Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 161136
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
635 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 12Z TAFS through 12Z Thursday morning)
Issued at 635 AM CDT Wed Aug 16 2017

Cigs/WX/vsby: MVFR-VFR cigs associated with scattered shra/ts moving
toward the TAF sites early this morning-between 13-16z. Better shot
at KLSE to get some rain from this, although meso models suggest it
will be on a decreasing trend by the time it nears KLSE.

Post that, expect cigs to return to VFR, with some potential for
scattering out east of the Mississippi river for a few hours this
afternoon. Elected to keep bkn cigs for now.

Low pressure system approaches from the west this evening, sliding
its warm front, then cold front across the area. Expect shra/ts
development along both boundaries. Heavy rain likely with any of the
stronger storms. Vsby reductions to around 1sm in the heavy stuff.
Not enough confidence to add that tempo group to forecast yet - but
various models starting to hone in on the 00-05z time frame as being
the most favorable for these conditions. Cigs would also drop to at
least MVFR, with some IFR possible/probable overnight.

Winds: should hold southeast through the better part of the forecast
period, swinging southerly late tonight.

&&

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

$$

SHORT TERM...JM
LONG TERM...JM
AVIATION...Rieck



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