Area Forecast Discussion
Issued by NWS Cheyenne, WY

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373
FXUS65 KCYS 241707
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1107 AM MDT Tue Apr 24 2018

.SHORT TERM...(Today - Wednesday Night)
Issued at 423 AM MDT Tue Apr 24 2018

An area of snow has developed early this AM, roughly along & south
of a Laramie-Scottsbluff line. This is in response to good mid-lvl
forcing/jet energy associated w/ a vigorous shortwave now tracking
across Wyoming. Do not expect to see a lot of accumulation, but we
may see a few tenths of an inch on grassy surfaces. Mid-level cold
pool moves overhead later today, setting the stage for instability
snow showers over much of the area through this afternoon. Decided
to keep at least isolated PoPs intact through 00z Wed. Impacts are
expected to be minimal, other than the potential for black ice for
higher elevations over the I-80 summit due to recent rain/snow and
colder temperatures overnight. Winds will be a bit gusty today due
to mixing with significant cold advection aloft, as well as strong
H7-H8 flow over the high plains. A brief warm-up on Wednesday with
shortwave ridging aloft. Models suggest another disturbance likely
arriving late Wednesday night, with precipitation chances expected
to spread from north to south late in the period.

.LONG TERM...(Thursday - Monday)
Issued at 423 AM MDT Tue Apr 24 2018

Main weather highlights late week and into the weekend will be an
initial cold front and increasing precipitation chances on Thursday
followed by a well-above normal warm-up expected over the weekend.
Temperatures over the weekend look to be 10 to 20 degrees above
climate normals. Models diverge Sunday and beyond with lower
forecast confidence towards the end of the period.

Global models have good agreement on a briskly moving shortwave
trough shifting southeast through Wyoming early Thursday morning. An
attendant cold front will shift through the area as a result and
cool temperatures down from the previously warmer Wednesday.
Increasing divergence aloft and progged 700-300mb Q vector
convergence suggest enhanced synoptic lift shifting over the area.
Orographic lift up the Snowy and Laramie Ranges and lowering snow
levels should allow for some light snow across the higher mountain
elevations. Accumulations will be quite light with only 1-2 inches
at best given the quick moving system and lack of deep moisture
present. For lower elevations, light showers will be likely but
again, accumulations will be limited. Winds will also increase from
the north behind the front, especially across the high plains of
southeast WY and NE panhandle upwards of 35 mph.

The colder temperatures Thursday will be short-lived as a longwave
H5 ridging shifts over southeast WY and west NE late week and into
the first part of the weekend. Warm air advection on increasing H7
SW flow will help surface temperatures warm considerably above
normal. H7 temperatures will warm from -7C Thursday to +3C Friday to
+7C Saturday. This should allow surface temperatures to reach the
50s in the mountains and into the mid- to upper-60s in lower
elevations east of I-25 Friday. Saturday should be even warmer with
mid- to upper-50s mountains and low- to mid-70s lower elevations
with ample sun.

Global models diverge on the synoptic pattern late weekend as GFS is
more progressive with a cut-off low moving on-shore across CA vs.
the ECMWF that is much slower and slightly farther south. GFS
ensembles suggest decent variance also vs. the operational GFS run
and have sided with more of an GFS ensemble mean/ECMWF blend for
Sunday into early next week. One possible hazard to watch for will
be increasing winds Sunday if the GFS pans out as pressure gradients
tighten. However, this hazard could be delayed until Monday per the
ECMWF due to slower progression. An H7 front will shift through the
area with the passage of the closed low and bring temperatures back
towards normal values early next week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1105 AM MDT Tue Apr 24 2018

Widespread low clouds seen on satellite imagery across southeast
WYoming and Nebraska Panhandle late this morning. Mainly MVFR
conditions expected to slowly improve through the afternoon. By
00Z, most airports should be VFR with the exception of KSNY. WIll
continue to monitor through the afternoon.

&&

.FIRE WEATHER...
Issued at 423 AM MDT Tue Apr 24 2018

Despite breezy conditions, no fire weather concerns for today with
cooler temperatures keeping RH values well outside of our critical
thresholds. Considerably warmer and drier for Wednesday, but we do
not expect winds to be particularly strong. Minimum RH values west
of the Laramie range will likely be as low as 10 percent on Wed.

&&

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

$$

SHORT TERM...CLH
LONG TERM...JSA
AVIATION...GCC
FIRE WEATHER...CLH



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