Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000
FXUS65 KCYS 221210
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
510 AM MST Wed Feb 22 2017

.SHORT TERM...(Today - Thursday)
Issued at 421 AM MST Wed Feb 22 2017

...MAJOR WINTER STORM TO IMPACT THE REGION TONIGHT THROUGH FRIDAY
MORNING...

Our confidence continues to grow with respect to a widespread, and
significant winter weather episode across all of southeast Wyoming
and the western Nebraska Panhandle. The latest WV satellite images
and objective upper air analysis data indicate a deep upper trough
making landfall on the west coast early this morning. A cold front
moved across the CWA overnight and is now pushing into northern CO
and central NE.

The aforementioned trough will deepen and broaden over the next 24
to 36 hours as it tracks across the 4 Corners. The GFS and ECM are
still in excellent agreement w/ the synoptic pattern characterized
by a 120-130 knot H25 jet streaking across AZ/NM on Thursday and a
significant mid-level vort lobe extending from NV into KS. Surface
cyclogenesis will occur over southeastern CO tonight w/ impressive
difluence aloft. On Thursday, a 988-992 mb surface low will likely
track very slowly eastward across KS before lifting northeast some
time on Thursday night. A secondary surface low should evolve over
Carbon and Sweetwater counties on Thursday, beneath the closed low
aloft, enhancing low-level convergence and also increasing surface
pressure gradients across east central WY. MSLP progs from the GFS
ensembles continue to show high standard deviations in excess of 5
to 6 mbs to the west of the mean low, suggesting a slower/possibly
stronger storm and higher snow accumulations in our CWA. Expect QG
forcing to ramp up along/W of the Laramie Range this afternoon, w/
snow increasing in intensity through the evening. Snow is expected
to begin in earnest along/E of the Laramie Range after 06z as east
to northeast low-level upslope flow takes shape. The heaviest snow
will occur during the day on Thursday with the best combination of
orographics and upper-level divergence. Winds are also trending to
the up side with stronger low-level gradients, so we could see the
potential for blowing snow as well.

A few caveats to discuss. The GFS remains extremely modest, to say
the least with its QPF output despite an identical synoptic set up
compared to the ECMWF. This appears to be related to a mid-lvl dry
slot that does not belong according to other models. The NAM still
focuses precipitation north of I80 w/ an un-realistic wind pattern
at the surface (westerly downsloping winds). The ECMWF remains the
model of choice with consistent E-NE low-level flow along and east
of the Laramie Range with widespread moderate/heavy QPF farther to
the south. This still seems more realistic, and WPC seems to agree
with new guidance trending south. SREF plumes for CYS show amounts
anywhere from 0 to 14 inches with a mean around 6, but the bulk of
members are at the higher end of that spectrum. All guidance shows
a swath of 1+ inches of QPF over a large part of southeast Wyoming
and the western Nebraska Panhandle, so confidence in 12 to 18 inch
amounts is quite high north of I80 and over the summit. Changes to
the forecast were to increase snow amounts for most areas with the
slower system. We also lowered amounts at Laramie as a significant
easterly component to surface winds should result in a substantial
snow shadow. 4-6 inches looks good here with a period of northwest
winds creating at least some upslope.

Despite increasing confidence, no changes to headlines this AM per
coordination with neighboring WFOs. There are still some questions
with snow amounts, mainly along the I-80 corridor from Cheyenne to
Sidney. As stated earlier, the GFS and ECMWF have shown two rather
different solutions but have remained consistently inconsistent w/
one another. That said, it will not hurt to let the day shift look
at another model cycle before deciding to upgrade the Winter Storm
Watches to Warnings. Do not see why the majority of zones will not
necessitate upgrades, except Laramie/Saratoga.

.LONG TERM...(Thursday night through next Tuesday)
Issued at 500 AM MST Wed Feb 22 2017

***WILL MAINLY CONCENTRATE ON THURSDAY NIGHT AND FRIDAY DUE TO
WORK LOAD WITH THIS WINTER STORM***

Main forecast concern Thursday night and Friday will be when this
storm system moves east of the area and when snowfall will taper
off. Model ensembles continue to show a high standard deviation to
the NW of the surface/llvl low pressure system as it develops
across eastern Colorado and Kansas. These systems tend to slow
down with rapid surface cyclogenesis as they push into the plains.
Ensembles are hinting towards this so can not remove the potential
for snowfall until Friday or Friday night. Additional snow
accumulations Thursday night and Friday should not be too
high...but another 2 to 5 inches is not out of the
question...especially across the mountains and High Plains. If
this system lingers across central Nebraska as some ensemble
show...be prepared for heavier snow accumulations into early
Friday than what is forecast.

All models indicate that this system will push well east of the
area by early Saturday...with cold temperatures for the weekend.
The surface snow pack will influence temperatures as
well...especially at night. Highs will struggle to reach the upper
20`s to near 30 on Sunday after a cold Saturday. Overnight lows
should be in the single digits each morning this weekend with the
more sheltered valleys potentially below zero each morning due to
the snow pack.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 421 AM MST Wed Feb 22 2017

Conditions will gradually deteriorate from west to east today and
tonight. Periods of IFR visibilities in light snow showers are
possible through most of today at KRWL and KLAR. Do not expect
these snow showers to have too much of an impact further east
where it will be warmer. Widespread IFR or LIFR conditions are
expected this afternoon/this evening at KRWL. KLAR will be tricky
due to localized downslope winds...but MVFR to near IFR VIS and
CIGS are possible in the evening.

Further east...VFR today with maybe some light rain showers this
afternoon. Conditions will quickly deteriorate tonight...around
06Z at KCYS and closer to 09z to 12z further east across western
Nebraska.

&&

.FIRE WEATHER...
Issued at 421 AM MST Wed Feb 22 2017

No fire weather concerns with widespread significant snowfall.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Storm Watch from this evening through Friday morning for
     WYZ101-102-107-108-118-119.

     Winter Storm Watch from this afternoon through Friday morning
     for WYZ103>106-109>111-113-115>117.

     Winter Storm Warning until 11 AM MST Friday for WYZ112-114.

NE...Winter Storm Watch from this evening through Friday morning for
     NEZ002-003-019-020-095-096.

     Winter Storm Watch from late tonight through Friday morning for
     NEZ021-054-055.

&&

$$

SHORT TERM...CLH
LONG TERM...TJT
AVIATION...TJT
FIRE WEATHER...CLH



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