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afdarx

Area forecast discussion
National Weather Service La Crosse WI
505 am CST Tue Feb 21 2017

Short term...(today and tonight)
issued at 308 am CST Tue Feb 21 2017

Main fcst concerns this period are the continued unseasonably warm
temps.

Data analysis at 06z had the sfc trough/front moving east across the
area. Last of the rain had all but moved out of the east end of fcst
area with westerly flow and clearing already spreading into the west
end. Cooler temps spreading in as the sky clear, but even these at
06z were still above the normal highs for this time of year. Some
fog developing across central Minnesota into north-central Iowa where skies
were clear, but SW to west winds of around 5kts most locations
keeping the fog from becoming dense.

No issues noted with 21.00z model initializations. Solutions quite
similar for rising hgts/zonal flow over the region today as the
trough continues quickly exiting. Westerly flow continues over the
region tonight, with the main shortwave track well north of the area
across ND/northern Minnesota. Short-term fcst confidence for dry/warm
weather today/tonight is good.

Shortwave ridging at the sfc and aloft slides across the area this
with a return of SW low level flow/warm advection for this afternoon
and tonight. This south of a weaker sfc low to move into/across
central/northern Minnesota in response to the shortwave energy tracking
across ND/northern Minnesota. All this points to a dry and warming airmass
(sourced from the central/western plains) spreading across the area
for today/tonight. Little in the way of low cold advection behind
trough from Mon/Mon night, with 925mb temps in the +10 to +13c range
by 00z Wed, then continuing to warm a bit tonight. Model soundings
showing mixing to just shy of 925mb this afternoon. Approaching
these mixed 925mb temps with what should be plenty of sunshine, no
snow cover and SW winds 10-20mph this afternoon, would place highs
today in the upper 50s to lower 60s most locations - near the
record highs for Feb 21. Favored warmer of guidance highs today.
Some SW gradient winds of 5 to 10 mph tonight to the bl stirred for
most locations. Blend of guidance lows for tonight, still well above
the normal highs for late Feb, looks well trended.

Long term...(wednesday through monday)
issued at 308 am CST Tue Feb 21 2017

For Wednesday thru Thursday night: main fcst concerns this period
are near record/record highs Wed, cooling Wed night thru Thu night
and increasing precip chances/types thru Thu/Thu night.

21.00z model runs in reasonable agreement for a shortwave to track
across central/northern Minnesota Wed then hgts to then rise a bit for Thu
with shortwave ridging to quickly build across the region Thu.
General agreement on stronger troughing coming thru The Rockies Thu
and into the Central Plains Thu night. Fcst confidence remains good
Wed/Wed night, then falls to average by later Thu/Thu night with
model detail differences having impacts on the sensible weather into
Thu night.

Fcst area under the warm bubble south of the sfc low track Wed, with
925mb temps progged in the +10c to +15c range Wed afternoon. Model
soundings showing a bit deeper mixing Wed afternoon. Sfc gradient
winds of 10-15mph for mixing as well. Some mid/high clouds may
impact temps highs Wed, especially across the north, but across the
south end of the fcst area, potential to hit the 70f mark for highs
in the WI River Valley given the continued dry SW low level flow.
Highs Wed could set all time record highs for Feb at some
locations. Trended toward warmer of guidance high Wed. Wed low
does drag a cold front southeast across the area Wed night, with a
stronger round of lower level cold advection spreading across the
area Wed night/Thu. Cooling temps for Wed night/Thu, but still
well above the normals. By Thu, clouds/moisture/lift ahead of the
troughing coming thru The Rockies already starts to spread into
the area, with increasing rain chances thru the afternoon. Plenty
of detail differences, but precip chances to continue increasing
Thu night. Question by Thu night and more-so toward Fri morning
become precip types as lower level colder air continues to filter
into the region. Sizable detail differences among the models by
12z Fri with the location of the sfc low: GFS - NE IA, NAM
-central IL, European model (ecmwf) - southeast IA, Gem - northwest MO. This has impacts on how
quickly the deeper cold air arrives and changes any precip to
snow. NAM would be the fastest changeover, GFS the slowest. Given
all these differences by Thu night, stayed with the model/ensemble
consensus at this time, which in itself has and continues present
a lot of spread among members on the late-week storm system.

For Fri thru Mon (days 4 to 7): main fcst concerns this period are
the impending winter storm Fri/Fri night, colder temps for Sat thru
Mon.

