Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 232028
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
228 PM MDT MON MAY 23 2016

.SHORT TERM...(Tonight - Wednesday)
Issued at 300 PM MDT Mon May 23 2016

Showers and thunderstorms should develop shortly as recent visible
satellite imagery shows widespread cumulus along the I-80 corridor
between Rawlins and Cheyenne. No severe weather expected late this
afternoon or evening with weak instability; however, an inverted-v
appearance to forecast soundings and high cloud bases suggests the
potential for gusty winds especially with decaying storms. Hail is
also possible with more robust cores. Focus for convection through
the evening hours should be in the vicinity of a Pine Bluffs WY to
Alliance NE line, closer to the H25 speed max and the llvl theta-e
ridge axis. The HRRR has been rather consistent with tstm coverage
in this area later today, so opted to include likely PoPs into the
early evening. Expect convection to diminish quickly with the loss
of daytime heating.

Severe thunderstorms are looking more likely for Tuesday afternoon
with excellent instability/shear and the presence of significantly
better forcing than what we had on Saturday. The 1730z installment
of the SPC Day 2 outlook nudged the Enhanced Risk farther westward
into Kimball, so this could be a higher-end setup for the cwa. The
models are in good agreement with a vigorous/fast-moving mid-level
shortwave lifting across the southern Laramie Range and eventually
across the high plains after 18z, likely originating from a parent
upper trough currently off the CA coast. This will promote surface
cyclogenesis over northeast CO and moist/southeast upslope flow in
the low-levels. The timing of the lead impulse and backing of flow
near the surface suggests convective initiation in the vicinity of
the Laramie Range around 18z, quickly spreading onto the plains as
cinh rapidly weakens. Likely PoPs have been introduced along and e
of the surface dryline. BUFKIT soundings suggest MUCAPES over 3000
J/kg over the southern NE Panhandle and around 1000 J/kg near KCYS
around 21z. 45-50kt 0-6 km shear vectors oriented perpendicular to
the dryline suggests potential for discrete supercells potentially
capable of very large hail, and maybe a couple of tornadoes w/good
directional shear in the effective layer. Thinking that high cloud
bases may mitigate the tornado threat, but still expect widespread
severe storms. Expanded T+ wording westward into Cheyenne as model
guidance usually tries to mix the dryline east too quickly. Threat
for convection should be east of the CWA by 03z Wed.

An unsettled pattern will continue into Weds with active southwest
flow in place in the mid and upper-levels. Widely scattered shower
and thunderstorm activity can be expected once again, although the
threat for severe weather should be low.

.LONG TERM...(Wednesday night - Monday)
Issued at 300 PM MDT Mon May 23 2016

Cool and unsettled weather will continue in the long term with
almost daily chances for afternoon and evening showers and
thunderstorms. A low pressure system continues to track across
Colorado Thursday and Friday that could bring substantial rainfall
to the area. Right now though...its looking like the mountains
will see overnight snow above 9K feet Thursday and Friday with
rain during the day.

Cool and unsettled weather to continue into the weekend. No major
warm ups expected.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1108 AM MDT Mon May 23 2016

Another round of convection expected this afternoon...mainly along
and east of the Laramie Range. Looks like the convection today
will be confined to areas along and southeast of a line from
Alliance to Cheyenne. Did go with VCTS wording in the 18Z TAFs for
airports in this area this afternoon through sunset.

&&

.FIRE WEATHER...
Issued at 300 PM MDT Mon May 23 2016

No major fire weather concerns over the next several days with rhs
well outside of critical thresholds. Daily chances for showers and
thunderstorms will exist through the week. Thunderstorms should be
quite numerous on Tuesday afternoon and evening, mainly along/east
of the Laramie Range. Some storms will be severe, with very large/
destructive hail and damaging winds.

&&

.HYDROLOGY...
Issued at 110 PM MDT Mon May 23 2016

Cool temperatures continuing through the week with morning lows at
or below freezing and afternoon highs in the mid 40s to near 50
for the mountains. This should slow the snow melt into streams and
tributaries of the Upper North Platte and Laramie Rivers. Need to
be watching Thursday and Friday though as a low pressure system
tracks across Colorado. Latest forecast guidance does show fairly
widespread precip in the Snowy and Sierra Madre Ranges. Need to be
watching how this rain on snow event will impact river levels
going into the weekend.

&&

.CYS Watches/Warnings/Advisories...
WY...None.
NE...None.
&&

$$

SHORT TERM...CLH
LONG TERM...GCC
AVIATION...GCC
FIRE WEATHER...CLH
HYDROLOGY...GCC



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