Severe Thunderstorm Warning 225, NWS Cheyenne, WY

Area Forecast Discussion
National Weather Service Cheyenne WY
637 PM MDT Wed Jul 5 2023

.UPDATE...
Issued at 635 PM MDT Wed Jul 5 2023

We are sending out a quick update to issue a dense fog advisory
for the I-80 foothills and central Laramie county. Latest webcams
were showing the low cloud deck continuing to lower, due to
continued upslope flow and strengthening inversion.

&&

.SHORT TERM...(This evening through Thursday night)
Issued at 300 PM MDT Wed Jul 5 2023

KEY MESSAGES:

1. Low clouds continuing over portions of southeast Wyoming and
the Nebraska panhandle are keeping temperatures well below average
for July. Many sites will likely replace the records for coldest
high temperatures for July 5th.

2. Slight Risk (level 2 of 5) in effect tomorrow for the severe
thunderstorm potential. All modes of severe weather are possible:
large hail, strong wind gusts, heavy rainfall, and a tornado
cannot be ruled out. It looks to be a later show, starting around
5 PM, and not ending until around 2 AM. It will be multiple
rounds of storms, so there will be breaks in the storms.

DISCUSSION: The stratus deck across southeast Wyoming and the
Nebraska panhandle has remained in place all day, keeping
temperatures well-below average for July. It feels more like an
October day than July 5th! Cheyenne, Scottsbluff, Chadron,
Alliance, and Sidney will all most likely break their records for
coldest high temperatures:
	Cheyenne: 60 deg F
	Scottsbluff: 68 deg F
	Chadron: 64 deg F
	Alliance: 68 deg F
	Sidney: 62 deg F

With these cold temperatures and cloud deck in place, and
continued moisture with upslope flow, fog is once again possible.
Did not have confidence in how widespread it will be, so opted not
to do a Dense Fog Advisory.

Now the big story, what is going to happen tomorrow?! SPC
maintained their Slight Risk (level 2 of 5) for almost all of
southeast Wyoming and the Nebraska panhandle. There is a 15-29%
chance of severe wind gusts, 15-29% chance of significant hail
(greater than 2 inches in diameter), as well as a 2% chance of
tornadoes. Hires guidance is showing isolated to scattered severe
thunderstorms initiating late afternoon, between 3 and 5 PM over
the higher terrain, staying near the I-80 corridor, moving east.
Then, a line of thunderstorms swing south and east through
Converse, Niobrara, Sioux, Dawes, and Box Butte counties, clipping
Platte and Goshen. However, not all hires guidance is in
agreement in the timing or locations.

Soundings show a strong cap in place across many areas
east of the Laramie Range, but once that cap breaks, the
environment is progged for severe weather. Dewpoints in the 50s
to low 60s, shear values 45 to 60 knots, and 1000 to 2000 J/kg of
CAPE are prime conditions for strong severe thunderstorms. In
addition, it's definitely concerning seeing all HREF members
showing a long UH track through east central Wyoming and the
northern Nebraska panhandle. With all these ingredients coming
together, an upgrade to Enhanced would not be surprising, but the
question will be how widespread the severe storms are. Aside from
the severe potential, temperatures will return back closer to
normal with highs in the 70s and lows in the upper 40s to mid-50s.
Remember to have multiple ways to receive weather forecasts and
warnings to ensure you and your family are always prepared!

.LONG TERM...(Friday through Wednesday)
Issued at 300 PM MDT Wed Jul 5 2023

Unsettled weather Friday as surface front lays roughly across the
front range of Colorado...northwest to Laramie to southwest
Wyoming. Northwest upper level flow as a series of shortwaves move
through this flow. Could be similar to the pattern we are seeing
today as low level northeast upslope flow being shown on GFS along
with 700mb temperatures ranging from +5C across the northern
Panhandle to around +8C along the Laramie Range. GFS soundings
for southeast Wyoming really not showing a lot of instability
Friday and for the Panhandle...showing a mid level capping
inversion just above 700mbs. Looks like mainly showers with maybe
an isolated thunderstorm or so. Will see how guidance pans out as
we get closer to the event. SPC has a Marginal Risk over much of
the eastern CWA into the Panhandle Friday afternoon.

CWA stays in that upper northwest flow Saturday into Sunday. Maybe
better dynamics Saturday night as a strong shortwave tracks
across the area...lifting the surface boundary northeast into
Converse/Niobrara Counties and the northern Panhandle. Upper jet
near 100kts in the northwest flow would help with shear and lift.
Could be a late show for convection Saturday evening that could
persist overnight into Sunday.

Guidance hinting at drier conditions next week as upper ridge
builds into the area. GFS mainly dry across the CWA...but given
moisture already in the ground...hard to not go for afternoon and
evening showers on a daily basis.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 534 PM MDT Wed Jul 5 2023

The main concern with this TAF issuance is the timing when the
LIFR/IFR ceilings will clear out along and east of the I-25
corridor.

Latest visible imagery continues to indicate stubborn low clouds
not showing any signs of improving this afternoon. In fact, some
of the low cloud decks have dropped at KCYS and KSNY this
afternoon. These TAF sites will need to be watched for dense fog
overnight especially at KCYS where ideal upslope flow will be in
place along with fog top cooling. Another site to watch will be
KBFF due to their proximity to the North Platte River Valley and
upslope flow. The exception may be KCDR where downslope flow off
the Pine Ridge may tend to scour out the lower clouds a bit. These
low clouds should tend to improve tomorrow afternoon, but not
confident that the inversion will break. We are the most confident
that areas west of the Laramie Range including KLAR and KRWL will
stay mostly cloud free with only a few passing showers this
evening around KLAR.

&&

.FIRE WEATHER...
Issued at 122 PM MDT Wed Jul 5 2023

Low fire weather concerns due to recent wetting rainfall and
continued chances for rain. Drier conditions possible early next
week with gusty winds.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Dense Fog Advisory until 10 AM MDT Thursday for WYZ117-118.

NE...None.
&&

UPDATE...REC
SHORT TERM...LK
LONG TERM...GCC
AVIATION...REC
FIRE WEATHER...LK

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