Medium range model runs of 21.00z in reasonable enough agreement on
a strong shortwave trough to lift thru the region Fri/Fri night, but
plenty of detail differences to impact the sensible weather outcome
these periods. Decent agreement for colder northwest flow aloft over the
upper Midwest for Sat/sun, with what detail differences there are
having less impact on the sensible weather thru the weekend.
Surprisingly good agreement for broad mid level ridging to build
across the north-central conus Mon, ahead of stronger troughing
moving into the western conus. Fcst confidence is average for Fri
thru Mon.

Still a signal for a strong winter storm system to move across the
the region Fri/Fri night, with potential to drop significant
snowfall somewhere across the area. However this portion of the fcst
looked keyed on 3 stronger pieces of shortwave energy currently
rotating thru the troughing off the pac coast and how they interact
with each other as they cross The Rockies Thu and eventually reach
the upper Midwest Fri. Quite a bit of spread remains among the
various ensembles/members, and among the deterministic models. The
sfc low/lower level thermal profile differences coming out of Thu
night continue Fri, with eventual stronger cooling Fri night as the
sfc-mid level trough axis pass. European model (ecmwf) has been one of the more
consistent solutions for the Fri/Fri night timeframe, and remains
closer to the middle of the pack/consensus of ensembles/other
deterministic runs. A warm advection/f-gen band of precip to move
across the area Fri, with the wrap-around deformation band of precip
to swing across the area late Fri/Fri night behind the passing mid
level low. Consensus has snow falling across roughly the northwest 1/3 of
the fcst thru Fri, with a change from rain to snow moving across the
central part thru the day, then across the southeast end of the fcst area
Fri evening. Again, plenty of timing differences among the models on
this. The earlier the changeover to snow, the greater the potential
accumulation, as models continue to show potential of 1 to perhaps
as high as 2 inches of quantitative precipitation forecast from Thu night thru Fri night. Strong
signal for a band of heavy snow, just where and how much is not
nailed down yet given all the between model and ensemble
variability. Will hold off on any winter storm watches at this point
but stay tuned. Tight pressure gradient over the area Fri/Fri night
as well, for plenty of wind to go along with the snow. Drier/colder
can high pressure builds in for the weekend, with temps trending
near to even below normal. Low level warm advection returns for Mon
under the rising hgts/ridging aloft. Consensus highs Mon mostly in
the 30s look good at this time.

&&

Aviation...(for the 12z tafs through 12z Wednesday morning)
issued at 505 am CST Tue Feb 21 2017

Cigs: low clouds right on the edge at klse, and should clear by 12z
or shortly after. Fog was developing Post this low cloud shield
though, and some low cigs associated with low vsbys are likely
before mixing out by mid/late morning. Moreso for krst. Rest of the
day and tonight then look VFR with a few high clouds.

Visibility/wx: light winds, saturated sfcs resulting in areas of fog that
will impact the taf sites through mid morning. Expect quick
improvement with sun/mixing. Klse likely to hold more MVFR while
krst will have to deal with LIFR/IFR for a few hours.

Winds: look for a shift to the south this morning as gradient
tightens a bit and speeds pick up. Southerly flow will hold through
tonight.

&&

Hydrology...
issued at 308 am CST Tue Feb 21 2017

Widespread rainfall amounts of 0.5 to 1.5 inch Mon/Mon evening have
produced rises on area streams and rivers. The most significant
rises are expected along the black, yellow, and Wisconsin rivers,
whose headwaters are co-located with any remaining snow pack. Minor
to possibly moderate river flooding will be possible at some
locations along these rivers. Minor flooding may also occur along
portions of the Trempealeau and Kickapoo rivers. Any ongoing river
issues may be exacerbated by the potential winter storm late this
week. See the latest hydrologic products for more details on
specific rivers and forecast stages.

&&

Climate...
issued at 308 am CST Tue Feb 21 2017

After a briefly "cooler" day today, high temperatures will
return to near record territory Tuesday and Wednesday at La
Crosse and Rochester. Lows will also be near record high minimums
at times through mid-week. The warmest day in this stretch is
looking to be Wednesday. Good warming should occur ahead of the
surface low that will be moving across the area with southwest
winds and the potential to mix up to about 900 mb. Some locations
could top the 70 degree mark that day.

Below are the records for La Crosse and Rochester through
Wednesday February 22nd:

Record highs
------------

La Crosse Rochester
----------- -----------

Tue (21st) 64/1930 Tue (21st) 60/1930
Wed (22nd) 62/1984 Wed (22nd) 59/1930

Record high lows
----------------

Mon (20th) 41/1930 Mon (20th) 34/1954
Tue (21st) 48/1930 Tue (21st) 39/1930
Wed (22nd) 39/1930 Wed (22nd) 42/1930

&&

Arx watches/warnings/advisories...
WI...none.
Minnesota...none.
Iowa...none.
&&

$$

Short term...rrs

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