Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 220552

Area Forecast Discussion
National Weather Service Cheyenne WY
1152 PM MDT SAT MAY 21 2016

Issued at 931 PM MDT Sat May 21 2016

For the mid evening update, we dropped thunder mention across the
southern zones and kept a chance going Converse into Niobrara
counties...but even there the risk for a thunderstorm is
diminishing rapidly. We have some fog and likely drizzle across
the southern half of the western Nebraska Panhandle this evening.
Gusty winds over there to 35 mph makes me think it may be more
very light drizzle versus fog. Sidney has gone down periodically
to 1/4 mile when the winds drop below 25 mph. Kept areas of fog in
the forecast over in the southern half of the Panhandle for the
rest of tonight and mentioned patchy fog around Cheyenne after
midnight. Winds at Cheyenne projected to turn more southwesterly
after midnight...which would be a downslope flow for them and not
conducive to fog although some drizzle with the passing front a
possibility towards morning. of forecast

UPDATE Issued at 639 PM MDT Sat May 21 2016

Mesoscale Update: Fair amount of speed shear has essentially been
tearing apart any significant towers that could get going over SE
Wyoming through the course of the mid to late afternoon.
We are currently watching the best tower we have seen all day
go up west of Glendo, WY...but it appears on radar and visible
satellite imagery to be spreading out...and spotter reports in
the area indicated some significant tilting of the tower as it
started to break. This is one of those conditional setups that IF
we get significant development some of our parameters that we are
watching would suggest rapid development into a significant
supercell. That so far does not appear to be happening from
everything we see. So...we will continue to watch it a bit
further for severe potential along the I-25 corridor north of
Cheyenne. From Cheyenne eastward the Convective Inhibition values
have increased over the past 3 hours a bit more significantly with
modest increases north along I-25. The potential is still there
for severe storms into the mid evening hours, but he potential
will decrease as we get past sunset.


.SHORT TERM...(This evening through Sunday)
Issued at 227 PM MDT Sat May 21 2016

Strong to severe thunderstorms are still expected this afternoon and
evening, mainly across the high plains of southeast Wyoming into the
far western Nebraska panhandle. Storm coverage is still the question
mark as the degree of capping is not clear, but any development will
have a good chance of being severe. Good surface heating w/ temps in
the 70s and steep mid-level lapse rates in excess of 8 deg C/km will
continue to support MLCAPES to 2000 J/kg along the I25 corridor. SPC
Mesoscale Analysis shows 35-45 knots of effective bulk shear, mainly
due to considerable veering in the lowest 3 km. The mesoscale
environment is favorable for supercells with large hail/damaging
winds. Llvl jet is progged to ramp up after 00z with the GFS
showing H8-H85 flow in the neighborhood of 40-50 knots. The models
show this resulting in 0-1km helicity values between 200-400
m2/s2, so an isolated tornado or two is definitely not out of the
question as long as storms are surface- based. LCLs are not ideal,
but are low enough around 1500 m.

Severe weather threat should end around 03z or shortly after. We may
have some fog from Cheyenne-Torrington and points southeast as moist
southeast flow keeps the boundary layer saturated. Surface winds may
stay high enough to limit fog development, but there should be a bit
of a window between 06z-12z where we should see at least patchy fog/
drizzle. Strong upper low will sweep over the CWA Sunday along w/the
surface fropa. Slightly cooler temperatures and drier low-levels are
expected to limit instability enough so that severe weather will not
be a major concern; however, with a 40-50 kt 500 hpa jet overhead, a
stronger storm is definitely possible. Still, we anticipate isolated
to widely scattered convection across the CWA in the right rear quad
of a 100+ kt H25 jet.

.LONG TERM...(Sunday night through Saturday)
Issued at 227 PM MDT Sat May 21 2016

Cooler...but active weather pattern in the extended with
rain/thunder chances just about everyday.

Starting off Sunday night...upper shortwave moves east of the CWFA
after 00Z with showers ending in the Panhandle shortly after 03Z.
700MB temperatures drop down close to freezing sunday night into

Next chance for convection looks to be Monday afternoon/evening
with the approach of our next trough tracking into Utah. This
should mainly be confined to the mountains and could fall as snow
given the cold 700mb temperatures.

Do not see a warming trend in 700mb temperatures until Tuesday
afternoon and evening when the GFS forecast +4C over southeast

Next significant rainfall appears to be Thursday as both the GFS
and ECMWF track a fairly strong shortwave through central Colorado
that tracks into western Nebraska as a closed low.


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1145 PM MDT Sat May 21 2016

Latest HRRR model guidance indicates IFR cigs at CYS improving to
VFR after 09Z as winds shift to west-northwest. Could see a brief
periods of MVFR cigs at RWL between 09Z and 13Z. IFR cigs will
likely persist through 13Z for most of the western NE Panhandle
along with gusty southeasterly winds of 25-35 kt.


Issued at 227 PM MDT Sat May 21 2016

No fire weather concerns through the weekend w/green fuels and winds
and humidities remaining outside of critical thresholds. Showers and
thunderstorms will occur this afternoon/evening, and again on Sunday
afternoon. A few may be severe along/east of the Laramie Range today
with large hail/damaging winds. Breezy conditions will continue over
Carbon county late this afternoon with gusts of 35 to 45 MPH and rhs
around 20 percent. A cold front will sweep across the region Sunday,
bringing cooler temperatures. Near average temperatures and isolated
to widely scattered showers/thunderstorms are expected next week.


Issued at 227 PM MDT Sat May 21 2016

Rivers responding to recent runoff and should see the Upper North
Platte River at Saratoga rise to above minor flood stage Sunday
night and Sinclair by noon Monday. Cooler temperatures on the
horizon that will slow the rate of mountain snowmelt. A cold front
will move through southeast Wyoming early Sunday with freezing
overnight temperatures and highs only in the 40s for the mountains
through mid week.


.CYS Watches/Warnings/Advisories...


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