Area Forecast Discussion
Issued by NWS Cheyenne, WY

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117
FXUS65 KCYS 230908
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
308 AM MDT Sat Jun 23 2018

.SHORT TERM...(Today through Sunday night)
Issued at 207 AM MDT Sat Jun 23 2018

Water vapor Imagery showed a compact upper low moving over
northern Idaho and Montana this morning with another low over the
Great lakes. Observed Radiosondes showed westerly mid level flow
across Wyoming and Nebraska at 45 to 50 kts and increasing
further wets near the base of the upper low. Model guidance shows
the center of the upper low and vort max moving slow to the east
northeast through the the day Saturday. A lead shortwave trough
will move through Wyoming and Nebraska this afternoon and evening.
At the surface, an approaching cold front will slowly move
through western Wyoming. Southeasterly low level flow will advect
increasing moisture across the plains helping to fuel showers and
thunderstorms. Warm advection will be relatively weak, limiting
daytime high temperatures to about 5 degrees below normal. Buffer
soundings show moderately steep mid level lapse rates
contributing to increasing instability with 500 j/kg of MLCAPE
from the Wyoming foot hills east into the high plains. The
combination of CAPE and vertical shear of about 35 kts will lead
to isolated organized thunderstorms capable of severe weather.
Current SWODY1 outlook shows a large Marginal risk over eastern
Wyoming and western Nebraska. This matches well with the current
line of thinking that storm coverage will be small and severe
potential relatively low. The greater threat as it stands now will
be the potential for heavy rain through Sunday. QPF values over
an inch across Converse and Niobrara counties suggest a flooding
threat. A Flood Watch was issued from 00z Sunday to 06z Monday to
account for the threat of isolated flooding.

.LONG TERM...(Monday through Friday)
Issued at 305 AM MDT Sat Jun 23 2018

Medium to Long Range models in reasonable agreement for the early
to middle part of the week, but then solutions begin to diverge a
bit after Wednesday morning. The slow moving upper level low,
which will impact the weather this weekend, will still be located
just east of eastern Wyoming on Monday, and gradually drifting
northeast through the day. Can not rule out a continuation of rain
showers and isolated thunder east of I-25 and possible the
mountains due to unstable conditions and ample boundary layer
moisture. Some dry air aloft will attempt to move into the area
but still expect some isolated activity further west into Albany
and possibly Carbon county Monday afternoon. Once the upper level
low exits and becomes an open wave as it approaches the Great
Lakes region, the upper level flow then transition to a more zonal
pattern for Tuesday and Wednesday with the jet stream retreating
northward into Montana and the Dakotas. However, model differences
start to appear around Wednesday with the GFS showing a cool
front moving across the high plains late Tuesday night and into
Wednesday morning, stalling along the Laramie Range. This should
be enough convergence to at least initiate isolated thunderstorms
as subsidence aloft is not too strong. However, other models do
not show this front progressing across the area until later on
Wednesday and it is much weaker. Added a 15 to 25 percent chance
of thunderstorms along and just east of the Laramie Range even
though consensus models do not show much activity. Should be a
more favorable environment for thunderstorms on Thursday as the
front, or whats left of it, progresses northeast through the day.
Low and upper level forcing appears slightly more favorable,
especially further east. Kept forecast dry on Friday as a dry-line
like boundary sets up over southeast Wyoming and progresses
eastward into the western Nebraska panhandle through the day,
which will limit thunderstorm development. The ECMWF solution is
the outlier here, and shows a similar setup to the GFS on
Wednesday, with a cold front stalled across the Laramie Range.
Kept POP below 10 percent for now with above normal temperatures.

&&

.AVIATION...(For the 12Z TAFS through tonight)

Mainly VFR conditions over the next 18 to 24 hours. Another round
of showers and thunderstorms are expected after 18z today through
this afternoon and this evening. This convection should be pretty
widespread north of I-80, and more scattered along and south of
I-80. Will continue VCTS for most location, earlier in the
afternoon near KLAR and KCYS, and later in the afternoon/evening
for the western Nebraska terminals.

&&

.FIRE WEATHER...
Issued at 207 AM MDT Sat Jun 23 2018

Fire weather concerns are low for the period as showers and
thunderstorms are expected this weekend. Wetting rain potential
will increase tonight through sunday. High relative Humidity and
low winds will keep fire weather quiet.

&&

.HYDROLOGY...
Issued at 207 AM MDT Sat Jun 23 2018

Widespread showers and thunderstorms will develop this afternoon
and evening across southeastern Wyoming and western Nebraska. 700
mb mixing ratios over 7 g/kg will be more than adequate for heavy
rain with the stronger cells. Most models showed accumulations
over an inch through Sunday with several rounds of thunderstorms.
Although motion will not be particularly slow, training storms
and already saturated ground may cause possible flooding across
the watch area.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...

WY...Flood Watch from this evening through Sunday evening for
     WYZ101>103.

NE...None.
&&

$$

SHORT TERM...ADL
LONG TERM...TJT
AVIATION...TJT
FIRE WEATHER...ADL
HYDROLOGY...ADL



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