Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 201852
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1252 PM MDT Thu Jul 20 2017

.UPDATE...
Issued at 1252 PM MDT Thu Jul 20 2017

Forecast remains on track for today`s cooler temps and chances for
showers/thunderstorms. Temps across the forecast area are in the
70s and 80s with dew points in the 40s/50s across Wyoming, and
50s/60s across the Panhandle. Mesoanalysis already shows a fairly
unstable atmosphere with a slow but steady weakening of the cap,
although lapse rates are still overall pretty marginal. Satellite
imagery shows convection developing over the high country, and
should see this activity intensify over the next couple of hours.
Some storms should persist eastward into the high plains through
the late afternoon and evening as lapse rates steepen. Still not
impressed with current and progged shear values overall, but 20-30
kts of progged bulk shear could contribute to production of a few
stronger pop up storms when combined with CAPES in excess of 1000
J/kg. Weak steering flow will keep the biggest threat from storms
heavy rain which could result in areas of flash flooding. The
focus for heavier storms looks to remain across southeast Wyoming
where the cap is expect to weaken the most. A stronger shortwave
looks to move across Wyoming late this evening and overnight,
which should support at least isolated showers/storms through
early Friday morning.

&&

.SHORT TERM...(Today and tonight)
Issued at 315 AM MDT Thu Jul 20 2017

A broad area of rain, and a few embedded thunderstorms has plagued
much of the northern NE Panhandle through the early AM hours. KCYS
radar imagery finally shows this activity decreasing in coverage &
intensity as a significant midlevel shortwave pushes eastward into
central NE and southwest/south central South Dakota. Subsidence in
the wake of the aforementioned wave will likely keep convection at
bay for much of the day outside of the high country, but we expect
another surge of monsoonal moisture by 18-21z ahead of yet another
weak disturbance aloft. This should promote convective development
over the lower elevations of southeast Wyoming/western Nebraska by
late afternoon or early evening. Some questions regarding coverage
of storms with GFS/NAM soundings showing considerable CINH through
the day, but do believe we will see at least scattered coverage w/
the excellent fetch of mid/high level moisture. It will be quite a
bit cooler today as H7 temperatures fall about 2 to 5 C from those
values seen on Wednesday, but still on the warm side.

A lot of low-level moisture to work with today w/many areas seeing
dew points in the upper 50s/lower 60s. PW values are still progged
to be quite high in the 1.0-1.5 inch range for a large part of the
CWA, especially east. Heavy rain remains a concern, along with the
threat of flash flooding w/ weak steering currents aloft. The most
recent SPC Day 1 Outlook has a SLGT Risk for severe convection for
mainly northern portions of our high plains zones. CAPE of 1000 to
2000 J/kg may be sufficient for hail but the tall/skinny nature of
the profiles would suggest warm rain processes vs contributions to
large hail growth. In addition, midlevel flow will not be all that
strong today with only around 20 knots at 500 mb. Despite the weak
wind profiles, forecast soundings show plenty of veering w/ around
30 knots of effective bulk shear which pushes 0-3 km helicity over
150 m2/s2 in the SLGT risk area. This should be sufficient for the
development of a few rotating updrafts capable of supporting large
hail. An isolated brief tornado would not be surprising given some
of the 0-1 km EHI progs from the GFS/NAM, but definitely prefer to
see stronger low-level flow to be confident in that. Thunderstorms
will continue well into tonight, with a diminishing threat for any
severe storms after 03-06z.

.LONG TERM...(Friday through Wednesday)
Issued at 315 AM MDT Thu Jul 20 2017

With the main shortwave moving east of the region on Friday, a
weakening cold front will drop southward across the area early
Friday with the main plume of monsoonal moisture shifting to the
southeast through the weekend. This pattern should result in a
diminished coverage of showers and thunderstorms over the weekend
with relatively cooler temperatures. The models have also backed
off on the deep moisture return across SE Wyoming and western
Nebraska Monday into Tuesday as a ridge of high pressure builds
over the 4 corners area putting. Overall the pattern would
suggest low PoPs with warmer temperatures in the upper 80s to
middle 90s again next week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1246 PM MDT Thu Jul 20 2017

Vfr conditions will prevail through the taf period. Scattered
showers and thunderstorms will develop after 20Z in areas near the
Laramie Range and then spread eastward through the evening. The
best coverage of storms should be from LAR to SNY. Some pockets of
brief heavy rain and associated mvfr vis will be possible. Winds
should mostly be below 15 kts outside of gusty outflow winds near
any storms.

&&

.FIRE WEATHER...
Issued at 315 AM MDT Thu Jul 20 2017

Scattered thunderstorms are expected today, w/ the primary hazards
being lightning and gusty/erratic surface winds. Minimum RH values
will be well outside of critical thresholds. A generally warm, and
dry weather pattern prevails for Fri-Sun, but light winds preclude
any significant risk for fire growth.

&&

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

$$

UPDATE...RJM
SHORT TERM...CLH
LONG TERM...JG
AVIATION...ZF
FIRE WEATHER...CLH



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