Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 210943

Area Forecast Discussion
National Weather Service Cheyenne WY
343 AM MDT Fri Jul 21 2017

.SHORT TERM...(Today through Saturday night)
Issued at 343 AM MDT Fri Jul 21 2017

Showers and thunderstorms are once again the main forecast concern
for today as a cold front drops from north-to-south across the CWA
over the next few hours. All models bring this front south into CO
by 18z today, suggesting limited convective coverage for areas out
side of the higher elevations this afternoon/evening w/the arrival
of drier and more stable air. However, there is potential for this
front to stall near the Colorado border, which would keep the low-
level convergence in the vicinity of the I-80 corridor and support
shower/thunderstorm development a bit further north. A few earlier
HRRR runs were showing this, but that guidance has trended further
south. Meanwhile, the NAM has trended further north. That said, we
have a very low confidence PoP forecast today but believe the best
chances will be in the mountains and the I-80 corridor between LAR
and CYS. Limited instability and weak shear should preclude a risk
for strong/severe storms today.

Saturday is expected to be quite a bit cooler in the post-frontal
air mass w/ H7 temperatures falling to around +12 deg C. Isolated
afternoon/evening thunderstorms will be possible once again, but
overall chances appear limited as some of the best monsoonal
moisture becomes focused south and east of the CWA.

.LONG TERM...(Sunday through Thursday)
Issued at 343 AM MDT Fri Jul 21 2017

Sunday into Monday still looks mostly dry across the area with a
dry airmass in place on the northern side of the upper ridge,
except for possibly a few isolated thunderstorms in the afternoon
forming up near the mountains of southern WY. Next significant
chance of rainfall not all that impressive coming on Wednesday as
some model guidance wants to bring a stronger shortwave across
Wyoming potentially bringing up some deeper monsoonal moisture,
but there is some discrepancies in the longer range models with
some showing a greater coverage of diurnal convection with others
less so. For now will carry a chance of thunderstorms in the
afternoon and evening Wednesday along and east of the Laramie
Range, but confidence not all that high for appreciable amounts
of rainfall to put a significant dent in our continued mostly dry


.AVIATION...(For the 12Z TAFS through 12Z Saturday)
Issued at 343 AM MDT Fri Jul 21 2017

Lingering light patches of rain/showers gradually dissipating
across the area early this morning...mostly light rain falling out
of the leftover mid-deck from earlier convection. Expect VFR
through the morning with scattered showers and isolated
thunderstorms developing this afternoon around Rawlins, Laramie
and Cheyenne near a slowing cold frontal boundary that will settle
just south of the Colorado border. Expect lesser coverages today
across the Nebraska Panhandle, so no mention of thunderstorms in
the aviation forecast, but will mention mid to late afternoon
thunderstorms in the vicinity of Rawlins, Laramie and Cheyenne
where models are hinting on some convective buildups along and
just north of the frontal boundary.


Issued at 343 AM MDT Fri Jul 21 2017

Scattered thunderstorms are expected today over the mountains, and
areas along/south of I-80 from Laramie to Sidney. The main hazards
will be lightning and gusty/erratic surface winds. RH will fall to
near 15 percent over east central WY and the northern NE Panhandle
this afternoon as dry air works in behind a cold front. Otherwise,
expect RH values between 20 and 30 percent. Light winds are likely
to keep fire weather concerns very low through the weekend.




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