Area Forecast Discussion
Issued by NWS La Crosse, WI

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000
FXUS63 KARX 131951
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
251 PM CDT Wed Mar 13 2024

.KEY MESSAGES...

- Precipitation Chances Remain On Tap Late Tonight Through
  Thursday, Limited To Rain With Limited To No Thunder Chances

- Highest Confidence For Near 0.5" Along & South Of Interstate
  90 with 1"+ Totals Possible In Narrow Swaths

- Today Wraps Up The Highest Potential For The Record Breaking
  Warmth As Highs Return Back To Normal For Thursday, Remain
  Slightly Above Normal Into The Weekend, & Become Coldest
  Sunday Night Into Monday With 5 to 10 Degrees Below Normal
  Temperatures

&&

.DISCUSSION...
Issued at 142 PM CDT Wed Mar 13 2024

Initial Precipitation Chances Overnight Into Thursday Morning:

A multi-faceted forecast on tap for the Upper Mississippi River
Valley over the next 24 hours. Morning observations (GOES/METARs)
have exhibited the stationary moisture boundary that was south of
the forecast area earlier this morning slowly returning north with
diurnal heating. A near 90th percentile PWAT this morning at
DNV`s RAOB exemplified the strong moisture gradient with a
climatological mean PWAT at MPX (13.12Z) sounding. For the
anomalous warmth lovers, best warm air advection remains west
and south of the forecast area through today.

An upstream surface low undergoing cyclolysis while sagging
southeast through the Northern Plains early this morning will
advect the narrow filament of more anomalous temperatures
southwest of the local area. This trajectory will continue to
strengthen the theta e gradient along our southern periphery as
an appendage of the Canadian anticyclone behind this low`s
associated frontal boundary persists northeasterly flow from
central to northern Wisconsin. This theta e gradient expected to
zonally drape and bifurcate the forecast area through tonight.
Any leftover energy from this weakening low will be consumed
into the main surface low in the South-Central Plains into
Thursday.

Given the repeatedly mentioned drier air mass to the north,
precipitation onset continues to push back in time. Previous
high resolution model runs (Fv3,ARW,HRRR, RAP) exhibited a start
time near 9pm tonight, while most recent runs (13.12Z) are in
near 100% agreement with a later onset near 3am Thursday morning
as the initial, original deformation band remains to the
southwest from northern Iowa into eastern Nebraska. Not until
additional energy and increased troughing from the Northern
Plains surface low does confidence for saturation increase.
Lobes of mid level vorticity accompanying the mid level height
depression advect northeast through the area Thursday morning
as the exit and entrance regions of dual upper level jet streaks
provide optimal synoptic forcing.

Further Precipitation Timing Through Thursday:

Rain chances continue through much of Thursday as the elongated area
of lower surface pressure progresses just south of the forecast area
through eastern Iowa. Initial isentropic descent within the drier
air mass is overcome with meager upglide and enhanced low level
frontogenesis north of the surface low. Therefore, a band of higher
rainfall amounts expected to pivot across the Upper Mississippi
River Valley. Other than being directly dependent on saturation
location (i.e., 25th percentile remains 0.0"), HREF mean 1-hr
rainfall reaches 0.05" in this deformation band from 14.12Z -
14.14Z with highest confidence along the Iowa/Minnesota border
and points east in western Wisconsin. The same hourly, albeit
short lived, rainfall rates also concurrently graze far
southwest Wisconsin associated with the low center. The main
deformation band pivots cyclonically near the IA/MN/WI border
into the afternoon, eventually exiting east through the
afternoon hours.

Precipitation Amount, Type, & Impact:

While confidence in amounts has slightly increased, dependency on
the narrow band of frontogenetical forcing limits determining
the exact location of higher amounts. High resolution guidance
varies from a maximum 2+" within the NAM Nest from northeast
Iowa into southwest Wisconsin to near 1" within the most recent
HRRR (13.12Z). Although, this is twice as much as its previous
run (13.06Z), albeit not as widespread with the 1" isohyet as
the next previous run (13.00Z).

Have broad brushed near 0.5" storm rainfall totals as placing
tight, increased amounts would be misleading with current
forecast. These totals line up with the higher, albeit
moderate, confidence (60-80%) for the same amount in HREF and
same in long term global ensembles (60-100%). Most recent long
term global inter-model guidance has reached this same 100%
confidence threshold for 24-hour rainfall too. Although, the EPS
(13.12Z) exhibits this confidence in a very tight band from
northeast Iowa into southwest Wisconsin, similar to high
resolution guidance, while the GEFS (13.12Z) has broad brushed
100% to the southeast from eastern Iowa into northern Illinois.
Irregardless, given the the tight gradient, especially on the
northern periphery, expect sharp cutoffs between potentially
higher amounts.

A quicker solution for the subsequent upstream upper level trough,
over the Canadian Rockies at discussion issuance, resulting in a
quicker exit for precipitation and therefore removing frozen
precipitation chances locally. The colder airmass does

Temperatures Today Into Next Week:

Today appears to be the final day of record breaking warmth for
the Upper Mississippi River Valley as a general cooling trend is
expected through the weekend into early next week. Initial
temperatures today rebounded quicker than previous days, as the
colder, drier air mass remained shallow. The colder, sinking dry
air did limit valleys from bouncing back however. This, in
combination with the northern extent of the warmer air, has
resulted in Rochester breaking the high daytime temperature late
this morning while La Crosse has just recently tied its high
record near AFD issuance (2 pm CDT).

Temperatures remain throttled tomorrow as precipitation chances
through much of the day keep high temperatures near normal, in
the 40s for much of the area. A slight delay in the longer wave
cold tongue advecting farther south through the Upper
Mississippi River Valley keeps temperatures slightly above
normal, in the 50s, for Friday and Saturday. Long term global
ensemble confidence (80-100%) has remained persistent for this
cooling trend, bringing slightly below normal temperatures, 40
and below, for Sunday. The cold spell remains transient though,
coldest Sunday night with 5 to 10 below normal temperatures in
the teens to low 20s, remaining similar daytime highs into the
new week.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1126 AM CDT Wed Mar 13 2024

CIGS: gradual increase in high clouds tonight with lowering to a
MVFR/IFR deck after 12z Thu (50-60% shot for IFR). The low cigs
should hold through the afternoon with improvement likely Thu night.

WX/vsby: rain showers start to spread in from the south near/after
12z Thu. The pcpn will battle the dry airmass currently in place
across the local area, serving as a northward deterrent to the rain
chances. A few of the meso models suggest the pcpn might not make
the I-90 corridor - a trend that will be monitored. For now, will
ride -shra for at least the morning hours of Thu. If -shra are
realized, there could be periods of MVFR vsbys before shifting east
by 00z Fri.

WINDS: sub 10 kt easterly winds becoming northeast/northernly toward
12z Thu and increasing. Gusts in the low to mid 20 kts Thu morning,
into the afternoon. Winds will drop off Thu evening with relaxing of
sfc pressure gradient.

&&

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

&&

$$

DISCUSSION...JAR
AVIATION...Rieck


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