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FXUS63 KARX 171746
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1246 PM CDT Mon Jun 17 2024

.KEY MESSAGES...

- Thunderstorms likely this morning with a locally heavy rain
  threat, shifting north of the area by later this afternoon.
  Storms may also produce winds up to 60 mph and hail around 1
  inch in diameter, but confidence remains on the lower side.

- Continued warm and humid through Tuesday, with at or above
  normal temperatures for much of the seven day forecast.

- An active and wet weather pattern shaping up for mid to late
  week with repeated rounds of storms that have the potential to
  bring heavy rainfall.

&&

.DISCUSSION...
Issued at 400 AM CDT Mon Jun 17 2024

Today - Tonight: Convective and Temperatures Trends

Strong moisture transport and 310K isentropic ascent over a
quasi-stationary boundary draped across northern Nebraska
sparked an elongated complex of thunderstorms shortly before
midnight, with the steering flow and cold pool propagation
allowing for the complex to work its way eastward. As the
complex transects south-central Minnesota, it will outrun the
strongest 850-mb moisture transport and enter a region where the
low level flow becomes more boundary-parallel. Therefore, a
general weakening trend is expected as the system moves through
this morning.

That all being said, there has been an uptick in the
organization of the cold pool at the leading edge of the now
bowing line segment over southwest MN and northwest IA, with
multiple wind reports of 60 to 70 mph reported since 2:30am.
While the overall deep-layer kinematic environment isn`t ideal
for a prolonged wind event, these wind storms can persist for
longer than the environment may suggest and has prompted the
issuance of a severe thunderstorm watch for this morning across
much of the forecast area. In addition, hodographs are more
elongated north of the surface boundary (mostly just north of
the forecast area), so one cannot rule out the potential for
larger hail with any storm that develops ahead of the main line.

Given the potential for a period of cell training along the
boundary, localized corridors of 1-2 inches of rain may fall
over southeast Minnesota into west-central Wisconsin through
early this afternoon. A few HREF members even have pockets of
2-3 inches of rain from these cells, but the probability of
these higher amounts is very confined in space and time.

The various 00-06Z HREF members differ on how much convection
remains for the afternoon hours along the lower tropospheric
boundary with weak upper level ridging passing through, so have
maintained broad-brushed 20-30 percent PoPs through the
afternoon. Forecast soundings/hodographs from the RAP/HRRR could
support severe weather if there is enough recovery in the wake
of this morning convection to realize the 2000-3000 J/kg of
SBCAPE. As the boundary finally marches northward in response to
the next subtle upper tropospheric wave ejecting from the Four
Corners region, the threat for precipitation shifts northward
and out of the forecast area by late this evening.

High temperatures today--most notably along a line from roughly
Charles City, through La Crosse, and up to Wisconsin Rapids--
hinge substantially on convective trends and whether the cold
pool from the morning MCS can push the synoptic boundary any
further south. Recovery of the airmass won`t take place until
late in the day, so locales north of the front will likely stay
10-15 degrees cooler than points just to their south.

Tuesday: Hottest Day of the Week

Short and medium range solutions are locked in on Tuesday being
the warmest day of the forecast with a very tight clustering of
NBM members around the 90 degree mark. Did nudge highs a few
degrees warmer in favored valleys, which bring heat indices to
around 100 degrees at peak heating. Will assess any need for a
targeted advisory on upcoming shifts.

Tuesday Night - Saturday: Repeated Rounds of Storms and Rain

An ejecting PV lobe drives the surface boundary back south
Tuesday afternoon and night as a cold front. The mid to upper
level flow should take on a more line-parallel orientation as
the front sags southeastward, promoting a slower frontal passage
and more training of individual cells along the front. Deep
moisture will be plentiful ahead of this front with a wide open
Gulf enabling PWATs to exceed the 97-99th percentile of the ENS
model climatology with values of over 2 inches possible. Such a
setup would be favorable for heavy rainfall, with the greatest
amounts early on in the event before the cold pool begins
outrunning the convective updrafts as complex nears the
Mississippi River.

Upper level ridging remains steadfast east of the region
through the week, even retrograding westward Thursday into
Friday. This causes the front to eventually stall out and
meander for Wednesday and Thursday before lifting back northward
on Friday, then swinging through as a cold front on Saturday.
Where the front sets up for midweek and subsequent timing
details remain a source of variability in the medium range
guidance and are heavily driven by convective influences each
day, yet nearly half of the NBM guidance have rainfall amounts
exceeding 3 inches over southeast Minnesota and western
Wisconsin between today and the weekend. Each round of storms
will further saturate the soils and could increase the risk of
river and flash flooding through the week.

While the passage of the cold front will bring some relief from
the heat, highs in the upper 70s to mid-80s remain on the docket
for the rest of the week with dewpoints in the 60s still making
it feel very much like summer.

Sunday into Early Next Week: Cooler and Drier

Indications at this stage in the forecast is that the upper
level ridge finally flattens with the arrival of a northern
stream wave over the weekend with either zonal or northwesterly
flow in its wake. Guidance is understandably well-spread in the
various details of the forecast, but only about 20 to 30 percent
of the members have highs above 80 degrees for Sunday and
Monday. Drier weather sets in with the passage of the surface
high pressure cell, but the timing of this dry period hinges on
the flow pattern as a more zonal setup would usher the ridge
through faster versus a more amplified pattern.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
-- Changed Discussion --
Issued at 1246 PM CDT Mon Jun 17 2024

Showers and storms will be the main aviation concern through the
rest of the afternoon and evening. Currently, observing showers and
isolated storms pushing north with a boundary, as it pushes north
instability will push into the region allowing for storms to
potentially initiate later this afternoon. The trouble with
this is the short-range convective guidance is showing
differing solutions with how storms will manifest later this
afternoon. Consequently, deciding to hold with a VCSH group at
both TAF sites through the evening and may need to amend to
include thunder mention as needed. Otherwise, should see
improving conditions into the overnight hours with increasing
southerly winds during the day on Tuesday to around 10-20 kts
with gusts to around 30 kts, mainly in unsheltered areas of
southeast MN and northeast IA.
-- End Changed Discussion --

&&

.HYDROLOGY...
Issued at 400 AM CDT Mon Jun 17 2024

Repeated rounds of heavy rain are expected between today and the
upcoming weekend, with each round of rain increasing the risk of
flash flooding and river flooding. Much of southeast Minnesota
into western Wisconsin will likely see over 3 inches of rain by
the end of the week with a 20-30 percent chance of 4-5 inches.
River flooding concerns, especially along the Mississippi River
and its Minnesota tributaries will need to be monitored closely.
Many locations along the Mississippi River will approach flood
stage towards next weekend (50-70% chance) if rainfall unfolds
as forecast.

&&

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

&&

$$

DISCUSSION...Skow
AVIATION...Naylor
HYDROLOGY...Skow