Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 260359

Area Forecast Discussion
National Weather Service La Crosse WI
1100 PM CDT Sun Jun 25 2017

.SHORT TERM...(Tonight thru Monday night)
Issued at 220 PM CDT Sun Jun 25 2017

Main fcst concerns this period are SHRA/TSRA chances lingering this
evening and again Mon afternoon, continued cool temperatures.

18z data analysis had broad NW flow across the Upper Midwest. WV
imagery showed deep cyclonic flow aloft over the region, with
embedded shortwaves moving SE thru the flow. One of these dropping
across MN, along with diurnal warming/mixing, producing sct SHRA
across much of the NW half of WI and NE 1/3 of MN. A few of these as
far south as the KLSE area. Abundant lower level moisture over much
of the region today. Along with the diurnal warming/mixing under the
cool temps and cyclonic flow aloft plenty of cu/strato-cu clouds
over the Upper Midwest this afternoon, helping hold temps down.
Early afternoon temps across parts of MN/WI as much as 15-25F
below normal, only in the 50s to lower 60s.

25.12z model runs initialized well. Solutions quite similar as one
shortwave in NW flow slide by this evening, then for another bundle
of shortwave energy to drop SE, mainly across WI, on Mon. Good
consensus for hgts to then rise Mon night but flow aloft to remain
NW. Short-term fcst confidence is generally good this cycle.

In the short term, shortwave energy passing late this afternoon/
early this evening takes the bulk of the 925-700mb moisture
east/south with it. Any lingering SHRA/isolated TSRA over the NE
side of the fcst area should exit/end by mid evening, with a
decrease of clouds from W to E with the drying and loss of diurnal
heating. A weak shortwave dropping ND this afternoon drops into
western IA tonight, spreading some 800-700mb moisture/moisture
transport across southern MN/IA during the late evening/overnight
hours. The main lift with this progged to stay SW of the fcst area,
and will leave NE IA dry tonight. But it does look to spread some
clouds over the SW side of the fcst area overnight. Lower levels are
drier with the next shortwave to pass across WI Monday, but model
soundings showing diurnal heating develops some CAPE Mon afternoon
along/east of the MS river. This with little CIN due to the cool
temps aloft. 20-40% SHRA/TSRA chances with passage of the shortwave/
mid level trough axis along/east of the MS river Mon afternoon quite
reasonable for now.

NAEFS indicates 850mb temps 2-2.5 std deviations below normal
tonight/Mon, moderating to 1-1.5 std deviations below normal by 12z
Tue. Temps tonight-Mon night to remain well below normal. With
lighter winds and clear-partly cloudy skies tonight and again Mon
night, blend of guidance lows mostly in the 40s both nights look
good. With the sfc ridge axis nearby/overhead Mon night, will have
to watch the normally colder, low laying areas along/NE of I-94 for
a few lows dipping into the 30s by Tue morning. Model soundings
indicating diurnal mixing to near 850mb Mon, with these temps
supporting the blend of guidance highs mostly in the mid 60s to
lower 70s across the fcst area.

.LONG TERM...(Tuesday thru Sunday)
Issued at 220 PM CDT Sun Jun 25 2017

For Tuesday thru Wednesday night: main fcst concerns this period are
increasing SHRA/TSRA chances/heavy rain threat centered on Wed.

Model runs of 25.12z in good agreement for rising hgts/shortwave
ridging to build across the region Tue/Tue evening, ahead of
stronger shortwave troughing to move into the northern plains.
Detail differences increase but consensus brings this trough across
the region Wed/Wed night. Fcst confidence good Tue, then average by
Wed/Wed night.

Tue still trending to be the dry day this upcoming week as sfc high
pressure and the shortwave ridging aloft build into the region. A
warmer day Tue with a return of southerly flow/low level warm
advection. Even with plenty of late June sunshine expected, mixed
850mb only supporting highs mostly in the mid/upper 70s, still about
5F below the normals.

