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775
FXUS63 KARX 191040
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
540 AM CDT Wed Jul 19 2017

.SHORT TERM...(Today and tonight)
Issued at 242 AM CDT Wed Jul 19 2017

Impactful weather in the short term will be highlighted by a
severe thunderstorm threat tonight with heavy rainfall possible.

The very juicy airmass that supported the locally heavier rainfall
with showers/storms yesterday will be shunted somewhat southward
today as the surface frontal boundary gets dragged south ahead of
weak high pressure. As a result, most of the day will be dry with
less humid air at least making it into northern portions of the
area.

Attention will be focused out to the west later today as
thunderstorms should develop across the northern plains in
conjunction with an upper shortwave trough/speed max and mid-level
warm advection near a frontal zone. Eventually, storms are expected
to congeal into one or more MCSs that dive southeastward along a
pronounced instability gradient/mid-level cap late today/tonight,
which currently looks to be set up near the Mississippi. This
aligns well with SPC`s day 1 enhanced risk area near and west of
the Mississippi. High res models do indicate some differences
with the timing of storms into the area, with some uncertainty how
storms earlier in the day across the Dakotas into Minnesota will
evolve prior to subsequent redevelopment/reintensification. Will
have to watch where the instability gradient sets up later in the
day.

With nearly 50 kts of 0-6 km bulk shear expected, including 30 kts
of 0-3 km shear, a long-lived severe MCS may persist well into the
night as a strengthening low-level jet assists with strong moisture
transport into the area. The linear, possibly bowing MCS, would
likely race southeastward across the area later during the
evening/overnight with potentially an enhanced damaging wind threat
and potential for embedded mesovortices (nonzero tornado risk)
given the 0-3 km shear. Some hail is possible, but damaging winds
should be the greatest threat, especially later on overnight.

Heavy rainfall potential certainly exists given the deep warm cloud
depths/high precipitable water values. The low-level jet is expected
to veer westerly later in the night with the better moisture
transport quickly shifting eastward, helping to keep the system
progressive. However, with the potential for high rain rates,
will have to watch for any training convection that does attempt
to focus on prior outflow which could lead to local flood
concerns.

.LONG TERM...(Thursday through Tuesday)
Issued at 242 AM CDT Wed Jul 19 2017

Behind the convective system, the surface boundary may again be
suppressed southward temporarily with weak surface high pressure
again potentially keeping most of the daylight hours dry with warm
highs in the 80s.

However, the active pattern will continue late this week as models
indicate a few upper shortwave troughs will crest the upper ridge
axis and interact with the low-level boundary and instability
present on the edge of the stronger capping to produce continued
showers/storms Thursday night through Friday night. While exactly
where the focus for storms during this time may be impacted by
mesoscale details that are tough to pin down because of the impact
of earlier convection.  That said, potential flood risks will
increase with each successive round of storms, depending on where
and how much rain falls in any given location, as conditions will
remain favorable for heavy rainfall (deep warm cloud depths, high
precip water, etc) until the front clears the area and drier air
gets dragged southward. However, some severe potential may exist
on Friday as well given sufficient shear/instability.

Showers and storms will remain possible into Saturday ahead of a
stronger upper trough diving into the Midwest later Saturday into
Sunday. The 19.00Z GFS is a little slower and holds some mid-level
shortwave energy farther west, keeping precip chances going into
Sunday, while the more progressive ECMWF brings mainly dry weather
into the area as surface high pressure builds southward. A few dry
an cooler days are possible into early week before southerly flow
develops behind the departing high and deeper moisture returns
northward.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 540 AM CDT Wed Jul 19 2017

After any remaining patchy fog burns off early this morning, VFR
conditions are expected today with light winds this morning
gradually becoming southeasterly through the day. The primary
aviation impact through this period will be the potential for a
line of thunderstorms to impact the terminals later this evening
into the early overnight period. Have kept VCTS for now given
uncertainty in timing and location, but as confidence in these
details increases, a TEMPO group likely will be needed. Brief
visibility restrictions into the MVFR/IFR range could occur with
heavier rainfall while strong winds may also accompany the
storms.

&&

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

$$

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