wy discuss
Office: CYS
FXUS65 KCYS 201126
AFDCYS
Area Forecast Discussion
National Weather Service Cheyenne WY
526 AM MDT Sat Apr 20 2024
.KEY MESSAGES...
- Freezing drizzle and freezing fog will be the main concerns
today around the Cheyenne Ridge, South Laramie Range, and
South Laramie Range foothills. Ice accumulations up to one
tenth of an inch on elevated surfaces, bridges, and overpasses
will be possible. Slick road conditions can be expected.
- Snow will be more favorable in the southern Nebraska panhandle
with accumulations around 1 inch possible.
- Much warmer temperatures are expected for Sunday through
Thursday.
&&
.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 445 AM MDT Sat Apr 20 2024
Pretty quiet across the CWA as much of the action remains south of
the Wyoming/Colorado border. Dense freezing fog has developed across
much of the South Laramie Range early this morning as expected with
the moist low-levels and southeasterly winds. Models like the HRRR
and HREF keep the fog here throughout much of the day, as well as
expanding it into the foothills and central Laramie County this
morning as surface winds become more easterly. A webcam just south
of the Wyoming/Colorado state line does show light snow, however,
per Hi-Res guidance, snow likely won't move into the CWA until
around daybreak.
The forecast for today remains tricky regarding precipitation types.
Strong 700 mb southerly flow will advect warmer air aloft into the
CWA this morning. Before this warmer air arrives, precipitation will
likely fall as snow as multiple model soundings show a saturated
layer colder than -10C. As this warmer air pushes northward, the
saturated layer will warm into the -10C to 0C range. This is when
precipitation will transition to freezing drizzle. Most likely area
for freezing drizzle will be the Cheyenne Ridge and the South
Laramie Range and foothills. The strongest warm air advection will
be in these locations, with the transition to freezing drizzle
possible by mid-morning, if not a bit sooner. Accumulation of ice is
possible on elevated surfaces, as well as bridges and overpasses.
This will lead to slick and slippery road conditions. Ice
accumulations up to one-tenth of an inch are possible in these
areas. The advancement of warmer air looks to be a bit more delayed
in the southern Nebraska panhandle. Most precipitation this morning
should fall as snow, with accumulations generally around an inch or
less. The warmer air looks to arrive by the afternoon, which could
lead to a brief period of freezing drizzle, although the bulk of
precipitation will be done by then. Winter Weather Advisories remain
in place for the South Laramie Range, foothills, and central Laramie
County for snow, fog, and freezing drizzle. These Advisories run
through noon today, but could need an extension if precipitation
continues into the afternoon. But, the worst of the winter weather
will likely occur during the morning hours. Precipitation will taper
off by the evening.
Model soundings do show the potential for fog developing from the
South Laramie Range eastward into Sidney Saturday night. Low-levels
will be sufficiently moist due to the precipitation during the day
Saturday with calm winds. Conditions aloft also look to dry out and
clear, leading to radiation fog. Models like the HRRR are also
hinting at fog with surface visibility drops. However, lingering
clouds in the southern Nebraska panhandle may limit the fog
potential.
Looking like a much nicer day on Sunday as weak ridging builds over
the CWA, allowing warmer and drier air to infiltrate the area.
Precipitation chances will be minimal with sunnier skies
expected. High temperatures will warm up about 20 degrees
compared to Saturday's highs. High temperatures will be around
average to slightly above average for late April.
&&
.LONG TERM /MONDAY THROUGH FRIDAY/...
Issued at 204 AM MDT Sat Apr 20 2024
Mainly dry cold front moves through the CWA Monday bringing
breezy conditions to the forecast area. GFS advertising 700mb
winds across Carbon and Albany Counties up near 40kts out ahead
of the front. Local WRKHGT wind guidance showing Craig to Casper
850/700mb height gradients near 35mtrs...so for now...wind
headlines are not anticipated. Could see critical fire weather
conditions out by Rawlins Monday afternoon though. But with
fuels rated NO at this time out there...fire weather headlines
are not expected either. ECMWF paints some light QPF for the
mountains Monday afternoon and evening behind the front.
Warming trend for Tuesday and Wednesday as a ridge of high
pressure dominates the weather. GFS 700mb temperatures climb
from -4 to +4C Monday...to +4 to +10C Wednesday. Expecting to
see afternoon highs in the 70s Wednesday.
Next chance for any appreciable precipitation looks to be
Thursday when a low tracks across northeast Colorado. Low draws
a frontal system south across the Laramie Range with 700mb
temperatures falling to around +2C. Would think most of this
precipitation will fall as rain. Unsettled weather continues
through Friday into the weekend as we see another low pressure
system tracking across Colorado. For now...still too warm for
outside the mountains.
&&
.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 524 AM MDT Sat Apr 20 2024
Deteriorating flight conditions at KLAR...KCYS and KSNY as a
band of snow moves north out of northern Colorado this morning.
Used HRRR guidance on timing of lowered conditions. IFR/LIFR may
persists at KCYS all day with southeast upslope flow. VFR
conditions at KRWL...KCDR. KBFF may see MVFR conditions briefly
as they will be on the northern edge of the snow.
&&
.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Weather Advisory until noon MDT today for WYZ116>118.
