Area Forecast Discussion
Issued by NWS La Crosse, WI

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774
FXUS63 KARX 252237
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
535 PM CDT Tue Jul 25 2017

.SHORT TERM...(Tonight thru Wednesday Night)
Issued at 245 PM CDT Tue Jul 25 2017

Main fcst concerns this period are SHRA/TSRA chances late tonight
into Wed evening and severe threat with them mainly Wed
afternoon/evening.

18z data analysis had high pressure retreating into the eastern
great lakes/OH valley. Low pressure was moving towards Hudson`s Bay
and into the central plains, with a frontal boundary between the 2
near the MN/ND border. First surge of roughly 925-700mb moisture
transport ahead of the plains low has been responsible for
SHRA/TSRA spreading across much of the fcst area this morning into
early this afternoon. Clouds with this convection have held temps
down across the area so far today, but clearing now working in with
temps looking to make some recovery toward the upper 70s/low 80s as
the afternoon wears on.

No issues noted with 25.12z model initializations. Solutions similar
as the mid level low north of lk Winnipeg moves east across central
Can tonight/Wed and shortwave energy over WY/UT/CO moves into the
region. These features to move east of the area Wed evening, with
good agreement on a secondary NW flow shortwave to drop into
northern MN by 12z Thu. Short-term fcst confidence is generally good
this cycle.

For the short term, The main low level warm advection/isentropic
lift and moisture transport is progged to set up west and north of
the fcst area tonight, mainly across SW MN into NW WI ahead of the
cold front as it moves to near a KDLH-KOMA line by 12z Wed. This
evening to be dry across much of the fcst area, with an increase of
SHRA/TSRA chances across the NW side after midnight as front and
thermo-dynamic forcing gradually approach. Bigger concern from all
of this is the convective debris clouds expected to spread across
the fcst area later tonight and for Wed as the mid/upper level flow
becomes W-NW and the WY/UT/CO shortwave energy approaches. Expected
ongoing convection west of the fcst area Wed morning expected to
continue feeding the debris cloudiness, with Wed trending to be a
mostly cloudy/cloudy day. This to have an impact on how much CAPE
can build across the area and potential strong/severe TSRA chances
Wed afternoon/evening. For now, severe threat per SWODY2 appears
conditional that some sunshine/diurnal heating/CAPE development
occurs. Greater likelihood of some sunshine Wed is over the SE end
of the fcst area, and given timing of the front across the area, a
few strong/severe TSRA mainly south of a KCCY-KVOK line, where deep
shear would be stronger and CAPE higher, cannot be ruled out. TSRA
Wed/Wed evening will also have the potential to produce locally
heavy rains, but this threat may end up focused south of the fcst
area as stronger of the low level moisture transport is progged
across east-central IA into NW IL Wed afternoon/evening. See hydro
section below for more details. Continued with 60-80% SHRA/TSRA
chances across much of the area Wed afternoon/evening as the front
would be moving across the area.

Mid level shortwave energy/trough axis and cold front pass Wed night
with drier/cooler Can high pressure building behind the low/front.
Trend of SHRA/TSRA chances diminishing/ending from NW to SE across
the area thru the night looks good. Stayed with a blend of the
guidance lows/highs for tonight thru Wed night.

.LONG TERM...(Thursday through Tuesday)
Issued at 245 PM CDT Tue Jul 25 2017

For Thu thru Fri night: main fcst concern this period is
temperatures.

Model runs of 25.12z in good agreement sweeping the shortwave
trough/mid level low quickly SE across the western great lakes Thu,
with shortwave ridging/NW aloft to follow it for Thu night-Fri
night. Some detail differences with weak shortwaves rippling SE thru
the flow Thu night-Fri night, but this more-or-less expected in NW
flow. Fcst confidence is good this period.

