Special Weather Statement, NWS Cheyenne, WY

Area Forecast Discussion
National Weather Service Cheyenne WY
540 AM MDT Thu Mar 23 2017

.SHORT TERM...(Today - Friday)
Issued at 428 AM MDT Thu Mar 23 2017

An extremely active weather pattern will prevail over the next 24-
36 hours with numerous forecast challenges to address. Water Vapor
satellite imagery was indicative of a significant upper-lvl trough
digging into southern California early this AM. The main stream of
mid/high level moisture extends from north central AZ northward to
the western third of Wyoming w/ much drier air to the east over CO
and southeast WY. We cut back on afternoon PoPs across much of the
area (except the far western zones) given a notable absence of mid
and high level moisture observed on the latest WV loop. This could
also result in some fire weather concerns today as BUFKIT sounding
data from the GFS/NAM shows potential for deep mixing, which could
tap into the dry layer aloft and force RHs below 10 percent. It is
likely that winds will become gusty this afternoon as well, so the
consensus was to issue a Red Flag Warning for FWZ 310/313, despite
the main concern being pre-frontal along I-80. A few thunderstorms
are possible today with slightly negative LI values across a large
part of the area, greatest coverage west of the Laramie Range. The
threat for severe weather is low, but some may produce gusty winds
with the well mixed boundary layer and inverted-v profiles. Precip
will increase in coverage later tonight w/improved dynamic support
and a developing TROWAL from northeast Colorado, into the southern
Nebraska Panhandle. H7 temps falling to -4C to -8C by 12z Fri will
allow a change over to snow for elevations above 5500 feet w/a mix
of rain and snow elsewhere. Questions remain w/ snow amounts as we
are concerned with the rapid occlusion of the upper low between 0z
and 6z Fri and surface cyclogenesis occurring well to the south in
extreme southeast CO.

700-300 mb moisture fields continue to suggest the rapid occlusion
effectively chokes off the moisture supply to the system and keeps
the deep moisture confined to the TROWAL area. Model QPFs continue
to trend lower through Friday AM, with the consensus less than one
quarter of an inch over the I-80 summit and much higher amounts in
Cheyenne county where the PTYPE should mainly be rain, but if cold
air filters in any faster this could change. Concerning the Winter
Storm Watch for the I-80 summit, blowing snow is a real concern w/
even a little snow given wind gusts to 40 MPH. However, the trends
in guidance continue to be lower with QPF and the favored area for
precipitation enhancement is further east. That said, there is not
enough support to upgrade to a Warning at this time. The day shift
can take a look at one more model cycle and decide if a Warning is
warranted and what adjacent areas will need an Advisory. There may
be some high wind concerns as well along I-80 between CYS-SNY late
tonight through Friday AM, especially near Sidney w/model sounding
data showing adiabatic lapse rates under the frontal inversion and
50+ knot flow just off the surface. MAV guidance suggests 30 to 35
knot sustained winds between 09z and 18z, so we cannot rule out 60
MPH wind gusts. Did not issue any wind highlights right now as the
NAM has slightly lower winds aloft, but it is possible. This could
be very problematic if precipitation chances to snow over Cheyenne
county which would almost certainly result in Blizzard conditions.

.LONG TERM...(Friday Night - Wednesday)
Issued at 428 AM MDT Thu Mar 23 2017

Temps will moderate as we head into Saturday with a shortwave ridge
moving over the CWA.  An upper level trough will move eastward from
the Great Basin on Saturday into southern Colorado by Saturday
night. The best forcing appears to be over Colorado, however will
keep 30-50 percent PoPs going for Saturday night through the day
on Sunday. Slightly cooler temps will occur on Sunday with a weak
cold front pushing across the region. Another shortwave ridge is
progged to be over WY on Monday in advance of the next system
digging into Utah. Llvl upslope combined with divergence aloft
will bring the next chance of precipitation Monday night into
Tuesday. The GFS digs the upper low further into Arizona by
Tuesday whereas the ECMWF has the midlvl low center near the Four
Corners region. It does look like an unsettled period from Tuesday
into Wednesday with cool temps, widespread cloud cover, and
chances of rain/snow.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 537 AM MDT Thu Mar 23 2017

Vfr conditions will prevail through 18Z. Southwest winds will
increase by late morning at the southeast WY sites with gusts to
25-30 kts through the early afternoon. A strong cold front will
push across the sites between 21-03Z with gusty north winds behind
it. Widespread IFR/MVFR conditions in rain and snow will develop
behind the front during the evening.

