Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 080958

Area Forecast Discussion
National Weather Service La Crosse WI
500 AM CDT Thu Jun 8 2023

.DISCUSSION...(Today through Wednesday)
Issued at 248 AM CDT Thu Jun 8 2023

Key Messages:

 - Similar Conditions Today as Yesterday, Very Dry and Warm,
   Relative Humidities in the teens to 20s with Light Winds

 - Storm Chances Increase Through Much of Saturday, Lingering Into
   the New Week East of the Mississippi River

 - Potential Breakdown in Blocking Pattern Through Midweek

Persistent Pattern Provides Another Warm, Dry Day:

Similar conditions expected today as seen yesterday across much of
the area. Drier air mass remaining in place will provide very low
relative humidities, especially east of the Mississippi River where
RHs may bottom out in the teens. While near surface smoke
unlikely today, upper level smoke expected to provide hazy skies
across the area, stronger west of the Mississippi River. Other
than the smoke/haze, which is efficient at limiting incoming
radiation, a lack of clouds will provide a thick low level mixed
layer. Concentrations of upper level 2.5 um particulate matter
are not expected to be enough to fully limit incoming radiation
and decrease PBL mixing substantially. With the dry conditions,
aforementioned RHs bottoming out in the teens, elevated fire
concerns continue today, primarily east of the Mississippi River,
albeit lighter surface winds. Practice fire and burn safety today.

Much Needed Precipitation Possibilities Return:

The local forecast area remains within this drier air mass for much
of the day Friday, with increased sky cover advecting from northwest
to southeast. Current timing of increased cloud cover will allow
thick PBL mixing through the day Friday before meager low level
moisture advection Friday night. Precipitation chances tied to
upper level jet streaks, mid level positive vorticity advection,
and meagerly increased low level theta e will increase late
Friday night into Saturday. Initial onset overnight Friday into
Saturday morning will be limited by this dry air with better low
level moisture transport well to our south and west through the
Central Plains.

Precipitation Onset Specifics:

Model guidance at the 48 hour forecast window limited to SREF and
long range global ensembles. SREF probabilities for a hundredth
of an inch of rainfall exhibit the limiting dry air in place,
staying <30% into Saturday. GFS (08.00Z) deterministic on Friday
night much more saturated than the coincidental near-terminus
forecast of high resolution models (RAP/HRRR 08.03/06Z).
Differences stem in speed of trough and northwest flow advecting
meagerly increased low level theta e. GFS appears the quickest,
advecting 50 degree dewpoints, while RAP/HRRR much slower to let
go of residual surface high pressure, keeping 30 degree dewpoints
for much of the area. While RAP/HRRR have been much better with
precipitation over the last week in the given pattern, PBL scheme
tends to mix higher than observations, and could be too low (i.e.,
dry). Irregardless, when the synoptic northwest flow can overcome
residing anticyclonic flow, saturation likely shortly thereafter and
subsequent precipitation. Have continued with National Blend
guidance at this time, placing "Slight" PoPs along our northwest
counties as early as Friday night. Will be something to keep an
eye on as more high resolution convective allowing models are

Weekend Precipitation Chances, Much Needed Rainfall:

Precipitation chances increase through Saturday as the upper level
trough digs and phases through the Northern Plains, advecting a
bowling ball of vorticity across the area. Overall impacts appear
limited with highest confidence for most areas to receive a quarter
of an inch of rainfall with maxima near a half of an inch.
Instability will keep storm chances through much of Saturday as
limited shear limits any severity.

New Work Week:

Precipitation chances linger east of the Mississippi River into the
new work week as the upper level trough phases into a closed low
over the forecast area. The upper level closed low is slow to
advect downstream through the Great Lakes, leaving the local
forecast area in a tight surface pressure gradient and cold air
advecting northwest flow saturating low level lapse rates. Higher
confidence for any precipitation lies to our east, in eastern
Wisconsin, painting a very low 10-40% probabilities for greater
than a hundredth to a tenth (EPS/GEFS 08.00Z). Otherwise, gusty
northeast to northwest winds expected Sunday to Monday, respectively,
nearing 30 mph. Through the new work week, appears the lingering-
neverending upper level blocking pattern temporarily breaks down
with upper level diffluent flow through midweek before PNW troughs
traverse the Northern Plains. Daytime temperatures may peak into
the high 80s to low 90s through midweek, but much disagreement and
spread between global model ensembles at this time.


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 500 AM CDT Thu Jun 8 2023

CIGS: mostly sct high/mid level clouds through the period as a sfc
high wavers across the area. Milky skies from Canadian wildfire

WX/vsby: No impacts currently expected. Morning convection over MN
should hold west of KRST in ribbon of upper level forcing, weak
instability-moisture plume.

WINDS: light, mostly easterly in direction.




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