Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 211744

Area Forecast Discussion
National Weather Service La Crosse WI
1244 PM CDT Wed Mar 21 2018

.SHORT TERM...(Today and tonight)
Issued at 303 AM CDT Wed Mar 21 2018

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

Data analysis at 06 had high pressure over northern Ont and ridging
SW into MN and Neb. Lead shortwave energy was moving across western
WI to eastern IA, with the -SN that fell across much of MN/IA had
dissipated as drying above 700mb behind the lead wave spread across
the region. WV imagery showed more shortwave energy upstream over
southern Man into Alb, helping to maintain abundant lower clouds
over the eastern Dakotas into western MN.

No issues noted with 21.00z model initializations. Solutions similar
as the Man/Alb shortwave energy slides SE across the region today
then NW flow aloft and rising hgts tonight. Short-term fcst is
generally good this cycle.

For the short-term, a rather quiet period as the high pressure ridge
remains over the Upper Midwest. Mid/high clouds to continue exiting
off to the east early this morning, leaving a period of mostly clear
skies thru much of this morning. Thru the afternoon, 925-850mb flow
backs and becomes more northwesterly to advect the Dakotas/western
MN moisture/clouds into the fcst area, at least into areas west of
the MS River. With a period of sunshine today, model soundings
showing mixing to near 850mb and mixed 925mb temps supporting highs
mainly in the upper 30s to mid 40s. 925-850mb ridge axis progged to
approach tonight, but models showing 925-850mb moisture may be
rather tenacious over the area at last thru this evening. With
subsidence generally progged in the lower levels, will continue with
the trend of decreasing clouds/clearing for much of the area
tonight. With mostly clear skies and light winds overnight, blend of
guidance lows looks good but did lower mins a bit in the normally
colder low laying areas along/NE of I-94.

.LONG TERM...(Thursday through Tuesday)
Issued at 303 AM CDT Wed Mar 21 2018

For Thursday thru Friday night: main fcst concerns this period are
snow chances/amounts with the system Fri night.

21.00z models in good agreement for ridging aloft to build across
the region Thu-Fri. Agreement continues to improve on a lead
shortwave out of the western trough to come thru the mid CONUS
ridging Fri and into the Upper Midwest Fri night. Trend/consensus is
toward a stronger wave/energy surviving the trip thru the ridging
and into the region Fri night. Fcst confidence this period is good
Thu-Fri the average to good Fri night.

A period of dry/quiet weather continues for Thu/Fri as hgts rise and
the sfc high pressure ridge axis slowly drifts east of the region.
Should be plenty of sunshine Thu with 925mb temps supporting near
normal highs in the 40s. 925mb temps a bit warmer on Fri, but
mid/high clouds to be increasing thru the day as upper level
moisture/lift/divergence ahead of the shortwave start to arrive.
Blend of guidance highs for Fri similar to those of Thu is quite

850-500mb warm advection/isentropic lift along with PV advection and
250-300mb ageostrophic flow/divergence between jets maxes spreads
rather quickly NE across the area Fri afternoon. With drier sfc-
700mb airmass in place question becomes how quickly do the lower
level saturate to allow precip to reach the ground. Tightening
consensus shows moisture advection/lift to overcome the dry lower
levels across the SW end of the fcst area by later Fri afternoon.
Model soundings/X-sections showing BL to be warm enough for any
precip Fri afternoon as -RA. After that, thru Fri evening,
evaporative cooling looks to win out with column cooling enough for
precip mainly as snow by later Fri evening and thru the overnight
hours. Given initial dryness of the lower levels, will have to watch
for a faster change over to snow Fri evening with sfc-850mb
evaporative cooling, which would equate to more of the Fri evening
QPF falling as snow. For now will stay with the model consensus and
can detail Fri evening precip types in later fcst cycles. Latest
model consensus now produces 2" to 6" of snow across the SW 2/3 of
the fcst area, with little if any precip NE of I-94. Given this
event is driven by a shortwave crashing thru a mid level ridge,
still plenty of time for the track/strength to change by the time it
arrives Fri night. Stay tuned to later forecasts.

For Saturday thru Tuesday (days 4 to 7): main fcst concerns this
period are lingering SN/-RA chances Sat then precip types/chances
with next system mainly Mon into Tue.

Medium range model runs of 21.00z in decent agreement for the
shortwave to quickly drop SE of the area Sat and ridging aloft to
quickly build in for Sat night/Sun. Models in surprisingly good
agreement for broad SW flow aloft over the central CONUS Mon, then
for the flow to flatten as troughing tracks across south-central Can
Tue. Day 4-7 fcst confidence is average to good this cycle.

With quick passage of the mid level trough axis Sat, lift is also
set to quickly exit. Trend of Sat precip chances mainly in the
morning, across the SW side of the fcst area and mainly as snow
looks good. Anti-cyclonic flow and high pressure for Sat night/Sun
should result in mostly clear skies. A warmer day Sunday with what
should be plenty of sunshine but temps looking to be held down by
the snow fall from Fri night/Sat morning in areas west of the MS
river. Deep sfc-500mb SW flow by Sun night into Monday looks to pull
a plume of Gulf of Mex moisture (PW values up to an inch) northward
into the region. This ahead of low pressure/troughing in the central
plains Mon and ahead of a cold front Mon night/Tue. Details far from
certain by Mon/Tue but consensus -RA/-SN chances mainly in the 20-
50% range OK for now, as are the consensus highs in the 40s both


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1244 PM CDT Wed Mar 21 2018

Main taf concerns are location of MVFR/VFR condition/
stratocumulus deck of clouds...which are currently over much of
Minnesota. Latest metars show clouds are MVFR over central
Minnesota and VFR elsewhere. Latest 21.12z deterministic models
suggest MVFR conditions will remain west of both RST/LSE taf sites
through the period. This is based on dry easterly flow near the
surface allowing for some erosion on the eastern periphery of the
stratocumulus deck. Confidence remains high VFR conditions will
prevail at both taf sites through taf period. With surface ridge
entrenched over northeast Minnesota and northwest
Wisconsin...winds will remain less than 10 miler per hour through
taf period.




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