Special Weather Statement, NWS La Crosse, WI

Area Forecast Discussion
National Weather Service La Crosse WI
638 AM CDT Wed May 17 2017

.SHORT TERM...(Today and tonight)
Issued at 416 AM CDT Wed May 17 2017

main fcst concerns are SHRA/TSRA chances thru tonight, heavy rain
and severe TSRA threats this afternoon/evening.

Data analysis at 06z had a 993mb low near the OK/CO/KS border with
troughing to the NE to another low lifting NE across Ont. Sfc
front with the trough combined with shortwaves rippling thru the
broad SW flow aloft and the low level jet bringing PW values of 1.5+
inches northward and MUCAPE in the 1K to 2K j/kg range, SHRA/TSRA
rather widespread across western WI/southern MN and central/western
IA. Convection has subsided from earlier in the evening, but
stronger TSRA still producing 1 to 2 inch/hr rain rates and gusty
winds. A summery night across much of the region with early morning
temps mostly in the mid 50s to lower 70s.

Other than GFS again too high with its sfc dew points across IA,
17.00z model runs initialized quite well. Solutions quite similar
as the mid level low near the OK panhandle lifts into western IA by
00z Thu, opening up and lifting into northern WI by 12z Thu. Short-
term fcst confidence is generally good this cycle.

In the short term. One lead shortwave in the broad SW flow lifts
north of the area by 12z, with a relative lull in the convection/
precip over the area before the sfc-mid level low moving into
IA this afternoon. Still some SHRA/few TSRA around this morning.
More widespread SHRA/TSRA set to spread into spread NE across the
fcst area this afternoon into this evening, ahead of the approaching
strong sfc-mid level low. MUCAPE progged in the 1K to 2K j/kg range
ahead of these features this afternoon/early this evening, with deep
shear progged in the 50-70kt range. Plenty of convergence/lift
present and divergence aloft as the system approaches, with a arc of
convection expected to move rather quickly NE across the fcst area
in roughly the 20z-01z period. Per SWODY1, appears most of the
parameters are present for TSRA later this afternoon/early this
evening to be strong to severe. With the mdt/strong approaching
sfc-mid level low will be deep cyclonic flow and enhanced 0-1km
helicity. Appears TSRA this afternoon/evening will be more inclined
to rotate, especially if they are rooted in the boundary layer.
Slight to enhanced risk of severe TSRA and possiblity of tornadoes
in SWODY1 looks quite reasonable. Plan to cancel existing flash
flood watch with package issuance this morning. Storms this
afternoon/evening will also be capable of heavy rains with PW values
still in the 1.5 inch range, but storms are expected to be fast
moving with that 50-70kts of deep shear. At this time, will not be
issuing another flash flood watch for this afternoon/evening.

With passage of the sfc-mid level trough axis late this evening into
the overnight hours, the CAPE and deeper moisture is quickly swept
out of the area as the deeper flow becomes west then NW. Decreasing
precip chances after midnight to be mainly -SHRA as colder air
spills SE across the region.

.LONG TERM...(Thursday through Tuesday)
Issued at 416 AM CDT Wed May 17 2017

For Thursday thru Friday night: main fcst concerns this period are
lingering -SHRA chances Thu morning then returning SHRA/TSRA chances
Fri/Fri night, and much cooler temperatures.

17.00z models in decent agreement on transient shortwave ridging to
build across the region Thu/Thu night, ahead of the next mid level
low to move into eastern CO by 12z Fri. The ridge axis is quickly
pushed east of the area by later Fri with deep southerly flow/
falling hgts returning Fri night as the mid level low/strong
troughing moves out into the central/northern plains. Fcst confidence
this period is generally good this cycle.

This period starts out with deep cyclonic flow still over the area,
but on the west side of the departing low/trough. The consensus 20-
50% -SHRA chances Thu morning (highest NE of I-94). A bit of an 850-
700mb trough lingers across the area thru Thu afternoon, but this
looks to do little more than delay the decrease/clearing of clouds
until Thu night. Stronger of the low level cold advection spreads
across the area Thu afternoon/night. Highs Thu may well be in the
morning, with mixed 925mb temps by Thu afternoon only supporting
temps in the low-mid 50s. Highs Thu in the 50s to low 60s, with a
brisk N wind going to feel like a bit of a shock after the past few
days. 925mb temps by 12z Fri in the +1C to +4C range. With lighter
winds and clear to partly cloudy skies by later Thu night, the
consensus lows in the mid 30s to lower 40s appear reasonable. Will
have to watch the colder low laying areas along/NE of I-94 for
patchy frost around 12z Fri. Sfc low/mid level low/trough ejecting
into the plains Fri quickly spreads increasing moisture back into
the region Fri, moreso Fri night. PW values over the fcst area
progged back into the 1-1.25 inch range by 12z Sat. Some detail
differences on how quickly this moisture returns Fri and how much
instability will lift north into the area with it. Initially the
moisture is at/above 800mb thru Fri, but deepens/lowers with some
weak CAPE when lifting elevated parcels Fri night. Left precip
chances Fri afternoon/night as -shra but introduces a small TSRA
mention later Fri night as moisture deepens and lift increases as
troughing advances across the plains. Increasing/thickening clouds
to hold temps down Fri, but provide a blanket and keep Fri night
lows warmer than those of Thu night.

