955
FXUS63 KARX 071803
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
103 PM CDT Fri Jun 7 2024

.KEY MESSAGES...

- Rain chances move in for parts of southeast MN/northeast IA
  later this afternoon, but more widespread overnight/Sat
  morning. Only a low end thunder threat (30%) with rain
  amounts mostly 1/4" or less.

- Relatively cool weekend into the start of new work week
  (highs 60s to low 70s for most) but turning warmer for the
  latter half.

&&

.UPDATE...
Issued at 100 PM CDT Fri Jun 7 2024

Current surface analysis shows low pressure centered over the pan
handle of Nebraska with southerly flow advecting moisture north
in a somewhat narrow ribbon 3/4ers the width of the state. The
60F dewpoint line has rapidly shot north to near I-80 with radar
showing elevated /10ft/ showers and storms across SD into nwrn
IA early this afternoon. This area is associated with a strong
area of 600-700mb frontogenesis within the MuCAPE gradient of
250-750 J/Kg per SPC, with the heart of the MUCAPE pool further
southwest over wrn NE/CO. Per latest RAP guidance, this elevated
forcing /fgen/ translates east today in swift zonal flow,
grazing northeast IA closer to I-35 early this evening, however
without the elevated instability /which spreads in late
tonight/. With the low-level ridging and dry air mass holding
firm across the Upper Miss Valley, would think this will
translate into thick clouds at 10kft and scattered very light
showers or sprinkles.

GOES water vapor showing a well-formed shortwave trough in
northwest flow dropping southeast through srn Saska/Alberta at
18Z. Short term model consensus has moderate 300-500mb QVector
convergence along and north of I-90 Saturday morning with
moderate isentropic lift through the low-levels as well with
this wave. As the low-level southwest flow increases late
tonight as the ridge axis pushes east, low MUCAPE instability
/100-300 J/Kg/ does shift in near and south of I-90, and is
speed convergent, isentropically upgliding. Have a hard time
believing showers would not be present south of I-90 with
isolated thunder too after about 3 am. At that same time, a cold
front with the northwest flow trough is heading through MN,
moving through the area Saturday morning into very early Saturday
afternoon. This will act to prolong the period of showers Saturday
and there is some timing spread among the CAMS. So, forecast
will paint a little bit bigger window than may actually occur
and hopefully timing can be refined in the next 12 hours. So,
continued very high rain chances near and I-90 and north late
tonight into Saturday, and increased them southward into
northeast IA and southwest WI /60%/...with clearing trying to
work in post-frontal in the afternoon from NW->SE.

&&

.DISCUSSION...
Issued at 200 AM CDT Fri Jun 7 2024

* OVERVIEW:

Upper level longwave trough over the great lakes/new england states
will persist as various shortwaves drop into the trough, re-
enforcing/carving it out. This setup will continue to funnel
relatively cool air southward from Canada while the shortwaves and
afternoon instability work in concert to spark a few showers
from time to time. Thunder chances relatively low moving into
next week.

The long range guidance remains steadfast in breaking down the
blocking pattern as we move into the middle part of the new week,
with broad upper level ridging/zonal flow taking over. Heights climb
as does low level warming. How strong the ridge becomes, and where
it moves/centers itself is not clear. WPC cluster analysis suggest a
few different ways this may play out - which could result in steamy
temps locally, or just more moderate warming. The EPS and GEFS
members don`t have much of a preference at this time, dropping
members in all 4 clusters. Will let the model members details temps
for the time being.


* WEEKEND RAIN CHANCES - most likely late tonight/Sat (60-80%). Only
  a low end thunder threat (10-20%).

A bit of upper level energy is progged by the short/medium range
range guidance to slide east out of the southern rockies today,
shifting across IA by late afternoon/evening. Some low level warm
air advection and mid level frontogenetic forcing with the feature.
Good fetch of low level moisture return into the shortwave as 30 kt
850mb jet noses into northcentral IA. Pool of instability holds
southward of the systems associated sfc warm front.

CAMS models develop a northwest-southeast running bkn line of
showers in response to the forcing/moisture interaction, but mostly
mid level variety. Higher convective threat lies south/southwest of
this band.

Moving into the overnight another shortwave trough looks to drop out
of southern Canada and move across the upper mississippi river
valley. Weak low level thermodynamics but some upper level QG
convergence to work with. Sensible weather impacts come in the form
of widespread showers, move favored across parts of MN and WI.

Other, small bits of upper level energy and weak afternoon
instability could continue spotty shower chances from time to time
Saturday afternoon through Sunday, but not a lot of consensus in the
models on how this will play out.


* RAIN CHANCES FOR NEW WORK WEEK? mixed messages in the models,
  lower confidence

How the rain potential plays out for next week is unclear as the
upper level ridging works in. Some of the GEFS and EPS bring rain
chances to the region while other members favor dry conditions. Not
a clear signal either way in movement/track of any upper level
disturbances, and confidence isn`t high in any particular period
over another. Will hold with the blend for the details for now.


* COOL THIS WEEKEND INTO EARLY NEXT WEEK - but looking warmer by
  week`s end

The cool low level air that settled across the region Thu looks to
hang around through the upcoming weekend and into the early part of
the new week. Still looking at highs approx 5 to 10 degrees below
the early June normals - mostly topping out in the upper 60s to
lower 70s.

With broad ridging building in as we move into the middle part of
the new week, increasing heights with corresponding bump in temps
promises a return to the seasonable normals, if not exceeding them
by 5 to 10 degrees. Consistent signal in the long range guidance to
warm back into the 80s while the GEFS and EPS suggest a 5 to 10%
chance to push 90 for a few locations by the new weekend.

&&

.AVIATION /18Z TAFS THROUGH 18Z SATURDAY/...
Issued at 100 PM CDT Fri Jun 7 2024

Moisture will be on the increase overnight with probabilities
will increase toward sunrise for some flight restrictions. A
weather system moving in from the northwest will provide
widespread rain showers later tonight and Saturday morning. It
looks like visibilities should remain VFR except small periods
of time where more intense rainfall cores may hit the airfields.
However, the ceiling probabilities increase rapidly for either
MVFR or IFR ceilings after 11Z. NBM guidance suggests IFR would
be reached at RST by 14Z, with MVFR guaranteed. Probabilities
are more favorable for lower MVFR at LSE after 11Z per the NBM
guidance.

For this TAF cycle, have decided to play a bit more conservative
with MVFR ceilings at both sites during the morning. Probabilities
for IFR will be monitored in future forecasts as there is some
chance of that occurring /25%/ at KRST. Moisture is increasing
across the area versus the dry air in place, however confidence
is a bit lower on the degree and depth to the saturation.

&&

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

&&

$$

UPDATE...Baumgardt
DISCUSSION...Naylor/Zapotocny
AVIATION...Baumgardt