598
FXUS63 KARX 160817
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
317 AM CDT Mon Sep 16 2024

.KEY MESSAGES...

- Above normal temperatures are expected for much of the week
  with high temperatures in the 80s for most.

- Relatively dry conditions are expected for the first half of
  the week with increasing chances for precipitation later this
  week and into the weekend.

&&

.DISCUSSION...
Issued at 316 AM CDT Mon Sep 16 2024

Today - Wednesday: Quite Warm With Relatively Dry Conditions

GOES-16 water vapor imagery and 16.06z RAP 500mb heights show the
general synoptic pattern over the CONUS early this morning with
a deepening trough situated over the Western Rockies with high
pressure sitting to our east. As we continue through the early
part of this week, much of our weather will be dictated by a
building ridge and a drier airmass attempting to move into our
region from the east, shown in the precipitable waters across
the recent RAP/NAM being generally under 1" across eastern
portions of the local area early this week. Consequently, looks
like a fairly favorable day to dry out the low-levels when
considering diunral mixing and reasonably steep hydro-lapse
rates at 700-800mb in 14.06z RAP soundings. As a result, should
certainly feel a bit less humid in spite of the well above
normal temperatures which when given the overall drier setup
likely will overachieve the NBM some. Therefore, have trended
temperatures towards the upper-quartile of the NBM for today
with highs generally in the middle to upper 80s, with an
isolated 90 being possible. Similar conditions can be expected
for both Tuesday and Wednesday as upper-level ridging remains
firmly in place with 850mb temperatures holding steady (around
15-18C) in deterministic guidance through Wednesday.

Thursday - Sunday: Precipitation Chances Return

As we head into late week, a sharp axis of 850mb moisture transport
accompanied with a parent broader troughing regime attempts to nudge
its way westward, eventually reaching the western periphery of
the local area. As this occurs, a shortwave trough is progged to
pivot through north-central MN on Thursday, providing some
ascent with a downstream surface frontal passage into Thursday
evening. While there is some disagreements between the
deterministic models on the exact orientation of this shortwave,
the overwhelming majority of ensemble guidance paint high
measurable precipitation probabilities (80-90% in the 16.00z
EC/GEFS) across southeast Minnesota. Generally noting lower
precipitation chances (50-80%), particularly in the GEFS, as you
head eastward with weaker lift and more minimal instability. As
a result, thinking the NBM precipitation chances for Thursday
evening are fairly appropriate with the highest rain chances
furthest west. The threat for more organized severe weather
with this frontal passage appears to remain west of the region
at the current moment with the 16.00z GFS generally waning
instability as the front move towards our region. Additionally,
with our region currently expected to be further displaced from
the parent shortwave and stronger mid-level flow in the current
state of guidance, our bulk shear would likely be limited as
well. As a result, very low probabilities (under 10%) are
present in the joint CAPE/Shear probs (>500 J/kg of SBCAPE and
>30 kts of bulk wind shear) in the grand ensemble (GEFS/EC
ensemble/Canadian ensemble).

The upper-level flow pattern becomes far more uncertain as the
weekend approaches with disagreements on how a secondary shortwave
will eject towards the region for the weekend and if any upper-level
ridging can manifest ahead of it. As a result, noting a large degree
of spread in ensemble guidance for high temperatures heading into
the weekend with the 10th to 90th range in the grand ensemble
across the local area having roughly 15 degrees of spread on
Sunday with median temperatures generally in the 70s areawide.
Additionally, with the low confidence in how the overall
synoptic pattern will pan out, noting more broad brushing of
precipitation chances with higher probabilities (50-70%) spread
across the local area in the grand ensemble for the weekend.
This would suggest that while confidence is lower on exact
details for precipitation, there is a reasonable chance that our
area will get some rainfall into the weekend.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1045 PM CDT Sun Sep 15 2024

VFR conditions are forecast through the period with current satellite
showing mid to high level clouds moving into the area. No
significant change in thoughts from the previous forecast as
increased flow aloft/increase in clouds may work to limit fog
development at KLSE. Otherwise, winds remain south/southeast
through the period with some diurnal cumulus through the
afternoon.

&&

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

&&

$$

DISCUSSION...Naylor
AVIATION...EMS