcst area in the afternoon as the sfc-850mb
circulation moves across the central plains and moisture transport
increases. Lower/mid level thermo-dynamic forcing strengthens/
deepens thru the afternoon as well with approach as hgts fall across
the plains into Upper Midwest ahead of the shortwave. Dry easterly
low level flow holds over much of the fcst area thru Fri, but decent
consensus for increasing moisture/lift overcome it across the SW
side of the fcst area by late afternoon. 925-850mb temps generally
cold enough for precip as snow by later Fri afternoon, but BL to
perhaps 925mb layer looks to remain warm enough for precip mainly as
-RA before 00z Sat. Precip then looks to quickly change to SN with
BL/evaporative cooling and some 925mb cold advection. Models then
maximize the lift over the fcst area Fri night with a mid-level Fgen
band and strong divergence aloft ahead of the shortwave and between
jet maxes SW and NE of the area. However, all this lift continues to
fight the easterly feed of dry sfc-850mb air out of the high over
the great lakes across the area. Present model consensus has this
lower saturation/non-saturation tension zone along roughly at KRGK-
KLSE-KOVS line. Strong signal for a very tight snowfall gradient on
the east side of the snow band, somewhere NW to SE across the fcst
area. NE of the KRGK-KLSE-KOVS line, little if any snow may fall,
while models producing around an inch of QPF over the SW side of the
fcst area and upwards of 6" of snow. With the shortwave still well
off the CA coast and plenty of time/distance for track changes,
still uncertainty on location of that sharp east side snowfall
gradient. For now will issue a winter storm watch west of the KRGK-
KLSE-KOVS line 00z-17z Sat. Dry, quiet weather returns by Sat night
as sfc-mid level high pressure build in. Blend of guidance highs/
lows for Fri-Sat night appear to have them well trended for now.

For Sunday thru Wednesday (days 4 to 7): main fcst concern this
period are rain/snow chances mainly Mon thru Tue.

Medium range model runs of 22.00z in good agreement for the pattern
to progress Sun/Mon with the ridge axis moving east of the region
and broad SW flow aloft over the central CONUS Sun into Tue. Even a
good consensus late Tue into Wed for a northern stream wave to track
across the north-central CONUS and transition the flow to more
westerly and drier by mid-week. Day 4-7 fcst confidence is above
average this cycle.

Area to be under the influence of dry, seasonable high pressure Sun,
with the deep, broad SW flow spreading warmth and a plume of Gulf of
Mex moisture (PWs in the 1" range) northward into the area by Mon.
This ahead of waves of low pressure in the central plains and a cold
front to push across the region Tue. Plenty of deep layered lift
progged across the region centered on Mon night. Timing could still
shift, but given the high PW airmass and good model consensus,
precip chances as high as 60-80% centered on Mon night OK for now.
Column with this early next week system looking warm enough for the
bulk of any precip Mon into Tue to be -RA. Abundant cloud cover
Mon/Tue, and potential precip looking to keep highs these days
mainly in the 40s to lower 50s. Tue front to usher drier, but
seasonable high pressure into the region for Wed.


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 546 AM CDT Thu Mar 22 2018

A band of MVFR clouds over Minnesota has been slowly moving east
overnight and has spread over KRST. The main concern becomes how
long these clouds will persist. Looking at forecast soundings from
the 22.08Z RAP, this would suggest the clouds will stay MVFR
through about the middle of the morning before either lifting to
VFR or scattering out as the depth of the moisture layer
decreases. Plan to carry the MVFR ceilings until 15Z but concerned
they could persist longer. Will also include the possibility of
these clouds reaching KLSE this morning with a VFR ceiling. VFR
conditions expected for the afternoon and night with high
pressure extending from Ontario into the region.


WI...Winter Storm Watch from Friday evening through Saturday morning
     for WIZ054-061.

MN...Winter Storm Watch from Friday evening through Saturday morning
     for MNZ079-086>088-094>096.

IA...Winter Storm Watch from Friday evening through Saturday morning
     for IAZ008>011-018-019-029-030.



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