&&

.FIRE WEATHER...
Issued at 428 AM MDT Thu Mar 23 2017

A warm, dry, and breezy day is expected ahead of the cold front in
areas along and south of I-80 between Cheyenne and Sidney. This is
expected to result in critical fire weather conditions with RHs as
low as 8 percent and wind gusts of 25-35 MPH. We have issued a Red
Flag Warning for FWZ 310 and 313 from 11 AM until 6 PM today. Fire
weather concerns will be low for Friday w/ widespread rain or snow
expected late tonight and early Friday.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Storm Watch from this evening through Friday morning for
     WYZ116-117.

     Red Flag Warning from 11 AM this morning to 6 PM MDT this
     evening for WYZ310.

NE...Red Flag Warning from 11 AM this morning to 6 PM MDT this
     evening for NEZ313.

&&

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

Hazardous Weather Outlook
National Weather Service Cheyenne WY
443 AM MDT Thu Mar 23 2017

NEZ002-003-019>021-054-055-095-096-WYZ101-102-107-108-119-241045-
Dawes-Box Butte-Scotts Bluff-Banner-Morrill-Kimball-Cheyenne-
North Sioux-South Sioux-Converse County Lower Elevations-
Niobrara County-East Platte County-Goshen County-
East Laramie County-
443 AM MDT Thu Mar 23 2017

This hazardous weather outlook is for portions of western
Nebraska...east central Wyoming and southeast Wyoming.

.DAY ONE...TODAY AND TONIGHT

Isolated thunderstorms will be possible this afternoon. No severe
storms are expected.

A strong storm system will move through Colorado through Friday
morning. This system will bring colder temperatures, widespread
rain and snow, along with gusty north winds this evening through
Friday morning. At this time, snowfall accumulations below 6000 ft
are expected to be light, generally less than one to two inches.
Some impacts to travel due to falling and blowing snow will be
possible.

.DAYS TWO THROUGH SEVEN...FRIDAY THROUGH WEDNESDAY

The probability for widespread hazardous weather is low.

.Spotter information statement...

Spotter activation will not be required.

WYZ103>106-109>118-241045-
North Laramie Range-Ferris/Seminoe/Shirley Mountains-
Shirley Basin-Central Laramie Range and Southwest Platte County-
Central Carbon County-North Snowy Range Foothills-
Southwest Carbon County-Sierra Madre Range-
Upper North Platte River Basin-Snowy Range-Laramie Valley-
South Laramie Range-South Laramie Range Foothills-
Central Laramie County-
443 AM MDT Thu Mar 23 2017

This hazardous weather outlook is for portions of south central
Wyoming and southeast Wyoming.

.DAY ONE...TODAY AND TONIGHT

Isolated thunderstorms will be possible this afternoon. No severe
storms are expected.

A strong storm system will move through Colorado through Friday
morning. This system will bring colder temperatures, widespread
rain and snow, along with gusty north winds late this afternoon
through Friday morning. At this time, snowfall accumulations of
one to four inches look possible from Arlington to Cheyenne. A
Winter Storm Watch is in effect for the southern Laramie Range and
adjacent foothills. Hazardous travel conditions will occur across
southeast Wyoming, and especially along Interstate 80 between
Laramie and Cheyenne, due to the combination of falling and
blowing snow.

.DAYS TWO THROUGH SEVEN...FRIDAY THROUGH WEDNESDAY

The probability for widespread hazardous weather is low.

.Spotter information statement...

Spotter activation will not be required.

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