Scientific Forecaster Discussion
fxus63 karx 140510
Area forecast discussion
National Weather Service La Crosse WI
1111 PM CST Wed Dec 13 2017
Short term...(tonight through Thursday night)
issued at 226 PM CST Wed Dec 13 2017
Main short-term fcst concerns are cloud/temperature trends thru the
period and some small -sn/flurry chances Thu night.
18z data analysis had a 992mb low centered south of kmsn, moving
rather quickly E-se. Bulk of the precip/snow with this low had
exited east of the area with the trailing edge now NE of I-94 and
moving off to the east. Greater snow amounts this morning were
mainly confined to Taylor co. Tight pressure gradient around the
low encompassed much of WI/MN/IA. Northwest winds 15-30 mph gusting 30-45
mph were common across much of Iowa/southern Minnesota in stronger low
level cold advection and pressure rise behind the low. These
stronger winds spreading east into WI as the low moves toward
Southern Lake Michigan.
No issues noted with 13.12z model initializations. Solutions quite
similar moving the northern stream shortwave quickly southeast of the
area this evening then weak shortwave ridging aloft tonight. Next
stronger shortwave to drop south thru the northern into Central
Plains Thu, with another northern stream wave to drop south into
Minnesota/WI Thu night. Given rather good model similarity on the
shortwave details, short-term fcst confidence is generally good
For the short-term, with some -sn still falling across the north end
of the fcst area as of 20z, will continue with the Winter Weather
Advisory for Clark/Taylor ctys thru 00z. The brisk/gusty northwest winds
to diminish rather quickly this evening as the strong low exits
into the eastern Ohio Valley by midnight. Otherwise, tonight thru
Thu night shaping up to be a quieter, drier and colder period
under the more northwest to north flow aloft. Weak sfc-500mb anti-cyclonic
flow and low level cold advection should keep tonight dry. Even
with this flow, models hang onto considerable 925-700mb moisture
over the region tonight, for generally mostly cloudy skies. Cloud
cover to temper lows tonight, but consensus lows mostly 5 above to
mid teens appear quite reasonable. A weak shortwave to drop into
WI Thu does deepen the moisture over the area thru the day. Given
the cold column, depth of moisture, weak low level warm advection
and lift with the wave, will add a sct flurry mention to much of
the area, especially by the afternoon into the evening. Some small
-sn/flurry chances thru Thu night reasonable the weak trough
axis/deeper cyclonic flow and sfc-700mb cooling work to squeeze
some moisture out of the column. Any snow accumulations Thu night
looking to be quite light and mainly NE of I-94, in the colder air
and more on the east side of the mid level jet axis. Blend of
guidance highs/lows for Thu/Thu night look good.
Long term...(friday through wednesday)
issued at 226 PM CST Wed Dec 13 2017
For Friday thru Saturday night: main fcst concerns this period are
-sn chances Fri afternoon into Sat, temperatures.
Model runs of 13.12z in good agreement for the strongest northwest flow
aloft and yet another northwest flow shortwave to move east of the area Fri
as troughing digs into the northern rockies and shoves ridging aloft
east across the northern plains. Reasonable agreement for hgts over
the region to continue rising Fri night/Sat, with the ridge axis
passing and SW flow aloft ahead of plains troughing by Sat night.
Fcst confidence this period is average to good.
Next stronger shortwave to drop across the central Great Lakes
Fri as forcing over western noam progresses the pattern. Area
under the left entrance region of a 500-300mb jet Max with this
wave later Fri/Fri night, as a round of stronger 925-700mb warm
advection ahead of a sfc-850mb trough/warm front spreads in. Precipitable water
values progged to increase into the 0.5 inch range for Fri
evening. Models developing a consensus for a band of -sn (column
cold enough with cloud tops around -15c) to develop and lift NE
across at least the NE 1/2 of the fcst area later Fri/Fri night
into Sat morning. The 20-30% consensus -sn chances Fri afternoon
thru Sat morning good for now but if present trends continue these
chances will need to go up, especially Fri night into Sat
morning. Will have to watch precip type Sat, as potential to lose
ice in the clouds before any precip would end. Warm front pushes
in Fri night then progged to stall over the area Sat. Area looking
to be stuck under plenty of clouds Sat, but highs Sat should
still be a category or two warmer than those of Fri. Deeper SW
flow spreads over the area Sat night. This may not be much help
for the lower level moisture/cloud cover looking to be stuck
across much of the region under a Stout inversion just below
900mb. Blend of Sat night guidance lows some 5-10f above normal
looking good at this point.
For Sunday thru wednesday: main fcst concerns this period are
warming temperatures and any small precip chances.
Medium range model runs of 13.00z/13.12z in reasonable agreement for
the mid level flow to have more of a westerly component sun-Mon,
then transition back to more northwesterly by later Tue and Wed.
Plenty of between model and run-to-run differences on the
timing/strength details of shortwave thru the faster, more zonal
flow. This variability not unexpected in the day 4-7 timeframe.
Overall fcst in the day 4-7 period is average this cycle.
Some consistency for a stronger trough in the plains to move into/
across the upper Midwest sun/Sun night, but trend is for energy
in this trough to split with the northern portion outracing the
southern portion across the Continental U.S.. trend is also for this system
to have limited moisture as it crosses the region as a positive
tilt trough and 850-700mb flow is sourced from the central/
northern plains and eastern rockies. Some small -ra/-sn chances
sun into Mon OK for now. The main impact is looking to be the S
to west lower level flow bringing some warmer air and above normal
temps to the area for sun/ Mon. The passing trough Mon swings the
flow back to northwest for Tue/Wed, allowing another can high and colder
airmass to slip south into the region. Again, shortwave detail
differences and may yet need some small -sn chances somewhere in
the Tue/Wed period but given the low confidence on this at day
6/7, generally dry blend of models into the middle of next week,
along with temps back to near normal by Wed is reasonable.
Aviation...(for the 06z tafs through 06z Thursday night)
issued at 1111 PM CST Wed Dec 13 2017
Cigs: Big Holes have opened up in the cloud cover, in an area of
subsidence Post a departing upper level trough. Satellite trends
suggest the skc/sct conditions will hold for several hours into the
overnight, with low clouds returning between 09-11z. Might be VFR at
first, but models still suggestive that MVFR will move back in by
12z-ish or so, and then hold across the taf sites into Friday.
Not completely sold that VFR conditions won't hold longer into the
morning, but will follow trends for the moment.
Weather/vsby: a few ripples in the upper level flow and/or low level
warming could result in scattered flurries, perhaps a few snow
showers, over the next couple days. Nothing that looks impactful,
nor "cut and dry" enough to add to the forecast. Friday night
showing more Promise for an elongated light snow band that could
bring minor accumulations and some vsby restrictions, but models
favor keeping north of the taf sites for the moment.
Winds: sfc pressure gradient has slackened this evening as low
pressure exited east, and winds will continue to drop off. Mostly
north/NW, then variable toward 12z. Winds should increase a bit from the
west by 18z Thu, mostly holding 10kts or less.