110
FXUS63 KARX 180336
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1036 PM CDT Tue Sep 17 2024

.KEY MESSAGES...

- Unseasonably warm weather will continue through Thursday with
  highs into the 80s for most areas.

- The next risk of storms comes Thursday with a low (5 to 15%)
  chance for a strong or severe storm, mainly near and west of
  the Mississippi River during the afternoon and early evening.

- Gradual cooling along with low to medium (30 to 50%) rain
  chances expected for the weekend into early next week, but
  forecast confidence, including timing of any rain is lower.

&&

.DISCUSSION...
Issued at 230 PM CDT Tue Sep 17 2024

OVERVIEW:

A blocking ridge with anomalous mid-level heights across eastern
Canada will persist through late week with stronger shortwave energy
largely deflected north across the northern plains/southern Canada.
By the weekend, several troughs embedded within split flow will
attempt to move east into the Midwest/Great Lakes, but
predictability lowers with the evolution of these troughs, related
to uncertainty in the downstream flow pattern/strength of the ridge.
Unseasonable mid-September warmth will give way to some cooling
through early next week, while shower/storms chances increase
ahead of a front on Thursday, and persist through the weekend,
although confidence/predictability lower substantially during
this time.


TEMPS:

The trend of warm September days will continue through Thursday with
persistent low-level southerly persisting on the western periphery
of the ridge. There is high confidence for temps at or above 80 (>70-
80%) for much of the area through Thursday with low chances (~20%)
for 90 degree readings, mainly across southwest Wisconsin and in the
typically warmer valleys. An increase in high clouds, though, is
expected on Wednesday and despite stronger warm advection Thursday
with breezy southerly flow, clouds and any showers/storms could also
impact temps. That said, overall model spread in temps is low
through Thursday. By the weekend into early next week, a gradual
cooling trend is anticipated, although the potential range of
outcomes increases due to uncertainty in the large scale flow
pattern.


RAIN/STORM CHANCES:

Northward moisture transport will be focused west of the Mississippi
River through Wednesday with low-level easterly flow keeping drier
air in place eastward across Wisconsin allowing afternoon RH to fall
below 30% in spots. As a result, showers/storms will remain focused
farther west through this time.

By Thursday, a trough will be forced north and east across the
northern plains into the Canadian prairies with a southward
extending cold front sliding east towards the Upper MS Valley. Warm,
moist advection will occur ahead of the front, with some potential
for showers/storms during the morning within the area of stronger
warm advection. The greater synoptic forcing likely will pass by to
the north of the area later Thursday as the primary shortwave gets
shunted north and east into Canada, but there is expected to be some
overlap between a fairly narrow plume of instability ahead of the
front and enhanced low to mid-level flow. Depending on the degree of
low-level moistening, capping could be an issue, but storms are
probable along the front during the afternoon/evening, then
likely weakening as they progress eastward across the area
through the evening as instability wanes, given the modest
support aloft. The most likely location for any strong to severe
storms looks west of the Mississippi River where timing of the
front and higher instability would potentially support more
intense updrafts.

Otherwise, for this weekend into early next week, separate troughs
across the southwest US and north across Canada will progress
eastward, although the predictability of these troughs is relatively
low, given the influence of the downstream ridge and overall
uncertainty in the evolution of the flow pattern. Consequently low
to medium (30-50%) rain chances extend from Saturday into early next
week. These chances will inevitably be refined as predictability
increases through the week.

&&

.AVIATION /06Z TAFS THROUGH 06Z THURSDAY/...
Issued at 1035 PM CDT Tue Sep 17 2024

VFR conditions and south to southeasterly winds are forecast
for the TAF period. Winds tonight range from calm to 5 kts east
of the Mississippi River to 5-10 kts to the west. These winds
increase to around 10 kts areawide with gusts near 20 kts west
of the Mississippi River for the afternoon.

&&

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

&&

$$

DISCUSSION...JM
AVIATION...Skow