719
FXUS65 KCYS 190333
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
933 PM MDT Sat May 18 2024

.KEY MESSAGES...

- Warm & breezy conditions are expected again for this afternoon.
  Overall chances for showers are low, but an isolated storm
  cluster may develop during the late afternoon and/or early
  evening along and southeast of a line from Cheyenne to
  Alliance.

- The risk for showers and thunderstorms is expected to increase
  from Sunday afternoon through Monday as significant upper-
  level troughing deepens and digs into the 4 corners region. A
  Marginal risk for severe weather exists Sunday afternoon.

- More widespread rain showers with high elevation snowfall
  expected for Monday PM through Tuesday night. Exact snow
  levels remain uncertain at this time.

- Cooler air behind Tuesday`s storm system may bring near
  freezing temperatures to portions of the area Wednesday
  morning.

&&

.SHORT TERM /THROUGH MONDAY NIGHT/...
Issued at 300 PM MDT Sat May 18 2024

The quiet weather pattern continues early this afternoon as
breezy winds and low relative humidity readings remain present
across portions of southeast WY. A weak boundary is evident on
the radar as of 20Z this afternoon, and is likely to continue
lifting north ahead of an approaching weak weather disturbance
from the south. This will bring chances for isolated rain
showers and even a couple rumbles of thunder along the I-80
corridor by early evening, and the NE Panhandle.

A longwave trough will dive south on Sunday towards the Four
Corners area, brining an increased amount of moisture,
instability, and forcing to our cwa by Sunday afternoon. The
best locations for this to result in organized convection is
along and east of the Laramie Range. SPC Day 2 has a Marginal
Risk for isolated severe weather to occur from the I-25 corridor
toward western Nebraska. There is a Slight Risk from the SPC
just to our east. We will have to monitor CAMs in the coming 24
hours as current model guidance shows the strongest convection
initiating in western Nebraska and pushing east from Sunday
afternoon in the evening time frame. Main hazards would be
strong, damaging winds and hail up to 1" in diameter of in areas
of the Marginal Risk. At this time of inspection, model support
doesn`t show widespread chances of strong convection. However,
model soundings have the infamous "inverted-V" feature, so rain
showers that develop will have the potential to gust to severe
thresholds easily.

The aforementioned longwave trough will continue to dig toward
the Four Corners on Monday, with an attendant surface cold
FROPA heading towards our cwa. Model guidance has an aggressive
approach towards 700mb and 850mb temperatures decreasing rapidly
behind the surface cold front, well below 0C. Cold fronts
typically translate faster than model guidance across our cwa,
so have gone with a decrease in daytime highs for our northern
forecast zones slightly, and nudged temperatures down slightly
along the I-80 corridor also. Looking at scattered to numerous
rain showers as this FROPA advects southward, with isolated to
scattered thunderstorms as well. Will begin to see a rain and
snow mix in the higher terrain by Monday evening, after dark.
Colder temperatures will increase in coverage overnight, with
sub-freezing temperatures likely in the higher terrain. Have
nudged down the overnight lows for Tuesday morning across the
entire cwa to reflect this. Overall, expect a period of active
weather to ramp back upward beginning Sunday afternoon into
early next week.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
Issued at 300 PM MDT Sat May 18 2024

A cooler and more unsettled weather pattern will continue through
much of next week. A broad longwave over the western CONUS will
dominate the weather pattern, with numerous shortwaves rotating
around the trough through the area over the course of the week. The
first shortwave ejects to the east Monday night, but a secondary
shortwave just behind will likely bring the coldest air of this
cycle. Due to the two shortwaves in close proximity and the second
vorticity maximum tracking more or less directly over the area, the
lifting mechanism is somewhat muddled. This reduces confidence that
widespread rainfall will continue through the day Tuesday, but
several ensemble members still show a more southerly track of the
second wave which allows heavier precipitation to continue longer.
Precipitation rate will also have an impact on precipitation type.
There`s pretty good confidence this will be cold enough to support
some snow flakes mixing in above 7000 ft and accumulation above 8500
feet. However, the atmosphere is expected to be cool enough that
heavier precipitation rates could locally drop snow levels to around
6000 ft, which would include Cheyenne. The current most likely
scenario of lighter precipitation rates may lead to a few flakes
mixing in, but we`d need the heavier scenario to lead to more
noticeable snow event. This scenario is shown by approximately 30%
of ensemble members at this time. Precipitation should come to an
end sometime Tuesday night as drier air works in behind the second
shortwave. We`ll be watching temperatures into Wednesday morning, as
the potential for near freezing temperatures exists. Many locations
are a week or more past their climatological last freeze date, so
would likely need frost/freeze products in this scenario. Currently,
a freeze looks likely for areas along and west of the Laramie range
(but this isn`t very unusual for those areas), with about a 30-50%
chance of a freeze for the higher elevations east of the Laramie
range, including the corridors from Cheyenne to Kimball near I-80
and Douglas to Harrison near US-20.

The cool snap should be pretty short-lived, with a weak ridge
pushing back in by Wednesday. Highs will probably remain below
normal Wednesday, but climb back above normal for Thursday. Models
are starting to come into better agreement for the end of the week,
leaning towards a sharp trough passing through Thursday night or
Friday. However, most guidance keeps the upper level low north of
our area, and thus the precipitation potential is not very
impressive. Nonetheless, looking at another shot of cooler air (but
probably not as significant as the early week front), with continued
chances for afternoon showers and thunderstorms each day through the
end of the week.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 928 PM MDT Sat May 18 2024

Southwest flow aloft will develop overnight and continue on
Sunday.

Wyoming TAFS...Ceilings will range from 8000 to 15000 feet.
Expect thunderstorms in the vicinity from 20Z to 00Z. Winds will
gust to 25 knots at Cheyenne after 15Z Sunday, and to 35 knots
at Rawlins and Laramie from 14Z to 00Z.

Nebraska TAFS...Ceilings will range from 8000 to 15000 feet,
except for occasional fog reducing visibilities to 4 miles and
ceilings near 1500 feet from 09Z to 15Z. Expect thunderstorms in
the vicinity from 20Z to 01Z. Winds will gust to 30 knots at
Chadron, Alliance and Sidney until 01Z, and to 23 knots at
Scottsbluff from 20Z to 01Z.

&&

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

&&

$$

SHORT TERM...BW
LONG TERM...MN
AVIATION...RUBIN