070
FXUS63 KARX 161907
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
207 PM CDT Sun Jun 16 2024

.KEY MESSAGES...

- Storms This Evening & Overnight. Heavy Rainfall Will Be
  Primary Hazard With Limited Potential For Hail

- Warm Into Midweek

- Temporary Break In Rain & Storms On Tuesday, Likely The
  Warmest Day of The Week

&&

.DISCUSSION...
Issued at 1242 PM CDT Sun Jun 16 2024

Observations This Afternoon:

The active pattern continues this evening and through the
overnight hours with separate rounds of storms. Surface
observations show 2-3 surface frontal boundaries this afternoon
that will impact precipitation and storms tonight. Pronounced
boundary lies to the west near the Red River Valley exhibiting
15 degree dewpoint differential and 5 to 10 degree temperatures
differential. This frontal boundary will be the main forcing
mechanism for initial storm potential tonight. Another frontal
boundary drapes zonally through central Wisconsin, associated
with the warm sector. The anomalous airmass within the warm
sector, 90th percentile SPC climatology; 2+ standardized
anomalies NAEFS/ENS, has quickly increased temperatures after a
slightly cooler morning than. Also of note are the increased
winds and increased 70 degree dewpoint temperatures within this
airmass.

While ACARs and high resolution forecast soundings have shown
building instability sufficient for potential storms (2000-3000
J/kg) early this afternoon, satellite imagery has shown
building billow and cumulus clouds, evident of stable air off
the deck. This elevated mixed layer has been building east
through the early afternoon, coming into play for storm
potential locally tonight. Satellite imagery and surface
observations also exhibit minimal shear.

Initial Precipitation & Storm Chances Late This Evening:

Local precipitation chances are expected to graze the
northwester periphery from southeastern Minnesota into central
Wisconsin this evening, shifting southeast with the
aforementioned more pronounced surface boundary. Concurrently,
the driving surface low lifts north through the late evening
stranding the increased low level theta e gradient from the
west-southwest to east-northeast. The EML, a low level
inversion, and little to no CAPE limits initial local impacts
from these storms. Primary impact will be isolated areas of
increased rainfall with amounts near 0.25" in select areas.

Overnight Rain & Storm Chances:

Subsequent precipitation chances form on the nose of the
intensifying and more meridional oriented low level jet during
the early overnight hours. The moisture transport greatly
increases with a Midwest Atmospheric River of 500-1000
kg/m-1/s-1. Initial overnight, locally impactful, storm
formation occurs along a shrinking-stretching axis from
southwest Iowa into east-central Minnesota. An anticyclonic
rotation in the nocturnal low level jet and best moisture
transport, highly evident in the AR orientation, advects this
area of storms from west to east across the forecast area
through the overnight hours.

The location of the previously mentioned low level theta e
gradient suggests highest storm and rainfall potential north of
the local forecast area. While instability builds from
steepening mid level lapse rates, a low level inversion and EML
limits stronger storm confidence locally. The departure of the
low level jet limits overall shear as CAPE deepens concurrently
in forecast soundings. In summary, expect limited strength and
duration for these storms as well.

Confidence & Potential Solutions Overnight:

Confidence remains low due to the location of the narrow
convergence zone and exact location of the frontal boundary.
Evidently there is a near split in solutions between high
resolution model guidance. Nearly half of the CAMs (NAM4km/ARW/
NSSL) are more progressive with the frontal boundary, advecting
a southwest to northeast oriented line of storms from the
northwest to southeast early in the overnight while remaining
CAMs delay to the early morning hours dependent on the low level
jet.

Expected Hazards:

Primary hazard will be heavy rain as the tropical air mass
contains anomalous moisture. HREF paints 3 hour amounts near
0.5" by midnight with an additional 1" grazing our western
periphery. Highest confidence (50+%) for 1" per hour rainfall
rates also lie along our northwest periphery in south-central
Minnesota to west-central Wisconsin. Total amounts vary in the
HREF from a maximum of 3+" and a probability matched mean of
 2"- 2.5" in a narrow band in these same areas. Confidence in
other hazards remain quite limited. Although, with a deeper,
stronger CAPE profile, can`t rule out some hail. Higher freezing
levels, lack of shear, and anomalous temperatures/moisture
resulting in melting hail limit overall confidence.

Monday Storm Potential:

Storms are expected to linger through Monday morning as the low
level jet continues to push north through Minnesota. The associated
quasi-stationary surface boundary advects north, resulting in
perpetual storms along and north of the local forecast area from
west-central into central Wisconsin. Exact location does remain in
flux however with the most southern solution lingering locally
earlier in the morning. Select CAMs suggest a limited 3 hour
window of higher storm potential of deepening CAPE after
meeting convective temperatures near 90 degrees. However,
there`s lower confidence in temperatures tomorrow given the
increased sky cover from morning storms. Also, besides the
lowest levels, which likely won`t be realized, little to no
shear to sustain stronger storms. Have also decreased surface
temperatures for tomorrow away from National Blend in
collaboration with neighbors and due to the increased cloud
cover.

Through Midweek:

Through Tuesday morning, an upstream amplified trough is
expected to lift through the Central Plains, remaining well west
of the forecast area. As a result, a break in precipitation
expected locally through Tuesday morning as the increased low
level theta e axis pushes into southern Manitoba. This lingering
moisture source eventually separates from its Gulf of Mexico
source through Tuesday, eventually advecting the narrow corridor
of precipitation and instability across the forecast area from
northwest to southeast late Tuesday into Wednesday morning.
While the short break in precipitation may be beneficial for
some, there`s higher confidence for it to be the warmest day of
the week. Long term global ensembles, GEFS; EPS, both suggest
temperature anomalies greater than 10 degrees. The EPS has a
slightly warmer solution, grazing the 90 degree isotherm farther
north while the GEFS keeps the same parameter well to the
southwest in the Central Plains. Probabilities increase for
greater than 0.5" through midweek in both the EPS and GEFS from
50%-80%.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1232 PM CDT Sun Jun 16 2024

Increased southwest winds are expected to continue into this
evening across the forecast area from southeast Minnesota,
northeast Iowa, and western into central Wisconsin. Highest
winds have been observed west of the Mississippi River. Clear to
scattered skies also continue into this evening.

Initial limited storm chances return this evening, confidence
lies primarily in central Wisconsin. Higher confidence storm
chances move from west to east during the overnight hours.
Thunderstorms likely to impact TAF sites (KLSE & KRST). Have not
included flight restrictions given lower confidence but will be
subsequent forecast detail to monitor.

&&

.HYDROLOGY...
Issued at 646 AM CDT Sun Jun 16 2024

Due to the repeated storms, we`ll need to assess the flood
potential with each shift. The 3hr FFG remains around 2 to 3
inches. Although heavy rain fell northwest of the local area
Saturday night, locally, amounts were generally less than an
inch. Some areas could see another inch this morning before
this wave exits the area. We`ll need to monitor the rainfall
rates due to the high efficiency. The storms continue lifting
northeast at a pretty good clip, thus heavy rain should be
localized. Tonight the heavier rainfall is again forecast north
of the local area with more rain Tuesday night. With each rain
event over MN, the probabilities for flooding increase for the
Mississippi River. The Mississippi River should remain elevated
for the next couple of weeks. There is a 30% chance that the
Mississippi River reaches Minor Flood Stage over the next 10
days through Winona (and 50% chance at Wabasha).

&&

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

&&

$$

DISCUSSION...JAR
AVIATION...JAR
HYDROLOGY...Zapotocny