Flash Flood Warning 18, NWS Cheyenne, WY

Area Forecast Discussion
National Weather Service Cheyenne WY
318 PM MDT Wed Aug 21 2024

.KEY MESSAGES...

- Scattered to numerous showers and thunderstorms are expected
  on a daily basis for the foreseeable future, some with locally
  heavy rainfall. Isolated strong to severe thunderstorms will
  be possible at times as well.

- A drying trend will arrive by Sunday with the potential for
  increased risks of Fire Weather concerns through early next
  week.

&&

.SHORT TERM /THROUGH THURSDAY/...
Issued at 257 PM MDT Wed Aug 21 2024

Today and tomorrow will feature active weather over southeast
Wyoming and western Nebraska. A classic monsoon pattern is evident
over the southwestern CONUS with a ridge of high pressure in place
and seasonably high PWAT values around 1-2 standard deviations above
the normal. Forecast soundings this afternoon indicate that much of
that moisture is above a deeply mixed boundary layer present
anywhere along and west of I-25. Deep inverted-V soundings generally
favor strong wind gust potential and that threat will be present
especially if any stronger storm cores can form over higher terrain
and descend eastward through the afternoon and evening hours. There
is also a dry lightning threat given large T/Td spreads at the
surface, highlighted as well by the SPC Astride outlook over
east- central Wyoming today. Farther east and south over far
southern Wyoming and into the Nebraska Panhandle, a very modest
amount of west-southwest mid and upper-level flow is present
resulting in bulk shear values in the 20-30 knot range. Given
this weak shear overlap with better surface moisture (Tds in the
mid to upper 50s) and SBCAPE values near 2000 in the NE
panhandle, one or two severe thunderstorms are possible later
this afternoon after any MLCIN is eroded. CAM guidance supports
the idea of eastward propagation of any convection which will
likely end before around 8pm this evening. Overnight, expect
more mild temperatures with scattered to broken cloud ceilings
helping to prevent better radiational cooling.

A very similar setup is expected on Thursday over southeast Wyoming
and western Nebraska. Convection is first expected to initiate over
the higher terrain of the Snowy, Sierra Madre, and Laramie ranges by
19z or so and spread eastward throughout the afternoon. Height rises
at 500mb and associated warming mid-level temperatures will help
sustain capping throughout the early afternoon, perhaps resulting in
a bit weaker convection east of I-25. However, a subtle vort
max/weak wave of faster flow aloft moving through by sunset may help
touch off a few storms farther east in the NE panhandle. Cloud cover
should help reduce afternoon highs somewhat from past days. Overall,
HREF ensemble guidance is in good agreement through the forecast
period. Primary adjustments to the forecast on Wednesday and
Thursday were to raise overnight low temperatures given the cloud
cover forecast and nudge PoP values slightly upward.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Issued at 257 PM MDT Wed Aug 21 2024

The monsoonal pattern is expected to continue with plenty of
moisture. With a southwesterly flow, Carbon and Albany county could
begin to see a slight chance of scattered thunderstorms after noon.
Convection could then spread east to the rest of southeast Wyoming.
The Nebraska panhandle seems to look pretty dry until 6 p.m, when
scattered storms could impact the area. So far, the chances of these
storms impacting the CWA being severe is on the lower side due to
minimal wind shear, though not impossible.

Minimum temperatures look to hover in the mid 60’s in the Nebraska
Panhandle and upper 50’s to low 60’s in Southeast Wyoming. The
higher elevations of the Medicine Bow and Sierra Madre Mountain
ranges have an 80% chance of seeing minimum temperatures in the low
to mid 40’s. As for maximum temperatures, the mountainous areas will
likely see highs into mid to upper 50’s. Carbon and Albany county
will see cooler temperatures than the rest of the CWA around the
upper 70’s, with the rest of southeast Wyoming and the Nebraska
Panhandle seeing upper 80’s and low 90’s.

The weekend forecast will center around a trough of low pressure
moving through the northwest CONUS and how it interacts with
residual monsoonal moisture to the south. As of the 12z ensemble
guidance cycle today, southwest flow aloft will increase on
Saturday and Sunday as this trough approaches. Forecast
guidance indicates that the majority of deeper surface moisture
will be shunted eastward into the high plains, east of the NE
panhandle thus limiting the overall threat of severe weather
despite better wind shear/flow aloft. However, if the speed of
this trough slows enough, Saturday afternoon could feature a
more robust severe threat in the eastern plains/NE panhandle if
enough surface moisture/upslope surface flow can remain in
place. By Sunday, most ensemble members drive a hot, dry and
deeply mixed southwesterly flow environment over the entire CWA
resulting in a mainly dry day. Fire weather/Red Flag concerns
may become relevant on both days if RH values can remain below
20%. Dry weather will continue on Monday and Tuesday over
southeast Wyoming and western Nebraska as a westerly flow regime
remains in place. Expect temperatures to remain above average
through the period. Forecast confidence remains high through the
long term with slight adjustments made to PoP grids during the
afternoon hours.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1105 AM MDT Wed Aug 21 2024

VFR conditions will prevail for most of the day at all
terminals. This afternoon, some isolated showers and storms will
develop mainly in far southeast Wyoming and western Nebraska.
CYS, BFF and CDR will have the best chance of storms which may
briefly reduce vis and cig to MVFR. However, any reductions will
likely only last 15 mins or less. An isolated strong storm is
possible at AIA and SNY after 0z. Storms will diminish after 0z
in Wyoming and after 2z in Nebraska with partly cloudy skies
expected overnight.

&&

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

&&

SHORT TERM...MAC
LONG TERM...MN/MAC/KD
AVIATION...MAC

Hazardous Weather Outlook
National Weather Service Cheyenne WY
512 AM MDT Wed Aug 21 2024

NEZ002-003-019>021-054-055-095-096-WYZ101>119-221115-
Dawes County-Box Butte County-Scotts Bluff County-Banner County-
Morrill County-Kimball County-Cheyenne County-
Northern Sioux County-Southern Sioux County-
Converse County Lower Elevations-Niobrara County-
North Laramie Range-Ferris/Seminoe/Shirley Mountains-
Shirley Basin-Central Laramie Range and Southwest Platte County-
East Platte County-Goshen 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-East Laramie County-
512 AM MDT Wed Aug 21 2024

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

.DAY ONE...TODAY AND TONIGHT

Scattered to numerous showers and thunderstorms are expected
across much of the area this afternoon and evening. These storms
may produce locally heavy rainfall as well as gusty and erratic
surface winds. Small hail is also possible mainly with storms over
the western Nebraska Panhandle.

.DAYS TWO THROUGH SEVEN...THURSDAY THROUGH TUESDAY

A persistent monsoonal weather pattern will support daily chances
for thunderstorms for the foreseeable future.

.Spotter information statement...

Spotters are encouraged to report significant weather to the
National Weather Service in Cheyenne.

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