Some detail differences on how quickly the sfc low/cold front clear
the area Thu. A small lingering consensus SHRA, perhaps TSRA, chance
over the SE end of the fcst area early Thu morning okay for now.
Otherwise, this period to be dominated by dry and seasonable Can
high pressure building SE across the region under the NW flow/
shortwave ridging aloft. Bulk of Thu thru Fri night trending to be a
couple of dry/quiet/comfortable late July days. Trend for sfc dew
points to fall into the 60s Thu, then into the 50s by Fri as the
center of the high would be over the region looks good. Model
soundings showing mixing to around 850mb both Thu/Fri. Under what
should be sunny/mostly sunny skies, blend of guidance highs mostly
in the upper 70s-lower 80s, about a category below normal, appear
well trended at this time.

For Saturday thru Tuesday (days 4-7): main fcst concerns this period
are small SHRA/TSRA chances Mon/Tue, temperatures thru the period.

Medium range model runs of 25.00z/25.12z in good agreement on
western NOAM ridging and eastern NOAM troughing thru this period,
which leaves the Upper Midwest under NW flow aloft. Some consensus
for shortwave ridging aloft over MN/IA/WI Sat, with shortwave
troughs to approach/pass Sat night thru Tue. Typical between model
and run-to-run detail differences on these waves in the day 5-7 time
frame. Overall fcst confidence for near to slightly below normal
temps is above average this period, but confidence on any small
SHRA/TSRA chances during the day 4-7 period is below average.

Sat continues to trend as a dry day with a sfc-mid level ridging
over the region. Should one of the NW flow shortwaves speed up, may
yet need a small SHRA/TSRA chance by Sat afternoon. By Sun, detail
differences with the NW flow shortwaves impact timing of and
SHRA/TSRA chances across the fcst area. Given low confidence in any
one deterministic solution, the small consensus SHRA/TSRA chances
Sun-Tue will have to do.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 535 PM CDT Tue Jul 25 2017

Cigs: high broken clouds expected through the night, with a gradual
lowering as a cold front sags south/southeast toward the TAF sites
by Wed afternoon. Could see a drop into MVFR for a few hours as the
boundary moves through, but will hold VFR for the moment. Expecting
the cigs to hold into Wed evening, with clearing looking likely
overnight.

WX/Vsby: shra/ts ongoing around a sfc front across southern MN, with
the low level jet and instability maximum lending a hand. Meso model
trends favor shifting this north during the evening, with a decrease
in intensity. No impact to the TAF sites. Not entirely sold that
KRST is going to miss out, but will keep dry for now and monitor.

Better shot for shra/ts as the front sags south/southeast Wed. Could
see a shower threat late morning, but the better chances could come
in the afternoon - if instability can near the taf sites. How much
cloudiness lingers (hindering instability) will be a big factor.
Will go with VCTS to highlight the threat for now, detailing as
timing/threats become more apparent.

Winds: Southerly fetch swinging to the northwest/north Wed
afternoon/evening as that front slips south.

Some threat for LLWS tonight with the low level jet streaking
overhead. Borderline criteria wise, but will continue mention.

&&

.HYDROLOGY...through Wednesday night
Issued at 245 PM CDT Tue Jul 25 2017

Flood Warnings continue along the Kickapoo River from Soldiers Grove
to Steuben and along the Trempealeau River at Dodge. These levels
will continue to fall tonight and early Wed.

From mainly late tonight into Wednesday evening, additional rain is
expected across the fcst area. Warm cloud depths will be near or
above 4km with PW values in the 2 inch range, making any TSRA
efficient rain makers. Potential exits for locally heavy rains from
storms, but plenty of detail differences between models makes where
difficult to pin-point. At this time the greater threat of heavy
rains from TSRA is looking to be just NW of the fcst area late
tonight/Wed morning, then across parts of NE IA/SW WI Wed
afternoon/evening. Given the passing low and cold front, storms
should be more on the progressive side Wed/Wed night, keeping the
widespread heavy/excessive rain threat on the lower side.

However, if locally heavy rains would fall on areas hit by heavy
rains last week, additional river rises and perhaps minor flooding
would be possible.

Thursday through Sunday are trending dry.

&&

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

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION.....Rieck
HYDROLOGY....RRS



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