Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 251044
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
444 AM MDT Thu May 25 2017

.SHORT TERM...(Today - Saturday)
Issued at 328 AM MDT Thu May 25 2017

The main concern during the next 24-48 hours will be the threat for
strong to severe convection across portions of southeast WY and the
western NE Panhandle on Thu/Fri. A strong upper-level shortwave was
tracking across central MT early this morning, with a series of sfc
frontal boundaries pushing southeast across WY. Models have trended
further north w/ the position of the primary surface boundary later
today, most likely resulting in high temperatures being a degree or
two warmer and contributing to a more unstable environment. Low-lvl
moisture return will be good this afternoon as surface flow becomes
more ENE, enabling dew points to climb into the lower 50s. CAPES in
the neighborhood of 1000 J/kg and 45-50 knots of 0-6 km shear would
suggest potential for stronger storms, but still expect the overall
best instability/severe chances to be over northeast CO. We did add
mention of small hail/gusty winds to coincide with the SPC Marginal
Risk on Day 1.

A more organized threat for severe storms appears likely for Friday
as a secondary shortwave tracks from ID into western WY, leading to
subtle dynamic forcing over the CWA along w/ lee cyclogenesis along
the Front Range of CO/WY. Southeasterly low-level upslope flow will
lead to excellent moisture return, and low-level convergence should
be more than enough to overcome any capping which should not be all
that significant with fairly cool thermal profiles. Main concern is
morning fog/stratus over the high plains which may be tough to burn
off per forecast soundings, but models suggest clearing by mid-late
morning from CYS-SNY. CAPEs up to 1500-2000 J/kg and strong low-lvl
and deep layer vertical shear (0-3 km SRH approaching 300 m2/s2 and
0-6 km shear of 45-55 knots) should be very supportive of supercell
development. Large hail/damaging winds would be the primary threats
but very strong low-level directional/speed shear and LCLs possibly
only around 1000 feet AGL suggests potential for isolated tornadoes
as well, especially in the late afternoon and early evening. We may
see some additional development and/or growth into one or more MCSs
in the evening/overnight as low-level WAA strengthens. Another cool
and unsettled day on Saturday as the upper trough axis swings south
east across the area.

.LONG TERM...(Saturday night through Wednesday night)
Issued at 415 AM MDT Thu May 25 2017

Models continue to show a slow warming trend beginning for the
Memorial day weekend with temperatures likely returning to near
normal for this time of the year by the middle of next week. The
stubborn upper level low over southern Canada and the Great Lakes
region will gradually weaken and drift eastward towards the
eastern seaboard. This will allow a ridge axis, currently over the
eastern Pacific, to move into the Rockies. High temperatures will
remain slightly cooler than normal Sunday and Memorial day, with
a return to the upper 60`s to mid 70`s by the middle of next week
as 700mb temperatures increase over 5c with temps up to 10c
further west. However, most of southeast Wyoming and especially
western Nebraska will likely be under the influence of northwest
flow aloft. With plenty of llvl moisture across the area, combined
with afternoon instability and convergence along the Laramie
Range, can not rule out isolated to widely scattered showers and
thunderstorms each afternoon through Wednesday. Not much dynamic
forcing in this pattern, so it is difficult to nail down a day
which seems more favorable for strong to severe thunderstorms. For
now, kept POP between 20 to 40 percent across the high valleys
and high plains with the highest values over the mountains and
adjacent plains of southeast Wyoming.

&&

.AVIATION...(For the 18Z TAFS through Friday morning)
Issued at 328 AM MDT Thu May 25 2017

VFR conditions will prevail today with scattered showers and
thunderstorms developing after 17z and moving eastward through the
afternoon. Thunderstorms with heavy rainfall may impact most
terminals south of KCDR and KAIA into this evening. Brief MVFR
conditions are expected with heavy rainfall and erratic gusty
outflow winds. This activity is expected to linger through tonight
along the Interstate 80 corridor from KCYS to KSNY.

&&

.FIRE WEATHER...
Issued at 328 AM MDT Thu May 25 2017

No fire weather concerns. A cool and unsettled weather pattern will
prevail over the next several days, likely leading to a good chance
for widespread wetting rains.

&&

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

$$

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



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