Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 240542 AAA

Area Forecast Discussion...UPDATED
National Weather Service Cheyenne WY
1142 PM MDT MON MAY 23 2016

Issued at 921 PM MDT Mon May 23 2016

Latest radar loop shows the bulk of the showers had ended across
the area. As a result, we removed the shower chances during the
remainder of the night. The next concern is the jet energy moving
out of the desert southwest. This jet energy is expected to lift
northeast into northern Colorado Tuesday afternoon while the jet
energy from the convection earlier this evening begins to pull out
into the Northern Plains. These two features will induce will good
upper level diffluence over our region tomorrow along with good
surface pressure falls. In response to these pressure falls, the
low level flow will turn southeast and bring a good surge of low
level moisture from northwest Kansas where dewpoints are in the
low to mid 60s. These dewpoints will really help boost the
instability over the Nebraska Panhandle during the next 12 to
18 hours. This in combination with good good upper level forcing
will induce convection over the Laramie Range early tomorrow

It is still difficult to ascertain the amount of coverage with
the convection, but the chances of discrete supercellular
convection is still looking pretty favorable especially for areas
east of I-25 where the cape may range from 1500-2500 j/kg along
with 40 to 50 kts of effective shear. Not real impressed with
0-1km shear due to limited turning, but there is a decent 0-3km
cape taking place in the southern Nebraska Panhandle which will
help enhance the stretching potential. Thus, still cannot rule
out an isolated tornado in that region. Shear vectors are also
somewhat perpendicular...but this complex could also evolve
quickly to a squall line depending on how soon the low level flow
veers, and push out of the forecast area by 9 pm. Stay tuned to
further updates.


.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 most unstable
CAPES 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 06Z TAFS through 06Z Tuesday night)
Issued at 1135 PM MDT Mon May 23 2016

VFR outside isolated to scattered showers and thunderstorms Tuesday
afternoon and evening, mainly along and east of the Laramie Range.
Expect breezy southerly surface winds with gusts up to 30 kt. Winds
will be variable and stronger invof thunderstorms. Convection will
taper off by 03Z southeast Wyoming, but linger through late
evening western Nebraska.


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.


Issued at 110 PM MDT Mon May 23 2016

Cool temperatures will persist through the week with morning lows
at or below freezing and afternoon highs in the mid 40s to mid 50s
for the mountains. This should slow the snowmelt into streams and
tributaries of the Upper North Platte and Laramie Rivers. Need to
be watching Thursday and Friday as a low pressure system tracks
east across Colorado. Latest forecast guidance does show fairly
widespread precipitation over 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...


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