Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 190642 AAA
AFDCYS

Area Forecast Discussion...UPDATED
National Weather Service Cheyenne WY
1242 AM MDT Sat May 19 2018

.UPDATE...
Issued at 910 PM MDT Fri May 18 2018

WSR-88D reflectivity from Cheyenne and nearby radars indicates
widespread showers and isolated thunderstorms across southeast
Wyoming along and east of a Douglas to Cheyenne line and over the
northern Nebraska Panhandle. The 00Z NAM progs the upper trough
over northern Colorado to move slowly eastward to far northeast
Colorado by daybreak with a cold front plunging southward across
our counties as winds turn to the north and increase in speed. NAM
QPF indicates the bulk of precipitation falling overnight across
our northern counties of Converse and Niobrara and also over our
southern Nebraska Panhandle counties with the bulk of QPF due to
the mid level trough moving across northeast Colorado. Have
therefore adjusted rain chances, POPs for the rest of the
overnight hours. Our Flood Watch is in decent shape and no changes
are needed at this time.

&&

.SHORT TERM...(This evening through Saturday)
Issued at 202 PM MDT Fri May 18 2018

In the very near term, we will need to be on the lookout for a few
stronger storms initially developing across the higher terrain
west of I-25 where steeper midlevel lapse rates could result in
some larger hail and strong wind gusts. This potential for
stronger storms will edge east onto the plains through the late
afternoon and early evening hours, but overall threat not as high
as what we saw yesterday.

A large, negative tilt upper trough will continue to push across
the central Rockies through Saturday with a deepening upper low
still projected over northeastern Colorado late tonight through
Saturday. The NAM, GFS and ECMWF have the band of heaviest
rainfall on the north side of the upper low a bit farther west
along I-25 late tonight into Saturday, while the latest HRRR
continues to advertise the heaviest rainfall axis to be across the
southern Nebraska Panhandle into eastern Colorado. With the
uncertainty in play as to exactly where the heaviest precip axis
will set up, we decided to extend the watch farther west to
highlight the potential threat for areas along and east of the
south Laramie Range. As for other hazards, we continue to
advertise 8 to 12 inches of new snowfall tonight above 9000 ft
elevation on the west facing slopes of the Snowy Range and will
continue to Winter Weather Advisory that area tonight through
Saturday.

.LONG TERM...(Saturday night through Friday)
Issued at 202 PM MDT Fri May 18 2018

As the upper low deepens, it will stall late Saturday along the
Colorado, Nebraska border then will make slow progress
northeastward into the northern Plains through late in the
weekend. We start ti taper PoPs down Saturday evening keeping
chance PoPs going over the Nebraska Panhandle, then we see a
drying trend Sunday into Monday. Another slow moving upper low
deepens over the desert southwest by Tuesday with several
shortwaves ejecting northeastward into our area mid to late week,
bringing increased chances of showers and thunderstorms starting
Tuesday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1226 AM MDT Sat May 19 2018

difficult flying conditions are ongoing across southeast Wyoming
and the Nebraska Panhandle this evening as TSRA/SHRA and low
ceilings are present over many portions of the area. TSRA/SHRA
activity is west of CYS and into the NE Panhandle where IFR and
LIFR conditions are being observed due to low visibilities and
ceilings with the +RA/TS. This activity will persist through the
next 6 hours with a slow precipitation shift to the west. MVFR to
IFR ceilings will persist through the day Saturday as north to
northeast winds increase upwards of 20 to 30 mph. Higher MVFR to
VFR ceilings will be likely for LAR/RWL. Precipitation will linger
through Saturday night and then tapper off into Sunday morning.

&&

.FIRE WEATHER...
Issued at 202 PM MDT Fri May 18 2018

Driest areas this afternoon where green-up has been delayed a bit would
be around zone 306, however winds and RH values remain outside
critical thresholds for Red Flag Warnings. Fairly wet pattern
expected over the next 24 hours with numerous showers and isolated
thunderstorms moving across the districts...with high elevation
snow expected for the Snowy Range and parts of the Sierra Madre
Range this weekend. Drier conditions expected for Sunday into
Monday, however increased chances for showers and thunderstorms
return by Wednesday.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Weather Advisory until midnight MDT tonight for WYZ114.

     Flood Watch through this afternoon for WYZ106>108-116>119.

NE...Flood Watch through this afternoon for NEZ019>021-054-055.

&&

$$

UPDATE...Rubin
SHORT TERM...JG
LONG TERM...JG
AVIATION...JSA
FIRE WEATHER...JG



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