Area Forecast Discussion Issued by NWS Cheyenne, WY
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000
FXUS65 KCYS 272335
AFDCYS
Area Forecast Discussion
National Weather Service Cheyenne WY
535 PM MDT Wed Mar 27 2024
.KEY MESSAGES...
- Occasional wind gusts up to 60 MPH are possible near Arlington
and Elk Mountain this evening and overnight. High Wind
Warnings are in effect.
- Warmer Thursday. Widespread wind gusts 45-55 MPH expected
along and west of I-25 during the afternoon.
- A slow moving weather system will bring an extended period
with chances for precipitation from Friday through Monday.
Specific details remain uncertain at this time.
&&
.SHORT TERM /THROUGH FRIDAY NIGHT/...
Issued at 221 PM MDT Wed Mar 27 2024
A High Wind Warning remains in effect for Arlington/Elk Mountain
from 6 PM MDT this evening until 9 AM MDT Thursday. Winds should
continue to increase into the late afternoon & early evening, as
H7/H85 CAG-CPR gradients climb to 55 to 60 meters in response to
surface pressure falls across eastern MT ahead of a weak clipper
system. H7-H8 flow over ARL is still not particularly impressive
through tonight. However, WAA aloft should generate a sufficient
inversion near mountain top to support enhanced gap flows. Still
seeing near equal probabilities of elevated (45-57 MPH) and high
wind (58+ MPH) gusts portrayed by in-house random forest models,
so a marginal high wind episode still looks probable. Winds will
likely remain sub-warning for the other wind corridors (BRX/VDW)
as the gradient orientation is less-than-optimal, despite decent
flow aloft. The threat for 60+ MPH gusts is likely to come to an
end by sunrise Thursday, but may actually persist beyond the 15z
expiration as lapse rates steepen by late Thursday morning. Will
need to consider potential for 55-60 MPH gusts impacting Rawlins
as well on Thursday afternoon as surface pressure gradients seem
to get quite strong across Carbon County per the GFS in response
to a strengthening surface high over western Colorado. Generally
would prefer to see stronger flow aloft and MAV/MET guidance at/
above 30 knots for sustained winds, but it is certainly possible
to get a rogue gust or two. Will continue to max out gusts at 55
MPH for now, especially w/ the potential for greater coverage of
low and mid-level cloud cover. A warming trend is likely through
Friday as upper-level ridging builds into the region. Mainly dry
on Thursday, with the exception of a few rain or snow showers in
western areas, especially in the high country. Rain/snow chances
should begin to increase by Friday afternoon given increasing SW
flow aloft ahead of a deepening western US trough. Deterministic
models vary greatly in the initial evolution of this wave, but a
trend toward cooler and more unsettled weather appears likely as
we head into the weekend.
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.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
Issued at 221 PM MDT Wed Mar 27 2024
A complex storm system is expected to traverse through the area this
weekend as a split trough moves across the western CONUS. The first
phase of the storm is described above in the short term section. As
the strong closed low digs down just off of the California coast on
Saturday, downstream amplification of the ridge over the high plains
will push the stationary frontal boundary back to the north and
usher in decent warm air advection. Southerly flow overrunning the
frontal boundary should keep precipitation going over the boundary,
but this will shift north through the day, decreasing PoPs across
our area into Saturday, particularly further south. There may be
some weak vorticity maximums ejecting along this boundary over the
weekend, which reduces confidence somewhat in specific timing of
precipitation. A boundary straddling or just north of the area also
results in a fairly low confidence temperature forecast for Saturday
and Sunday. For now, leaning that warm air will make it into the I-
80 corridor, so nudged to the NBM 75th percentile here, while nudged
to the 25th percentile for the northern portion of the forecast area
which may remain on the other side of the boundary. Additionally,
the relative lull with the northward frontal shift will mainly apply
to areas east of the Laramie range. Further west, moist westerly
flow and frontogenesis will keep precipitation going across Carbon
and Albany counties Saturday and Saturday night, with some models
show fairly intense banded precipitation during this period. Thus,
while the 18z Sat to 18z Sun is a relative minimum in 24-hour
precipitation totals for Cheyenne, this is the maximum 24-hour
period for Rawlins.
On Sunday into Monday, the closed low will traverse across the
desert southwest. Models differ somewhat still in the strength and
degree of negative tilt of the southern branch trough, as well as
its proximity to the northern branch trough. All of these will
impact our precipitation outcome for Sunday into Monday. The overall
setup is that the trough moving east of the Rockies will support lee
cyclogenesis over east central Colorado. As the surface low
strengthens, it will draw the frontal boundary back to the south and
bring colder temperatures through our area, along with a change over
back to snow, for areas of the high plains that warmed up enough for
rain on Saturday or Sunday morning. Northeasterly upslope flow with
some wrap-around moisture will lead to more widespread precipitation
Sunday night into Monday over the high plains. However, even though
we have yet another Colorado low, its disorganized nature is likely
to cut into precipitation totals. The development of a secondary
closed low somewhere over Wyoming or Colorado will also be key in
determining if the band of heavier precipitation on Sunday/Monday
will be across east central Wyoming into SD and northern NE, or more
focused along the I-80 corridor. With uncertainty still quite
considerable, trended QPF down somewhat below a fairly aggressive
NBM, and closer to the ECMWF ensemble and GEFS mean. Ensemble mean
700-mb temperatures drop to around -8C late Monday in the wake of
this system, so expect temperatures to drop back below normal, but
not as cold as last week`s cold snap. Expect a fairly quick recover
also, as the low moves out by Tuesday and strong ridging edges in on
top of it from the northwest. Look for highs climbing back close to
normal on Tuesday, and then above normal by Wednesday.
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.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
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Issued at 530 PM MDT Wed Mar 27 2024
The flow aloft will turn westerly tonight, then west
southwesterly on Thursday.
Wyoming TAFS...Scattered to broken clouds from 7000 to
15000 feet will occur. Winds will gust to 40 knots until 03Z,
then to 38 knots after 14Z Thursday.
Nebraska TAFS...Scattered to broken clouds near 8000 feet will
occur early this evening, then skies will be mostly clear with
scattered clouds near 15000 feet after daybreak on Thursday.
Winds will gust to 35 knots until 01Z, then to 25 knots at
Scottsbluff and Sidney after 16Z Thursday.-- End Changed Discussion --
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.CYS WATCHES/WARNINGS/ADVISORIES...
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WY...High Wind Warning until 9 AM MDT Thursday for WYZ110.
NE...None.-- End Changed Discussion --
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$$
SHORT TERM...CLH
LONG TERM...MN
AVIATION...RUBIN