Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 260528
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1128 PM MDT Tue Jul 25 2017

.UPDATE...
Issued at 848 PM MDT Tue Jul 25 2017

Evening GOES-R water vapor animation showed the center of the
upper ridge over the south-central CONUS and upper low near the
northern CA coast. Abundant mid/upper level subtropical moisture
was streaming north-northeast across WY/CO/NE. A cold front was
located from south-central WY to northwest NE and eastern SD.
Regional radars indicated mostly light to moderate showers with
isolated t-storms. Convection allowing models maintain this
precipitation trend through most of the nighttime period. Also,
moistening low level upslope flow north of the front will promote
patchy fog and low ceilings later tonight into Wednesday morning,
especially along and east of the Laramie Range. Zones were updated
earlier this evening to lower t-storm coverage to isolated and
tweak PoPs.

&&

.SHORT TERM...(This evening through Wednesday)
Issued at 147 PM MDT Tue Jul 25 2017

Thunderstorms will likely become more numerous across southeast WY
and the western NE Panhandle over the next few hours w/the passage
of a strong mid-level short wave tracking across southwest WY late
this afternoon. GOES-16 Water Vapor satellite imagery shows plenty
of monsoonal moisture spreading northeast from Utah and western CO
on the back side of upper-level ridging anchored over the southern
high plains. The GFS and NAM have been somewhat meager w/ coverage
of storms this evening, but the HRRR has shown very strong run-to-
run consistency with widespread showers/thunderstorms mainly after
21z. We lean toward the HRRR given the amount of available mid and
high level moisture and dynamic support. Main hazards through this
evening will be locally heavy rain w/ tall and skinny CAPEs, but a
risk for strong/gusty winds is also there with stronger flow aloft
and an inverted-V signature on forecast soundings. It appears that
the overall threat for severe weather is low, but will need to pay
close attention to the northern Panhandle w/ a surface low lifting
northeast early this evening. This will likely enhance directional
shear and lead to a ribbon of stronger instability with CAPES over
1500 J/kg courtesy of dew points in the mid 60s. If LCLs were just
a bit lower, this would be a concern for a tornadic threat but the
main concern appears to be hail if a discrete storm can form. Mode
of storms will likely be too messy and disorganized to support any
kind of substantial severe threat. Widespread precipitation in the
next few hours will contribute to enhanced boundary layer moisture
early Wednesday morning, so anticipate low stratus over the plains
along with the possibility of some fog. Forecast soundings suggest
this burning off by mid morning. For Wednesday, expect to see less
storm coverage outside of the mountains with the upper-level ridge
axis building into the CWA through the day.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 230 PM MDT Tue Jul 25 2017

We maintain a fairly active pattern for afternoon and evening
showers and thunderstorms Thursday and Friday as we remain under
southwesterly monsoonal flow aloft. Friday would appear the most
active for sheer coverage of storms east of the Laramie Range as
a shortwave moves across the region. By Saturday, it would appear
that upper ridging will be extending northward and pushes the
deeper moisture to our southeast, so we will advertise a little
less coverage of showers and thunderstorms over the weekend into
next week.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1122 PM MDT Tue Jul 25 2017

Latest short range models are not handling current precipitation
very well so confidence is a bit lower than normal in precip and
resultant ceiling/vis restrictions tonight. Expect the area of
showers over western Carbon County to continue to affect KRWL with
periods of MVFR in heavier showers through 10Z. Showers will
expand over the northern Panhandle into the early morning hours as
well. Elsewhere expect ceilings to lower into the early morning
hours with widespread MVFR to IFR ceilings developing in moist
upslope flow over the plains east of the Laramie Range. This low
stratus may persist into the late morning hours in the favored
locations such as along the Cheyenne Ridge near KCYS. Low ceilings
should eventually lift to VFR by Wednesday afternoon.

&&

.FIRE WEATHER...
Issued at 147 PM MDT Tue Jul 25 2017

No fire weather concerns with widespread showers and thunderstorms
likely this evening and overnight. Thunderstorms may produce gusty
and erratic surface winds. Afternoon RH values should remain above
30 percent for most areas over the next several days.

&&

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

$$

UPDATE...MAJ
SHORT TERM...CLH
LONG TERM...JG
AVIATION...DEL
FIRE WEATHER...CLH


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