Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 171953
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
253 PM CDT Mon Jul 17 2017

.SHORT TERM...(Tonight through Wednesday)
Issued at 252 PM CDT Mon Jul 17 2017

High Impact Weather Potential: Thunderstorm chances ramp up later
Tuesday afternoon into Tuesday night. There remains a risk that some
could be severe with a risk for damaging winds and perhaps large
hail.

One more quiet weather day ongoing out the window, as low level
ridging continues to drift east of the area but mid level shortwave
ridging dominates. While dew points east of the Mississippi River
are again in the more comfortable range, signs of change appear to
the west with a ribbon of increasing return flow/moisture advection
through Iowa and Minnesota, heading northward into a frontal
boundary draped from northwest MN into southwest SD as of 18Z.
Weaker shortwave riding the Canadian/U.S. border has helped drive
some convection along that front since late this morning, with that
activity likely to dwindle with eastward extent into this evening as
additional storms fire near the front across northern MN into
western SD, perhaps drifting into northern WI on the nose of
stronger moisture transport after 06Z. At the moment, don`t
anticipate any of this activity impacting us as our airmass will
just be recovering, but can`t rule out a rogue shower/storm perhaps
across Clark or Taylor Counties late.

Bigger story remains later Tuesday into Tuesday evening (night?)
with continued hints that the remnants of upstream convection will
drift in our direction, perhaps upscaling during the afternoon/
evening hours as they encounter increasing instability across our
area and increased moisture transport centered on the Mississippi
River. Still some disagreement regarding overall available shear
with the NAM most robust and indicating a stronger convectively-
enhanced shortwave crossing the region and an associated uptick in
deep shear toward 45 knots (GFS/ECMWF weaker toward 30 knots). Given
the degree of building instability with MLCAPE values pushing 2000+
J/kg and moisture pooling with dew points well into the 70s ahead of
the front, can`t rule out some more organized convection with a
possible wind or hail threat, though with high freezing levels,
we`ll certainly need the better shear for a larger hail risk.

What happens into Wednesday remains lower confidence, but should
convection get better organized as it sweeps through on Tuesday,
suspect our driving frontal boundary will be pushed well to the
south of the area. Recent runs of the NAM have strongly agreed with
this scenario, as have recent ECMWF Runs, and per pattern
recognition, it seems guidance typically is too fast to return
fronts north in these types of setups. As such, agree with the
previous forecaster that much if not all of Wednesday may end up dry
(and probably less humid in many areas) as transient low level
ridging works through the area.

.LONG TERM...(Wednesday Night through Monday)
Issued at 252 PM CDT Mon Jul 17 2017

A very active stretch of mid summertime convective weather appears
likely somewhere across the region heading into mid/late week and
possibly part of next weekend as well, though confidence in the
specific details remains low. As discussed for many days now, the
basic pattern across the CONUS is a good one for nearby MCS tracks
across our area, featuring a broad mid level heat ridge centered
over the central Plains toward the mid Mississippi River Valley and
several hard-to-time weaker shortwaves likely to roll over the
northern periphery of that ridge. The trick is nailing down both the
synoptic and eventually mesoscale details regarding the placement of
better moisture/instability as well as an elongated frontal boundary
draped somewhere across the local area, likely to be influenced each
day by various rounds of potential convection.

With all that said, there remains a pretty good signal toward
Wednesday night for an upstream MCS to get rolling across
North/South Dakota, riding a strong instability gradient somewhere
toward the area. Not really sure how far north moisture/instability
recovery will reach into our area Wednesday night, with persistent
hints that the better instability gradient may bisect the area,
suggestive that any organized MCS may "turn" down into northern or
even central Iowa. However, shear during this time is quite robust,
owing to a corridor of 50-70 knots of mid level flow working into
the area, thus will really need to watch the wind potential with any
convective complex.

Beyond Wednesday night, the details become rather murky (par for the
course in this type of patter), but much of Thursday may again end
up being dry with subsidence behind Wednesday night`s convection.
There are then signals of better chances for convection to redevelop
sometime toward Friday and perhaps into Saturday as well with a
better upper wave possibly dropping toward the area. To reiterate
though, confidence is far from high in the details this far out, and
while each forecast period will carry some precip chances, it
certainly won`t rain for the entire time. With that said, am a
little concerned about the possibility for repeated rounds of
convection and heavy rainfall, with current WPC 5-day totals in
excess of 4 inches certainly possible, though much will depend on
the track of each convective complex. Thankfully, per multi-day
guidance trends, the Sunday-Monday time frame looks to end up dry as
we briefly return to thermal troughing aloft for a time as high
pressure ushers in notably less humid conditions.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1133 AM CDT Mon Jul 17 2017

Expect VFR conditions through the period. Overnight the LLWS will
be borderline for inclusion into the TAFS with about 20kts of
shear in a thin layer near 1500 ft AGL. Have chosen not to include
it at this time.

Thunderstorm chances begin to increase Tuesday afternoon at the
TAF sites. Future TAFS will probably need a period of TEMPO TSRA
later Tuesday afternoon and evening.

&&

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

$$

SHORT TERM...Lawrence
LONG TERM...Lawrence
AVIATION...Baumgardt



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