Area Forecast Discussion
Issued by NWS Cheyenne, WY

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624
FXUS65 KCYS 240346
AFDCYS

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

.UPDATE...
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
afternoon.

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 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 00Z TAFS through Tuesday afternoon)

Area of thunderstorms which developed over SE Wyoming have moved
into southern Nebraska this evening. These showers and Tstorms will
linger until 02z and then dissipate or slide to the east into
central Nebraska. Kept mention of VCTS and gusty convective winds
over the next few hours...and then clearing skies overnight with
light and variable winds.  Another round of thunderstorms is
expected Tuesday afternoon. These thunderstorms may be stronger and
include large hail and strong winds through Tuesday evening...mainly
east of the Laramie Range.


&&

.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.
&&

$$

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



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