Area Forecast Discussion
Issued by NWS Cheyenne, WY

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000
FXUS65 KCYS 280611
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1211 AM MDT Fri Jul 28 2017

.UPDATE...
Issued at 948 PM MDT Thu Jul 27 2017

Issued a quick update to continue trends of weakening storms over
the eastern zones although some of them east of I-25 through late
evening could be strong with wind gusts to near 55 mph, frequent
lightning, small hail and brief but heavy rainfall. Also watching
a line of storms moving southeast out of northeastern Wyoming, but
current thinking is the southern extent of that will weaken a bit as
it moves across Niobrara county into the northern Panhandle...but
some convection will linger northeastern zones after midnight.
Question is how far west will the strongest convection fire tomorrow
afternoon? Current thinking is a bit farther east than what we saw
today, so coverage should be a bit less west and similar east, and
we will have to watch for strong to severe storms in our Panhandle
zones and possibly far eastern Wyoming zones tomorrow afternoon.

&&

.SHORT TERM...Tonight through Saturday...

The main concern through the afternoon/evening hours will continue
to be the potential for a few severe thunderstorms across portions
of southeast Wyoming, and possibly western Nebraska. Convection is
expected to increase along the Laramie Range through the afternoon
hours, within zone of moist low-level upslope and enhanced low-lvl
convergence in the vicinity of the higher terrain. We would expect
a few of these storms to spread east into the I-25/US-85 corridors
during the late afternoon/evening. More widespread development may
occur over east central WY as stronger dynamic support overspreads
the area with the passage of a more pronounced shortwave. CAPE has
increased to around 2000 J/kg over the SLGT Risk area per the most
recent (19z) SPC mesoanalysis. This, combined w/ bulk shear around
40 knots in the effective layer will likely support a few rotating
updrafts/supercells capable of primarily large hail. LCLs are also
quite low this afternoon with surface dew points in the 60s across
much of the high plains in the presence of considerable veering in
the lowest 2 to 3 km, so would not be surprised to see an isolated
tornado in this environment although the main limiting factor will
be a lack of speed shear in that same layer. PWAT values over 1.25
inches will support efficient rainfall production with storms this
afternoon/evening as well.

In general, a similar synoptic environment will exist on Friday w/
scattered afternoon/evening thunderstorms likely once again. There
will also be a few ongoing storms over northeastern zones early in
the day. Yesterday, it looked like Friday had better potential for
more widespread convection on the second day of return flow, but a
few changes as models have started to show more inhibition because
of warmer temperatures aloft during the day. However, some mid-lvl
cooling occurs after 00z which should aid in convective initiation
mainly for areas along I-80 east of Laramie, and areas east of US-
85. Strong to severe thunderstorms are a possibility once again as
convective parameters will be similar to those seen today. We will
remain on the northern fringe of upper-level ridging thru Saturday
with continued shots of monsoon moisture. That said, Saturday will
be quite active as well.

.LONG TERM...(Saturday night through Thursday)
Issued at 330 PM MDT Thu Jul 27 2017

Long range models in good agreement with the overall weather
pattern, showing the upper level high across the southern plains
gradually dissipating this weekend and rebuilding further west
near the eastern Great Basin region/four corners area by early
next week. Models show PW`s between 1 to 1.5 inches in addition to
upslope flow, which suggests moisture will be plentiful across
most of the forecast area through Sunday and early next week.
Models have trended a little further north with the shortwave
energy ejecting out of the Arizona/New Mexico Saturday and
Saturday night. This shortwave is expected to stall as the upper
level high rebuilds further west across the eastern Great Basin.
Will have to monitor this feature closely since this pattern
suggests very heavy rains and flash flooding for the Front Range.
Models are starting to show QPF amounts between 1 to 2 inches along
and east of the Laramie Range. Increased POP between 40 to 60
percent across the area for scattered to numerous showers and
thunderstorms Saturday night. Kept some POP for the evening and
nighttime hours with models showing organized convection sliding
southeast across the high plains.

Unsettled weather is expected to continue into early next week as
northwest flow aloft develops over the area. A more defined upper
level trough will move through the area Tuesday and Wednesday, and
may result in a more widespread light rain event. Otherwise, kept
temperatures near average for this time of the year with highs in
the upper 70`s to mid 80`s along and west of I-25, and near 90
across western Nebraska and far eastern Wyoming. Expect a slight
drying trend as we get into late Wednesday and Thursday as the
upper level high nudges eastward.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1211 AM MDT Fri Jul 28 2017

A line of thunderstorms will impact KCDR-KAIA through roughly 08Z
before moving to the east. May see a few showers redevelop
through the early morning hours over the Panhandle. Stratus may
fill in behind this activity due to continued moist upslope flow,
but confidence is not high at the moment at height of cigs nor
how widespread the cloud deck will be. Will amend as needed.
Generally light winds will prevail through Friday morning, and
will switch to the north through the day as a leeside pressure
trough develops and shifts to the east. Showers/storms look to
develop in the late afternoon evening across the panhandle ahead
of the trough.

&&

.FIRE WEATHER...
Issued at 139 PM MDT Thu Jul 27 2017

Fire weather concerns will be low over the next several days given
good chances for wetting rains, light winds, and minimum RH values
above 20 percent.

&&

.HYDROLOGY...
ISSUED AT 210 PM MDT FRI JUN 5 2015

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.
&&

$$

UPDATE...JG
SHORT TERM...SML
LONG TERM...ZF
AVIATION...RJM
FIRE WEATHER...SML
HYDROLOGY...ZF



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