Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 191142
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
642 AM CDT Wed Apr 19 2017

.SHORT TERM...(today and tonight)
Issued at 344 AM CDT Wed Apr 19 2017

Main fcst concerns this period are SHRA/TSRA chances and heavy rain
threat today/tonight, severe TSRA chances/threats mainly this
evening.

Data analysis at 06z had low pressure over eastern Lk Superior with
a cold front SE across eastern WI to southern IA. Front making
steady progress south/east across the region early this morning as
Can high pressure builds into the northern plains. Plenty of lower
clouds behind the front, trapped in/under an inversion based near
900mb. These under plenty of mid/high clouds streaming NE ahead of
the next low taking shape in eastern CO. Moisture flowing north
ahead of the CO low and over the west end of the cold(warm) front
producing increasing SHRA/TSRA across NEB into SD.

Model runs of 19.00z initialed quite well. Solutions similar in that
they all bring the broad array of shortwave energy over the Rockies
early this morning across the central plains today then into the
western great lakes tonight. Detail differences remain, which
look to originate from the handling of the pieces of shortwave
energy over the Rockies early this morning. These differences
impact the sensible weather outcome this afternoon and tonight.
Overall trend is another northward shift in the track of features
to impact the area today/tonight. Short term fcst confidence is
average this cycle.

In the short term, trend is toward a more progressive system across
the area this afternoon/tonight with lowered heavy rainfall
potential. The northward shift now brings the sfc low into/across
the area along roughly a KCCY-KDEH-KDLL line tonight, allowing the
warm front and `warm sector` to nose into the south end of the fcst
area as the low passes. This for an upward trend in the severe TSRA
risk over the south end of the fcst area centered on this evening.

Even though Nam/GFS/Gem/ECMWF vary on the details for today/
tonight, surprising agreement among the hi-res/CAMs models on the
SHRA/TSRA evolution/timing across the area this period. Hi-
res/CAMs consensus of evolution would lift a band of SHRA/few TSRA
N across the fcst area later this morning thru the afternoon as a
lead shortwave rotates NE across the area and the deeper/stronger
925-800mb moisture transport/theta-e convergence pushes north
over the low level cool dome behind the cold front. The hi-res
model consensus then develops a line of SHRA/TSRA near/just west
of I-35 in the 22- 00z time-frame late this afternoon as a lift
from the main shortwave and sfc-850mb trough axis/front would
approach. Model then quickly move this line east across the area
between 00-07z. With the main precip production period trending
toward a N-S line moving E-W across the fcst area, QPF
expectations are lower, with most rain totals now looking to be
3/4 to 1 1/4 inches. Still cannot rule out locally heavy rains of
2 to 3 inches, but overall risk looks to have diminished. Will not
be issuing any hydro related watches at this time.

At the same time, the more northward track of the sfc low across
the fcst area allows for the potential of more favorable severe
weather parameters of CAPE and shear, along with the warm/cold
fronts into the south end of the fcst area this evening. Sfc
low/warm front progged well south of the fcst area at 00z, with a
rather stout inversion near 900mb and plenty of clouds expected
thru the day. MUCAPE arrives with the 1.25+ inch PW plume late in
the afternoon. Until then soundings showing a rather stable
airmass over the area. Model vary on erosion of the inversion over
the south end of the fcst area this evening. Timing more into the
mid/late evening as CAPE would be waning not the most favorable,
but the presence of strong deep shear, deep forcing/lift and the
low/warm front in the area may well overcome that. If the
inversion/capping can/does erode (some conditionality here) over
the south end of the fcst area for this evening, severe TSRA with
hail, wind, and even a tornado threat would be possible. See
SWODY1 for more details.

For today/tonight, did adjust precip chances to line up with the hi-
res/CAMs model consensus, with highest SHRA/TSRA chances this
evening then tapering SW to NE overnight. Cooler consensus highs
today under E/NE BL flow and plenty of clouds reasonable, as is the
blend of lows for tonight.

