Area Forecast Discussion
Issued by NWS La Crosse, WI
FXUS63 KARX 190919
Area Forecast Discussion
National Weather Service La Crosse WI
419 AM CDT THU MAY 19 2016
.SHORT TERM...(Today and tonight)
Issued at 419 AM CDT Thu May 19 2016
Main fcst concerns this period: temperatures and cloud trends.
Data analysis at 06Z had broad high pressure centered over the
western great lakes with ridging southwest into MO/KS/OK. Tighter
pressure gradient on the west side of the high was over the Dakotas
into far western MN where south to southeast winds 5-15 mph were
keeping the boundary layer stirred and early morning temps in the
50s. Closer to the center of the high winds were light/calm with
little to no boundary layer stirring and temperatures cooling into
the mid 30s to mid 40s.
For the most part, 19.00Z models initialized well. The exception
being the GFS that initialized sfc dew points some 10-15F too high
across WI/MN/much of IA. This appears to perhaps impact the GFS QPF
output across MN/IA/WI this afternoon and again Fri. Otherwise,
models in generally good agreement as shortwave energy near lake
Winnipeg slowly sinks southeast into the Upper Midwest thru tonight.
This and downstream ridge building ahead of strong troughing digging
down the west coast shunts the southwest U.S. trough/shortwave more
eastward across the plain into mid MS valley, well south of the fcst
area. Overall fcst confidence for this period remains above average.
For the short term: sfc-850MB ridge axis is pushed a bit east today
as lee troughing strengthens in WY/MT. Southerly flow on the west
side of this ridge axis look to pull the airmass with more 850MB
moisture that was over IA/MO northward over the fcst area today.
Thus expecting more diurnal cumulus this afternoon vs. the past
couple days. This moisture along with diurnal mixing should also
help keep sfc dew points a bit higher and minimum RHs this afternoon
more in the 25-35% range vs. the 15-25% range of the past couple
days. Model soundings showing a mdt inversion above the mixed layer
(around 750MB) this afternoon, limiting CAPE and cloud depth.
Suspect GFS with its sfc dew points initialized 10-15F too high
across the region is the reason it tries to produce some light
precip in/around the area this afternoon. Soundings/X-sections do
not support the GFS light rain chances. Discounted its solution and
sided with the drier models. 850MB temps this afternoon progged to
be similar to those of Wed. Some south winds for mixing today
looking to off-set the increase of diurnal cumulus, with highs today
similar to those of Wed. Some south/southeast winds tonight to keep
boundary layer stirred tonight and most lows a category or warmer
than those of this morning. Finally a night without mention of frost
somewhere in the fcst area. Trended toward warmer of guidance highs
today, then stayed near a blend of the guidance lows for tonight.
.LONG TERM...(Friday through Wednesday)
Issued at 419 AM CDT Thu May 19 2016
For Friday thru saturday night: Main fcst concerns this period are
continues to be temperatures and cloud trends.
19.00Z model runs in good agreement for the shortwave energy over
ND/northern MN at the start of the period to slowly drop south
across the region Fri/Fri night as strong ridging aloft builds north
behind it. Trend is slower/stronger with this feature. Models remain
in good agreement Sat/Sat night as rather impressive troughing
progressing across the western CONUS pushes the mid level ridge axis
into the Upper Midwest. Fcst confidence this period is above
Even as the shortwave energy is progged to drop south across MN/IA/
WI Fri/Fri night, the sfc-850MB ridging retrogrades into the western
great lakes/WI. Lower level anti-cyclonic flow with this ridging
does little to help with lower level thermo-dynamic forcing under
the shortwave. Model soundings Fri maintain the mdt inversion on top
the mixed layer (near 750MB), limiting mixing/cloud depth and any
precip chances. Again discounted GFS on Fri and maintained a dry
fcst across the area. With a continued increase of moisture around
850MB over the area, Fri may well end up more of a partly sunny vs.
mostly sunny day with diurnal clouds. 850MB temps Fri perhaps 1C
warmer than those of Thu but with more clouds, highs similar to
those of Thu look good. Lighter winds but more clouds should keep
Fri night lows similar to those of tonight. Strong ridging aloft/
hgt rises finally build over the region Sat/Sat night. Sfc-850MB
ridge axis is over the area Sat with the rising hgts/deep layered
subsidence aloft. 850MB temps progged around 10C Sat afternoon and
with a mostly sunny day expected, most highs should be in the mid/
upper 70s. Still a cooler night Sat night with the center of the
high near/overhead with light winds and mostly clear skies. Used a
blend of the guidance highs/lows for Fri/Fri night then sided with
warmer of guidance highs Sat and cooler of guidance lows Sat night.
For Sunday thru Wednesday (days 4 to 7): main fcst concerns this
period are SHRA/TSRA chances from late Sun night thru Wed.
Medium range model runs of 19.00Z trend progressive with the western
CONUS trough Sunday. Mid level ridge axis is already pushed east of
the region late Sun/Sun night with the area coming under deep
layered south/southwest flow. Models remain consistent lifting a
lead piece of energy out of the western trough northeast across the
northern plains Mon then into Ont Tue. Reasonable model consistency
for weak shortwave ridging to build across the upper MS valley Wed,
but region remains under general southwest flow aloft ahead of
troughing/shortwave energy lifting into the central plains. Fcst
confidence this period is average this cycle.
Sfc high is pushed east Sun as the flow/ridging progresses but
maintains its influence on the area. Sunday trending to be one of
the warmer days with plenty of sunshine, an increase of south winds
for mixing and 850MB temps of +12C to +14C. Consensus has highs
mid/upper 70s Sun but would not be at all surprised to see a few
lower 80s pop up. tighter gradient spreads over the area Sun
night/Mon as the sfc trough/front approach. Deeper moisture plume/
higher PW airmass now progged to arrive late Sun night then move
across the fcst area Mon into Mon night. Arrival of small consensus
SHRA/TSRA chances into the west end of the fcst area later Sun night
reasonable, with 30-50% chances Mon/Mon night as the moisture plume
and thermo-dynamic forcing would move across the fcst look good as
well. Given the detail differences by Tue/Wed and potential for
more lower level moisture/lingering troughing to remain over the
region under southwest flow aloft, small SHRA/TSRA chances these
days okay for now. One or more of the day 6/7 periods may well end
up dry as details become clearer. Other than Sunday highs looking
a bit cool, model/ensemble consensus highs/lows for days 4-7
appear well trended at this time.
.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1138 PM CDT Wed May 18 2016
Clear skies overnight will give way to late morning/afternoon
cumulus in the 5000 to 7000 ft agl layer. Could see some VFR
ceilings develop mid-late afternoon with increasing mid-level
moisture moving northward along southerly return flow on backside of
departing surface high. Winds will persist from the south-
southeast...generally 10 kts or less at both TAF airfields.
WI...Frost Advisory until 7 AM CDT this morning for WIZ034-042>044.