207
FXUS63 KARX 070752
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
152 AM CST Thu Nov 7 2024

.KEY MESSAGES...

- Patchy dense fog this morning, mainly in river valleys, should
  break up with improving conditions towards mid morning.

- Rain threat returns to the area late Saturday into Sunday,
  with another one towards the middle part of next week. No real
  signal for higher rainfall totals at this point.

&&

.DISCUSSION...
Issued at 151 AM CST Thu Nov 7 2024

TODAY THRU FRIDAY: Fog impacts this morning

Extent and impact of fog will be main concern this morning with area
between weather systems and lingering saturated airmass. Even with
decoupled boundary layer /calm surface air/ seeing gradual increase
of 20-30 kts just above this which has made extent of fog and/or
stratus more variable. GOES nightime microphysics imagery shows a
fairly typical valley fog signal with hit and miss observations in
METARs. Even area webcams show a very shallow layer so would assume
it will break up fairly quickly this morning.

Expect far more sunshine today and again Friday with temperatures
climbing to at or above seasonal normals. Boundary layer relative
humidity fields in higher resolution models / ~900mb / still remain
high this morning and with some possible lift taking place ahead of
wave across southern Canada, could still see some intermittent
stratus before a more clearing trend sets in. Probability of lower
cloud cover in grand ensemble remains highest /70-90%/ across
northern half of forecast area mainly this morning.

SATURDAY INTO NEXT WEEK: Rain threat returns by Saturday night
and then again mid part of next week

Strong upper low over the southwest CONUS will be our next main
weather producer this weekend. Movement will be slow next 24 hours
but then begin to eject into the Central Plains Friday and into our
region later Saturday. As discussed yesterday, precipitation onset
can be a challenge with this pattern and usually a bit slower than
expected. Seeing some trends of a slower arrival, especially in
clusters that have more ECMWF members. GFS members actually may be a
tad faster. Model blends did slow, or lower, rain chances a bit for
Saturday afternoon, especially closer to Interstate 94, which looks
reasonable and remains persistent with earlier thinking.

Although this is a dynamic system, still not seeing much signal for
higher rainfall amounts, even with overall categorical rain chances
/90%/. Surface frontal structure will be well occluded and ensembles
suggest higher precipitable water values />1.2 inches/ get shunted
or pinched off across Illinois. So even with large area of
isentropic lift expected late Saturday, a majority of ensemble
members have 0.25 inch or less from passing showers.

After weekend system passes, upper flow across CONUS goes into
another amplification process. Some differences in medium range
guidance with possible short wave trough crossing ridge already
Monday but overall it appears mean height rises will occur with
above normal temperatures to start the week.

Evolution of larger trough next week will be worth noting,
especially with potential for more prolonged north moving moisture
advection and overall higher precipitable water values leading to
higher rainfall potential. Way too far out for details, especially
with differences in trough placement in Day 7 cluster analysis of
mean heights and anomalies. Most hazard discussions that far out
involve diverging storm tracks with a higher risk for heavy
precipitation passing by to our south, which appears to be most
likely outcome at this point.

&&

.AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
Issued at 952 PM CST Wed Nov 6 2024

Satellite imagery shows that skies have been generally clear,
however moisture is increasing from the southwest across Iowa aloft,
and stratus is increasing across central Minnesota. Forecast
soundings increase moisture around 925mb from the northwest in
the 08-10Z through 12-15Z. In the clear/areas with cirrus
clouds are beginning to see some reductions in visibility. With
dewpoint depressions narrowing, a period of fog will be possible
until the stratus moves and winds aloft further increase. As a
result, it could be a messy forecast at KRST with periods of fog
and stratus, but they will be transient. Did include MVFR/IFR
conditions developing at KRST, but becoming VFR by 15Z. For now
due to low confidence at KLSE related to fog, included BCFG
with scattered stratus then an MVFR deck developing for a few
hours Thursday morning.

&&

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

&&

$$

DISCUSSION...Shea
AVIATION...Zapotocny