-- Discussion containing changed information from previous version are highlighted. --
157
FXUS65 KCYS 161755
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1155 AM MDT Sun Jun 16 2024

.KEY MESSAGES...

- Scattered thunderstorms possible this evening, mainly east of
  the Interstate-25 corridor. Storms could contain hail and
  strong winds.

- A cold front will bring cooler temperatures for Tuesday and
  Wednesday, along with a chance for late day showers and
  thunderstorms. A warming trend will develop for Thursday
  through Saturday, along with daily chances for afternoon and
  evening showers and thunderstorms.

&&

.SHORT TERM /THROUGH MONDAY/...
Issued at 410 AM MDT Sun Jun 16 2024

Looking at a fairly quiet afternoon on this Father`s Day. A trough
over the Pacific Northwest will gradually move southeastward over
the next several days. This will push a cold front south into
northern Colorado where it will likely stall out this
afternoon, becoming more of a stationary front. With the passage
of the cold front, high temperatures this afternoon will be
noticeably cooler, especially east of the Laramie Range. Here,
highs will be in the 70s and 80s, about 10 to 15 degrees cooler
than Saturday. Because of the cooler temperatures, surface-based
CAPE appears almost nonexistent, particularly across the far
northern zones where highs in the 70s will lead to a more stable
environment. There is a bit more surface instability further
south, closer to the stationary boundary where temperatures will
be warmer. As the front weakens, winds will turn more out of
the southeast. This will lead to increasing dewpoints and
increasing MUCAPE values late this afternoon and through the
evening hours. All this to say that some scattered convection
could be possible this evening. Hi-Res guidance continues to
show limited convection across the CWA, keeping most of the
storm activity in the northern-most zones where a weak
convergence boundary is located. However, most models show a
strongly sheared environment in this area with 0-6 km shear
values around 60 kts. This strong of shear may work against the
longevity of any storms that develop. Model sounding profiles
actually look much more promising for convection further south
in the CWA, and a bit later in the evening. MUCAPE values over
1500 J/kg, 0-6 km shear around 45 kts, and dewpoints in the 50s
lead to a better environment for convection. However, there is a
lack of forcing further south, especially with the decaying
cold front so storms may not form at all, but they cannot be
ruled out. Any storms that do develop during the evening,
regardless of where they are, will have the potential to be
severe. The primary concern with any elevated convection will be
large hail with the higher MUCAPE values, but an isolated
strong wind gust or two could also be possible.

A slight warm-up is expected Monday as southwesterly flow advects
warmer 700 mb temperatures into the CWA. Looking at mostly dry
conditions throughout the day, but a few weak vorticity maxes ahead
of the strong Pacific Northwest trough could spark an isolated storm
or two across the area.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Issued at 410 AM MDT Sun Jun 16 2024

Pacific cold front is still on track to arrive late Monday night
and early Tuesday morning, with the front pushing through the
far southeast counties (Kimball, Cheyenne Co.) by mid morning
Tuesday. Models continue to be in excellent agreement with this
solution and have continued to trend slightly cooler with each
run. 700mb temperatures are now down to -1c to -3c, mainly for
Carbon, Converse, and Albany counties...which will likely translate
to freezing temperatures over 9000 feet. The high valleys
(including Rawlins, Laramie, Saratoga, Shirley Basin) have a
chance to see some late-season near-freezing temperatures or
some frost early Tuesday morning and early Wednesday morning
once the colder air settled over the area. Models show little or
no precipitation as this front moves through the area, so high
elevation snow looks unlikely at this time. Tuesday will be the
coolest day for areas west of the I-25 corridor and east central
Wyoming as this is when the midlevel cold pool will lift
northeast across the region. For the rest of the forecast area,
Wednesday looks like the coolest day due to increasing cloud
cover over the eastern plains and a better chance for scattered
rain showers and isolated thunder. Highs on Tuesday will likely
reach the low to mid 70s over the plains due to ample sunshine,
especially earlier in the day, while areas west of I-25 and east
central Wyoming struggle to reach the low to mid 60s. Wednesday
will be similar, but most of the eastern high plains will likely
be stuck in the mid to upper 60s or near 70 during the day while
Rawlins, Laramie, and Douglas moderate into the low to mid 70s.
Low temperatures are going to be tricky since deterministic
models and ensemble forecasts indicate solid cloud cover,
especially Tuesday night/Wednesday morning. Therefore, did not
trend forecast towards the colder MOS guidance and kept lows
roughly between the ensemble mean and the 25th percentile. Even
the 10th percentile NBM and ENS/GEFS are still above freezing,
but do show a low-end chance of frost for Rawlins, Laramie (and
vicinity) and up towards Douglas and Lusk. Forecast
temperatures are still 8 to 15 degrees below normal for this
time of the year. Thankfully, probability for strong or severe
thunderstorms appears very low both of these days due to a
relatively stable boundary layer.

For late this week, models show a slowly building upper level
ridge across the central plains, which should lead to a general
warming trend across southeast Wyoming and western Nebraska.
Models show another weak Pacific trough moving onshore early
Thursday, with weak disturbances ejecting northeast into Wyoming
as the main trough weakens. The subtropical jet will become more
active during this time. NAEFS mean PWAT are generally in the
90th to 98th percentile with values around 1.00 inch over the
Wyoming eastern plains and around 1.25 to near 1.50 for western
Nebraska. This will result in a greater chance for afternoon
thunderstorms across most of the area, with a chance for some
nocturnal convection across the eastern high plains depending on
the timing of the shortwave activity/vort maxes. Increased POP
towards 50 percent for high-end scattered coverage of showers
and thunderstorms through Friday and into next weekend.

&&

.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
-- Changed Discussion --
Issued at 1140 AM MDT Sun Jun 16 2024

Man concern with this TAF issuance is the convection potential
this evening and overnight.

Latest satellite loop was showing mainly clear skies over the
majority of the TAF sites. Latest obs/radar data was showing the
frontal boundary that passed through earlier this morning was
now near the Palmer Divide in eastern Colorado. This front
packed quite a punch today with cooler air filtering into areas
east of the I-25 corridor. The short range models are showing an
abrupt wind shift to the east this afternoon, but based on the
latest trends we did delays this shift a couple of hours from
the previous forecast. Later this afternoon and early evening,
we should begin to see a decent surge of moisture back into
KCYS/KSNY by 02z or so with this moisture surge continuing into
the remainder of the Panhandle TAF sites by 06z. Depending on
the strength of this surge and any forcing that occurs we may
see a rogue thunderstorm or two. However, confidence in this
occurring remains rather low. Regardless, the cap will remain
in place with the potential for MVFR ceilings developing
overnight across most TAF sites along and east of the I-25
corridor.
-- End Changed Discussion --


&&

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

&&

$$

SHORT TERM...SF
LONG TERM...TJT
AVIATION...REC