Area Forecast Discussion
Issued by NWS La Crosse, WI
FXUS63 KARX 181029
Area Forecast Discussion
National Weather Service La Crosse WI
530 AM CDT WED MAY 18 2016
.SHORT TERM...(Today and tonight)
Issued at 358 AM CDT Wed May 18 2016
Main fcst concerns this period: slowly warming temperatures.
Data analysis at 06Z had broad, cool and dry Canadian high pressure
centered over the Upper Midwest. Skies were generally clear across
the region. With light/calm winds, the drier boundary layer airmass
and strong radiational cooling, temperatures had already cooled to
the 30s across much of northeast MN and the northeast 2/3 of WI.
Frost/ freeze headlines for much of WI and nearby areas looks on
track early this morning.
No issues noted with 18.00Z model initializations. Solutions very
similar as mid level troughing exits east of the region today and
shortwave ridging builds over MN/IA/WI tonight. Trend is toward
slower of the earlier runs on the exit of the trough today, then a
faster/stronger with the next shortwave trough/energy into MAN/ND by
12Z Thu. These trends look to have little impact on the fcst for
today/tonight with short-term fcst confidence good this cycle.
In the short term, a very quiet weather period for today/tonight as
hgts rise aloft and the sfc high remains nearly stationary. Model
soundings/X-sections showing deep mixing again today, to around
800MB, with some near saturation around the top of the mixed layer.
Few/sct high-based cumulus expected again today, much like Tue.
With plenty of sunshine today, the low level airmass continue to
slowly warm in place (with the sfc-850MB ridge nearby/overhead
advection remains weak). 850MB temps at 00Z Thu progged to be 1C to
2C warmer than those at 00Z Wed, with highs today looking to be 2F
to 4F warmer than those on Tue. 925MB temps at 12Z Thu progged to be
2C to 3C warmer than those at 12Z this morning. Another cool night
tonight under light winds, dry air and clear skies. However lows
should be more in the mid 30s to lower 40s eliminated much of the
frost threat. May yet see some patchy frost in the usual cold low-
laying areas along/northeast of I-94 and added this to the grids for
late tonight. With the model consensus highs now having been 2-3F
too cool the past 3 days, again sided with the warmer of the
guidance highs today. With another strong radiational cooling night
favored cooler of the guidance lows, especially along/northeast of
.LONG TERM...(Thursday through Tuesday)
Issued at 358 AM CDT Wed May 18 2016
For Thursday thru Friday night: Main fcst concerns this period
continue to be temperatures and cloud trends.
18.00Z model runs offer improving agreement as the ND/MAN shortwave
drops slowly southeast across the region Thu thru Fri night. Wave
generally weakens as the strong troughing/mid level low near the
west coast amplifies the ridging over central NOAM that the ND/MAN
wave is moving thru. Run-to-run consistency generally good this
period with fcst confidence for Thu thru Fri night also good.
Generally tranquil/seasonable weather continues this period with the
sfc ridge axis moving little. In fact, sfc ridge axis progged to
retrograde and be centered back over WI Fri night. With the ND/MAN
shortwave fighting the building ridging around it and the sfc-850MB
ridge axis nearby/overhead for little in the way of advections, fcst
continues to trend dry Thu thru Fri night. 850MB ridge axis does
shift far enough east of the area Thu thru Fri for some southerly
flow and a slow moisture increase, at least around 850MB. Model
soundings/X-sections showing this moisture increase. Trend would be
for more sct cumulus Thu afternoon then a sct to perhaps bkn diurnal
cumulus deck on Fri. Lower level airmass continues to modify in place
with 850MB temps rising a degree or 2C each day with high by Fri
mostly 70-75F, which would be above the normals. With the bit of
southerly 850MB flow/moisture sfc dew points to gradually rise as
well with the afternoon airmass not quite so dry by Fri. Favored
warmer of the guidance highs Thu/Fri. With the airmass drier than
normal and light winds favoring strong radiational cooling at night
the cooler of the guidance lows were favored Thu/Fri nights.
For Saturday thru Tuesday (days 4 to 7): Main fcst concerns this
period are SHRA/TSRA chances returning Mon thru Tue, warmer
temperatures thru the period.
Medium range model runs of 18.00Z offer improving agreement as
ridging aloft builds over the plains Sat the moves east into the
lower thru upper MS valley Sun. Trend favors a compromise of the
earlier runs Sat/Sun. Improving agreement for the first stronger
piece of energy out of the western CONUS troughing to eject into the
northern plains Mon. This to push the ridge axis east of the region
for southwest flow aloft over the area for Mon/Tue. Fcst confidence
for Sat/Sun is good, then average Mon/Tue under southwest flow
aloft and more details to be concerned with.
The sfc ridge axis to remain over the Upper Midwest Sat/Sat night as
the ridging aloft builds over the plains. As the lead shortwave
begins to eject into the plains Sun and lee troughing strengthens
the high is pushed east and lower level southwest flow begins to
strengthen over the region. The moisture plume in the plains begins
to move east as well. This moisture along with an increase of
instability is slated to move into the fcst area Mon then across the
fcst area Mon night/Tue. lower level thermo-dynamic forcing/lift
increases over the Upper Midwest as well, along with broad
divergence aloft in the deep southwest flow ahead of the trough.
Timing is likely to shift with later cycles, but present consensus
of SHRA/TSRA chances spreading into the west side of the fcst area
Mon then across the fcst area Mon night/Tue reasonable at this time.
By Sun, and for Mon/Tue 850MB temps in the +10C to +15C range,
which would support highs in the mid 70s to lower 80s. Model/
ensemble consensus highs mostly in the mid 70s reasonable Mon/Tue
with expectations of more cloud cover. However Sun still looking to
be a mostly sunny day, and consensus highs mostly in the mid 70s are
likely a category or 2 too cool. Left Sun highs as is for now as
they blend well with neighboring grids. Consensus highs Sat may be a
few degrees too cool as well given a mostly sunny day.
.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 530 AM CDT Wed May 18 2016
High pressure meanders slowly eastward across the region
today...gradually shifting across the eastern Great Lakes late Thu
night. A deep light wind layer should result for today...favoring
light/vrb winds. Winds should start to steady into a more southerly
direction later tonight. Could see few/sct vfr afternoon cu - in 5-6
kft range. Elected to not add to forecast at this time.
WI...Frost Advisory until 8 AM CDT this morning for WIZ017-029-