Area Forecast Discussion
Issued by NWS Cheyenne, WY

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

FXUS65 KCYS 212115

Area Forecast Discussion
National Weather Service Cheyenne WY
315 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

Showers will end Monday night with the arrival subsident air aloft
in the wake of the shortwave. Tuesday will be drier most areas.
The exception will be the Nebraska Panhandle where weak moisture
convergence and instability may trigger isolated convection, mainly
from Alliance and Sidney eastward. 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 still 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


.AVIATION...(For the 18Z TAFS through 18Z Monday morning)
Issued at 1127 AM MDT Sun May 21 2017

A passing disturbance will bring some showers and a few tstms to the
area mainly this afternoon with clearing conditions this evening. VFR
expected to prevail though some patchy MVFR cigs around KRWL early
this afternoon.


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.


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.




HYDROLOGY...MAJ is the U.S. government's official web portal to all federal, state and local government web resources and services.