Scientific Forecaster Discussion
fxus63 karx 181946
Area forecast discussion
National Weather Service La Crosse WI
246 PM CDT Wed Oct 18 2017
.Short term...(tonight thru Thursday night)
issued at 246 PM CDT Wed Oct 18 2017
The main short-term fcst concerns this period are temperatures.
18z data analysis had the cold front near a kdlh-kmsp-ksux line,
making steady progress E/se. MDT/strong pressure fall/rise couplet
ahead of/behind it to keep the front moving today. Per sfc obs and
satellite imagery, little more than sct-bkn cirrus with the front
this afternoon. Main thing with the front is the wind shift from S-
SW to W-NW. Noticeably cooler air behind the front lags by some 200
to 300 miles. Ahead of the front low-mid 70s, mid-upper 60s behind
it with 50s across ND and far northwest Minnesota.
No issues noted with 18.12z model initializations. Solutions
similar as man/saskat shortwave trough tracks toward hudson's Bay
tonight. Solutions then remain quite similar as the quasi-zonal
flow across noam buckles, troughing digs into the western Continental U.S.
And downstream ridging builds over the mid/upper MS valley Thu/Thu
night. Short- term fcst confidence remains good for tonight-Thu
night to remain a dry, quiet and seasonably warm period.
For the short term, a weakening cold front will be dragged into/
across the area late this afternoon/this evening. Given the
initially deep, dry airmass over the region, little more than a wind
shift and a period of mainly high clouds expected with this frontal
passage. Area to get a glancing blow of weak to MDT low level cold
advection behind the front, with 925mb temps only cooling 1c to 4c
from 00z to 12z Thu. More importantly, pressure gradient with the
front relaxes rather quickly thru the evening as a weak ridge of
high pressure builds in behind the front, with the ridge axis over
the fcst area by 12z Thu. Light winds and generally clear skies
after midnight, lows should be cooler tonight, with a blend of the
guidances low reasonable. Did trend lows downward in the normally
colder low-laying areas along/NE of I-94 where winds most likely to
decouple by/after midnight. Even so, most lows tonight still looking
to remain above the Oct 19 normals.
Low level warm advection quickly returns on Thu as the mid level
ridge building commences. Mixed 925mb temps Thu afternoon indicating
highs in the upper 60s-lower 70s. A mild night Thu night as gradient
sfc winds expected to remain in the 5-15 mph range to keep the bl
stirred. Blend of guidance lows in the 40s to near 50 for Thu
night looks good.
Long term...(friday thru wednesday)
issued at 246 PM CDT Wed Oct 18 2017
For Friday thru Saturday night: main fcst concerns this period are
the unseasonably warm temps on Fri and rain showers/thunderstorms and rain chances Sat/Sat
Model runs of 18.12z in good agreement for the pattern to be
progressive after the flow buckles, with the western trough axis
over eastern Montana to nm by 12z Sat then to Minnesota-western Iowa/OK by 12z
sun. Overall trend favors a compromise of the earlier runs on the
timing/strength of the trough as it crosses the plains toward the
region Sat/Sat night. Fcst confidence for Fri-Sat night remains
average to good this cycle.
An even warmer bubble of air is lifted northward across the region
in increasing lower level S/SW flow Fri. 925mb temps over the area
by Fri afternoon in the 16.5c to 18c range. Mixing these to the sfc
with the expected brisk/gusty S/SW winds produces highs in the
lower/mid 70s, perhaps a few upper 70s in the WI/MS river valleys.
Speaking of winds, 925mb winds progged in the 30kt range at 18z Fri
and around 40kts at 00z Sat. Increased winds/gusts in the fcst grids
above the model consensus centered on Fri afternoon and hope have
raised them enough.
Leading edge of the 850-700mb moisture return/moisture transport
spreads into/NE across the fcst area Fri night, with some 850-700mb
warm advection and 305-315k isentropic lift. Precipitable water values in the 1 to
1.5 inch range progged to spread in near/after midnight, along with
some weak MUCAPE. The consensus 20-40% rain showers chances late Fri night
quite reasonable, with the main question how far east to spread them
by 12z Sat. For now will stick with the consensus of late Fri night
chances over the northwest 2/3 of the fcst area.
Tightening consensus (except for 18.12z can gem) for the sfc-700mb
trough/front to push into/across the area later Sat/Sat evening.
This with increasing lower level Thermo-dynamic forcing/lift and
divergence/falling hgts aloft ahead of the approaching mid level
trough. This with the precipitable water values in the 1 to 1.5 inch range and
MUCAPE values of as much as 250-500 j/kg ahead of the front Sat into
Sat evening. Consensus rain showers/thunderstorms and rain chances increasing Sat morning,
mainly west of the MS river, with 60-80% rain chances and scattered
thunderstorms and rain spreading east across the fcst area ahead of/with the front Sat
afternoon/evening, then diminishing from west to east behind
the front later Sat night look good.
For Sunday thru Wednesday, (days 4 to 7): main fcst concerns
Timing differences exist with the exit of rain along the cold front
on Sunday, with the 18.12z GFS moving the rain out of the area by
Sunday morning, while the 18.12z European model (ecmwf) keeps it in the eastern parts
of the forecast area through mid-day. The 18.00z European model (ecmwf) had then
become the faster model on Monday, amplifying a long wave trough and
bringing the trough axis, a cold front, and associated precipitation
through the area by Monday afternoon. However, the 18.12z European model (ecmwf) came
in more in line with the 18.12z GFS, which has this same feature
moving through the area Monday night/Tuesday morning. The timing
differences also lead to uncertainty on the high temperatures for
Monday, but only 5f or so (upper 50s/lower 60s vs. Mid 60s). The
trough will continue to deepen on Tuesday, with guidance even
hinting at the possibility of a cutoff low forming near lower
Michigan. Otherwise, cold air will move in behind the front, with
850 mb temperatures in the -6 to -3 c range, along with spotty
precip chances. With lows in some of the favored cold spots on
Tuesday night in the lower to mid 30s, could see a few snowflakes,
but most of the pops for this time period look to come from the
18.12z Canadian, which is an outlier.
Aviation...(for the 18z tafs through 18z Thursday afternoon)
issued at 1243 PM CDT Wed Oct 18 2017
VFR conditions are expected through the period. A cold front will
move through this afternoon. The main concern with the frontal
passage will be winds gradually switching from south to northwest
through the afternoon. Sustained wind speeds of 15-20 kt with gusts
around 25-30 kt will gradually diminish from mid afternoon into the
evening. High pressure will build into the region tonight, giving
way to light winds and a continuation of mostly clear skies.