Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 161730
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1230 PM 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 18Z TAFS through 18Z Thursday afternoon)
Issued at 1230 PM CDT Wed Aug 16 2017

Good VFR expected until late this afternoon and the area is between
front ahead of a strong sfc-mid level system in the central plains.
As the main low level cold front/trough axis lifts NE across the
area late this afternoon/this evening a band of SHRA/TSRA with MVFR
and IFR cigs/vsbys expected to accompany it across the area. The
front/band of SHRA/TSRA to be progressive, with the lower cigs/vsby
mainly impacting the taf sites for a couple hours. Model consensus
tightening that this will likely be in the 23z-01z time frame this
evening and added a tempo group for TSRA this period at both
KRST/KLSE. Less consistency for SHRA behind the main line of
convection and only carried a period of VCSH behind it thru 05z.

Concern then becomes lower clouds/cigs in the deformation band of
moisture to wrap around the passing sfc-mid level low for later
tonight/Thu morning. Consistent signal among models for this and
carried IFR or lower MVFR cigs at both taf sites after 05z. Deep
cyclonic flow and the sfc-mid level trough axis near/over the area
Thu morning, included a VCSH at both sites in the 14-18z period.
Winds increasing from SW to W behind the system as well Thu morning,
with some gusts 20-25kt at KRST by mid morning.

&&

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

$$

SHORT TERM...JM
LONG TERM....JM
AVIATION.....RRS



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