Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 221730

Area Forecast Discussion
National Weather Service Cheyenne WY
Issued by National Weather Service Riverton WY
1130 AM MDT Wed Mar 22 2017

.SHORT TERM...(Today through Friday)
Issued at 410 AM MDT Wed Mar 22 2017

An active forecast period ahead as a vigorous upper-level shortwave
tracks across the Great Basin and eventually into the central Rocky
Mountains later in the week. Today, we enjoy one more warm day with
ridging aloft contributing to H7 temperatures of +4C to +6C. Expect
most areas to remain dry through early this afternoon w/ the deeper
moisture confined to western CO/WY, but would expect precip chances
to increase this evening/overnight with strengthening southwesterly
flow aloft. Westerly flow along/west of the Laramie Range, along w/
the exit region of the H25 jet bisecting the I-25 corridor suggests
higher PoPs farther west over western Carbon/Converse counties thru
12z Thursday. A lee trough is progged to strengthen on Thursday and
move east across the high plains. Moisture will almost certainly be
limited, but increasing dynamic support aloft combined w/ low-level
convergence along the trough may yield afternoon convection for the
high plains. CAPEs are not particularly high, but slightly negative
LI values suggests the potential for isolated thunder. The Day2 SPC
outlook does have portions of the NE Panhandle under a MRGL risk on
Thursday afternoon, but believe chances for stronger convection are
higher a bit further east. Our attention then shifts to Thurs night
and early Fri with the rapid intensification of the upper low along
with significant surface cyclogenesis over southeast Colorado. This
system is still expected to occlude very quickly, no doubt limiting
the amount of cold air available from the cold fropa on Fri AM. The
models have also been trending farther south and east w/ the TROWAL
signature. The consensus is for the majority of the QPF enhancement
to be along/southeast of a line from the I80 summit to AIA. The GFS
and NAM show a considerable amount of dry air in the H7-H5 layer at
06z-12z Friday, and as a result have trended a lot lower w/ overall
QPF. The ECMWF/GEM show some fragmentation in the moisture field as
well, but continue to produce over one inch of QPF, mainly over the
southern NE Panhandle. The GFS and NAM are now showing little or no
precip in areas where both models hinted at 0.75+ inch, just 24 hrs
ago. That said, confidence is just too low to warrant any headlines
right now but still expect accumulating snow above 5500 ft. Biggest
concern will be the I-80 summit. Winds likely will be a factor with
a strong pressure gradient and deep, 30+ knot flow aloft on Fri per
the GFS. Precipitation should quickly end from west to east between
15z-21z Fri.

.LONG TERM...(Friday night through Tuesday)
Issued at 345 AM MDT Wed Mar 22 2017

00Z/06Z medium range model consensus points to an active progressive
flow aloft as a series of Pacific low pressure troughs will impact
the regional weather during the extended forecast. A upper ridge
will promote dry and mild weather early this weekend. The first of
two troughs will track east across the Intermountain West Saturday
and Rockies Sunday. Rain and high elevation snow will develop along
and west of the Laramie Range Saturday evening, spreading east
across southeast Wyoming and the Nebraska panhandle Saturday night
and Sunday morning. Precipitation amounts will be light. Sunday
will trend slightly cooler and breezy in the wake of a Pacific
frontal passage, with precipitation lingering over parts of
southeast Wyoming through early Sunday evening before a brief dry
period with the passage of another upper ridge. A stronger trough
moves ashore into the West Monday, with light precipitation
developing across much of southeast Wyoming along a leading
disturbance aloft. There are model differences with the track and
position of the trough. The GFS is much slower as it forms a closed
low over the southern Great Basin Tuesday. The ECMWF is much faster
with a trough over the south central high plains. The Canadian is
in between the two extremes. Regardless, expect widespread rain and
snow with near normal temperatures. The upside of the long term will
be opportunities for much-needed moisture.


.AVIATION...(For the 18Z TAFS through 18Z Thursday)
Issued at 1130 AM MDT Wed Mar 22 2017

MVFR ceilings at a few sites in western Nebraska have dissipated this
morning, allowing VFR conditions throughout the area.  Breezy gusty
south to southwest wind will continue this afternoon at all terminals,
with gusts over 30 kts likely.  Expect isolated thunderstorms over the
SE WY mountains this afternoon and possibly into the evening hours.
KRWL may see some surrounding showers and thunderstorms as well with
the better instability over SW WY.  Winds should decrease overnight,
generally around 10 kts, with strong SW flow picking up across SE WY
before 18Z Thu. An advancing cold front will push through the area
between 00z-03z Fri bringing widespread precipitation.


Issued at 410 AM MDT Wed Mar 22 2017

Elevated fire weather conditions are expected this afternoon across
much of the area w/ widespread RH values of 20-25 percent and winds
gusting to 30 MPH especially west of I-25. Otherwise, concerns will
be on the low side w/ widespread rain/snow and cooler conditions to
arrive on Thu/Fri.




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