Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 220348
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
948 PM MDT Sat Apr 21 2018

.UPDATE...
Issued at 941 PM MDT Sat Apr 21 2018

We are sending out a quick update to bring the fog in a little
earlier to areas west of the Laramie Range. Latest webcams are
showing some dense fog developing along I-80 between Cheyenne and
Laramie. However, the latest HRRR guidance does not show this fog
sticking around very long. At this point, we are not real
confident on how widespread the fog will be tonight. The valley
fog along the North Platte River still looks like a good bet, due
to upvalley flow developing in that area. The rest of the Nebraska
panhandle and parts of southeast Wyoming may end up being very
patchy, but dense in a few spots.


&&

.SHORT TERM...(This evening through Monday night)
Issued at 230 PM MDT Sat Apr 21 2018

Forecast challenges deal with fog tonight into Sunday along with
rain chances returning Monday afternoon.

Currently...Upper low on water vapor imagery lays across western
Kansas this afternoon. Warp around moisture to the north and
northwest of this low brought some pretty good snow here at
Cheyenne this morning, but is quickly decreasing this afternoon.
IR imagery showing warming cloud tops across the Panhandle, so
would expect precip to come to an end later on this afternoon. Our
next system seen on water vapor imagery off the coast of northern
California at 1PM, still a couple days away.

For tonight into Sunday...A lot of low level moisture in place as
parts of the southern Panhandle received close to 3/4ths of an
inch of moisture and Cheyenne received close to a half inch. Soils
are wet. Low level winds are pretty light on forecast soundings in
the Panhandle and possibly here at Cheyenne, though GFS low level
wind directions not particularly favorable. Do have a low level
inversion setting up on forecast soundings, along with the
saturated soils. Can`t rule out fog tonight and introduced into
the forecast across much of the Panhandle, up the Platte River
valley to Douglas tonight. Here at Cheyenne, went with patchy fog.
HRRR guidance showing lower conditions developing overnight as
well, so confidence is moderate on this happening.

Next frontal system on track to begin moving into the area Monday
afternoon/evening. Precip becomes pretty widespread Monday evening
and have fairly high PoPs in the forecast for that time. Rain
changing to snow late evening into Tuesday morning as 700mb
temperatures fall to -7C across Converse/Niobrara Counties, -4C
here in Laramie County and -2C over the southern Panhandle.

Two last areas of concern are thunderstorms ahead of the front
Monday afternoon and wind behind the front. SPC has a General
Thunder area identified over southeast Wyoming and southern
Panhandle Monday afternoon. Lifted Indices down to -2C Monday
afternoon across the Panhandle with afternoon CAPE around 800
J/KG. Can`t rule it out and will add isolated Thunder to the
forecast Monday afternoon/evening ahead of the front.

Behind the front, GFS showing 850mb winds from the northwest at
35-40kts across the Panhandle and eastern Plains of southeast
Wyoming. Did increase winds from forecast guidance.

.LONG TERM...(Tuesday through Saturday)
Issued at 230 PM MDT Sat Apr 21 2018

Tuesday...Shortwave trough moves southward from northern Wyoming in
the morning to northern Colorado in the late afternoon. Expect
isolated to scattered morning showers associated with the shortwave
over our northeast half, and over the Nebraska panhandle in the
afternoon. Considerably cooler compared to Monday with cloud cover
and in the wake of the cold frontal passage.

Wednesday...Decent warming trend with a flattening ridge aloft and
with the flow aloft becoming northwest.

Thursday...Continued mostly dry, and cooler in the wake of a passing
cold front.

Friday...Dry northwest flow aloft prevails, with a warming trend as
700 mb temperatures rise to near 7 Celsius, yielding maximum
temperatures from the upper 50s to upper 60s at lower elevations.

Saturday...Continued mild as the ridge axis aloft moves overhead.
Isolated showers possible as some low and mid level moisture streams
northward from Colorado, though most locations will remain dry.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 551 PM MDT Sat Apr 21 2018

Main aviation headline overnight will be the lingering MVFR
conditions for CYS and western NE TAF sites along with an IFR to
patchy LIFR visibility drop 08-15Z due to fog development. HRRR
and NAM model guidance suggests pockets of dense fog being
possible, especially in the vicinity of the North Platte River
Valley early Sunday morning. Recent snow and rain has saturated
the near-surface layer and as temperatures cool overnight with
low winds across the high-plains, fog development appears likely.
Lower confidence in fog development for LAR/RWL as greater wind
mixing should reduce fog potential.

VFR conditions are expected after 14-17Z across all TAF sites
Sunday with general northwest winds at 10 to 15 knots for the
mountains and southeast winds across the NE Plains by the
afternoon.


&&

.FIRE WEATHER...
Issued at 230 PM MDT Sat Apr 21 2018

Minimal fire weather concerns as recent rain and snow has the
ground pretty much saturated across a large portion of the
forecast area. Rainfall amounts in the Panhandle range from a few
hundredths over northern portion with close to an inch south.
Across southeast Wyoming, our souther FWZs have seen close to a
half inch in FWZ 310 to well over a tenths west. Next wetting rain
event taking shape for Monday as another cold front moves into
the area from the northwest, spreading rain and mountain snow
towards the southeast through Monday night/Tuesday morning. No
hazardous fire weather conditions expected.

&&

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

$$

UPDATE...REC
SHORT TERM...GCC
LONG TERM...RUBIN
AVIATION...JSA
FIRE WEATHER...GCC



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