.LONG TERM...(Thursday through Tuesday)
Issued at 344 PM CDT Wed Apr 19 2017

For Thu thru Fri night: main fcst concerns this period are lingering
SHRA chances Thu and cooler temps thru the period.

19.00z model runs in decent agreement on troughing to remain over
the region Thu/Thu night as a mid level low develops and northern
stream energy drops into the west side of the trough. Hgts to then
rise Fri/Fri night as the trough exits east and the next energy digs
into the lower/mid MS valley. Fcst confidence this period is average
to good this cycle.

The northward shift of features for today/tonight now places the
wrap-around/deformation band of -SHRA across the north half of MN/WI
Thu morning, lifting NE thru the day as the sfc-mid level trough
axis moves further NE of the area. Consensus -SHRA chances as high
as 50-70% over the north end of the fcst area Thu morning,
diminishing in the afternoon look good and blend well with
neighboring grids. W/Nw winds behind the departing low spread
spread mdt/strong low level cold advection across the area Thu.
With generally cloudy skies and brisk/gusty winds, Thu to be a
raw/cool day compared to the past few. Coldest low level airmass
to be over the area Thu night, but NW winds of 5 to 10 mph and
some lingering clouds (especially NE side of the fcst area) should
keep lows from bottoming too far. Even so, some lows in the
favored cold spots along/NE of I-94 could dip to 32F. High
pressure continues to build in Fri then is more centered over the
area Fri night, under what should be clear/mostly clear skies with
light winds. Warmer/deeper mixing on Fri with plenty of late
April sunshine should return highs to near the normals. Even with
a warmer airmass over the area Fri night, light winds and mostly
clear skies favor strong radiational cooling. Consensus Fri night
lows similar to those of Thu night appear well trended for now.

For Saturday thru Tuesday (days 4 to 7): main fcst concerns this
period are temperatures thru the weekend then SHRA chances early
next week. Day 4-7 fcst confidence is average this cycle.

19.00z Medium range model runs in good agreement for rising
hgts/mid level ridging and sfc high pressure to be over the region
Sat/Sun, for a rather pleasant, dry weekend with highs near the
seasonal normals. Reasonable model agreement for a shortwave
trough to move across the northern plains Mon then into/across the
Upper Midwest Mon night into Tue. SHRA chances mostly 30-50%
centered on Mon night look good for now. Depending on instability,
may yet need a TSRA mention in the Mon afternoon/evening time-
frame but plenty of time to detail that as needed. Passing
low/front Mon night would drag a cooler airmass into the area
behind it, for what would be cooler/slightly below normal temps
for Tue.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 642 AM CDT Wed Apr 19 2017

MVFR ceilings at KRST/KLSE this morning should gradually give way
to rising cloud bases by 19.16-19.18Z. A narrow band of showers
will then lift northeastward across TAF sites this afternoon with
little overall impact to ceilings/visibility. Of greater concern
are more widespread showers and possible thunderstorms later this
evening with a return to at least MVFR ceilings. Timing/coverage
of storms still somewhat uncertain, so will keep thunder out of
12Z TAFs, but something that bears watching. Once showers/storms
move east of TAF airfields, expect a continuation of MVFR
(possibly IFR) ceilings through the rest of the period. North-
northwest wind early this morning will slowly veer through the
day, becoming east-northeasterly this afternoon. Additional
turning to the southeast is possible this evening and overnight
before eventually shifting sharply to the west behind a cold
front.

&&

.HYDROLOGY...Today and tonight
Issued at 344 AM CDT Wed Apr 19 2017

This afternoon and evening there remains potential for widespread
3/4 to 1 1/4 inch to locally 2 to 3 inches of rainfall. It remains
unclear where in the fcst area the heaviest rain threat lies, but
the overall trend is toward a more progressive system and overall
lower rainfall totals thru tonight. Given the continued uncertainty
on where any heavy rain may fall, we will reissue the ESF. If the
details come into focus on where any heavy rain is more likely, a
flood or flash flood watch may yet be needed. Continue to monitor
rainfall forecasts closely.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION...Rogers
HYDROLOGY....RRS



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