Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 240956
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
356 AM MDT Thu Jun 24 2021

.SHORT TERM...(Today - Saturday)
Issued at 321 AM MDT Thu Jun 24 2021

A few light rain showers across mainly far southeast Wyoming & the
southern Nebraska Panhandle early this morning, likely in response
to a lead mid-level short wave currently tracking ENE along the I-
80 corridor. This activity has a history of a few brief wind gusts
around 40 MPH over the past few hours. Widespread showers & storms
are expected later today with the greatest coverage between around
21z and 03z as the main trough axis tracks across the CWA. Highest
potential for strong/severe storms will be along & east of I-25 w/
soundings showing potential CAPEs up to 1000 J/kg along with up to
30 knots of deep layer vertical shear. Main hazard should be down-
burst winds up to 60 MPH given fairly large T/Td spreads, but some
potential for hail will exist as well with any organized updrafts.
Thermodynamic & kinematic profiles are more muddled for areas west
of the Laramie Range, with much more limited CAPEs and fairly weak
low & mid level flow (generally less than 20 kts below 500 hpa) in
the mid/late afternoon. While gusty winds will be a threat w/ some
of the initial activity, expect the main concern to shift to heavy
rainfall as the low levels saturate. Substantial warm cloud depths
and PWATs well above climatology at 0.8 inches should support very
efficient rain producers. The aforementioned modest flow below 500
mb and expected lower cloud tops in a warm rain environment should
yield fairly slow moving storms, up to 15 knots at best. Could see
some training as well with storms hanging up on the terrain. Areas
near the Mullen Burn Scar could see some action today, so this may
have to be monitored closely.

Widespread precipitation will continue Friday with the trough axis
moving very little through the end of the week. Low-level cold air
advection and extensive cloud cover should keep temperatures quite
cool w/ many areas struggling to get out of the 60s for their day-
time highs. Severe potential will be a bit lower on Friday as cool
temperatures will yield weaker instability profiles, but still the
potential will exist for a few small hail producers. This cold air
mass will remain in place through Saturday w/ H7 temps expected to
continue falling to around +4 to +6 deg C by 00z Sun. Showers will
not be quite as widespread, but with the continuation of active NW
flow aloft & another approaching cold front we can anticipate more
rounds of convection.

.LONG TERM...(Saturday Night - Wednesday)
Issued at 321 AM MDT Thu Jun 24 2021

Key Messages:

1) Below normal highs late weekend by 10F degrees as scattered
clouds and showers continue behind a cool front.

2) Return to near normal temperatures mid-next week with some
isolated showers possible with limited overall coverage.

Weather Discussion: Interesting changes in the run to run global
models and ensembles compared to yesterday as the degree of warm-up
through early- to mid-next week looks more subdued at this time.
While we will start below normal for high temperatures Sunday and
even into Monday, a warning trend is still on track mid- to late-
week. The degree of warming however, now looks to be more in limited
given model solutions of a large omega block forming across the
Canadian West coast with WY/NE more locked under the downstream
trough wavelength.

For Sunday, H7 temperatures will be the coolest of the period at
only +2C east to +4C west, with this being impressively -2 to -3
standard deviations below normal for this time of year. In fact,
through the entire week, the region stays at near to almost one
standard deviation below normal for H7 temperatures per the latest GFS
deterministic solution. ECMWF is a bit warmer but also showcases the
blocking omega pattern with only subtle H7 temperatures differences.
Despite the cooler conditions, showers and a few thunderstorms will
be likely Sunday with 500-700 J/Kg MUCAPE. More limited area of
showers Monday closer to the WY/CO border and will need to monitor
nuances of next week for a closed low possibly forming nearby at the
base of the downstream omega trough. This could aid in some shower
activity - but confidence is low overall and forecast details will
need to be refined.

This nearby troughing was once advertised to be farther east and
progressive early next week now appears more stagnant with deeper
northerly flow over the region next week. The more persistent
northerly H7-H3 flow will limit overall warming amounts depending on
where the thermal ridge sets up to our west. Will still need to
maintain watch on elevated fire weather conditions but relative
humidity recoveries should be decent each night despite still
falling into the teens for western high terrain areas in the
afternoon.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1143 PM MDT Wed Jun 23 2021

VFR conditions ongoing across the region as SHRA/TSRA diminishes
while it shifts east into central Nebraska. No aviation concerns
overnight till about 19-20Z Thursday afternoon. At that time,
anticipate scattered to numerous SHRA with isolated to scattered
TSRA develop across high terrain of Wyoming initially and then
shift east into west Nebraska by 22Z-01Z. Brief MVFR conditions
could be possible if stronger TSRA directly impacted airport
locations with lower visibility for heavier rainfall. Storm tops
will likely reach FL400. SHRA will linger through the overnight
but be more isolated to scattered before another round pick up
again Friday early Friday and into the afternoon. Surface winds
will be variable overnight from the west or north and then
increase in speed Thursday afternoon from the west up to 20-25
knots. Erratic and higher winds could be possible near SHRA/TSRA
Thursday afternoon.


&&

.FIRE WEATHER...
Issued at 321 AM MDT Thu Jun 24 2021

No fire weather concerns w/ cooler temperatures & good chances for
widespread wetting rains over the next few days. Humidity values &
general winds will not exceed critical thresholds over the next 24
hours. However, widespread showers and storms should be capable of
brief/erratic surface wind gusts in excess of 50 MPH.

&&

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

$$

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


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