Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 262152
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
352 PM MDT SUN JUN 26 2016

.SHORT TERM...(This evening through Tuesday night)
Issued at 237 PM MDT Sun Jun 26 2016

Quasi-zonal flow aloft remains in place across the area today, along
with numerous disturbances traversing the flow. A few showers / weak
thunderstorms are possible through early evening with modest low-lvl
convergence along the I80 corridor between Cheyenne/Sidney. Marginal
instability with SBCAPE around 1000 J/kg with effective shear in the
25-35 knot range could support organized/rotating updrafts with hail
production, but the overall threat is low. Gusty winds should be the
main hazard with inverted-v soundings and LCLs near 15k feet. Do not
have much confidence in convective development today as the HRRR has
shown little or no activity with weak forcing. Frontal passage later
tonight allows winds to shift to the southeast across the western NE
Panhandle, resulting in increasing boundary layer moisture. Some fog
cannot be ruled out between 06z-15z Monday, mainly in the Platte Riv
valley with light up-valley winds.

Increasing potential for severe thunderstorms on Monday. GFS/NAM/ECM
are all in excellent agreement with general ENE flow in the low-lvls
through much of the day, resulting in strong moisture advection. The
GFS continues to show considerable mixing of the dryline with dewpts
at Cheyenne only in the upper 40s, while the NAM shows dewpts in the
low 60s. Believe the middle-ground is the best option, as it is hard
to believe there will be that much mixing in the anticipated upslope
scenario. As such, expect CAPES between 1500 and 3000 J/kg with very
strong 0-6 km shear in the 40-50 knot range. Low-level veering could
be strong too, resulting in 0-1 km EHI around 2 south of I-80 mainly
in the Panhandle. If the NAM has a better handle on moisture, BUFKIT
soundings show LCLs falling to 1500-3000 feet by early evening. Some
potential for a couple of tornadoes in this scenario. Tuesday should
be the more active day with models showing a vigorous mid-level wave
diving southeast along the edge of the ridge. It appears instability
and shear parameters will be even stronger with better forcing, so a
higher-end severe day is possible. Pattern recognition and model QPF
signals would suggest the best chance over east central Wyoming into
the Nebraska Panhandle, with a possible severe MCS moving rapidly to
the southeast during the evening. SPC Convective Outlooks for Day 2/
3 both highlight a SLGT Risk for the high plains.

.LONG TERM...(Wednesday through Sunday)
Issued at 237 PM MDT Sun Jun 26 2016

The main concern with this portion of the forecast is the
potential of convection through the remainder of the week.

Wednesday-Friday:
Upper level pattern is progged to be fairly stagnant through the
week with an omega block centered over Central Canada. This omega
blow is sandwiched between the upper level trof across the
British Columbia coastline and a upper level trof in Eastern
Canada. This will set the stage for an active weather pattern in
the Rockies through the remainder of the week, due to a northwest
flow regime. Moisture profiles are showing precipitable water
values approaching the 90th percentile for this time of year which
is quite a bit above normal. Therefore, we may see pretty good
chances of thunderstorms through the week. The low pressure trof
is also progged to remain mainly west of the Continental Divide
which should favor easterly surface flow and yield to higher
dewpoints in the 50s and 60s along and east of the Laramie Range.
Not only are we looking at higher dewpoints, but the effective
wind shear values may range from 40 to 50kts which is a good setup
for severe weather chances through Friday.

Saturday-Sunday:
The Ecmwf/GFS are showing the pattern trying to break down as the
upper level trof tries to undercut the omega block. If this
occurs, we may see the surface trof track further east into
Eastern Wyoming which may favor the convection developing mainly
in the Nebraska Panhandle on Saturday with even the potential and
drier and warmer conditions on Sunday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1142 AM MDT Sun Jun 26 2016

Latest Local WRF was indicating the potential for a few isolated
thunderstorms this afternoon/evening mainly around the CYS/SNY
after 21z. Any storms that develop will move east/southeast around
20kts and should exit the area around sunset. Otherwise, only
expecting some high based cumulus this afternoon at heights 8-10kft.
A weak boundary is expected to pass through the area tomorrow


&&

.FIRE WEATHER...
Issued at 237 PM MDT Sun Jun 26 2016

Warm and dry conditions continue along and west of the Laramie Range
but overall fire weather concerns will be minimal. Relative humidity
values will fall to 10 to 20 pct each afternoon, but winds should be
light. There will be increasing chances for thunderstorms across the
area on Monday and Tuesday, especially over the Plains.

&&

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

$$

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


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