536
FXUS63 KARX 041602
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1100 AM CDT Thu Jul 4 2024

.KEY MESSAGES...

- We`ll have some of nature`s own fireworks today as shower and
  thunderstorm chances increase through the day from west to
  east. Severe weather potential risk level is Slight/level 2 of
  5.

- Localized heavy rain possible due to storms; multiple rounds, thus
  storms may repeat over the same area.

- Seasonably cool for the weekend into next week. Rain chances
  increase again Saturday night into Sunday; probability for .5"
  or more is 20% to 40% toward DBQ.

&&


.DISCUSSION...
Issued at 400 AM CDT Thu Jul 4 2024

Overview:

Latest water vapor satellite imagery, 500mb heights, and lightning
showed a messy convective pattern with a large 500mb trough/areas of
storms over Saskatchewan/Manitoba southward into the Northern
Rockies into the Central Plains.  Also, storms were noted across the
Ohio River Vally into Kentucky and southern Missouri on the northern
fringe of the upper level ridge.  Meanwhile, a large ridge continued
over California.  Late evening 3-hr precipitation amounts were
variable across Nebraska from a few tenths to over an inch at McCook
and Cambridge.  In the western High Plains, some gusts 40 to 50kts
and an impressive outflow boundary were noted with the storms where
dewpoints  in the 50s dropped to the mid 40s. Locally, it was a
seasonably warm day with temperatures topping out in the 80s. The
showers and a few storms generally popped up across northern WI and
northern MN into northwest Iowa. The latest WSR-88D radar mosaic
showed the organized convection across KS/MO with a shortwave trough
and scattered convection farther northward closer to the area of
closed 500 mb low pressure over the Dakotas.

Today through Friday:

A 500mb trough of low pressure over the Central Plains will shift
eastward with lead energy moving across the local area this
afternoon, however the area of low pressure closes off over MN/Upper
Mississippi Valley overnight, slowly moving across Wisconsin
during the day Friday. The deterministic models the NAM/GFS/EC
have two 850mb circulations over a short distance which makes
the precipitation distribution a little messier to forecast. The
850mb front lifts into northeast Iowa and west central
Wisconsin during the afternoon, stalling then shifting east
across Wisconsin Friday morning. Shower and thunderstorm chances
increase through the day from west to east. Highest confidence
is from the afternoon into the evening with the approach of the
upper level trough and heating during the day with moistening
(precipitable water increasing to 1.4 to 1.6"). A seasonably
strong 95kt upper level jet tracks across Nebraska by 18Z with
left front exit region support across the forecast area. Drier
air shifts in from the west, however less confident on where
isolated-scattered thunderstorm re-development occurs ahead of
the dry slot over parts of southeast MN/northeast IA during the
evening. The closed low remains in the area for Friday with
wrap-around showers and thunderstorms.

Much of our local area is in a Slight Risk/risk level 2 of 5 for
severe storms. Surface pressure is somewhat ill-defined with low
pressure to the west.  By 18Z, the RAP shows surface low pressure
over southern MN with a warm from across southeast MN and the cold
front pushing across Iowa. The hi-res models show an uptick in
stronger storms as the afternoon progresses with an increase in
coverage. Generally, it appears one area of storms lifts northeast
with additional storms developing closer to the cold front, followed
by more isolated development behind the main area during the
evening. HREF mean MUCAPE increases late morning through mid
afternoon, with deep layer shear increasing during the afternoon.
Cloudcover will hinder maximum heating, however we still have highs
forecast in the upper 70s to mid 80s. Forecast soundings show dry
air initially in the lower levels with longer hodographs. MLCAPE
appears modest 500-1200J/kg. With the front in the area and
0-1km shear of 10-20kts and deep layer shear of 50-60kts will
want to monitor local boundaries, where instability is being
maximized and any thermal gradients. Strong to severe storms
possible with large hail and damaging winds possible.

Locally heavy rain is also possible with a 20-40% probability
of 2" or more per the EPS/GEFs/Canadian ensembles from Winona to
Trempealeau Co. Comparing total precipitation from model to
model there is a slight lowering of total rainfall across
northeast Iowa and a potential of 1 to 2" along and north of
I90. The EC was a bit farther northeast more north and east of
I94. Considered a Flood Watch due to multiple rounds of storms
and recent heavy rainfall. The current forecast has higher
amounts 1 to 2" forecast where the 6hr FFG is greater, along
and north of I90. There is lower confidence for general higher
amounts south of I90 where FFG is lower. Interestingly, the HREF
does have higher 1hr rainfall rates this afternoon just south of
the local area. The great probability for Friday exceeding .5"
is across Clark Co. At this time, storms appear progressive,
however, should they get tied to the 850mb boundary north of
I94, may need to reconsider the watch. Farther south confidence
is low in the storms putting down the general amounts needed for
a watch. Will continue to monitor Flood Watch potential trends
due to saturated soils.

Seasonably cool for the weekend into next week. Rain chances
increase again for Sunday.  The probability for .5" or more has
increased to 20% to 40% toward DBQ:

Saturday night into Sunday and into early next week, the unsettled
weather pattern continues with a weak wave, then a deepening trough
working into the area.  The EPS/GEFs/Canadian ensembles showed a
slight increase in higher rain potential for northeast Iowa 20-40%
chance of .5" or more.  The series of cool fronts continues into
midweek. Highs Saturday through Wednesday look to remain near or
cooler than normal with highs in the 70s to lower 80s.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 1100 AM CDT Thu Jul 4 2024

CIGS: mid level cigs will lower with an area of convection expected
to sweep east across the area from mid afternoon into early evening.
Short term guidance further lowers the cigs into IFR/MVFR with the
passage of the cold front, holding there through Friday morning.

WX/vsby: shra/ts set to shift west to east across the region roughly
from 21-02z time frame. A variety of different forcers could adjust
this main threat time, some of which may not show their hand until
an hour before storms spark. Messy environment. Will sketch pcpn
chances out with where the higher confidence lies and adjust
forecast as trends necessitate. Looking for a break later this
evening before scattered to areas of -shra rotate in from the
northwest, post the outgoing storm system. This could linger through
the better part of Friday.

WINDS: mostly light southerly today but look for enhanced westerly
winds with gusts into the mid/upper 20 kts with any TS later this
afternoon. Winds will swing west/northwest with the passage of a
cold front this evening, picking up as we move into late morning
Fri.

&&

.HYDROLOGY...
Issued at 400 AM CDT Wed Jul 3 2024

We continue to monitor river trends with the recent heavy rains
with most rivers continuing to fall, however flood warnings
continue for the Mississippi River and the Kickapoo at Viola.
General rains of .5 to 1.5" are forecast with the greater
confidence along and north of I90. Some spots per the 04.00Z HREF
could see 2"+ with a 20-40% chance for Winona/Trempealeau Co.
6-hr flash flood guidance is generally 2 to 3 inches for flash
flooding. Cannot rule out localized flash flooding with the
storms later today and tonight if storms repeat over the same
area. Be weather aware with outdoor activities and if
traveling, be aware of your surroundings.

&&

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

&&

$$

DISCUSSION...Zapotocny
AVIATION.....Rieck
HYDROLOGY...Zapotocny