Area Forecast Discussion
Issued by NWS La Crosse, WI

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722
FXUS63 KARX 291137
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
637 AM CDT Sat Apr 29 2017

.SHORT TERM...(Today through Tuesday)
Issued at 230 AM CDT Sat Apr 29 2017

Cool, showery into Tuesday.

A bit of upper level energy slated to spin out of the mean trough
(currently over the desert southwest) and lift through Iowa into
Wisconsin later this afternoon-evening. Good thermodynamics lead the
shortwave into the region (925-700 mb warm air advection) along with
a south-north sloping frontogenetic response. Saturation the main
deterrent to pcpn chances, and models north-south running time/height
x-sections suggest the deeper moisture creeps farther north compared
to yesterday. Pcpn chances will follow suit, although the higher
amounts will hold to northeast IA/southwest WI.

This system quickly departs overnight and there could be a few hour
break before the main pcpn show moves in, persisting through Monday.

The 500 mb trough to the southwest lifts northeast Sunday, becoming
vertically stacked with a negative tilt as it tracks into WI around
12z Monday.

There are 3 main areas of pcpn focus with this system:

1) west-east running band of favorable low level thermodynamics
coupled with sloping frontogenetic response ahead of the sfc/upper
level low, and running along the main push of shortwave energy, will
drive a broad area of showers Sunday-Sunday night. The bulk of the
pcpn associated with this forcing will exit north by 12z Monday.

Roughly 1/2 to 1 inch seems likely for much of the forecast area
with this push - locally higher can`t be ruled out if convection
develops. These storm chances will be higher across western WI where
an impinging 850 mb jet could fire off a few storms Sun afternoon-
evening.

2) Northeast-southwest running sfc boundary will interact with the
leading edge of the upper level trough/shortwave energy Sun/Sun
night. Nice fetch of moisture transport along/ahead of this, with
the main axis of instability also coming into play. This
interaction will be the best area for more widespread convection,
along with heavier rain.

Locally, aside from some of that low level jet support in the
afternoon/evening, the bulk of the convection will stay east of the
area - which has been a trend the past few days.

3) Low level moisture will wrap around the stacked low to feed its
deformation region. Models favor keeping the pcpn generally light in
this area, and mostly across MN into northwest WI. That said, enough
ripples in the flow that bits could rotate around the low, enhancing
the shower activity.

This deformation area is also the location where a rain, snow,
wintry mix question comes into play. Enough cold air for this spring
system to drop snow (accumulating) on the western edges of the
deformation area - with the current model solutions keeping
accumulations across MN. Locally, can`t rule out some mix across
southeast MN/northwest WI.

So, rain chances today through this evening, mostly along and south
of I-90, with the higher chances across northeast IA/southern
Wisconsin. Rain then becomes widespread for Sunday/Sunday night,
with isolated/scattered storms possible. Severe weather not
anticipated. Snow could mix in to the west and north, but no local
snow accumulations currently expected.

.LONG TERM...(Wednesday through Friday)
Issued at 230 AM CDT Sat Apr 29 2017

After the weekend/early week storm system exits east early Tuesday,
both the GFS and EC keep the Upper Mississippi River Valley under
northwest flow a loft, with the prospect of a couple ripples in the
flow sliding across the region Wed-Thu. No tap to gulf moisture in
this flow, so shortwaves will have to bring their own saturation for
pcpn chances. Some differences in timing/strength/locations of these
upper level features between the two models, although some mutual
leaning toward Thu as a decent shot for scattered to widespread
showers. Whatever would fall, amounts look minimal at this time.

Ridge building into next weekend promises warmer, more seasonable
temperatures.

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 637 AM CDT Sat Apr 29 2017

The ceilings will gradually lower this afternoon and evening...and
then become MVFR overnight. There will be even some scattered
showers toward sunrise on Sunday. However, they should not cause
any visibility restrictions. Northeast winds will gust into the
20 to 25 knot range at KRST through the TAF period.

&&

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

$$

SHORT TERM...Rieck
LONG TERM....Rieck
AVIATION...Boyne



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