000
FXUS63 KARX 041735
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1135 AM CST Mon Mar 4 2024

.KEY MESSAGES...

- Scattered thunderstorms today, mainly from northeast Iowa into
  southwest Wisconsin, with small hail and lightning the mail
  hazards.

- Narrow Bands Of Higher Rainfall Amounts Move From Southwest
  To Northeast Through The Forecast Area Today, Resulting 0.5"+
  of Rainfall In Spots.

- Overall confidence for severe storms with hail around 1" in
  diameter and gusty winds remains low, primarily from far
  southwest Wisconsin into northeast Iowa.

- Temperatures Stabilize Through The Week As Precipitation
  Chances Return For The Latter Half of The Week Into the Weekend

&&

.UPDATE...
Issued at 850 AM CST Mon Mar 4 2024

A rather complex forecast looms for the rest of the day as the
region finds itself situated on the colder side of the current
weather system and contending with a mixture of precipitation
types. A ribbon of 850-700-mb frontogenesis forcing intensifies
over the course of the morning and early afternoon hours from
south-central MN into west-central WI. HRRR/RAP forecast
profiles show a very shallow near-surface warm layer with snow
or sleet being generated aloft. Therefore, have pushed the
forecast more to snow within this fgen band--though overall
amounts are not expected to be much given the daytime solar
insolation and surface temperatures in the mid-30s.

A weak impulse moving through the southwesterly upper
tropospheric flow sparked a cluster of showers and thunderstorms
in central Iowa this morning that is moving into northeastern
Iowa. With very steep mid-level lapse rates through the hail
growth zone and dry lower level profiles, these storms have been
producing copious amounts of small (0.25" to 0.50") hail for
their size. Additional rounds of showers and storms work over
the region through the morning and early afternoon, with their
intensity increasing as the MUCAPE axis works northward. We will
have to watch the far southern forecast area for storms
approaching severe levels with the near surface front pivoting
near the IA/IL/WI border around 21-23Z.

&&

.DISCUSSION...
Issued at 418 AM CST Mon Mar 4 2024

Early Morning Frontal Boundary & Upper Levels:

Quite a progressive pattern overnight as the frontal boundary that
provided Sunday`s wind bag bifurcates the forecast area from
southwest to northeast. Surface high pressure rather
progressively traversing the forecast area has weakened surface
winds while rotating winds northwest. Upper level water vapor
imagery signifies a slight bump in heights advecting from the
Central to Northern Plains, supporting and advecting the
associated surface high pressure with an upstream satellite
derived jet streak quickly to follow.

Early Morning Low Precipitation Potential:

While minimally impactful, initial light precipitation chances this
morning expected to graze our western periphery from northeast Iowa
into southeast Minnesota. While overnight radar imagery has been
slightly more robust than select short term model guidance, the
colder, drier post-frontal airmass in these areas overnight
expected to limit initial saturation. Therefore, have favored
high resolution guidance with similar frontal placement,
resulting in a slower, lower PoP solution. As the high
progresses early this morning, a stretching axis within the
pressure saddle will narrow the filament of lowest level dry air
resulting in mid to low level saturation slowly winning out. As
a result, a very narrow band of precipitation, mostly northwest
of the forecast area, may need to be adjusted for with an
increase of PoPs.

Precipitation Chances Increasing Through Today:

Higher precipitation potential advects southwest to northeast later
this morning into the early afternoon as the surface low, seen near
the Missouri River Valley overnight, trudges into Iowa. The lowest
level aforementioned stretching axis works to advect the axis of
increased moisture northeast, before the low level frontal
boundary falls to the southeast, due to an upstream building
high pressure over the Northern Plains. As a result, areas of
rain showers progress from northwest to southeast through the
forecast area today. The best initial collocation of highest
moisture (HREF 0.75") and frontal boundary southeasterly sag
occurs this afternoon. As a result, a tighter band of rainfall
amounts are expected from northeast Iowa through through central
Wisconsin.

Expected Rainfall Amounts Today & Tonight:

While the location of highest rainfall amounts remain in a similar
area as previously forecasted, farther northwest push and
magnitude of the highest moisture content has increased local
rainfall totals. The result ion the HREF mean is a broader
brush of the 12-hour 0.5" localized probability matched mean
isohyet and an increase in frequency narrow 1" maximum isohyet
bands. While HREF confidence suggests the same for 6 hour
amounts, initially in northeast Iowa late morning - early
afternoon and central Wisconsin shortly thereafter, a straddle
of the 18Z forecast hour muddys exact timing. Have increased
forecasted rainfall amounts near 0.75" in this area from
northeast Iowa into central Wisconsin. Monitoring location of
the surface low and attendant increased moisture axis through
the morning hours compared to higher resolution forecast model
guidance will be important for exact location of highest
expected near term rainfall amounts today.

Local Storm Chances & Potential Impacts:

The probability of storm chances remains in question through the
afternoon and evening hours as the frontal boundary sags
southeast through northeast Iowa and southwest Wisconsin.
Earliest and warmest solution in the recent RAP (04.06Z) places
>500 J/kg of SBCAPE along our southern peripheral counties. This
solution is anomalous compared to other high resolution
soundings, due to the earlier passage, and higher surface
temperatures, well above current forecasts. Similarly timed run
of the HRRR (04.06Z) keeps lowest level boundary layer
sustaining the warm nose and its associated off deck inversion.
Therefore, no instability seen in the HRRR. HREF IQR (25-75th
percentile) is 0-500 J/kg grazing the very southern edge of the
forecast area. Again, will be important to monitor low level
frontal location and timing. Irregardless, given the frontal
proximity, and associated shear of 30+ kts, a linear mode with
hail as the primary threat would ne the most likely scenario.
Again, a very limited spatial and temporal window locally before
the front and attendant storms shift southeast.

Temperatures Through Work Week Into The Weekend:

A more stable pattern will dampen the recent fluctuatiyns in
temperatures through the end of the forecast period. DAytime
highs in the 40s and 50s expected with overnight lows in the 20s
and 30s. Long term global ensemble confidence for these
temperatures remains moderate at 50-80%.

Precipitation Chances Return For Latter Half of The Week:

Precipitation chances return through the latter half of the week as
open perturbation lobes sling around the upper level longwave trough
over Canada. Long term global ensemble confidence paints an
everlasting similarity as months of previous forecasts, a N/S
split in highest ensemble confidence over the local forecast
area. Initial confidence (20-50%) latter half of the week is
lower than subsequent, as the initial wave lifts through the
Northern Plains. The higher confidence (50-100%) comes into the
weekend as the Upper Mississippi River Valley is placed
northwest of a subsequent wave lifting through the Mississippi
River Valley.

&&

.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1128 AM CST Mon Mar 4 2024

Showers and thunderstorms will continue to occur throughout the
day. Some of these storms may have small hail with them.
Precipitation is expected to end between the late afternoon and
evening hours. With these storms, there is a high chance
(60%-70%) of MVFR conditions. There is also a low chance
(20%-30%) of IFR conditions to occur. The main threat to these
differing flight categories is the CIG height. While it is
precipitating, CIGS are expected to be between 1000ft and
3500ft. CIGS will lift during the overnight and skies will
become mostly clear by early tomorrow morning.


&&

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

&&

$$

UPDATE...Skow
DISCUSSION...JAR
AVIATION...Cecava

NWS ARX Office Area Forecast Discussion