Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 271735

Area Forecast Discussion
National Weather Service La Crosse WI
1235 PM CDT Thu Apr 27 2017

.SHORT TERM...(Today through Friday night)
Issued at 325 AM CDT Thu Apr 27 2017

An intense upper shortwave was lifting up towards Lake Michigan
early this morning. Strong mid/upper synoptic scale lift along with
low to mid-level frontogenetic forcing has led to a band of precip
streaking northeastward this morning up into the Upper Midwest,
with the western flank rotating across far northeast Iowa into
western Wisconsin. So far, temps have been warm enough for all
rain. However, with continued cold advection, thermal profiles
should become sufficiently cool for a wintry mix (sleet/freezing
rain/snow) especially across parts of central/north-central
Wisconsin. As any residual warm air aloft moves east after
sunrise, precip may end as snow in these areas, dependent on near
surface temps. Impacts should be minor overall with temps near
freezing, but will have to watch areas north of I-94, especially
into Taylor Co, if any locally heavier deformation bands lead to a
rather short period of enhanced precip rates. Some light icing on
elevated surfaces and light slushy snow accumulations could occur
before the precip ends by late morning creating some slick spots
on roads.

Surface ridging begins to work into the area tonight. As this
happens, model soundings indicate drying in the low-levels as the
upper trough begins to pull away. There is some question as to how
quickly the low-level moisture/clouds can be scoured out, impacting
just how low temps fall. However, with cool air already entrenched,
temps are expected to fall into the low to mid 30s, with 20s more
common across central/north-central WI. Will issue a frost advisory
for the Mississippi River Valley counties from La Crosse/Winona
counties southward for late tonight/early Friday. As of now, we
have not begun issuing frost advisories outside of these counties,
based upon the overall progression of vegetation so far.

Tomorrow an upper shortwave ejecting from a developing upper trough
to the west will get sheared out along an upper jet within confluent
flow. The upper jet/shortwave forcing along the established mid-
level baroclinic zone just to the south will lead to the
development of some showers. Model guidance has favored the area
south of I-90 for rain chances. The ECMWF has continued to remain
the driest solution with a weaker upper wave. With a lot of dry
air to the north of the boundary as the surface high slowly tries
to sink south, precip may struggle make much northward progress.
As a result, keeping the higher rain chances across northeast
Iowa/far southwest Wisconsin still looks good on Friday. Lows down
into the upper 20s to upper 30s are likely again Friday night.

.LONG TERM...(Saturday through Wednesday)
Issued at 325 AM CDT Thu Apr 27 2017

After a dry and cool start to Saturday under the influence of
high pressure, wet weather quickly returns by later Sat/Sat night.
A deep upper low will lift towards the central plains and close
off by Sunday afternoon. Ahead of the upper wave and surface low
Sat night/Sunday strong northward moisture transport and low-
level warm advection/frontogenesis will lead to widespread
rainfall developing. Plenty of Gulf moisture will be available to
be tapped ahead of the wave, with precipitable water values rising
back up to around 1.25 inches again. This rainfall could
potentially lead to some hydro concerns, especially river rises.
Current model runs show the strongest moisture transport east of
the area, though. Ensemble guidance does indicate a good amount of
spread in the the track of the surface cyclone, which would impact
where the main deformation zone precip band will set up and
greater threat for wintry precip, but current model runs right now
keeps that area to the west. However, even in this case some
snow/wintry mix could occur as the system pulls away. All that
said, the system is still several days out, and details, including
the precip amounts/types are bound to change heading closer to
the weekend.

As the system becomes stacked and slowly lifts northeast early next
week, wrap around precip could hang around into Mon/Mon night before
the system begins to pull away and northwest flow aloft develops.
Temps will be coolest Sun/Mon as the system impacts the area with
highs mainly in the 40s, before gradually rebounding heading into
the middle of next week.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1235 PM CDT Thu Apr 27 2017

MVFR cigs mostly 1500-2500 ft to gradually lift into the 2500-3500
ft range thru the afternoon. Then question becomes when will the
cloud decks dissipate/clear out. Latest set of model soundings and
lower level RH progs showing the clouds to linger thru the evening,
then decrease around and shortly after midnight as stronger
subsidence and a lower level ridge of high pressure builds east into
the region. Generally good VFR then expected thru the over night
hours and Fri morning with the drier sfc-700mb ridge axis over the
area. West-NW winds this afternoon will decrease late this
afternoon/early this evening, then generally be SW to NW 5-8kt or
less during the late evening thru morning hours of Fri.


WI...Frost Advisory from 1 AM to 8 AM CDT Friday for WIZ041-053-054-

MN...Frost Advisory from 1 AM to 8 AM CDT Friday for MNZ088-096.

IA...Frost Advisory from 1 AM to 8 AM CDT Friday for IAZ011-030.



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