NE...None.
&&
$$
SHORT TERM...SF
LONG TERM...GCC
AVIATION...GCC
Office: RIW
FXUS65 KRIW 201042
AFDRIW
Area Forecast Discussion
National Weather Service Riverton WY
442 AM MDT Sat Apr 20 2024
.KEY MESSAGES...
- Dry today, with warmer temperatures, though still below normal.
- Much warmer Sunday, with highs above normal. Elevated fire
weather concerns and precipitation chances (15% to 60%)
across northern Wyoming. Elevated fire weather concerns return
Monday.
- Next major system looks to arrive towards the end of the
workweek, though there is a lot of uncertainty in the details.
&&
.DISCUSSION...
Issued at 125 AM MDT Sat Apr 20 2024
A brief period of ridging is the weather story for today into the
start of tomorrow. Today, as the high pressure begins to move across
the area, temperatures warm. Highs will generally be in the upper
40s to around 60 degrees, the warmer of those in the Sweetwater
County area. For the most part (not including Sweetwater County),
these are only slightly below normal for this time of year. However,
with ample sunshine and a late April sun, it should feel fairly
pleasant outside. The ridging also acts to stabilize the air and
divert moisture away, resulting in less than 10% chance of
precipitation for the area; the northern Bighorn Mountains are the
exception, with about a 30% chance. Winds gusting to around 30mph
occur from the Rock Springs to Muddy Gap area early this morning,
decreasing by sunrise.
The ridge axis passes over Sunday. Temperatures climb to above
normal, with basin locations having nearly 100% chance for
temperatures in the 60s, with a 60% to 70% chance to hit the 70
degree mark. Jackson Hole is cooler, with temperatures more in the
mid 50s. This has been a fairly consistent forecast the past couple
days. One limiting factor may be increasing clouds ahead of the next
weather system.
Speaking of, a trough extending down from an upper low in Canada
sweeps through the state beginning Sunday afternoon. Models have
trended drier compared to yesterday. Precipitation chances pick up
the first couple hours of the afternoon for the mountains of
northwest Wyoming. Chances spread eastward through the afternoon as
the trough does. Precipitation chances have dropped for the lower
elevations, such as the Bighorn Basin, as a result of the drier
models. North of a Cody to Greybull line has the the best chances
(15%-30%) Sunday afternoon and evening. Northern mountains (YNP,
Absarokas, and Bighorns) have roughly 15% to 60% chances, the higher
of which for the Bighorns. All-in-all, not looking at any impacts
from this system when it comes to precipitation.
The other aspect to this trough is the wind. The HREF gusts have
come down a little, showing gusts more on the order of 20 to 30mph
across western Wyoming and the Wind Corridor. Gusts to 25mph are
possible elsewhere Sunday afternoon, with about a 50% chance for
gusts above 25mph. There is some uncertainty for some central
Wyoming locations, with the HREF showing a spread of 10 to 25mph
gusts in the Wind River Basin mid-afternoon.
This time of year, relative humidities (RH) also are on the mind.
The minimum RH deterministic forecast has values around 20% for most
of the lower elevations east of the Divide, as well as Sweetwater
County. There is currently a 20% to 40% chance RHs fall below 20
Sunday. These values, combined with the wind gust potential, lead to
elevated fire weather concerns for Sunday.
The main energy of the trough exits the area for Monday, but a
pressure gradient remains. There could be some lingering showers
over the northern mountains (30%-40%), which come to an end by
Monday night. RHs look to be as dry, or drier than Sunday. Wind
gusts also pick up again in the afternoon, especially for locations
along and west of the Divide. Elevated fire weather concerns once
again return for Monday.
Fairly zonal flow Monday through at least the first half of
Wednesday keeps precipitation chances low. Could be some isolated
(30%) showers in the mountains during this time. A more potent
system looks to be in the making for the end of the workweek.
Confidence is very low at this time on details, with the GFS and
ECMWF having effectively different synoptic patterns at this time.
Am confident in saying a pattern change is coming though, with
cooler and wetter conditions for next weekend.
&&
.AVIATION /12Z TAFS THROUGH 12Z SUNDAY/...
Issued at 440 AM MDT Sat Apr 20 2024
VFR conditions throughout the entire TAF period for all
locations. Mid level clouds through the morning and afternoon
and high clouds after sunset and into the overnight hours
through the end of the period. Light winds to start with a bit
of a breeze up to 18kt gusts at BPI, PNA, and RKS. Otherwise,
much of the day less than 10kts for all other locations. Dry
conditions with no other weather elements expected.
Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.
&&
.FIRE WEATHER...
Issued at 125 AM MDT Sat Apr 20 2024
Elevated fire weather concerns Sunday and Monday as RHs are most
probable to be around 20% for most lower elevation locations east of
the Continental Divide, as well as Sweetwater County. Currently, a
20% to 40% chances RHs fall below 20% Sunday. Winds gusts 20 to
30mph Sunday for much of the area. Of the zones with critical fuels
zone 280 would be most likely to have the RHs and wind gusts,
resulting in elevated fire weather concerns.
&&
.RIW WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
DISCUSSION...Wittmann
AVIATION...Lowe
FIRE WEATHER...Wittmann
