Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 220009
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
609 PM MDT Sun May 21 2017

.SHORT TERM...(Late this afternoon through Tuesday night)
Issued at 315 PM MDT Sun May 21 2017

The early afternoon GOES water vapor animation showed our region
wedged between a vigorous upper low over the Upper Midwest and a
stout upper ridge off the Pacific coast. Cold cyclonic flow aloft
resulted in steep lapse rates and marginal CAPEs. Scattered rain
showers and isolated tstorms developed west of I-25 this morning,
and have spread east into western Nebraska shortly after midday.
Brief gusty winds up to 40 mph and moderate rainfall may accompany
the tstorms through late this afternoon. Temperatures were 5-10
degrees milder than 24 hours ago with mid 50s to mid 60s. Winds
outside the convection were variable to westerly 10-20 kt.

There is good agreement with the 12Z short range models progging
the cyclonic flow to persist through the period. The broad upper
low pinwheels over the Upper Midwest, while the upper ridge out
west deamplifies as shortwave energy approaches the Pacific
Northwest. One shortwave producing today/s convection exits to
the southeast this evening. A stronger shortwave will track south
from eastern Montana across eastern Wyoming and western Nebraska
Monday. The combination of shortwave energy aloft and lift from
a surface cold front will produce scattered showers and isolated
afternoon tstorms as CAPEs rise between 500 and 1000 j/kg. The
stronger convection may produce small hail and brief moderate-
heavy rainfall. Winds outside the convection will be breezy from
the northwest between 25 and 35 mph. High temperatures Monday will
be a tad cooler across the northern CWA with the fropa during the
morning.

Showers will end Monday night with the arrival of subsident air
aloft in the wake of the shortwave. Tuesday will be drier most
areas. The exception will be the Nebraska Panhandle where most of
the model guidance progs weak moisture convergence and instability
for isolated convection. The pressure gradient remains a bit tight
east of the Laramie Range, with gusty north-northwest winds from
late morning through the afternoon. High temperatures Tuesday will
moderate a few degrees from Monday, but remain below seasonal
normals.

.LONG TERM...(Wednesday through Sunday)
Issued at 300 PM MDT Sun May 21 2017

Upper ridge moves across Wednesday providing dry and warm conditions
across the CWA, likely the warmest day of the week. Cooler and unsettled
conditions seen beginning Thursday and continuing through the weekend
as the area will be influenced by a large, positively-tilted upper
trough that will extend generally from southern Canada WSW over the
northwest part of the country. Several weak shortwaves will move through
the trough and across the region bringing nearly daily chances for
showers and a few storms. Severe potential looks low with no large
instability expected and temperatures a little cooler than average
overall.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 557 PM MDT Sun May 21 2017

Scattered shower and isolated thunderstorm activity will continue
through the early evening hours as a weak shortwave passes. May be
some brief MVFR ceilings and/or visibility with any showers.
Otherwise, VFR will prevail. This activity will subside through
the evening as we lose daytime heating and lift from the
shortwave. Monday will feature a strong cold frontal passage in
the afternoon. Shower and thunderstorm activity will be more
numerous especially along and east of the Laramie Range. Gusty
north winds will accompany the front and freezing levels will be
quite low so may see some pellets or small hail in a few cells
especially for KLAR, KCYS and KSNY. Ceilings will be dropping to
MVFR as the front moves in and scattered to numerous showers
develop.

&&

.FIRE WEATHER...
Issued at 225 PM MDT Sun May 21 2017

Fire weather concerns will be negligible, with showers early and later
in the week. Lingering snowpack from last week`s storm will continue
to melt as temperatures moderate closer to seasonal normals by mid-
week. Fuels remain moist and in greenup.

&&

.HYDROLOGY...
Issued at 225 PM MDT Sun May 21 2017

Concerns through the middle of this week will focus on runoff from
the recent heavy precipitation into area creeks and streams east of
the Laramie Range. Although flooding is not anticipated, many of the
creeks and streams may reach bankfull by the middle of the week.
Fortunately, cooler temperatures over higher elevations will continue
this week, which should slow the snow melt and resultant runoff into
the mainstem rivers. Will continue to monitor these water levels
closely over the next few weeks.

&&

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

$$

SHORT TERM...MAJ
LONG TERM...RE
AVIATION...DEL
FIRE WEATHER...MAJ
HYDROLOGY...MAJ


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