Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 151830

Area Forecast Discussion
National Weather Service La Crosse WI
130 PM CDT Fri Sep 15 2017

Issued at 130 PM CDT Fri Sep 15 2017

GOES water vapor imagery this afternoon showing an impressive mid-
level cyclonic circulation lifting from WY toward the Dakotas. This
was producing surface cyclogenesis over the Central Plains with
increasing moisture transport ahead of this low into our area. RAP
analysis has axis of 850mb moisture transport extending from eastern
KS northward into western MN. Have been watching an area of showers
and isolated thunderstorms lifting slowly northeast toward our
forecast area in northeast IA/southeast MN on the eastern periphry
of this moisture transport. Believe the bulk of this activity will
stay just west of the area through the remainder of this
afternoon/early evening. Otherwise, temperatures as of 2pm this
afternoon were in the 80s.


.SHORT TERM...(This evening through Saturday)
Issued at 130 PM CDT Fri Sep 15 2017

Going into tonight...low pressure will lift northeast across western
MN, drawing the surface cold front a bit further east into central
MN through northwest IA. Main axis of moisture transport/fuel for
convection looks to remain just west of our area but will likley see
isolated to scattered showers/storms nudge into western portions of
the forecast area (mainly northwest of a line from Charles City Ia
to Medford WI). This area will be on the very eastern fringes of
this better moisture transport. Otherwise, look for a rather muggy
overnight with lows in the middle/upper 60s with dew points in the
lower 60s.

The cold front oozes ever so slowly eastward toward the area on
Saturday, extending from northwest WI through south central MN by
late in the afternoon. This keeps bulk of showers/storms west of the
area through the day nwith continued warm air adevction across our
area. 925mb temperatures in the 23-25C range in the afternoon, so
showuld see unseasonably warm highs again in the 80s, coolest over
the northwest west where more cloud cover is expected.

.LONG TERM...(Saturday night through Friday)
Issued at 130 PM CDT Fri Sep 15 2017

Vigorous mid-level trough rotating northeast into the easter
Dakotas.northwest MN drives the surface cold front across our area
Saturday evening/overnight. CAPE will be waning going into the
nighttime hours and Bulk Shear is mostly post-frontal, so severe
threat looks minimal at this time. Howver, cannot rule out a few
stronger storms with small hail and gusty winds.

After some lingering morning shower chances, drier/cooler air will
filter intothe area Sunday. Look for mostly sunny skies by afternoon
with gighs in the upper 60 to the middle 70s, which is right around
seasonal normals.

High pressure strat shifting off to the east already by Sunday night
as a mid-level trough ejects out of the Central Plains toward our
area. This will produce a chance of showers west of the Mississippi
River after midnight.

For next week...southwest flow aloft will keep the area in
occasional shower/thunderstorm chances as several mid-level
disturbances eject out of the main trough out west into our area.
Otherwise, temperatures are expected to be slightly above normal
through this period with highs in the middle/upper 70s and lows in
the middle 50s/lower 60s.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1205 PM CDT Fri Sep 15 2017

The primary aviation concerns will revolve around the low level jet
tonight. This will increase shortly after sunset across the area
ahead of a short wave trough moving north/northeast across the
Missouri River Valley. The main axis of the low level jet will be to
the west of the area, but the eastern side of this will spread over
the region to create a dramatic increase in wind speeds to include
low level wind shear for both airports tonight.
The other concern from the low level jet is the development of any
convection on the convergent side of the low level jet. The 15.15Z
RAP also suggests there will be a weak mid-level short wave trough
ahead of the main short wave trough to provide some lift. Expecting
these two features to be enough to generate some convection west of
the Mississippi River and have opted to include a VCTS at KRST. This
activity should diminish around sunrise with just mid-level clouds
and VFR conditions Saturday morning for both airports.




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