Severe Thunderstorm Warning 311, NWS Cheyenne, WY

Area Forecast Discussion
National Weather Service Cheyenne WY
531 PM MDT Fri Jul 21 2023

.UPDATE...
Issued at 200 PM MDT Fri Jul 21 2023

The overnight forecast discussion remains on track for the
convective threat this afternoon. As of 20z, an area of agitated
cumulus cloud cover has developed over the northern and central
Laramie range with one particular updraft on the eastern side of
the range fully developed into a fresh thunderstorm. While
surface moisture along the Laramie range foothills has mixed
somewhat (Tds at 48 in CYS and 51 at EAN), sufficient boundary
layer moisture and depth of moisture remains in place to support
convection east of the range. Deeper moisture remains in place
just to the east (Td at TOR of 59 and 54 at Albin). As current
convection consolidates and moves south- southeastward, any
westward advection of the aforementioned higher dewpoints will
only serve to increase the severe weather threat this afternoon.
If a single dominant supercell can become firmly established,
southern Platte, Goshen, and most of Laramie Counties remain the
areas with the greatest risk with large hail of 1-2" and gusty
outflow winds the primary threats. Boundary- layer parameter space
including surface-2km wind fields, surface vorticity, and 3CAPE
remain relatively meager thus far this afternoon. Some increase in
both wind speed and enhanced backing in surface wind fields is
progged by CAM guidance later this afternoon in the CYS to EAH
corridor. Despite the lack of threat highlighted in the latest SPC
outlook, the tornado threat remains nonzero this afternoon -
conditionally dependent on an established supercell maintaining
itself south and eastward into Laramie County. Elsewhere, cumulus
fields west of the divide in Carbon and Albany county will likely
consolidate into a few high-based showers and thunderstorms this
afternoon. In the environment west of the Laramie range, surface
moisture is much lower and gusty outflow winds will remain the
most likely threat given larger inverted-v soundings. Area-wide,
heavy rain looks to be a much lower threat today given faster
storm motions (18-27kts) and an overall decrease in PWAT values.

&&

.SHORT TERM...(Today - Saturday Night)
Issued at 318 AM MDT Fri Jul 21 2023

The main forecast challenge over the next 12-24 hours continues to
be the potential for strong to severe thunderstorms this afternoon
into the early evening hours today. Models continue to struggle w/
the anomalously moist boundary layer in place across southeast Wyo
and the western Neb Panhandle, as suggested by a wide distribution
of progged dew points at KCYS at 18z today, ranging from the lower
to middle 40s (WRF-ARW/FV3/GFS) to the low/mid 50s (NAMNest/UKMET/
GEM/NSSL-WRF). Multiple models (global and high-res alike) suggest
either northerly or westerly boundary layer flow, which could help
contribute to the drier surface progs. However, given the expected
strong northwesterly mid & upper-level flow in place over the area
today, lee troughing should evolve along the Laramie Range through
the day. As such, expect surface winds to eventually become SSE as
increasingly cyclonic flow & resulting convergence develops across
the central Laramie Range near Chugwater. The only model currently
adequately predicting proper boundary layer wind profiles for this
pattern is the NAMNest. This is a classic mid-July pattern for the
development of storms over the northern and central Laramie Range,
which then move southeast and eventually turn near due south along
or paralleling I-25. This is supported by NAM progged right-moving
Bunkers vectors oriented nearly perfectly north-to-south. Overall,
CAMs are pretty quiet today, but those without substantial CI also
have northerly surface winds and a dry boundary layer. The NAMNest
suggests a classic "Chugwater" supercell tracking SSE over central
Laramie County near Cheyenne during the mid-afternoon hours. It is
worth noting that the overall synoptic pattern across the CONUS is
reminiscent of 7/29/18, which featured a scenario quite similar to
what seems progged by the NAMNest simulated reflectivity & updraft
helicity tracks. That event was associated w/ several reports of 2
inch diameter hail, and even a couple of tornadoes across portions
of Platte/Goshen/Laramie counties. While it is not a sure bet that
this will play out as it did in 2018, the pattern certainly favors
the more aggressive model guidance. As such, opted to increase the
PoPs to 50 percent over the higher terrain, into the I-25 corridor
south of Wheatland and including Cheyenne. Any storms that develop
today will have potential to produce very large/damaging hail with
progged MLCAPEs near 1500-2000 J/kg and 40+ kts of effective shear
courtesy of 35+ knot 500-mb flow and veering wind profiles. Threat
for a tornado is low, but non-zero especially where localized near
surface-flow becomes more backed.

