Scientific Forecaster Discussion
fxus63 karx 171940
Area forecast discussion
National Weather Service La Crosse WI
240 PM CDT Tue Oct 17 2017
Short term...(tonight through Wednesday night)
issued at 240 PM CDT Tue Oct 17 2017
Main short-term fcst concerns this period are temperatures.
Data analysis at 18z had broad high pressure over the southeast Continental U.S. And
low pressure in the Lee of The Rockies from alb to Colorado. Upper Midwest
was in the broad, dry gradient SW lower level flow between the two.
Another seasonably mild and mostly sunny early afternoon across the
region. Early afternoon temps across Minnesota/IA/WI generally in the 60s,
with some lower 70s across southern Iowa/SW Minnesota. These readings already
3f to 10f above the normal Oct 17th highs.
Model runs of 17.12z initialized well. Solutions similar with quasi-
zonal flow across the northern Continental U.S./Southern can and main storm
track shifted north of the border for the time being. Fcst
confidence for a dry/seasonably mild short-term period is good this
In the short term, 850mb dew point analysis this morning shows the
drier air has been pushed as far south as the northern Gulf of mex,
thus no deeper moisture available to be quickly pulled back north
into the region. A stronger shortwave to track across southern can
tonight/Wed, dragging a weakening cold front across Minnesota/WI/IA later
Wed/Wed night as the trough/wave heads for hudson's Bay. With the
deep, dry airmass over the central conus, little more than a wind
shift from SW to west/northwest and a period of MDT low level cold advection
expected Wed night. On Wed ahead of the front mixed 925mb temps
supporting highs in the upper 60s to mid 70s, warmest in the MS/WI
river valleys and across NE Iowa. Weak sfc ridge axis builds over
the area later Wed night, for some lighter winds with the cooler
Post-frontal airmass. Even so, the Post-frontal airmass is not
that much cooler, with the blend of guidance lows in the upper
30s to mid 40s looking good. If winds decouple, some lows in the
normally colder low laying areas along/NE of I-94 may dip into the
Long term...(thursday through tuesday)
issued at 240 PM CDT Tue Oct 17 2017
For Thursday thru Friday night: main fcst concerns this period are
the well above normal highs Fri then a returning rain showers chances later
17.12z model runs in good agreement for the quasi-zonal flow to
buckle this period with troughing over the western Continental U.S. And ridging
east by Fri. Some timing differences but the model consensus keeps
the pattern progressive with the western Continental U.S. Troughing ejecting
into the plains by 12z Sat. Fcst confidence in the Thu thru Fri
night period is generally good this cycle.
A bit of a cool-down expected Thu with 925mb temps 1c to 2c cooler
than those of Wed, mainly over the North/East parts of the fcst area. Even
so, the mixed 925mb temps for Thu still supporting highs around 10f
above the normal. As the mid level flow buckles, ridging aloft
builds over the mid/upper MS valley for Fri. Under this is a tighter
sfc-700mb pressure gradient and stronger S/SW low level winds for
warm advection and mixing. Friday looking to feel more like late
Summer/early fall than mid Oct with some brisk/gusty S/SW winds and
mixed 925mb temps allowing for highs of around 70 to the mid 70s.
Raised Fri highs a bit above the consensus in the valleys due to the
winds/mixing and drier airmass. By Fri night, increase of moisture
ahead of the lower level trough/front coming east across the plains
is progged to start spreading into the area. This with some stronger
850-700mb moisture transport and Theta-E convergence on the
leading edge of the moisture. Some 305-315k isentropic lift and
potential for some weak cape when lifting saturated parcels around
800mb exists as well. The small consensus rain showers chances across the
west and northwest ends of the fcst area OK for now. However, models
often struggle with forcing/originating precip above 800mb and
depending on timing trends, the later Fri night rain showers chances may
yet need to be expanded across the northwest 1/2 to 2/3 of the fcst area.
For Saturday thru Tuesday (days 4 to 7): main fcst concerns this
period are rain showers/thunderstorms and rain chances with a passing front Sat/Sat night,
more rain showers chances centered on Mon, cooler temps by Tue.
Medium range models runs of 17.00z/17.12z in reasonable agreement on
the passage of a stronger cold front late Sat/Sat night, then for a
stronger northwest flow shortwave to drop into/across the region later Sun
night/Mon. Fcst confidence in the day 4-7 period is average to good
Tightening model consensus for a stronger cold front, with an
increase of moisture and MUCAPE ahead of it to push into/across the
region Sat/Sat night. This with divergence/lift aloft ahead of the
approaching mid level trough/shortwave. Given the strength of the
forcing/lift signal and some building model consistency, along
with neighbors raised rain chances into the 60-70% range for the
west 2/3 of the fcst area Sat afternoon and east 2/3 Sat evening.
Fcst grids already include thunderstorms and rain chances Sat/Sat evening and this
also looks well trended given the cape and deeper forcing/lift.
After another day with highs around 10f above normal Sat, some
cooling arrives behind the front for sun. Timing differences as
expected by Mon, but rather consistent signal for a vigorous northwest
flow shortwave to drop into/ across the region centered on Mon.
This with some modest moisture but more importantly, much colder
air aloft for steeper sfc-mid level lapse rates, especially by Mon
afternoon. With the lift of the shortwave and deep cyclonic flow,
the consensus 20-30% rain showers chances Mon/Mon night are reasonable.
If the timing holds, consensus highs around 10f below normal for
Tue are looking well trended.
Aviation...(for the 18z tafs through 18z Wednesday afternoon)
issued at 1222 PM CDT Tue Oct 17 2017
Expect a tight surface pressure gradient across the region through
the period with broad high pressure across the southeast Continental U.S. And
an approaching trough moving across the northern plains. This will
result in elevated south-southwest flow, especially for the layer
immediately above the surface. As a result, added low-level wind
shear tonight at klse with 35 kt winds around 1000 ft agl. For now,
a stronger surface wind and slightly weaker winds aloft precludes
low level wind shear mention at krst. Skies will remain mostly
clear/sunny through the period, with perhaps a few wisps of cirrus.