FXUS65 KCYS 290606

Area Forecast Discussion
National Weather Service Cheyenne WY
1206 AM MDT Fri May 29 2020

Issued at 1203 AM MDT Fri May 29 2020

Went ahead and added areas of dense fog to the I-80 corridor
between Laramie and Cheyenne, along and near the Summit. Will have
to watch for further lowering and consideration of a dense fog
advisory should we get down into the quarter mile range over a
significant period of time. Will also keep an eye out for
continued easterly spread.

UPDATE Issued at 812 PM MDT Thu May 28 2020

Did a quick update to the forecast package for the overnight
hours. Lowered the temps a degree or two across the Panhandle to
match up better on our CWA borders with the LBF and BOU offices.
Current NBM guidance appeared to be 2-3 degrees warmer and was a
glaring opportunity to rectify while taking a look at projected
lows for that area. Warmest overnight lows still look to be in the
lower elevations along the North Platte River Valley in the NE
Panhandle region. One thing to take note of is that short term
guidance continues to hint at east/southeast/south winds overnight
in SE WY, east of the Laramie Range. Monitoring the decreasing
ceilings heights overnight will be recommended to the night shift
in the event that patchy fog or low visibility occurs. Current
HRRR guidance is only hinting at the Summit region potentially
having low ceilings/visibility, but wouldn't be surprised if it
spreads further east to encompass the city of Cheyenne if
widespread low ceilings and patchy fog occurs.


.SHORT TERM...(Tonight - Friday)
Issued at 202 PM MDT Thu May 28 2020

A few thunderstorms will be possible late this afternoon and early
evening, mainly over and in the general vicinity of the S. Laramie
Range where differential heating this afternoon has reinforced the
weak surface boundary draped across this area. SPC Mesoanalysis is
indicating upwards of 40 knots of deep layer vertical shear, along
with 1000 J/kg SBCAPEs where full sun has occurred today. A couple
of stronger storms are certainly possible with hail and gusty wind
the primary hazards, but overall forcing is quite weak with modest
upslope flow and limited low-level convergence. As such, we're not
expecting to see anything widespread but the ongoing SPC MRGL risk
is in good shape. Any activity should diminish after sunset.

Upper-level ridging remains in control Friday w/ the CWA remaining
on the northern periphery of the 4 Corners High. There is a chance
that weak disturbances riding along the edge of the ridge could be
foci for convective development on Friday, but in general we would
expect the better chances for showers & thunderstorms to be placed
to the north within stronger NW flow aloft. There is some question
regarding vertical shear strength, but 35 knots of 500 mb flow may
be sufficient for organized storms. The latest NAM Nest is showing
potential for dew points in the 50s & resulting CAPEs over 2000 J/
kg. Any storms that do develop could be strong to severe, but once
again it should be rather isolated.

.LONG TERM...(Friday night - Thursday)
Issued at 225 AM MDT Thu May 28 2020

Daily chances for showers and thunderstorms expected this week as
daytime high temperatures rise under the influence of a stubborn
upper level ridge, called an omega block, builds aloft. Surface
moisture will be pulled north under the ridge axis, though severe
potential looks limited by a lack of shear and capping aloft
beginning Saturday. This will be the story for the weekend and
into next week as the ridge stubbornly sits aloft and surface
temperatures increase into the 90s east of the Laramie Range by
MOnday. Some model discretion by mid-neat week with how quickly
this ridge will transition east, and how much moisture will be
pulled north under the ridge for widespread storm potential.
Either way, the region may be stuck under this warm, rather dry
pattern for the foreseeable future.


.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1130 PM MDT Thu May 28 2020

Favorable wind direction, decreasing dewpoint depression spread,
and HRRR are hinting at the potential for some patchy area fog
between KCYS and KLAR overnight. While the HRRR solutions do not
directly place fog on KCYS, trended CIGS down to 2 kft with
monitoring of deteoriating conditions. Otherwise conditions for
KRWL and NE should remain VFR with afternoon chances for showers
and thunderstorms.


Issued at 225 AM MDT Thu May 28 2020

Scattered showers and isolated thunderstorms expected this
afternoon, with gusty winds the primary threat. This may also be
the case tomorrow as a ridge of high pressure builds over the
Great Basin and transitions east over the High Plains. Will see
afternoon humidity drop into the teens from west to east,
beginning in Carbon County Saturday, and spreading east into the
High Plains by Monday. These humidities will pair with increasing
high temperatures, reaching into the 90s by next week. Daily
afternoon thunderstorm potential will pose a threat as chance of
wetting rains remains low for the period. This is a perfect setup
for elevated to locally critical fire weather conditions.




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