Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 181125
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
625 AM CDT Tue Jul 18 2017

.SHORT TERM...(Today and tonight)
Issued at 257 AM CDT Tue Jul 18 2017

Main focus of attention in the short term will be strong/severe
thunderstorm potential this afternoon/evening.

Early this morning, a cold front was oriented southwest to northeast
across northwest Minnesota. Water vapor imagery indicated several
low amplitude shortwave troughs withing relatively zonal flow
helping to sustain convection across portions of northern
plains/Upper Midwest west of the local area.

The cold front will slowly drop southward today, reaching southeast
Minnesota by late in the day. Stronger low-level moisture transport
will develop across the region through the day, with higher surface
dew points in the low 70s advecting into the area. With temperatures
climbing well into the 80s and low 90s, heat indices could reach the
95-100 range during the afternoon. Given only a weak cap,
thunderstorms should increase along the front and possibly along any
old outflow boundaries during the afternoon. MLCAPE values are
expected to rise into the 1500-2000 J/kg range, although mid-
level lapse rates aren`t expected to be terribly steep. In
addition, deep layer wind profiles will be modest, with 25 to 35
kts of 0-6 km bulk shear, including around 20 kts in the 0-3 km
layer. Strong to severe storms are possible within this
environment, but in the absence of stronger shear, the hail threat
may be limited by very warm freezing levels and the weak lapse
rates. A few strong/damaging wind gusts and locally heavy
rainfall appear to be the greatest threats during the mid-late
afternoon hours into the evening, before storm intensity begins to
decline with loss of daytime heating.

.LONG TERM...(Wednesday through Monday)
Issued at 257 AM CDT Tue Jul 18 2017

An active weather pattern will continue through the first half of
the extended period with several rounds of storms possible.

Model consensus suggests the surface boundary will at least
temporarily slide south of the area on Wednesday with weak high
pressure impacting the region. If this pans out, a generally dry
day with warm, but potentially less humid conditions, is
likely.

However, the break may be short-lived with potential for an
evening/overnight southeastward tracking thunderstorm complex to
impact the region Wednesday night. with strong ridging/capping to
the south with the upper ridge, thunderstorms would mostly likely
form farther north and west across the northern plains ahead of
an upper shortwave trough and track along the instability axis on
the edge of the warmer 700 mb temps. Still tough to get a handle
on the details of timing and placement, with the orientation of
the instability and placement of the surface boundary playing an
important role. However, there has been a consistent signal that a
convective system would track across or near the area. Deep layer
shear profiles would support upscale growth and maintenance of an
MCS with 40-50 kts of 0-6 km shear and 30+ kts potentially in the
0-3 km layer. With more of an overnight event instability will
likely be on the decline, so the threat for strong to severe
storms will depend on the overall organization of storms, with
locally heavy rainfall a threat.

Little change in the upper air pattern is expected into late
week with weak upper waves rounding the northern periphery of the
broad upper ridge to the south and the surface boundary likely to
remain across or at least near the area. As a result, an active
pattern with periodic thunderstorm chances will continue right
through Friday with seasonably warm temps in the 80s. Overall
model consensus favors increasing thunderstorm chances Thursday
night into Friday night. As has been advertised, this pattern
poses a risk for heavier cumulative rainfall totals over the
course of several days. Uncertainty in mesoscale details from day
to day and uncertainty in timing of weak upper impulses prevents
much confidence in the evolution of convection beyond a day or
two, but with the potential for some areas to receive locally
heavy rainfall on multiple occasions this week, hydro concerns may
be elevated this week, in addition to any severe threat with the
convective episodes.

A stronger upper trough is expected to move towards the upper Great
Lakes on Saturday dragging a cold front through with a continued
chance for storms. Finally by Sunday and Monday model guidance has
shown a consistent trend for surface high pressure building out
of Canada behind the front providing a break from the humidity and
higher thunderstorm chances.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 625 AM CDT Tue Jul 18 2017

An area of MVFR clouds has developed again this morning over the
MIssissippi River valley and was impacting KLSE. The 18.06Z NAM
captured this deck forming and suggests it should break up between
14Z and 15Z. After that, will await the arrival of the cool front
from the northwest. Most guidance sets indicate there should be a
band of showers and storms that move across the region with the
front, with the exception of the 18.09Z HRRR which is rather quiet
through the afternoon but shows some activity coming in during the
evening. For now, have maintained continuity and will bring in
showers and storms to both sites for the afternoon and evening.
The storms should have the potential to produce MVFR, if not IFR
conditions, have included this in a tempo group around what should
be the most probable times for the thunderstorm activity. However,
this could need to get pushed back if the trend of the HRRR is
picked up on by other models. Once the rain ends, there is the
potential for low clouds with for form again late tonight. For
now, do not plan to include this in the forecast as confidence is
not high that this will occur.

&&

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

$$

SHORT TERM...JM
LONG TERM...JM
AVIATION...04



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