Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 181540
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1040 AM CDT Wed Apr 18 2018

.UPDATE...Late This Morning into the This Evening
Issued at 1039 AM CDT Wed Apr 18 2018

RAP and HRRR models show that a tight 700 to 500 mb frontogenetic
band will pivot northward toward the Minnesota and Iowa border
early this afternoon and remain in that general area for the
remainder of the area into the early evening.

Due to this, we have expanded the Winter Storm Warning to include
Richland and Vernon counties in southwest Wisconsin and Houston
County in southeast Minnesota. Snow totals of 4 to 8 inches inches
will be found in southwest Wisconsin and 5 to 9 inches for
northeast Iowa and for the Minnesota counties along the Iowa
border. There may be some locally higher amounts. Meanwhile,
further north and south of this band, the snow totals were
tightened.

&&

.SHORT TERM...(Today and tonight)
Issued at 345 AM CDT Wed Apr 18 2018

Main short-term fcst concerns are the storm track today, where and
how much snow will fall.

Data analysis at 06z had 995mb moving across eastern KS. WV imagery
depicted a vigorous shortwave moving east along the KS/NEB border.
regional radar composite showing precip developing over eastern
Neb/NW IA were moisture/lift/saturation ahead of the shortwave are
finally coming together. Initial look shows this reflectivity
north of where bulk of models were putting it at 06-07z.
Otherwise, clouds increasing over the region with early morning
temps mostly in the upper 20s to mid 30s.

No issues noted with 18.00z model initializations. Solutions similar
with the big pictures as a mid level low develops and moves east
across northern IA and along WI/IL border today. However models
differing in the meso scale details, which have significant impacts
on the potential snowfall across the area today. Difference seem to
be tied to the fractured shortwave energy initialized in the trough
over the Rockies, and how the pieces congeal or rotate around the
trough/low as it crosses the region today. short-term fcst
confidence is not what one would like it to be given the fcst today.
Confidence for today/tonight is below average this cycle.

For the short-term, model detail differences raising havoc with
sensible weather outcomes across the area today. NAM/GFS/EC/Gem
remain more northerly with their solutions and spread significant
snowfall across roughly the south 2/3-3/4 of the fcst area. Most
high-res/CAMs models more southerly and keep bulk of the snow across
NE IA/far SW WI. Again, seems to be tied to the handling of the
shortwave energy in the trough/low and track of the mid level low
across the region today. May not know the outcome here until we can
see the whites of its eyes. I.E., keeping and eye on regional radar
trends early this morning to see where precip band(s) set up and
which group of models is verifying better. Signal remains for the
warm layer aloft to quickly erode this morning with dynamic forcing/
lift/cooling aloft as the mid level trough approaches. With some
delay in precip arrival, and the deep forcing/lift, where precip
occurs the column will be saturated thru the -10C to -15C layer
for ice in the cloud tops. Frozen precip loading into the shallow
warm boundary layer still points to most if not all precip across
the fcst area falling as snow. Just the big question of where and
how much snow falls as the system quickly crosses the Upper
Midwest today/this evening. NAM/GFS/EC/GEM consensus would say
current fcst for today in reasonable shape with only minor
realignment of the headlines, while CAMs would say a major revamp
is needed along/north of I-90. Based on 06z-08z radar trends vs.
model qpf output, appears more northerly solutions may still be
more correct than the southern CAMs grouping. For now will proceed
cautiously with headline/ expected snowfall adjustments. For now
generally shifted headlines south about 1 row of counties, and did
add Crawford/Grant counties to the warning with the heaviest snow
band looking to extend into SW WI. Reduced snow amounts near/north
of I-90, while will have to watch the heaviest snow band for snow
totals in excess of 10". For now a 6-8" band of snow in the
warning area blends well with neighboring grids. Previous fcst
grid highs today in the lower/mid 30s looking well trended with
the expected thick cloud cover and snowfall.

.LONG TERM...(Thursday through Tuesday)
Issued at 345 AM CDT Wed Apr 18 2018

For Thu thru Fri night: Main fcst concern this period is warming
temperatures over the snow cover.

18.00z model runs remain consistent with the Wed low/trough quickly
exiting Wed night and rising hgts/ridging aloft to build into/across
the Upper Midwest for Thu thru Fri night. Fcst confidence in this
period is generally good this cycle.

As ridging aloft builds in Thu-Fri, a ridge of sfc high pressure
parks itself across the Upper Midwest under it. Thu/Fri to start a
dry/quiet and warming period across the area. With the sfc-850mb
ridge nearby or overhead, little in the way of warm advection
expected with airmass having to modify in place. This over melting
snow cover. which will tend to slow the warming by late April
sunshine. Mixed 925mb temps support highs in the 40s to around 50
Thu and mid 40s to perhaps mid 50s Fri. Still cool at night though
with some snow cover, light winds and mostly clear skies.

For Saturday thru Tuesday (days 4 to 7): main fcst concerns this
period are continued warming temps Sat thru Mon then some rain
chances by Tue.

Medium range models of 18.00z in good agreement for ridging aloft to
remain over the Upper Midwest Sat into Mon. Lesser agreement for a
shortwave trough to drop into the region Tue. Fcst confidence is
average to good Sat-Mon then average Tue.

Under the ridging aloft Sat-Mon, sfc high pressure is progged to
remain over the great lakes with ridging westward across the region.
Airmass to continue modifying and the snow melting. Potential  for
some clouds to spread across the area Sat to temper the warming, but
by Sun/Mon skies expected to be partly cloudy/mostly sunny with
temps warming into the 50s to near 60 Sun then into the mid 50s to
lower 60s for Mon. Most of the snow cover should be gone by Sun/Mon.
The shortwave trough for Tue pushes a cold front and some moisture
into the region. The consensus small chances of rain Tue along with
temps a bit cooler than Mon look good for now.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 620 AM CDT Wed Apr 18 2018

Cigs: quick drop into ifr/mvfr this morning as storm system slides
south of the region. Should clear this evening as the system quickly
exits east.

WX/vsby: snow moves in this morning, roughly 14-16z. Expect
reductions to 1-2sm by 17-20z, likely holding there until 00z or so.
Still could see some 1/4-1/2sm snows if heavier snow bands shift
just a bit farther north. Something to watch closely. Bulk of
accumulating snows will be done by 00z.

Winds: northeast becoming north this morning, gradually becoming a
bit more northwest for Thu morning.


&&

.HYDROLOGY...(This Weekend)
Issued at 345 AM CDT Wed Apr 18 2018

By Thu/Fri, more sunshine and warming temperatures will produce
significant melting of the snowpack in place across the area,
leading to a good amount of runoff into area streams and rivers.
Besides the Trempealeau River at Dodge possibly exceeding flood
stage this weekend, the rest of the area streams and rivers are
expected to just see significant within-bank rises. Those with
interests or property close to streams and rivers will want to stay
up to date on latest river forecasts.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Storm Warning until 1 AM CDT Thursday for WIZ053>055-061.

     Winter Weather Advisory until 1 AM CDT Thursday for WIZ041>044.

MN...Winter Storm Warning until 1 AM CDT Thursday for MNZ094>096.

     Winter Weather Advisory until 1 AM CDT Thursday for MNZ086>088.

IA...Winter Storm Warning until 1 AM CDT Thursday for IAZ008>011-018-
     019-029-030.

&&

$$

UPDATE...Boyne
SHORT TERM...RRS
LONG TERM....RRS
AVIATION.....Rieck
HYDROLOGY....DAS/RRS



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