Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 201717
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1117 AM MDT Sun May 20 2018

.SHORT TERM...(Today and tonight)
Issued at 227 AM MDT Sun May 20 2018

Main headline this morning is the ongoing patchy to areas of dense
fog in Laramie County and higher elevations into the foothills
and summit of the Laramie Range along I-80. Once the fog mixes out
later this morning, a warming trend will kick off today and set
the stage for a warmer week ahead.

Weak upslope saturated low-level southeast flow across the High
Plains is aiding in low clouds and fog development along the I-80
corridor. As I-80 ascends into the Laramie Range the low clouds
become dense fog and caution is urged when traveling in these
areas as visibilities are reduced to less than one quarter mile.
Dense fog will mix out through the mid morning hours and more sun
is expected across the area this afternoon vs. days past.

Looking at the larger synoptic picture, the closed H5 low that
plagued the region with ample rain and cooler conditions the past
few days has shifted to the North-Central Plains with weak H5
shortwave ridging noted across western WY and UT. Scattered to
isolated mountain showers and thunderstorms will be probable out
west across the higher terrain but the High Plains will remain
dry.

Another round of ground fog will again be possible to likely
Monday morning near Cheyenne and fog prone areas into the Laramie
foothills and summit per BUFR soundings. Some fog could again
become locally dense.

.LONG TERM...(Monday through Saturday)
Issued at 227 AM MDT Sun May 20 2018

Main highlight in the long term will be the warming temperatures
through the period and afternoon shower and thunderstorm chances.
The more significant coverage and intensity day of storms could be
Wednesday for far east WY and moreso the NE Panhandle.

Weak shortwave ridging will be across SE WY and the Front Range
Monday but some weak perturbations in the flow rounding the ridge
axis and moderate instability could allow for one or two isolated
showers/storms to develop during peak heating in the afternoon.
The best environment will be across the NE Panhandle where deeper
moisture will exist but stronger capping should preclude greater
storm coverage Monday afternoon. By Tuesday, the H5 ridge axis
will shift east in to the Central Plains as a large western CONUS
closed low pivots into the Great Basin. Greater southwest flow
aloft for western areas in the mountains should allow for greater
spatial shower and storm coverage but instability and shear
combination looks to marginal at best. Going into mid-week on
Wednesday, the closed H5 low over the great basin will open up and
shift northeast across the Rockies. Stronger H3 diffluent flow
aloft along and instability over the High Plains (especially more
so NE Panhandle) should support stronger thunderstorm
development. A negating factor is wind shear as it continues to
look meager in the 25-35 knot range. Strong pulse to semi-
organized storms could be possible Wednesday afternoon into
evening.

By late week, the base of the transiting trough axis will shift
north of the region. Stronger west winds will develop and shunt
deeper moisture farther east into the Central Plains as drying
vertical profiles occur. Friday and into the early weekend, weak
northwest flow develops as the trough axis passes east and H5
ridging shifts in just west of the Rockies. Just enough
impulse/vorticity difluence aid and meager instability could help
scattered showers and isolated thunderstorms over the higher
terrain but overall coverage will be limited and the High Plains
will likely remain rain free.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1117 AM MDT Sun May 20 2018

Flying conditions much improved today as visible satellite imagery
shows residual ceilings from morning leftover moisture gradually
lifting where morning clouds persisted around Cheyenne and
Rawlins. Higher resolution models do depict another round of low
ceilings and some fog for the I-80 Summit between Cheyenne and
Laramie extending down the foothills into Cheyenne tonight. We
trended down to IFR ceilings late tonight into early Monday for
Cheyenne, but kept everyone else VFR. We do expect to see some
scattered showers work in from the southwest across the Rawlins
areas late this evening.

&&

.FIRE WEATHER...
Issued at 227 AM MDT Sun May 20 2018

Drier and warmer conditions expected by this afternoon as more sun will
break out after the round of morning fog and low clouds mixes out and
as the departing weather system shifts east.

Despite the warmer and drier conditions, fire weather concerns will
remain low as ample rain amounts have occurred over the past several
days for many areas. One areas that missed out on rainfall was Carbon
County. However, they should remain in the upper 20 to low 30 percentiles
relative humidity wise this afternoon. A few higher terrain showers and
isolated weak thunderstorms will be possible this afternoon but overall
coverage will be low to zero elsewhere.

A few isolated showers/storm could again be possible Monday across the
Laramie Range and into the NE Panhandle. Coverage again is expected to
be low. Relative humidity values will fall near 20 percent but winds
should remain below critical thresholds at 15 to 20 mph from the southwest.

Overall long range forecast appears to remain just moist enough during the
afternoons to preclude high fire concerns at this time.

&&

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

$$

SHORT TERM...JSA
LONG TERM...JSA
AVIATION...JG
FIRE WEATHER...JSA



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