Moisture return/transport ahead of the plains trough/low progged to
arrive later Tue night, then spreads across the fcst area in earnest
Wed morning. PW values over the west side of the fcst area in the
1.5 inch range by 12z Wed, with this spreading east across the area
thru Wed/Wed evening. Increase of CAPE generally lags the moisture
increase, but model soundings over the west side of the fcst area
showing lifting saturated 750-700mb parcels by late Tue night
results in 500-1000 j/kg CAPE and minimal/no CIN. SHRA/TSRA expected
to develop over the west end or just west of the fcst area by late
Tue night, then spread east across the fcst area Wed with the
moisture transport/theta-e convergence and lower level thermo-
dynamic forcing. Given the rather tight consensus on the forcing,
SHRA/TSRA chances in the 70-80% range Wed, diminishing Wed night as
the sfc front thru mid level trough axis would pass are quite
reasonable. Widespread convection firing/spreading across in the
morning to result in plenty of cloud cover and looks to keep any
severe TSRA risk to a minimum. Highs Wed may potentially struggle to
reach 70F. Bigger concern Wed may be locally heavy rains. See
hydrology section below for more info.

For Thursday thru Sunday (days 4 to 7): main fcst concerns this
period include SHRA/TSRA chances much of the period and continued
below normal temperatures into early July.

Medium range model runs of 25.00z/25.12z in reasonable agreement for
weak shortwave ridging to slide across the region Thu, to be quickly
followed by troughing for Fri/Sat then rising hgts/weak ridging for
Sun. However, as usual there are plenty of shortwave detail
differences with shortwave thru the flow for Fri-Sun. Fcst
confidence is average for day 4-7 precip chances and on the good
side for continued below normal temperatures.

The weak shortwave ridging and a sfc ridge axis sliding across the
area behind the Wed system should give the are a generally dry day
Thu. However, with potential shortwave detail/timing differences,
some small SHRA/TSRA chances in the afternoon per the model
consensus OK for now. With approach/passage of the next trough and
pieces of shortwave energy, SHRA/TSRA chances in the 20-50% range by
later Thu night into Fri evening, then diminishing thru Sat looking
reasonable. Present timing would bring another weak ridge/sfc high
across the area Sat night/early Sun, with more troughing coming
across the northern plains by later Sun. Given shortwave detail
differences, cannot argue against the consensus SHRA/TSRA chances Sat
night/Sun. Thu looking to be the drier, sunnier day of the day 4-7
period, also looking to be the warmest day. Weak low level cold
advection over the area Thu, followed by the troughing and what
should be plenty of cloud cover Fri/Sat, Thu=Sat highs mostly in the
70s look good. Potential for some warming Sunday, but given lesser
confidence in the precip chance details the consensus highs
reasonable for now.


.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1100 PM CDT Sun Jun 25 2017

Cigs/Wx: Monday another shortwave will slip across eastern WI.
Should trigger more afternoon clouds and scattered showers, but
higher chances for eastern parts of WI, away from the TAF sites.

Vsby: brief reductions in any heavier shower Monday afternoon.
Otherwise, see some potential for fog overnight Monday, maybe moreso
in the river valleys - which could impact KLSE. Something to keep an
eye on.

Winds: should hold west-northwest into Monday night. A bit more
south/southwest at KLSE tonight if it decouples. Will watch and make
adjustments if necessary. Not as breezy Monday as pressure gradient
slackens, but deep mixing could still lead to some afternoon
gustiness. Light winds Monday night, going more southerly.


.HYDROLOGY...(late Tue night into Wed evening)
Issued at 220 PM CDT Sun Jun 25 2017

Given PW values progged to be 1.5 to 1.8 inches and warm cloud
depths of 3500 meters or higher, TSRA centered on Wed looking to be
efficient rain producers. Main concern Wed may well be locally heavy
rains. Rains of the past week-10 days have left much of the local
landscape with wet soils, and most rivers and streams with continued
higher than normal flows. Potential exists for 1 to locally 3
inches of rain from late Tue night into Wed evening. If the
heavier rain would fall in a short period of time or on one of the
wetter parts of the landscape, some flooding or flash flooding
could occur.




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