Otherwise, a general trend toward warmer and drier conditions (for
most areas) will commence through the first half of the weekend as
a 595-597 dam H5 ridge becomes established across the Great Basin/
4 Corners region. As such, 700-mb temperatures will climb to +14 C
to +18 C across the CWA by Saturday afternoon w/ most areas seeing
highs in the upper 80s to lower 90s as a result. Most models still
show dry conditions for Saturday, but given recent tendencies with
our residual moisture, as well as the historical performance of NW
flow aloft in mid/late July, have added/maintained isolated/slight
chance PoPs in the vicinity of the Laramie Range.

.LONG TERM...(Saturday night through Friday)
Issued at 153 PM MDT Fri Jul 21 2023

Overall, the long term forecast looks much warmer and drier headed
into next week. Starting on Sunday, the ridge over the desert
southwest will gradually strengthen and slowly push eastward. By
Monday, a nearly 600 decameter high/ridge will be positioned over
the Rockies. This will usher in much warmer and drier air to the
CWA. The warmer temperatures stay locked over the region through
much of next week, even as the ridge flattens and becomes more
broad. During this time, temperatures will be on the hot side. On
Sunday, temperatures will predominately sit in the 90s for most
locations, with the warmest temperatures expected over the plains.
Monday and Tuesday will likely be the hottest days of the week as
700 mb temperatures make it up to the +16 to +20C range! As a
result, did nudge high temperatures upwards a bit above guidance,
especially Monday and Tuesday with the ridge axis almost directly
overhead. This will lead to most cities in the CWA having high
temperatures in the 90s and maybe even low 100s! As we head into
next week, will have to keep an eye on the temperatures as we enter
into county fair season.

The hot and dry weather will keep most storms away, but cannot rule
out an isolated thunderstorm or two during the afternoon hours as
weak vort maxes pass through the ridge aloft. Sunday looks like the
driest of the long term with maybe an isolated shower or two over
the high terrain. Precipitation chances are a bit better by mid-week
as both the GFS and ECMWF show a shortwave moving across the CWA and
northwest flow. A quick glance at model soundings suggest that
storms will stay sub-severe as instability and shear are limited.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 459 PM MDT Fri Jul 21 2023

VFR conditions likely through the forecast period for all
terminals. A few isolated thunderstorms will remain possible
throughout the afternoon and early evening with the current radar
imagery showing some weak storms across Carbon and Dawes County.
Once these storms clear out, quiet conditions expected across the
CWA as the ridge axis continues to shift east.

&&

.FIRE WEATHER...
Issued at 153 PM MDT Fri Jul 21 2023

Minimal fire weather concerns in the next 24 hours with plentiful
surface moisture and shower and thunderstorm chances through the
afternoon hours. A ridge of high pressure will build in over the
weekend which will usher in drier air and thus lower RH values. Winds
should remain generally light through the forecast period however and
critical fire weather conditions are not expected.

&&

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

UPDATE...MAC
SHORT TERM...CLH
LONG TERM...SF
AVIATION...MRD
FIRE WEATHER...MAC

Hazardous Weather Outlook
National Weather Service Cheyenne WY
156 PM MDT Fri Jul 21 2023

NEZ002-003-019>021-054-055-095-096-WYZ101>119-222000-
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-
156 PM MDT Fri Jul 21 2023

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

.DAY ONE...THIS AFTERNOON AND TONIGHT

Isolated afternoon showers and storms may produce large hail in
southeast Wyoming this afternoon. Gusty winds will also be a
possible threat in any thunderstorms that form.

.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY

Dry weather is expected through the weekend with temperatures
warming each day.

.Spotter information statement...
Spotters are encouraged to report any hail over 1 inch or gusty
winds to the National Weather Service.

Special Weather Statement
National Weather Service Cheyenne WY
719 PM MDT Fri Jul 21 2023

WYZ116-117-220145-
South Laramie Range WY-South Laramie Range Foothills WY-
719 PM MDT Fri Jul 21 2023

...A strong thunderstorm will impact portions of southeastern Albany
and southwestern Laramie Counties through 745 PM MDT...

At 719 PM MDT, Doppler radar was tracking a strong thunderstorm near
Buford, or 23 miles west of Cheyenne, moving southeast at 25 mph.

HAZARD...Winds in excess of 40 mph and penny size hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

Locations impacted include...
North Crow Campground, Granite Springs Reservoir, Vedauwoo, Granite
Springs Campground, Crystal Lake Reservoir, Crystal Lake Campground,
Vedauwoo Campground and Buford.

This includes Interstate 80 in Wyoming between mile markers 326 and
344.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

&&

LAT...LON 4120 10546 4127 10531 4100 10502 4100 10528
TIME...MOT...LOC 0119Z 321DEG 10KT 4107 10523

MAX HAIL SIZE...0.75 IN
MAX WIND GUST...40 MPH

Woodward