Area Forecast Discussion
Issued by NWS La Crosse, WI
FXUS63 KARX 112351
Area Forecast Discussion
National Weather Service La Crosse WI
551 PM CST Wed Jan 11 2017
Issued at 551 PM CST Wed Jan 11 2017
Per recent trends/reports, have made a couple of small tweaks to
the forecast the next hour or two, featuring a farther north
expansion of precip into southern Juneau/Adams Counties. Modest
axis of FGEN forcing is the culprit but should exit the next hour
or two at the latest, with dual pol radar data supporting RAP/HRRR
trends of a sleet/snow mix on the northern fringe and mainly just
rain/freezing rain over Grant County with a distinct lack of cloud
ice. Farther to the north, seeing some lingering flurries and even
just plain ice crystals here at the office (notice the neat drop
in correlation coefficient on radar products - kinda cool!). Will
likely be able to let the winter weather advisory go early, with
reports of only minor accumulation down across Grant County, but
will of course watch trends for a little while.
We will be closely watching the next fast-moving low amplitude
shortwave working through western Nebraska currently. That feature
will rapidly zip east toward our area later tonight with another
tightening FGEN response aloft. The question is the degree/
placment of that forcing as well as the degree of residual dry air
below 10kft. 18Z runs of the NAM/GFS/RGEM and 12Z GGEM strongly
hint at a narrow band of accumulating snowfall bisecting the CWA
after midnight, mainly along or south of the I-90 corridor.
Starting to see a nice flare-up of reflectivity across western
Nebraska the past hour or so, suggestive that these model runs
have some validity, with even the HRRR starting to fall in line
with that idea. Want to get another hour or to of upstream obs
under our belt before jumping in full throttle, but there is a
risk we will see a narrow band of perhaps 1-2" of snow
accumulation later tonight.
.SHORT TERM...(Tonight thru Thursday night)
Issued at 248 PM CST Wed Jan 11 2017
Main fcst concerns this period are -SN chance overnight tonight,
colder/below normal temperatures thru the period.
Data analysis at 18z had high pressure centered over the Dakotas
with trough/front from Lk MI across SE WI into southern IA.
Shortwave slipping by to the north produced a band of snow across
parts of central MN/northern WI this morning, with this band quickly
exiting into NE WI as of 18z. Another shortwave moving across IA
was producing another round of mid/upper level lift across
north/east IA into southern WI. Radar reflectivity increasing
across NE IA into southern WI, but sfc obs and model soundings
indicating little if any precip reaching the ground those areas
yet as 900-700mb dry layer proving difficult to overcome.
Elsewhere, some flurries across much of SE MN into central WI
between the 2 waves/rounds of lift.
No issues noted with 11.12z model initializations. Solutions quite
similar as the IA shortwave exits quickly this evening then another
bundle of energy out of the western troughing approaches/quickly
passes later tonight. Good agreement on the next shortwave (northern
stream this time) to drop toward Lk Superior Thu, opening the door
for an arctic high to settle into the Upper Midwest for later Thu
into Fri. Short-term fcst confidence is good this cycle.
For the short-term: winter weather advisory for the SE end of the
fcst area extended to 03z earlier today, to better match up with
neighboring headlines. Confidence for the advisory does remain on
the lower side for an advisory, however lower level thermal profiles
continue to indicate precip in/near Grant Co. WI late this afternoon
into very early this evening would be -FZRA/sleet. Road surfaces are
cold enough for glazing to occur rather quickly, thus will continue
with the advisory as issued. If precip ends up more spotty than
expected, may be able to cancel it early.
Next wave into the area later this evening/overnight quickly spreads
another round of upper level PV advection/divergence and some mid
level thermo-dynamic lift across the area, with upward motion
maximized across the -15C level. Trended toward a blend of the
higher guidance -SN chances overnight. Blends well with neighboring
grids but given model signal may end up on the low side for a low
QPF event and a tenth or 2 of snowfall over mainly the central/south
parts of the fcst area. Forcing/lift any -SN quickly exits by
sunrise Thu with attention turning NW as arctic high pressure builds
SE across the northern plains Thu then into the Upper Midwest Thu
night. Even with some sunshine Thu, consensus highs mostly in the
10F to 20F range looking good. The high ends up centered over MN by
12z Fri, with ridging SE toward lower MI. Resurgence of 925-850mb
cold advection spreads across the area tonight with an even stronger
shot later Thu/Thu night. 925mb temps in the -12c to -18c range at
12z Fri. This with what should be clear/mostly clear skies at least
Thu evening, over the fresh snow cover (especially across the NE 2/3
of the fcst area) and light winds near/under the high/ridge axis.
