FXUS65 KCYS 171751

Area Forecast Discussion
National Weather Service Cheyenne WY
1151 AM MDT Tue Jul 17 2018

.SHORT TERM...(Today and tonight)
Issued at 344 AM MDT Tue Jul 17 2018

Today...With mostly clear skies and temperature/dewpoints spreads
quite narrow, as well as the rainfall last night, we expect areas of
fog to form through the early dawn hours, especially east of
Interstate 25 and over the Nebraska Panhandle. Fog expected to be
more patchy over our far western counties.

Expecting another repeat performance of convective initiation and
development today with thunderstorms firing up over the southeast
Wyoming mountains by early afternoon and then spreading east and
increasing in coverage by late afternoon across the rest of
southeast Wyoming as a shortwave trough aloft draws near. Agree with
the Storm Prediction Center marginal outlook based on projected
shear and instability parameters.

Tonight...Widespread rainfall amounts of 1 to 2 inches fell last
evening over the Nebraska Panhandle with localized amounts of 2 to 5
inches in some spots. With another round of scattered to numerous
thunderstorms anticipated this evening, and possibly late tonight,
we have issued a Flash Flood Watch for the Panhandle for tonight in
coordination with our neighboring offices. The NAM and GFS are both
suggesting another possible MCS (Mesoscale Convective System)
developing this evening over far eastern Wyoming or southwest South
Dakota, and propagating southeast into western Nebraska overnight as
the moist southerly low level flow helps fuel the thunderstorm
complex. Some strong to severe thunderstorms likely again in the
evening based on expected shear and instability parameters. Showers
and thunderstorms will likely end again by late evening or in the
late night hours.

Wednesday...Expect a totally different weather regime to set up with
northwest flow aloft and significant drying noted at the low and mid
levels, producing a warmer day with no thunderstorms anticipated.

Wednesday night...Clear and mild overnight with northwest flow aloft.

Thursday...Warming trend continues as the airmass dries and
moderates with 700 mb temperatures warming. Airmass too dry to
support any shower or thunderstorm development.

.LONG TERM...(Thursday night through next Monday)
Issued at 340 AM MDT Tue Jul 17 2018

Not many changes made to previous medium-long range forecast as a
strong upper level ridge axis will dominate the weather pattern
across southeast Wyoming and western Nebraska. Models remain in
good agreement showing weak northwest flow aloft late this week
and into early next weekend. Main upper level high should remain
stationary near the four corners region through the period, with
little variability in strength and position. Therefore, mainly
expect a dry forecast late Thursday through Saturday across the
area. Believe a few thunderstorms are possible across the eastern
plains late Friday as a weak frontal boundary brushes northern
Nebraska, but confidence is too low to mention at this time. Very
warm temperatures are expected late in the week and Saturday, with
Saturday likely being the warmest day as the ridge axis slides
east of the region. High temperatures in the upper 80s to upper
90s are expected Saturday afternoon with thunderstorms chances
increasing in the evening.

Models have trended slower with the cold front on Sunday, showing
the front and associated upper level trough pushing into the
region on Monday. Showers and thunderstorms still look like a good
bet on Sunday though due to moist upslope flow and slightly cooler
temperatures aloft. Slowly lowered temperatures into early next
week back to near normal.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1141 AM MDT Tue Jul 17 2018

The main concern with this forecast package is the thunderstorm
chances again this afternoon and evening.

Latest water vapor loop was showing a shortwave tracking through
northern Wyoming. This feature was tracking east and should cause
some thunderstorms to flare across northern Wyoming this
afternoon. There is also enough moisture and upslope flow to
trigger some isolated convection along the southern Laramie Range.
Therefore, we went ahead and inserted some VCTS in the LAR/CYS TAF
sites early this afternoon. Not as confident during the early
evening at this point, but we will continue to monitor through the
afternoon. Further north in KCDR and KAIA thunderstorm chances are
looking fairly good this evening as an area of thunderstorms
should blossom northeast Wyoming and track east-southeast towards
the KCDR and KAIA taf sites between 04-06z. Meanwhile, KSNY and
KBFF may be on the southern edge of this complex of storms.


Issued at 303 AM MDT Tue Jul 17 2018

Concerns will increase Wednesday and Thursday as
minimum afternoon humidities range from 10 to 20 percent west of a
Douglas to Laramie line on Wednesday and similar percentages west of
Interstate 25 on Thursday, along with gusty winds approaching
critical thresholds.


Issued at 303 AM MDT Tue Jul 17 2018

Flash flood concerns increase tonight as a Flash Flood Watch has
been issued for the Nebraska Panhandle for tonight. After
receiving widespread rainfall amounts of 1 to 2 inches over the
Panhandle Monday evening, with localized amounts of 2 to 5 inches,
more scattered to numerous thunderstorms are expected this
evening and overnight, increasing the likelihood of flash flooding
from some thunderstorms. The heaviest rainfall amounts fell
Monday evening over southern Dawes and northern Box Butte Counties
as well as over portions of Scotts Bluff County.


NE...Flash Flood Watch from 6 PM MDT this evening through late
tonight for NEZ002-003-019>021-054-055-095-096.



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