Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
650 PM CDT Friday Jul 3 2015
Short term...(tonight through Saturday night)
issued at 309 PM CDT Friday Jul 3 2015
Main forecast concerns this period...small -shra/thunderstorms and rain chances this
evening and again Sat afternoon/evening...temperatures.
Data analysis at 18z had low pressure over east central ont was
trailing a trough/front across Lake Superior to near kdlh to northeast
South Dakota. Deeper moisture and diurnal cumulus were mainly near/behind the
front early this afternoon. Rain showers/thunderstorms and rain so far today confined to
northeast Minnesota near the shortwave...and over the u.P. Of Michigan in the
stronger low level convergence ahead of the trough/front. Visible
imagery showed the veil of Canadian wildfire smoke remained over
much of the upper Midwest...thickest over ND/northwest Minnesota...where
some surface observation reporting visibility restrictions in haze/smoke with mixing
behind the front. Early afternoon temperatures again on the cool side for
No issues noted with 03.12z model initializations. Models very
similar as one shortwave slides southeast across Lake Superior/Huron
this afternoon/evening and another moves from eastern Montana this
afternoon to southwest Minnesota by 00z sun. Tighter consensus as this wave
then lifts toward northwest WI by 12z sun. Trend favors faster of
the early runs with Montana/Minnesota wave by later Sat/Sat night. With the
tight consensus...short term forecast confidence is good this cycle.
In the short term...shortwave dropping across Lake Superior/Huron
drags the weak surface front/boundary in the north end of the forecast area
tonight...to southwest of I-94 by 12z Sat. This with some weak 925-
850mb moisture transport...300k isentropic lift and 250-500 j/kg of
MUCAPE through the late afternoon/evening hours over roughly the north
1/3 of the forecast area. Smoke and reduced diurnal warming may limit
the instability...but will leave a 20 percent -shra/thunderstorms and rain chance over
about the north 1/4 of the forecast area through this evening. With loss of
instability and stronger of any forcing well east of the forecast
area...trend forecast dry after midnight tonight and for Sat morning.
Boundary remains southwest of I-94 Sat morning...then lifts slowly
north Sat afternoon/evening as low pressure advances across the
northern plains and the lower level gradients/south winds increase
across MN/IA/WI. Some weak low level moisture transport continues
near the boundary...with MUCAPE progging in the 500-1500 j/kg range
for Sat afternoon into Sat evening. Continued a small -shra/thunderstorms and rain
chance in the 18z Sat to 03z sun period with the boundary in the
area...and the shortwave moving across southern Minnesota toward northwest
WI. Did shift the chances northward from the previous grid set to be
closer to the boundary lifting north. Lower level anti-cyclonic flow
and weak ridging over the area later Sat evening/overnight with loss
of the cape...and left the 03z-12z sun period dry even as the weak
shortwave moves into northwest WI. Used a blend of the guidance
lows/highs for tonight through Sat night.
Long term...(sunday night through thursday)
issued at 309 PM CDT Friday Jul 3 2015
For Sunday through Monday night...
Main concerns this period...passing front and rain showers/thunderstorms and rain chances
centered on Monday...temperatures.
03.12z models remain in good agreement on the next stronger trough/
shortwave energy digs through the northern rockies sun and into the
northern plains Sun night. However trend is slower/stronger with
this trough/energy by Sun night. Some detail differences by Monday but
decent overall consensus for a stronger trough axis to be near the
Minnesota/Dakotas border by late Monday...then swinging east across Minnesota/WI/IA
Monday night. Given the tighter consensus...sun through Monday night forecast
confidence remains on the good side.
Slowing trend of the models continues to delay the arrival of the
lower level trough/front...deeper moisture and stronger low level
Thermo-dynamic forcing. Sunday continues to trend dry and warm.
Model sounding showing diurnal mixing to near 850mb sun...with
rather strong capping in the 850-700mb portion of the column Sun
afternoon. Gradient tightens through the day on sun...for southerly
mixed layer winds of 10-20kts in the afternoon. Change from
northwest 700-400mb winds to more west and southwest will blow the
Canadian fire smoke out of the region. Column quite dry on Sunday
for what should be a sunny/mostly sunny day. 925mb temperatures in the 22c
to 24c range Sun afternoon...supporting highs in the middle...even a
few upper...80s across much of the area on sun. Slowing keeps the
main moisture/moisture transport and instability axis west of the
forecast area until later Sun night. Left sun through sun evening dry.
