Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000
FXUS65 KCYS 232007
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
107 PM MST Thu Feb 23 2017

.UPDATE...
Issued at 1050 AM MST Thu Feb 23 2017

Latest water vapor loop was showing plenty of jet energy moving
through Colorado. This jet energy is continuing to provide large
scale ascent across the area. In addition, the jet energy is
causing the surface low features to maintain their strength
which is helping aid in the gusty northeast wind. The good thing
is the main surface low that is developing now is further south in
southeast Colorado which may help limit the stronger winds in
areas east of the Laramie Range. However, near RWL where the
secondary surface low is in place, the winds may still provide
quite a bit of blowing snow/blizzard conditions.

We have also raised snowfall amounts across the area especially in
areas east of the Laramie Range due to the large scale ascent
persisting this afternoon, and good banding from extending from
Douglas to Chadron.

&&

.SHORT TERM...(Today through Friday)
Issued at 415 AM MST Thu Feb 23 2017

...MAJOR WINTER STORM IN PROGRESS THIS MORNING...

A major winter storm is in progress across much of southeast WY and
the western NE Panhandle this morning. The latest WV satellite loop
shows an impressive mid-level circulation along the Idaho/Utah line
at 09z. At the surface, strong cyclogenesis was underway over south
east CO. Strong pressure falls were observed W of the Laramie Range
with a strengthening inverted surface trough. Infrared satellite is
already showing rapidly cooling cloud tops along/northwest of a CYS
to AIA line, and radar and surface observations confirm snowfall is
already becoming quite widespread in those areas. Decent snow rates
occurring at CYS now with excellent dendritic growth, and favorable
low-level upslope.

Everything is on track today. The aforementioned mid-level low will
track from W-E along the CO/WY border with a 988-992 mb surface low
moving slowly across southern KS before lifting northeast. We still
have some concerns about cyclogenesis occurring somewhere along the
inverted trough axis later this morning with intense mid and upper-
level difluence. This will likely pull in some drier air, which may
cut into snow totals some, but of greater concern is the wind which
we will discuss below. Snow will continue to increase in coverage &
intensity in the next few hours w/ a difluent upper pattern and the
strong divergence associated with a 130+ knot H25 jet to the S over
AZ/NM. Strong QG ascent and mid-level frontogenesis, along w/ steep
lapse rates and orographic influences would suggest impressive snow
totals from this storm, especially north of I-80 given the track of
the low right over our southern zones. Models are generally in good
agreement with 0.8 to 1.0 inch of QPF with higher amounts north, so
believe we are on track for 15-20 inches, especially from the north
Laramie Range eastward across the northern NE Panhandle. Models are
trending toward solutions which keep the inverted trough west which
would favor persistent upslope into CYS east along I-80. GFS trends
in QPF have been higher further south, and GEFS plumes still show a
mean about 0.2 inch higher than the deterministic GFS. Have decided
to bump up snow totals another 1-2 inches from CYS-SNY. Laramie may
struggle to see 6 inches w/ shadowing, and influences associated w/
the inverted trough. In general, all headlines remain the same.

However, winds could be a big deal today. RWL has already seen wind
gusts over 35 MPH w/ MAV/MET guidance progging winds to increase to
28-30 knots sustained later today. This makes sense w/ a developing
surface low and locally enhanced pressure gradients across far west
Carbon county. Rawlins itself may actually see winds die off if the
low moves over them, but surrounding areas will remain windy. Since
web cams have already shown periodic blizzard conditions, there was
enough confidence to upgrade to a Blizzard Warning. Would expect to
see some of the worst conditions in Muddy Gap and areas immediately
along the Carbon/Sweetwater county line. We are also concerned that
areas from Converse county eastward into the northern Panhandle may
see winds strong enough to warrant a BZW, but lower confidence does
preclude an upgrade at this time. Will monitor trends closely. Snow
will gradually taper off during the day on Friday with the eastward
progression of the upper low, but it will remain cold beneath broad
deep troughing aloft.

.LONG TERM...(Friday night through Wednesday)
Issued at 330 AM MST Thu Feb 23 2017

00Z/06Z medium range model consensus progs a broad upper trough
across much of the CONUS, with several embedded shortwaves rotating
through the CWA. This pattern will promote occasional snow for areas
along and west of the Laramie Range through early next week. Below
normal temperatures this weekend will gradually moderate to near
normal Monday, then cool slightly Tuesday with a Pacific frontal
passage. Westerly winds will be breezy through much of the period.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 107 PM MST Thu Feb 23 2017

IFR to LIFR will prevail at all airports today into tonight with
short periods of MVFR where we get periodic breaks Laramie and
Rawlins closer to the surface to midlevel low pressure
circulation.

&&

.FIRE WEATHER...
Issued at 415 AM MST Thu Feb 23 2017

No fire weather concerns. Widespread significant snow accumulations
are likely today.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Storm Warning until 11 AM MST Friday for WYZ101>103-
     105>108-110-112>119.

     Blizzard Warning until 11 AM MST Friday for WYZ104-109-111.

NE...Winter Storm Warning until 11 AM MST Friday for NEZ002-003-
     019>021-054-055-095-096.

&&

$$

UPDATE...REC
SHORT TERM...CLH
LONG TERM...MAJ
AVIATION...JG
FIRE WEATHER...CLH


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