FXUS65 KCYS 212031

Area Forecast Discussion
National Weather Service Cheyenne WY
231 PM MDT Mon Aug 21 2017

.SHORT TERM...(This evening through Wednesday night)
Issued at 230 PM MDT Mon Aug 21 2017

West-northwest flow aloft prevails this afternoon under the
influence of a broad mid-upper level northern CONUS trough. A
shortwave was pushing southeast across WY and western Dakotas.
A surface front extended from northeast NE into extreme northern
CO. A slightly drier and more stable air mass should preclude
convective development this afternoon, except for scattered CU
over the mountains. Temperatures dropped between 7 and 12 degrees
during the solar eclipse. Lowered high temperatures a couple of
degrees. It was breezy, especially over the plains with northerly
winds gusting from 20 to 25 mph.

The short range models and ensembles are in good agreement progging
eastward progression of the northern CONUS trough and arrival of
a mid/upper level ridge Wednesday. Generally dry conditions will
prevail through the next 24 to 36 hours with precipitable water
values between 0.50" and 0.75". The exception will be the
southeast WY mountains Tuesday afternoon/early evening where
isolated convection may erupt due to favorable orographic lift.
High temperatures Tuesday will be near seasonal normals with mid
70s to mid 80s. Breezy south-southeast winds will develop across
the plains Tuesday afternoon.

The flow aloft becomes more west-southwest Wednesday, which will
advect some monsoonal moisture north-northeast into WY and CO.
Isolated convection develops by midday over the southeast WY
mountains, increasing in coverage as far east as the I-25 and
I-80 corridor into the evening. 700mb temperatures rise to 12-14
degrees Celsius Wednesday afternoon, yielding highs in the 80s to
around 90 degrees for lower elevations.

.LONG TERM...(Thursday through Monday)
Issued at 230 PM MDT Mon Aug 21 2017

More like August weather for the long term as upper ridge overhead
and we get impacted by shortwaves moving through the flow along
the northern periphery of the ridge. What this does for us here is
southeast Wyoming and Nebraska Panhandle this time of year is
create gusty west to southwest winds and dry conditions possibly
leading to fire weather concerns.

Starting off Thursday, this looks to be the case as the ECMWF
shows a shortwave over southwest Montana move northeastward
through the ridge. Southwest winds across Carbon County with
15-20kts mixing down to the surface. Some differences in the EC
and GFS on precip coverage. EC much more bullish on PoPs over the
GFS. Will go with the EC solution right now with fairly widespread
slight chance PoPs across the CWFA.

Secondary low pressure system tracks across northern Montana
Friday on the ECMWF with 700mb winds increasing to 20-25kts. Could
be a fire weather day Friday with afternoon humidities falling
into the upper teens/low 20s. We will need to keep an eye on this
as the event draws closer.

Montana shortwave then drops southeast over the weekend into the
Dakotas. When this happens, we see a backdoor cold front moving
into the area from the northeast. ECMWF 850mb winds shift to the
northeast over the Panhandle Saturday, eventually shifting to the
southeast as the ridge begins to build back into the west. Weather
looks dry over the weekend with precip well east and south of the


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1040 AM MDT Mon Aug 21 2017

VFR conditions this afternoon/evening with sunny skies and gusty
north to northeast winds. No weather impacts to aviation assets in
the Panhandle and across southeast Wyoming. Broken cirrus ceilings
towards 00Z for the Panhandle as clouds over northeast Wyoming
move south.


Issued at 230 PM MDT Mon Aug 21 2017

A cool front will push south of the districts this afternoon.
Afternoon humidities will fall to around 15 percent across lower
elevations of Carbon County, however winds will be light. Across
the plains, minimum humidities will range from 25 to 40 percent
with breezy northerly winds through early evening. Dry and warm
conditions are forecast Tuesday with lowest humidities across
lower elevations of Carbon County. It will be breezy east of the
Laramie Range Tuesday afternoon with south-southeast winds gusting
to around 20 mph. Moisture will increase Wednesday as monsoonal
flow from the southwest returns to the districts, and persists
through the end of the week. Higher humidities and better chances
of showers and thunderstorms can be expected at that time.




Back to Main Forecast Discussion Page