Stronger middle level troughing/shortwave now looks to send a wave of
low pressure up the front into southwest Minnesota by 12z Monday...lifting int
northern WI by 00z Tuesday. This with what looks be even deeper/stronger
forcing/lift over the area late Sun night into Monday evening. Spread
70-80 percent rain showers/thunderstorms and rain chances across the area on Monday. Severe risk
with thunderstorms and rain on Monday remains questionable...with stronger of the shear
into the northwest 1/3 of the forecast area and higher of the cape over
the southeast half of the forecast area. Convection timing is also
likely to limit severe potential...with debris clouds spreading
across the forecast area already Monday morning...limiting cape potential.
Precipitable water values prognosticated in the 1.75 to 2.25 inch range Monday...this with
warm cloud depths in the 4km range. Rain showers/thunderstorms and rain Monday should be
efficient rain producers. Heavy rain certainly a threat Monday if thunderstorms and rain
are slow moving or repeat over the same location.
Surface low/wave and frontal Boundary Pass later Monday afternoon/Monday
evening...with drier/cooler can high pressure building in. Precipitable water values
drop to less than 0.75 inch after midnight Monday night. Limited bulk
of the Monday night rain showers/thunderstorms and rain chances to the evening hours. Trended sun
highs toward warmer of guidance values...then favored a blend of the
guidance lows/highs for Sun night through Monday night.
For Tuesday through Friday...days 4 to 7...
Main forecast concerns this period...shra/tsra chances Wednesday night through
Friday...temperatures through the period.
Medium range model runs of 03.00z/03.12z in rather good agreement
for rising heights over the upper Midwest Tuesday/Tuesday night as the Monday
troughing moves into eastern can. Heights start to fall already on Wednesday
as the next trough moves across central can...however models begin
to differ on the progressiveness of the troughing. These differences
continue to increase through Thursday...with GFS having weak troughing over
the region while European model (ecmwf) forecasts broad ridging across the north-central
Continental U.S. On Thursday. Models in better agreement Friday with broad ridging
across the region...but get there by different routes. Forecast
confidence average to good Tuesday into Wednesday then below average for Wednesday
night through Friday.
Consistent signal for rising heights and cooler/drier can high pressure
to build across the region for Tuesday into Wednesday. Faster European model (ecmwf) troughing
by later Wednesday looks to struggle to bring deeper moisture back into
the area...with the Canadian high holding out through Wednesday. Models out
of phase /at least at 500mb/ for Wednesday night/Thu...however at the
lower levels some similarity for southwest flow and moisture
increase into the region. Models more similar at 500mb Thursday
night/Fri...with some form of southwest flow/moisture feed into the
upper Midwest. 20-30 percent consensus rain showers/thunderstorms and rain chances for much of
the area Wednesday night through Friday okay for now until the models converge
on a common solution and details in these later periods sort
themselves out. Even though Wednesday night through Friday precipitation chances may be
questionable...reasonably consistent signal for temperature to remain near
to a bit below normal next week. Tuesday trends to be the cooler of the
day 4-7 days with slow warming through the week. With drier high
pressure overhead Tuesday night...consensus lows that period still look
too warm. Otherwise model/ensemble blend of highs/lows for Tuesday through
Friday appear well trended for now.
Aviation...(for the 00z tafs through 00z Saturday evening)
issued at 650 PM CDT Friday Jul 3 2015
A weak cold front over northern Wisconsin and central Minnesota
will move slowly south overnight and stall just north of
Interstate 90 before retreating back to the north. Visible and
infrared satellite is showing a scattered to broken 4 to 8k deck
of clouds along this front. Soundings suggest that these clouds
will be in the taf sites between 04.08z and 04... then
skies will become clear for the afternoon.
Unlike the past couple of nights...the light winds only extend up
to 5k feet. This some clouds and the light wind depth being less
not as concerned about fog affecting klse. However further south
in the Kickapoo and Wisconsin river valleys...the the light winds
extend up to 10k feet...thus...will likely see valley fog once
again develop in these river valleys. This will likely occur
between 04.09z and 04.14z.
Another concern is the smoke from the Alaska and Canadian prairie
wild fires. This smoke is currently producing MVFR visibilities in
eastern North Dakota. Looking ath the air quality forecast...it
looks like these reductions to visibilities will not make it south
into our area. However aloft the smoke concentration will be
increasing on July 4th.