-- Discussion containing changed information from previous version are highlighted. --
039
FXUS65 KCYS 172327
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
527 PM MDT Fri May 17 2024

.KEY MESSAGES...

- Warm & dry conditions prevail through Saturday. Daytime highs
  today may reach the lower to middle 80s for areas along and
  east of I-25.

- Windy today as a strong disturbance tracks across the northern
  high plains. Wind gusts 45-55 MPH are expected for Arlington
  and Bordeaux. A couple brief gusts in excess of 60 MPH cannot
  be ruled out.

- Widespread precipitation expected Sunday night through Tuesday
  afternoon. Mostly rain below 8000 feet, but snow or a
  rain/snow mix cannot be ruled out between 6000 and 8000 feet.

&&

.SHORT TERM /THROUGH SUNDAY/...
Issued at 230 PM MDT Fri May 17 2024

Gusty winds and dry conditions are becoming widespread across
the forecast area early this afternoon. Much above average
temperatures are also spread across the area, with current
observations in the 70s and 80s east of the Laramie Range, and
slightly cooler temperatures for areas to the west of the
Laramie Range. This is approximately 10-20 degrees above normal
for mid May. Taking a look at upper level and surface analysis,
we have an upper level disturbance sliding south from northern
Montana, and a surface cold front approaching from northwest
Wyoming. The present surface low as of 20Z is located across
North Dakota, but a secondary surface low is setting up in
northern WY/southern MT. Gusty winds are expected to continue
into the late evening hours, especially the wind prone corridors
where 50-60mph gusts are possible. We will have a few rain
showers that push through the area this afternoon, ahead of the
surface FROPA, but they will likely be hit and miss. The frontal
boundary will be weak, so this will not create a large spread of
cooler temperatures behind it.

We will have another day of dry conditions for most of Saturday,
however there will be a weak disturbance ejecting out of the
Four Corners area by Saturday afternoon. This will bring
isolated chances of showers and thunderstorms from Colorado
near sunset to approximately the very early hours of Sunday
morning. Surface pressure gradients will be relatively steep on
Saturday, thus we can expect elevated winds along and west of
the Laramie Range to persist. Daytime highs will be slightly
above average, with the 60s and 70s anticipated across the area.

A more well-developed shortwave disturbance will propagate from
the Canadian provinces on Sunday, brining our next opportunity
for isolated showers and thunderstorms. Severe weather is not
anticipated with this next wave the sweeps across the area, but
isolated stronger gusts up to 50mph may be possible with some of
the rain showers and thunderstorms. Daytime highs will be a
couple of degrees warmer than Saturday, with upper 60s to lower
80s expected for the high plains. Cooler air will remain in the
higher elevations where daytime highs in the 40s and 50s are
likely, especially west of the Laramie Range. Overall, expect
breezy to gusty winds at times, especially during the afternoon
of today, tomorrow, and Sunday.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Issued at 230 PM MDT Fri May 17 2024

Minimal changes made to the long term forecast. The long term still
looks active with near daily chances for precipitation across
southeast Wyoming and western Nebraska. A low progresses into
northwest Wyoming by Monday morning, increasing precipitation
chances along and behind cold front. Precipitation type is mainly
expected to be rain below 8000 feet, but a rain/snow mix or just
snow cannot be ruled out between 6000 and 8000 feet. Precipitation
chances continue mainly through Tuesday as the trough swings
through, finally ejecting Wednesday afternoon. This trough is
quickly followed by a secondary low. Confidence remains low for the
impacts in this second low and there are major differences between
the GFS and ECMWF solutions. However, the deterministic model runs
have flipped since last night`s forecast. Now, the ECMWF brings the
second low in by Thursday morning while the GFS is around 24 hours
behind. They are somewhat in agreement with the low continuing north
of the forecast area, limiting overall precipitation potential, and
increasing the potential for stronger winds.

For more details, refer to the previous forecast discussion:

An active pattern expected for much of the long term forecast.
Several upper-level disturbances will pass overhead, leading to
afternoon shower chances nearly every day. Sunday night into Monday
a longwave, upper-level trough is progged to drag across much of the
western CONUS, with a tighter trough depicted in the GFS and a
broader trough in the ECMWF. As this upper-level trough pushes
towards the area, an attendant cold front will move across the
region. This cold front with concentrate the lift near the surface
on Monday and lead to decent precipitation chances area wide, around
40-60%. Winds are expected to increase behind the front.
Temperatures on Monday will be several degrees colder than Sunday,
with highs int he mid-50s to low-70s across the region.

On Tuesday, the upper-level trough will continue to swing through
the area, though the GFS is notably a few hours behind the ECMWF on
the propagation of the trough. Forcing will once again be present
ahead of the trough as reinforcing cool air remains overhead.
Temperatures are expected to be in the mid-50s to low-60s with the
reinforcing cold air aloft. Afternoon showers will be possible as
the trough slowly swings overhead and forcing out ahead of the
trough favors the development of showers. Zonal flow will move
overhead once the upper-level trough finally advects out of the
reason by early Wednesday. Riding at 700mb and 700mb temperatures
increasing to 0-2C will promote high temperatures on Wednesday in
the low-60s to mid-70s across the region. Minimal precipitation
chances will exist during the day Wednesday, but cannot rule out a
few isolated showers as weak forcing from cyclonic vorticity
advection at 500mb favors subtle lift across the region. Any showers
that develop will likely not be strong or long-lived.

For the remaining long term, a secondary upper-level is progged to
swing around the upper-level trough and impact the region Thursday
and Friday. There is significant disagreement between the GFS and
the ECMWF on the exact placement, speed, and timing of this
secondary low. The GFS places the low over northern Idaho Thursday
afternoon and over Montana and Wyoming by Friday morning. The ECMWF
on the other hand is about a day behind, with the upper-level low
still off the coast of the Pacific Northwest Friday morning and does
not approach the region until late in the weekend. Instead, the
ECMWF favors zonal flow across the region for Thursday and Friday.
Decided to drop temperatures just a little for Friday, as long range
model suggest 700mb temperatures just above 0C for Friday. Kept
precipitation chances low, around 20-30% for both days to account
for the uncertainty in the upper-level evolution.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
-- Changed Discussion --
Issued at 521 PM MDT Fri May 17 2024

A cold front will sweep eastward across the terminals tonight,
switching winds to west and northwest.

Wyoming TAFS...Scattered clouds near 10000 feet will occur this
evening, with occasional showers and gusts to 45 knots at
Laramie until 01Z, and showers in the vicinity at Cheyenne until
01Z. Clear skies will prevail after this evening. Winds will
gust to 44 knots at Rawlins until 02Z, to 30 knots from 02Z to
06Z, and to 35 knots after 15Z Saturday. Winds will gust to
38 knots at Laramie until 02Z, then to 30 knots after 12Z
Saturday, and to 35 knots at Cheyenne until 09Z and to 25 knots
after 15Z Saturday.

Nebraska TAFS...Clouds will be scattered near 10000 feet until
02Z, then skies will be clear. Winds will gust to 30 knots until
02Z, and to 25 knots after 06Z.
-- End Changed Discussion --

&&

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

&&

$$

SHORT TERM...BW
LONG TERM...LEG
AVIATION...RUBIN