Favored colder of guidance lows Thu night, especially across the NE
1/3 of the fcst area. Would not be surprised to see some Fri morning
lows around -20F in the colder parts of the counties along/northeast
of I-94 .
.LONG TERM...(Friday through Wednesday)
Issued at 248 PM CST Wed Jan 11 2017
For Friday thru Saturday night: main fcst concerns this period are a
small -SN chance Fri afternoon/night and temperatures.
Model runs of 11.12z remain in good agreement for hgts to rise Fri
then another piece of energy out of the western Conus trough to
approach/move across the region later Fri/Fri night. Reasonable
agreement for broad W-NW flow over the north-central Conus for
Sat/Sat night. Fri thru Sat night fcst confidence is good.
Rising hgts and retreating arctic high pressure allows low level
warm advection to return across the area by Fri afternoon. An
increase of clouds later Fri morning/afternoon ahead of the
approaching shortwave will temper Fri highs, but most highs in the
5F to 15F range are looking to be a 20F to 25F diurnal swing off the
morning lows. Forcing/lift with this next wave on the weaker side
and mainly mid/upper level. Airmass will be quite cold, with
dendritic growth zone on the relatively low side (700-600mb). Will
not take much to squeeze out some flurries or a little -SN. Small -
SN chances across mainly the south half of the fcst area Fri
afternoon/evening look good for now. Clouds and the warmer 9325-
850mb temps to keep lows closer to normal Fri night. Warmer start,
some continued low level warm advection and sunshine should allow
Sat high to approach normal for mid Jan. Another high settles over
the area for Sat night, with light winds and what should be
clear/mostly clear skies, favorable for strong radiational cooling
over the deeper/fresher snow cover. Consensus lows around zero to
10F above reasonable, even with 850mb temps approaching 0C by 12z
For Sunday thru Wednesday (days 4 to 7): main fcst concerns this
period include precip chances/types from Sun night into Tue,
temperatures thru the period.
Medium range model runs of 11.00z/11.12z in decent agreement Sun as
a mid level low would move across the southwest Conus and hgts
gradually rise over the Upper Midwest. Differences then begin to
increase for Mon/Tue on the northern-southern stream interaction and
eventual evolution of the southwest Conus low as it would lift
toward/into the mid/upper MS valley. Plenty of between model and run-
to-run phasing or not differences among the models. Loose consensus
for some form of mid level troughing to linger over mid/upper MS
valley/great lakes Wed. Fcst confidence average Sun, then below
average for Mon-Wed.
Slowly rising hgts aloft on Sun and retreating high pressure brings
at least weak low level warm advection to the area, with Sunday`s
highs returning to near normal. Low level warm advection would
continue/increase Sun night into Tue, ahead of the SW Conus low/
troughing as it would migrate into the plains then toward/into the
region. Moisture increases as well thru the period, along with
precip chances. Lower level thermal profiles would indicate precip
as snow/wintry mix Sun night, transitioning to mainly rain by Mon
night/Tue, then back to mainly light snow Tue night/Wed. Again,
plenty of warming/timing/strength detail differences in the day 5-7
period, contingent on when and how much northern-southern stream
phasing does or does not occur. Will continue with the model/
ensemble consensus for early next week until a tighter model
consensus develops and confidence increases. For now, consensus
highs 5F to 10F above normal for Mon-Wed look rather well trended.
However, consensus lows Sun thru Tue night lows looking as much as
5F to 10F too cool with what would be thickening lower clouds,
persistent low level warm advection and the potential of precip.
.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 551 PM CST Wed Jan 11 2017
Conditions at LSE/RST have improved to mainly VFR the past few
hours and should hold there, with some lingering flurries possible
at times the next couple of hours (actually getting just plain ice
crystals falling here at the office currently). Thereafter, we
will have to closely watch a fast-moving system approaching after
midnight. The latest trends suggest a window of snow is possible
with this feature, with some accumulation possible, though
confidence even at this closer range is still lower than average.
For now have included an MVFR mention at both sites, but it is
possible that a period of snow accumulation occurs, especially
between 07Z and 12Z. Worst case scenario would be about an inch at
LSE. Clouds should thin quickly through late morning/early
afternoon Thursday, with widespread VFR conditions as high
pressure noses into the area.
WI...Winter Weather Advisory until 9 PM CST this evening for WIZ061.