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Area forecast discussion
National Weather Service La Crosse WI
300 am CDT Sat Jul 20 2019

Short term...(today and tonight)
issued at 257 am CDT Sat Jul 20 2019

Early morning surface analysis reveals a couple weak surface lows,
one over eastern Wyoming and one near Marquette. A nearly stationary
front stretches between the two lows, draped across the northern
portion of our County Warning Area. A pool of oppressive dewpoints lingers south of
the front, with early morning readings still in the upper 70s to
lower 80s generally along and south of I-90. Temperatures south of
the front early this morning are still in the low to mid 80s in the
Mississippi and Wisconsin river valleys. Record warmest low
temperatures for July 20 are 78 degrees for La Crosse (1926) and 76
for Rochester (2011). While we're certainly on track to setting new
records early this morning, there's a decent chance we'll cool
sufficiently behind the front this evening to keep the current
calendar day records safely in place. It'll be another toasty, humid
day south of the front, across mainly northeast Iowa and far southwest
WI where highs will be near 90 and dewpoints in the mid-upper 70s.
Will keep excessive heat warning as is in this area. Northerly flow
will offer some relief north of the front with highs in the mid 70s
to mid 80s and dewpoints in the mid 60s to lower 70s.

A robust low level jet over the plains was feeding an mesoscale convective system over
western South Dakota early this morning. Current suite of convective
allowing models generally wants to bring this decaying mesoscale convective system into our
neck of the Woods towards midday, with only isolated pop up showers
and storms this morning. By midday the low levels are expected to
warm sufficiently to mostly erode any capping. The airmass south of
the front will remain sufficiently juiced to promote MLCAPES of 1500
to 4000+ j/kg, highest south of I-90. Moisture transport will be
considerably weaker than Friday evening, but with this kind of
instability it won't take much forcing to pop showers and storms
heading into the afternoon hours as the decaying mesoscale convective system approaches, and
the front will certainly lend additional support. Latest cams paint
fairly widespread convection crossing mainly the northern 2/3 of our
County Warning Area from late morning through the afternoon. Not as much deep layer
bulk shear to work with as Friday evening, but 30-45 knots in the
0-3km layer should support some organized storms (href shows some
respectable updraft helicity). Cams show potential for some discrete
cells popping up along the front initially before activity gets
swallowed up by the incoming MCS, perhaps morphing into some linear
segments. Damaging winds will be the main threat. Isolated large
hail and perhaps a brief tornado are also possible if updrafts can
become sufficiently organized.

From a Hydro standpoint, the good news is this afternoon convection
looks fairly progressive, but the bad news is the vast majority of
our County Warning Area has already been worked over by heavy rainfall over the last
several days and doesn't need any more at the present time. Pwats
will be in excess of 2 inches with deep warm cloud depths, so heavy
downpours are likely. Still too much uncertainty with regard to
convective evolution to warrant issuing a Flash Flood Watch at this
time, but day shift may need to consider a short term watch based on
how the mesoscale convective system evolves upstream this morning.

An incoming shortwave will nudge the front southward heading into
this evening, with perhaps another round of convection developing
along it after this afternoon's activity departs. Not as much
instability or shear with this potential later round, but heavy
downpours will remain a concern through late evening before activity
exits to our south overnight.

Long term...(sunday through friday)
issued at 257 am CDT Sat Jul 20 2019

By Sunday, continued light northerly surface flow will drop dew
points into the 55-65 degree range, bringing welcome relief from the
oppressive heat and humidity. Passage of the upper trough Sunday
night looks like it will be dry with not enough moisture to
generate any precip. Highly amplified northwest flow aloft through
mid week will reinforce the dry and seasonably mild trend through
late week. The next chance for rain looks like it will not be until
next weekend.

&&

Aviation...(for the 06z tafs through 06z Saturday night)
issued at 1233 am CDT Sat Jul 20 2019

A stationary front will remain the focus for additional shower
and storm potential heading into Saturday as it interacts with a
soupy, unstable airmass lingering over the region. Both taf sites
are expected to remain dry the rest of tonight outside of any
isolated pop up activity before daybreak. Hi-res models then
suggest convection currently developing over the Dakotas will
progress eastward Saturday morning, with remnants likely moving
into the upper Mississippi Valley towards midday. By that time,
daytime heating will promote a plenty unstable atmosphere, and
additional storms may Blossom along the front as it slowly drops
southward in the afternoon and evening. However, confidence in
convective evolution during the day Saturday remains low, so
currently handling with thunderstorms in the vicinity in the tafs. Other aviation concern
would be potential for MVFR ceilings developing along the front
towards daybreak, perhaps lingering for a good portion of the
day. Winds will turn north/northeasterly once the front drops
through lse/rst Saturday morning, perhaps becoming a tad gusty in
the afternoon.



&&

Arx watches/warnings/advisories...
WI...excessive heat warning until 7 PM CDT this evening for wiz054-
061.

Minnesota...none.
Iowa...excessive heat warning until 7 PM CDT this evening for iaz011-
018-019-029-030.

&&

$$

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