865
FXUS63 KARX 202342
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
642 PM CDT Fri Sep 20 2024

.KEY MESSAGES...

- Showers and storms return for Saturday afternoon and evening
  (50- 80% chance). An isolated strong to severe storm or two
  cannot be ruled out with gusty winds and hail.

- Shower and thunderstorm probabilities linger into Sunday and
  Monday, but are decreasing north of I90.

- More seasonal temperatures Sunday into early next week.

&&

.DISCUSSION...
Issued at 150 PM CDT Fri Sep 20 2024

Rest of Today - Saturday: More Showers and Storms For Saturday

Currently, water vapor imagery and 20.15z RAP 500mb heights show the
overall synoptic pattern with quasi-zonal flow over the local area.
To our west, a descending wave can be noted moving into portions of
Montana with an upper-level ridge to our south. Consequently,
temperatures today are relatively consistent with previous days with
high temperatures in the upper 70s to middle 80s. As we head into
tomorrow, the aforementioned upper-level wave pushes southeast with
an attendant surface cold front pushing eastbound. Ahead of this
front, weak warm air advection pushes northward into our local area
with some of the CAMs (mainly the FV3 and HRRR) have some showers
during the morning, however kept lower precipitation chances (10-
15%) for now with the weaker forcing regime.

The more substantial chance for any precipitation comes later into
the afternoon and evening as 850mb moisture transport increases,
aiding in advecting instability northward ahead of the cold front.
As a result, seeing many of the CAMs developing showers and storms
during the later afternoon hours. Overall the current thinking with
the 20.15z RAP/HRRR guidance keeps the better instability (2000-2500
J/kg of MLCAPE) across northeast IA where the strong moisture
advection is present. Shear profiles in model soundings are a bit
more questionable with 700mb winds being on the weaker side. This
can be noted with the 20.12z HREF ensemble with the soundings
showing weak effective inflow shear with median values around 15
kts. However, stronger synoptic flow at and above 500mb and
equilibrium levels eclipsing 200mb, storms could have some effective
shear to work with the inter-quartile spread in the 20.12z HREF
ensemble soundings varying between 30 to 35 kts which is on the
marginal side but could support some near-severe hail with
respectable mid-level lapse rates of 7 to 8 C/km in the 20.15z RAP.
Additionally with hints of some DCAPE (up to 1000 J/kg) along the
front during the afternoon, could see some gusty winds initially. As
a result, the SPC has our region in a Marginal Risk (threat level 1
of 5) for this lower-end potential.

Parts of the area received heavier rainfall of 1 to 3" Thursday
evening. Rainfall totals are mostly forecast to be 0.10 to 0.6"
through the weekend and the 3hr flash flood guidance is 2 to
3.5" for most spots. Some of the HREF guidance shows the potential
for localized higher amounts, thus will need to watch out for
those areas that are a little more primed and experience
stronger storms or storms repeating over the same area.

Sunday into Early Next Week:

Sunday morning, the 500mb pattern is progged to have an area
of closed low pressure over Ontario and another closed low over
northeast Colorado. The surface front is forecast to push
through the forecast area with surface high pressure building
into the the Dakotas. Drying is occurring from the northwest
with precipitation chances decreasing from the northwest.
However, the 850mb frontal boundary is still pushing through
parts of central and southern Wisconsin with deep moisture
nearby. The trend is southeastward during the day, then
returning northward Sunday night into Monday as the closed low
over Colorado tracks into Nebraska and pushes east as an open
wave Monday. Due to the latitudinal differences in the
deterministic models, the solutions vary from it being a washout
over parts of the forecast area south of I90 to being
completely dry Monday. Sunday morning to Monday morning, the
EC/GEFS/Canadian ensemble probability of 0.10" or more of
rainfall varies from 10% to nearly 80% near DBQ with the 20.00Z
run. Compared to the 19.12Z run, the northern gradient had
tightened and pushed southward slightly with it being drier
north of I90. Similarly, the trend from Monday morning to
Tuesday morning from the 12Z run yesterday to the 20.00Z run is
in the 20 to 40% range, but also trended southward of I90 with a
tightening gradient toward DBQ. We`ll see how this trends
through the weekend.

Our seasonal highs are in the lower 70s and we have highs in the
60s to lower 70s forecast Sunday through Tuesday. So, even
though it will be much cooler that recent days, it will be
closer to normal.

Mid to late-week:

The models diverge for again for mid-late week on where a
trough will evolve into a closed low over the Mid-Mississippi
River Valley or over the eastern Great Lakes. this will play
into the precipitation chances/cloud cover/winds/temperatures
for then. Temperatures are currently forecast to warm somewhat
with highs in the 70s to end the week.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 641 PM CDT Fri Sep 20 2024

Skies will be mainly clear tonight and then high and mid clouds
will move into the area on Saturday morning and early afternoon.
As a cold front moves toward the area during the mid- and late
afternoon, the CAMs are showing scattered showers and
thunderstorms possibly impacting the TAF sites. Since confidence
is not overly high, just introduced a PROB30 at KRST starting
at 21.21z and at KLSE starting at 21.22z.

&&

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

&&

$$

DISCUSSION...Naylor/Zapotocny
AVIATION...Boyne