For Saturday thru Tuesday (days 4-7): main fcst concern this period
are SHRA/TSRA chances/rain amounts Sat/Sat night, small precip
chances Sun-Tue, continued cooler temps thru the period. Fcst
confidence is good Sat then average at best Sun-Tue.

ECMWF/GFS runs of 17.00z in reasonable agreement Sat on the central
conus troughing/low to lift toward norhtern MN Sat/Sat night, with
broad, cool troughing to then remain over the north-central Sunday
into Tue. Plenty of shortwave and north-south stream interaction
differences by Sun-Tue. Favoring the more consistent GFS/ECMWF,
deeper moisture/lift and weak instability continue to spread NE
across the area Sat into Sat evening as the sfc- mid level low/
trough lifts across the region toward SW Ont by 12z Sun. Trend is a
bit faster with this system and toward lower total QPF mainly Fri
night/Sat. Given the recent rains, will have to watch this system
for additional hydro concerns with potential of widespread 1 inch
rains falling on a very wet/saturated landscape. Small consensus
mainly -SHRA chances Sun-Tue OK for now until the model detail
differences settle down. Plenty of ensemble variability by Sun as
well. With the broad troughing and general NW flow aloft over
the north-central conus, consensus highs/lows mostly 1 to 2
categories below normal for Sat-Tue looking well trended for now.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 638 AM CDT Wed May 17 2017

Strong low pressure will approach the region this afternoon and
early evening, bringing several rounds of showers and
thunderstorms. Rainfall this morning remains hard to time as it is
being driven by weak waves ahead of the main system, but a few
bouts of showers are likely around RST and perhaps LSE with
ceilings mainly VFR. Approach of the low toward 20Z should bring a
quick uptick in showers and storms for all areas, with a risk for
some of those to become severe. Confidence at this point remains
too low for any visibility or ceiling reduction in the forecast,
but any storms could produce brief IFR conditions. As that low
departs this evening and tonight, rain chances will end but with
much colder air arriving below 4kft allowing for the development
of MVFR stratus from west to east as winds shift to the northwest.

&&

.HYDROLOGY...(This afternoon and evening)
Issued at 416 PM CDT Tue May 16 2017

Some areas were hit by heavy rains again Tue evening, mainly
Bufflalo thru Jackson Co. WI, where radar indicates as much as 4
to 6 inches for rain may have fallen. Rivers and streams were
rising in these areas, with some flooding now expected along the
lower reaches of the Trempealeau river. With only lingering -SHRA
across the area early this morning, the flash flood watch that
was in effect thru 7 am has been cancelled. The next round of
SHRA/TSRA mainly this afternoon and evening, is expected to be
fast moving. TSRA this afternoon/evening may still produce higher
rainfall rates, but are not expected to be over any one location
very long, keeping rainfall amounts lighter than earlier expected.
Thus a new watch was not issued for parts of the area.

&&

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

SHORT TERM...RRS
LONG TERM....RRS
AVIATION...Lawrence
HYDROLOGY....RRS

Hazardous Weather Outlook
National Weather Service La Crosse WI
952 AM CDT Wed May 17 2017

IAZ008>011-018-019-029-030-MNZ079-086>088-094>096-WIZ017-029-
032>034-041>044-053>055-061-181100-
Mitchell-Howard-Winneshiek-Allamakee-Floyd-Chickasaw-Fayette-
Clayton-Wabasha-Dodge-Olmsted-Winona-Mower-Fillmore-Houston-
Taylor-Clark-Buffalo-Trempealeau-Jackson-La Crosse-Monroe-Juneau-
Adams-Vernon-Crawford-Richland-Grant-
952 AM CDT Wed May 17 2017

This Hazardous Weather Outlook is for portions of southeast
Minnesota, northeast Iowa, and southwest into central Wisconsin.

.DAY ONE...Rest Of Today and Tonight

Severe thunderstorms are expected this afternoon and evening,
mainly between 3 pm and 8 pm. The risks will be isolated tornadoes,
damaging winds and large hail. At this time, the tornado risk
looks to be across northeast Iowa into western and central
Wisconsin. Storms with damaging winds and large hail could occur
across the entire area. Locally heavy rain will be possible with
any storm, however, the storms should be fast moving limiting the
potential for flash flooding.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

Another round of widespread rain looks to spread across the area
Friday into Saturday night, with amounts of 3/4 inch to locally
1 1/2 inches appearing likely.

Isolated thunderstorms are also possible later Friday night and
Saturday. Severe weather is not expected with any of these
storms.

.SPOTTER INFORMATION STATEMENT...

Spotter groups should be prepared for activation from mid
afternoon into the evening hours. Severe weather reports,
including heavy rainfall and ongoing flooding, are appreciated
through this evening.

.EMERGENCY MANAGEMENT COORDINATION...

A webinar has been scheduled for 1 pm.

04/DAS

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