Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 231143

Area Forecast Discussion
National Weather Service La Crosse WI
643 AM CDT Thu Mar 23 2017

.SHORT TERM...(Today and tonight)
Issued at 333 AM CDT Thu Mar 232 2017

Main fcst concerns are increasing rain chances thru today/tonight
and TSRA chance later this afternoon/tonight.

Data analysis at 06z had Can high pressure centered near Detroit and
a broad area of low pressure from Saskat to NM. Broad area of
tighter pressure gradient and south flow between the two thru the
plains spreading warmth/moisture northward. Leading mid level
moisture/clouds were spread east across WI while leading lower
clouds/Gulf of Mex moisture was lifting into SE Neb/SW IA. Some
radar returns across central MN/northern WI with the warm advection/
moisture transport near 700mb but few if any report of precip
reaching the ground due to the very dry sfc-800mb airmass with the
Can high still over the region.

23.00z model runs initialized quite well. Solutions quite similar at
the larger scales as the troughing/energy coming thru the central/
southern Rockies and develops a 500mb low over western KS by 12z
Fri. Overall trend favors more progressive/further NE of the earlier
runs with the 500mb low position at 12z Fri. Smaller scale
differences as early as this afternoon (Nam starting to appear as an
outlier) have impacts on the sensible weather across the area thru
tonight and lower the overall short-term confidence. Short-term fcst
confidence is above average but shy of good this cycle.

For the short term, Gulf of Mex moisture spreads quickly NE
into/across the area today, with PW values climbing into the 1+ inch
range this afternoon. With this moisture comes some MUCAPE of as
much as 250-600 J/KG for the afternoon into early evening hours.
Model soundings showing potential for less CIN to convecting 850-
800mb parcels as some stronger 850-700mb warm advection/isentropic
lift and some FN-convergence in this layer would lift NE across the
fcst area with the elevated warm front. Evening crew introduced a
TSRA mention centered on 00z this evening. This looks good and
expanded/refined timing/aerial coverage a bit to match the
frontal/lift timing and neighboring grids. Other focus for today is
a bit more delay in precip chances spreading NE across the area, now
mainly during the mid to late afternoon hours, and more over the
NW half of the fcst area with the stronger low level moisture
transport/theta-e convergence. In fact, the SE end of the fcst
area may well stay dry thru today into this evening. stronger low
level forcing and moisture transport/theta-e convergence remains
focused over the N half of the fcst area tonight, closer to the
sfc-850mb frontal boundary. Some weak MUCAPE progged in the area
later tonight but model soundings showing this cape may be harder
to release with more warming/capping around 700mb and lesser
overall lift thru the column. Left TSRA mention out of the late
tonight into Fri morning grids for now.

Even lesser confidence on precip amounts this afternoon/tonight.
Models `all over the board` with potential QPF thru tonight, ranging
from totals of few hundreths to upwards of an inch. Not to
mention differences where the heavier QPF would occur. Will stay
with the model/ensemble consensus QPF thru tonight for now. Will
just have to plan on some shorter term adjustments/updates when it
become clearer where the potential heavier band of rain is going
to set up.

.LONG TERM...(Friday through Wednesday)
Issued at 333 AM CDT Thu Mar 23 2017

For Friday thru Saturday night: main fcst concerns this period are
rain chances and amounts mainly Fri thru Sat.

23.00z model runs remain similar enough at 500mb as the mid level
low slowly migrates to western MO by 12z Sat. Solutions then begin
to diverge Sat/Sat night as the mid level continues E/NE Sat/Sat
night. Even with these differences, the lower level features remain
rather similar, with all models progging the sfc low about 1009mb
over central IL at 12z Sun. Fcst confidence for Fri thru Sat night
is average to good this cycle.

Confidence is high that it will rain across much of not all of the
area Fri thru Sat. This as the main forcing/lift with the closed sfc-
500mb circulation approaches/moves into the mid MS valley. As
expected, plenty of detail differences when/where the stronger
surges of moisture/lift rotating around the low will move across the
area, so will stay with the model/ensemble consensus. Highest rain
chances across the north half of the fcst area Fri morning with/near
the frontal/F-Gen forcing, translating slowly south across the fcst
area Fri afternoon/night as the mid level circulation approaches. By
Sat afternoon/evening the sfc-700mb FN convergence north/east of the
circulation begins to weak and shift into the OH valley, along
with the stronger of the moisture transport and higher PW
airmass. Rain chances decreasing to the 20-50% range by Sat night
reasonable. Detail differences where the main F-Gen precip band
would be across the area from Fri into Sat morning, and where any
heavier QPF would fall. Most Individual models/ensemble members
produce near 2 inches or more of rain in their main F-Gen precip
band, but the issue is where. Again will stay with the
model/ensemble consensus QPF across the area these periods, which
produce totals of 1 to 1.5 inches from tonight thru Sat.

For Sunday thru Wednesday (days 4-7): main fcst concerns this period
are small/lingering rain chance Sun, above normal temps into the
middle of next week.

Model runs of 23.00z show decent agreement on the mid level low
to fill/open up Sun as it is pushed east by the next shortwave
troughs into both the central and northern plains. Enough lower
level moisture/cyclonic flow looking to remain over the area on
Sun for continued small -RA chances, mainly over the east side of
the fcst area. Drying Mon/Tue/Wed reasonable with area split
between one trough/shortwave passing north of region and the
southern stream wave crossing the mid/lower MS valley. Reasonable
model consensus for mid level shortwave ridging to build across
the region for Tue into Wed. Thru all this, the flow across NOAM
next week remains split, with the colder air held well north in
Can. Even with sfc high pressure nosing in from the north Tue/Wed,
temps into the middle of next week trending a category or two
above normal look well trended.


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 643 AM CDT Thu Mar 23 2017

MVFR stratus will overspread the TAF sites by this afternoon.
Showers and possibly a thunderstorm could impact the TAF sites
this afternoon into this evening. Then plan on ceilings lowering
overnight to IFR as steadier light rain develops. Visibilities may
also drop to IFR at times tonight. Low level wind shear will also
be a concern starting around 01Z at KLSE and around 06Z as
south/southwest winds increase to 40 to 50 kts at 2 KFT. The low
level wind shear will subside by 09Z. Look for IFR conditions to
then continue through 12Z.


.HYDROLOGY...(Thursday night through Wednesday)
Issued at 333 AM CDT Thu Mar 23 2017

It continue to look like the area will be in a prolonged rainy
period from tonight into Sat night as deeper moisture/lift spread
slowly across the region. PW values in the range of 2 to 3 std
deviations above normal across the area much of this period.
Depending on the track of the sfc-mid level low thru the mid MS
valley during this period, the potential exits for a band of
rainfall totals near/greater than 2 inches somewhere across the
fcst area. Otherwise, most rain totals will likely be in the 1 to
1.5 inch range. With this rain spread out over as much as 2 1/2
days, only anticipating within bank rises on area rivers at this




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