Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 132329
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
529 PM MDT Thu May 13 2021

.SHORT TERM...(This evening through Saturday)
Issued at 225 PM MDT Thu May 13 2021

Popcorn storms across southeast Wyoming and the west Nebraska
Panhandle this afternoon under a disturbed northwesterly flow
aloft. A few vorticity maxima will traverse the flow, aiding in
storm development, though the strongest vorticity maxima will
remain east of the forecast area. So far, we`re only seeing gusty
winds in the 30 to 40 mph range, with a deep dry boundary layer
up to about 600 mb; however, if a storm is capable to higher
vertical reach, winds could gust much higher with a unidirectional
flow and stronger winds aloft. The main focus of storms this
afternoon will be found along the I-80 corridor, and especially
across the northern Panhandle where CAPE near 1000 J/kg can be
found. Still, the strongest storms should remain to the east of
the west Nebraska Panhandle.

Looking for storms to diminish quickly this evening, by 9 PM. Cool
and calm weather expected tonight ahead of another day of
potential thunderstorms. As of this forecast iteration, have added
the potential for severe weather for FRiday afternoon from the
North Laramie Range, south towards the Colorado border, and east
towards Sidney. This is as bulk shear has jumped up from the 30kt
range to the 45 kt range, the dry layer has decreased in depth,
and CAPE has increased to near 1500 J/kg, higher even along the
I-80 corridor as a significant vort max pulses through the
northwesterly flow aloft and ejects east. With these conditions in
place, large hail and strong, gusty winds will be the primary
threat. To the west, a deep dry layer will remain. With increasing
mositure aloft as a deep upper low digs into the Intermountain
West and moisture is able to ride north of the digging low into
the upper levels, dry lightning will become the primary concern.
Again, expect the potential for storms to diminish by 9 PM Friday.

Cool and calm again Friday night with a few showers remaining east
of the Laramie Range. This will bleed into the next chance for
severe weather Saturday as yet another vorticity maximum pulses
through the northwesterly flow aloft. While CAPE will increase to
the 2000-2500 J/lg range east of the Laramie Range, and bulk
shear remains near 40 kt, dewpoints will play a larger role
increasing into the 50s east; west of the Laramie Range, dewpoints
will remain near 30 to 35 degrees. This could be enough as
diffluence aloft combines with the ingredients previously
mentioned to produce large hail with strong updrafts that develop.
There could be even be enough near-surface directional shear for
the development of a brief/weak tornado as well on Saturday.
Severe storm development will be more limited to the west of the
Laramie Range.

.LONG TERM...(Saturday night through Thursday)
Issued at 225 PM MDT Thu May 13 2021

Fairly active pattern moving into next week.  The strong to severe
based storms should start pushing off towards the east as the upper
level closed low digs further towards the southeast late Saturday
night into early Sunday morning. Next round of precipitation is
expected to resume again on Sunday, with another chance for strong
thunderstorms as the region remains within good moisture advection
and unstable air. Current forecast soundings are indicating MLCAPEs
around 900+ J/kg and a BRN Shear of +45 m2/s2 for the southern
portion of the CWA, with cooling aloft leading to a steepening mid-
level lapse rate around 7-8 degrees C/km. Still trending downward
from the previous day where the BRN Shear was around 75 m2/s2, but
still significant enough to support supercells, with locally intense
updrafts capable of larger sized hail. Nevertheless, will need to
monitor this weekend for the possibility of strong to severe storms
across the southern portion of the CWA. The chance of thunderstorms
remain through the forecast period as the closed low remains within
as a dominate influence across the CWA through Tuesday. The downward
trend remains fairly constant with the systems into Tuesday with
MLCAPE dropping to the 300-600 J/kg range, conducive for some
thunder and lightning, but with much less of a sheared environment
as the upper low drifts eastward.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 521 PM MDT Thu May 13 2021

Wyoming TAFS...VFR. Scattered to broken clouds from 8000 to
10000 feet AGL. Thunderstorms in the vicinity at Laramie and
Cheyenne after 19Z Friday. Wind gusts to 30 knots until 02Z, then
to 25 knots after 15Z Friday.

Nebraska TAFS...VFR. Scattered to broken clouds from 7000 to
10000 feet AGL. Thunderstorms in the vicinity at Chadron and
Alliance until 03Z. Wind gusts to 25 knots at Alliance,
Scottsbluff and Sidney until 03Z, then to 23 knots at Scottsbluff
and Sidney after 15Z Friday.

&&

.FIRE WEATHER...
Issued at 225 PM MDT Thu May 13 2021

Limited fire weather concerns with shower and thunderstorm
potential in the coming days, however, dry and gusty winds are
possible today across southeast Wyoming and the west Nebraska
Panhandle, as well as tomorrow with dry thunderstorm potential
west of the Laramie Range. Severe storm potential expected Friday
through Sunday.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.
&&

$$

SHORT TERM...AB
LONG TERM...MD
AVIATION...RUBIN
FIRE WEATHER...AB


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