Area Forecast Discussion
Issued by NWS La Crosse, WI

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687
FXUS63 KARX 160800
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
300 AM CDT Wed Aug 16 2017

.SHORT TERM...(Today through Thursday)
Issued at 300 AM CDT Wed Aug 16 2017

Surface analysis early this morning indicated a trough of low
pressure across the central high plains with a warm front draped
eastward across Nebraska and along the Missouri/Iowa border.
Relatively dry low-level air was in place to the north with
easterly surface flow under the influence of high pressure.

Showers/isolated storms have been lifting northward especially
across southeast Minnesota along the 850 mb warm front/moisture
gradient. A bit of development to the east may occur during the
early morning, although the strongest moisture transport is
focused mainly across eastern Minnesota. The broken band/s of
precip will gradually lift northward through the early morning
hours.

Once this lifts north of the area, much of the daytime hours
should be dry, under the influence of shortwave ridging. RAP
soundings would indicate some broken sun at times through the
day, allowing for MLCAPE to build into the 1000-1500 J/kg range.
However, vertical shear still looks unimpressive, with around 30
kts in the 0-3 km layer, but wind speeds in the 3-6 km layer are
only around 20 kts, so deep layer shear is lacking. Thus,
although the environment could support a few strong storms with
strong winds and small hail if sufficient CAPE builds, shear will
be a limiting factor for higher chances for severe storms.

Stronger forcing will arrive late this afternoon with increasing low-
level warm/moist advection ahead of the surface warm front. CAMs are
in pretty good agreement that one or more bands of convection
will lift north and eastward into the area by this afternoon and
tonight as forcing from the upper trough increases. Rainfall
parameters remain most impressive with this system, with potential
for high rainfall rates with stronger convection. Unless any
localized areas are impacted by persistent heavier thunderstorms,
would not anticipate any widespread hydro issues given the
antecedent dry conditions. The dry slot will push into the area
later tonight as the surface nears southeast Minnesota by early
Thursday.

The cold front will sweep through Thursday morning, but some showers
may linger during the day behind the surface low as the upper trough
slowly moves across northern. Winds will be breezy from the
northwest on Thursday before gradient slackens tonight.

.LONG TERM...(Thursday night through Tuesday)
Issued at 300 AM CDT Wed Aug 16 2017

On the heels of the exiting upper low, another piece of energy
diving across the central US Friday into Saturday will briefly
amplify northwesterly flow aloft into the start of the weekend. Some
rain chances will accompany the passage of the upper wave mainly
late Friday and Friday night. The flow will remain quite
progressive, though, with zonal flow evolving Saturday night through
early next week. A brief period of high pressure should result in a
fairly quiet end to the weekend, with temps near or a bit above
average. There remains general agreement among model guidance
that a shortwave trough and associated cold front will slide
southeastward later Monday into Tuesday, with a threat for
showers/storms.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1129 PM CDT Tue Aug 15 2017

Increasing moisture and weak lift will combine to cause a
challenging forecast period for the 06Z TAFs. It appears some
isolated SHRA/TSRA could be around through the night, especially
for KRST, with a decreasing trend after sunrise. Then, most of the
day looks good on Wednesday with increasing cloud heights and
possibly only a SCT cloud field.

Have minimized the impact of weather at the TAFS overall overnight
with KRST seeing a chance of TSRA at the field. Will monitor for
an upgrade to TAF and have included CB for now.

The most impactful weather comes in Wednesday evening where MVFR
conditions in RA BR are a good bet with diminishing trends
overnight. TSRA is a very good probability for Wed eve, but with
details on timing still uncertain, have not yet TEMPO`d this in
the TAFS after 17/00Z.

&&

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

$$

SHORT TERM...JM
LONG TERM...JM
AVIATION...Baumgardt



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