273
FXUS65 KCYS 091129
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
529 AM MDT Sun Jun 9 2024

.KEY MESSAGES...

- Areas of (locally dense) fog will persist through mid-morning
  across the high plains of southeast Wyoming and the western
  Nebraska Panhandle. There will be a risk for a couple of
  strong thunderstorms in the vicinity of the central and
  northern Laramie Range later this afternoon.

- A more widespread threat for showers and thunderstorms will
  occur with a cold frontal passage on Monday afternoon and
  evening. A few storms may be severe.

- Mainly dry and rather warm conditions will prevail for Tuesday
  through Thursday, with increasing chances for showers and
  thunderstorms for Friday and Saturday, along with slightly
  cooler temperatures.

&&

.SHORT TERM /THROUGH MONDAY/...
Issued at 352 AM MDT Sun Jun 9 2024

Widespread fog & low-level stratus has developed across the high
plains of southeast Wyoming & the western Nebraska Panhandle, in
response to southeasterly low-level upslope & residual low-level
moisture from earlier convection. Surface observations & webcams
indicate fog has become quite dense over mainly southern Kimball
County w/ visibilities being reduced to one quarter of a mile or
less. The lowest visibilities have been fairly localized so far,
but would not be surprised to see this expand in coverage across
the area over the next few hours. This could require a Dense Fog
Advisory prior to sunrise, but decided to hold off for now until
the area of fog becomes a bit more widespread. Visibilities will
most likely improve by mid-morning, but would expect to see low-
level stratus persist through at least early afternoon given the
favorable wind/moisture profiles.

The westward extent and temporal residency of the fog/stratus is
expected to significantly impact convective coverage today, with
high-res models generally keeping showers and storms confined to
the higher terrain. A few ripples in the mid & upper-level flow,
along with deep/moist southeasterly low-level upslope & enhanced
low-level convergence along the central & northern Laramie Range
should support convective initiation near Laramie Peak by mid to
late afternoon. This activity will most likely follow the narrow
instability axis just west of I-25, courtesy of the differential
heating resulting from earlier clouds. The overall severe threat
is fairly low with only around 1000 J/kg of CAPE and very modest
vertical shear, but any storm in this environment would have the
potential to become strong to marginally severe hail. There is a
MRGL Risk for this area, per SPC.

A more widespread convective threat is expected Monday, with the
passage of another cold front. Pre-frontal air mass will be very
warm and unstable w/ dew points in the lower to middle 50s along
with widespread surface temperatures in the 80s supporting steep
mid-level lapse rates & resultant CAPEs up to 1500 J/kg. Overall
shear profiles appear fairly modest, but decent veering profiles
should support 30+ knots of effective bulk shear & the resulting
potential for organized strong to severe storms. The DY2 outlook
from SPC has much of extreme eastern Wyoming and the western Neb
Panhandle in a SLGT Risk for Monday.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Issued at 352 AM MDT Sun Jun 9 2024

Monday night/Tuesday...Scattered showers and thunderstorms will end
from west to east Monday evening, mainly east of I-25.  Northwest
flow aloft remains, though with increasing heights aloft along with
warmer temperatures, and a decrease in low and mid level moisture,
we should see less coverage of late day showers and thunderstorms.

Wednesday...Even warmer as the flow aloft turns westerly, and with
700 mb temperatures near 17 Celsius, maximum temperatures will be in
the mid 80s to mid 90s. Warmer temperatures aloft will increase the
convective inhibition, and with less low and mid level moisture
around, it looks like it will be dry.

Thursday...Similar synoptic pattern as on Wednesday, though low
level winds will become northeast, and with an increase in low and
mid level moisture, we expect to see isolated late day showers and
thunderstorms. Perhaps a few degrees cooler than Wednesday with
upslope winds and more cloud cover.

Friday...Ridging aloft, over our forecast area, begins to break down
as the next shortwave trough aloft approaches, and with surface lee
trough convergence and an increase in atmospheric moisture, we
expect scattered late day showers and thunderstorms. Slightly cooler
with an increase in cloud cover and precipitation coverage.

Saturday...Looks like we will likely see a shortwave trough aloft
passage around peak heating, helping to spark scattered late day
showers and thunderstorms. Maximum temperatures look similar to
those of Friday.

&&

.AVIATION /12Z TAFS THROUGH 12Z MONDAY/...
Issued at 520 AM MDT Sun Jun 9 2024

Ridging aloft will develop and move over the area today, with
moist southeast low level winds aiding in keeping in areas of
low clouds much of the day.

Wyoming TAFS...For Rawlins and Laramie, scattered to broken
clouds from 6000 to 10000 feet will prevail. Winds will gust to
25 knots at Laramie after 18Z.

For Cheyenne, ceilings will range from 800 to 2500 feet until
20Z, with areas of fog reducing visibilities to 1 to 2 miles
until 16Z. Thunderstorms will be in the vicinity with ceilings
near 3500 feet from 20Z to 02Z, then scattered clouds near
10000 feet will occur. Winds will gust to 24 knots after 20Z.

Nebraska TAFS...At Chadron and Alliance, ceilings will range
from 300 to 900 feet until 16Z, with areas of fog reducing
visibilities to 1 to 3 miles until 15Z, then ceilings will be
near 8000 feet from 16Z to 21Z. From 21Z to 02Z, thunderstorms
will be in the vicinity with ceilings near 4000 feet, then
scattered clouds near 10000 feet will prevail after 02Z.
Winds will gust to 23 knots after 02Z.

At Scottsbluff and Sidney, ceilings will range from 300 to
1500 feet until 18Z, with occasional fog reducing visibilities
to 1 to 2 miles, then ceilings will be near 3500 feet until 20Z.
From 20Z to 02Z, thunderstorms will be in the vicinity with
ceilings ranging from 3500 to 5000 feet, then scattered to
broken clouds near 10000 feet will occur after 02Z. Winds will
gust to 23 knots from 15Z to 02Z.

&&

.HYDROLOGY...
Issued at 254 PM MDT Sat Jun 7 2024

Above normal temperatures have resulted in accelerated mountain
snowmelt across the Rocky Mountains. Temperatures in the Snowy
and Sierra Madre Ranges are expected to remain elevated this
weekend. Flood warnings for snowmelt continue for the Little
Laramie River west of Laramie, and the Encampment River south of
Saratoga, mainly due to minor flooding concerns. Emergency
Managers have reported some flooding in these areas over the
last 48 hours. Flood Watches continue for the upper North
Platte River valley and Medicine Bow River near Elk Mountain for
potential impacts this weekend. Current forecasts suggests
river levels will peak later this weekend and remain elevated
into early next wee. Will continue to monitor the potential for
locally heavy rainfall due to thunderstorms in and around the
mountains this weekend. However, the last round of thunderstorms
generally produced less than a quarter inch of rain Saturday
afternoon.

&&

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

&&

$$

SHORT TERM...CLH
LONG TERM...RUBIN
AVIATION...RUBIN
HYDROLOGY...WFOCYS