148
FXUS65 KCYS 102117
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
317 PM MDT Mon Jun 10 2024

.KEY MESSAGES...

- Numerous showers and thunderstorms will continue into the
  evening. Strong to severe storms are possible with the
  potential for hail, strong winds, and perhaps an isolated
  tornado with any quasi- discrete storms

- Mainly dry and rather warm conditions will prevail for Tuesday
  and Wednesday. Precipitation chances return Thursday and more
  widespread showers and thunderstorms possible Friday into
  Saturday. Sunday will see another warming trend
  with decreasing precipitation chances.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 300 PM MDT Mon Jun 10 2024

Thunderstorms are increasing in coverage this afternoon across the
area. On the synoptic scale, the upper level shortwave is moving
across eastern Montana, with some drier air in the mid to upper
atmosphere visible on satellite water vapor channels behind. The
surface front, located by a modest pressure trough and northerly
surface winds, is now pushing into Converse and Niobrara counties.
The dryline remains straddled fairly close to I-25, and is providing
the initial kick off zone for today`s storms. The latest
mesoanalysis indicates potent instability with SBCAPE exceeding 2000
J/kg. Modest MLCIN is continuing to erode, both along the US-20
corridor and US-85. We have fairly steep lapse rates in place and
abundant low-level moisture east of the dryline, above the 95th
percentile of climatology earlier this morning. The limiting factor
for severe convection is lackluster shear, ranging from around 25-
knots in the 0-6 km layer near the southern edge of the forecast
area up to 40-knots in the northern edge of the area. As the
shortwave trough approaches, expect upper level flow to increase,
which will increase shear values as the afternoon progresses. In the
absence of strong shear, storms are growing upscale and linearizing
fairly quickly, especially further south. Expect a linear system to
develop here shortly and progress across the NE panhandle
through about 5-6PM. While the main line will be out then,
models show modest isentropic lift holding for a few more hours
behind this. Additionally, the cold front is not very dry at
least on its initial edge, so low-level moisture will remain
fairly good even after the surface winds turn northerly. With
increasing shear through the evening, wouldn`t be surprised to
see a few discrete storms develop and become severe through the
evening. Storm activity should wind down by midnight.

Mid-level dry air will start to work into the area after midnight,
leading to a fairly rapid drop in precipitable water and clearing
skies towards Tuesday morning. Expect a much more tranquil day on
Tuesday in the wake of today`s shortwave. While this system will
drop temperatures slightly, the ridge over the southwest CONUS will
re-strengthen over the area fairly quickly. Thus, look for highs up
to around 5-10F above normal across the area. With the much drier
atmosphere overhead, we aren`t expecting much in the way of
precipitation activity. We may get some high altitude cumulus
clouds and virga showers, but the chance of anything reaching
the ground is very low.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Issued at 300 PM MDT Mon Jun 10 2024


With severe weather beginning over the area early this
afternoon, will keep the Long Range discussion focused on
Wednesday through Friday night with some weather impacts possible
on Friday.

Models in reasonable with some disagreement on the coverage and
extent of showers/tstorms Friday afternoon and Friday evening.
Hot temperatures are still expected on Wednesday with all models
showing a flat upper level ridge over the Front Range with
westerly flow over southeast Wyoming and western Nebraska under
westerly flow. 700mb temperatures are about a degree lower than
previous forecasts, mostly due to the apparent increase in
midlevel cloud cover and some convection over the higher
terrain, so kept forecast temperatures as is and generally in
the mid 80s to mid 90s over the area, with a few temperature
readings approaching 98 degrees over western Nebraska. Again,
may see a few late afternoon showers or thunderstorms, but
hardly any of this rain is forecast to reach the ground, so kept
POP below 15 percent for most areas into Wednesday evening.

For late this week, models show a backdoor shallow cool front
moving into the high plains for Thursday as the upper level
ridge axis amplifies northward west of the continental divide.
Not quite as warm on Thursday compared to Friday, but it will
still be pleasant with highs in the 80s to near 90, especially
further south along I-80. Models have trended a little more
aggressive with thunderstorm activity Thursday afternoon,
especially along and just east of the Laramie Range. Increased
POP up to 30 percent, but did not go too aggressive since the
boundary layer will still be relatively dry for most of the day.
Our main forecast concern will be Friday and Friday night as
models and most ensembles are in pretty good agreement on the
movement and timing of a central Pacific upper level low
tracking northeast across southern California and into Utah and
western Colorado. The 12z ECMWF is still struggling with
boundary layer moisture and thunderstorm coverage, but most of
the other deterministic models are in good agreement. NAEFS is
still showing 95th percentile PWAT with a few areas east of the
Laramie Range closer to 98th percentile with values over 1.25
inches. Although ensemble means are pretty modest with CAPE and
QPF, 10 to 20 percent of members do show a good signal for
considerable rainfall, mainly along and east of the Laramie
Range into the central and southern Nebraska Panhandle. Models
have also trended more unstable with higher CAPE and good 0-6km
shear. This is a favorable pattern for heavy rainfall and strong
to severe convection, so kept POP near 60 percent near the
Colorado border. Expect temperatures to lower closer to seasonal
averages for Friday and heading into Saturday, but still around
5 to 9 degrees above normal for this time of the year.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 1140 AM MDT Mon Jun 10 2024

A surface low across northern Wyoming and eastern Montana will move
east today as a cold front to the south of the low moves across
eastern Wyoming and into western nebraska later this afternoon
through this evening. Thunderstorms will start to develop around
19z, and should form a solid squall line as they move south and
east...impacting the western Nebraska terminals late this afternoon
through this evening.

HAZARDS/WEATHER TRENDS: VFR conditions will prevail over KRWL and
KLAR as most of the activity will remain to the east of these
terminals. However, can`t rule out an isolated shower/tstorm after
20z with gusty winds being the primary concern.

KCYS will see thunderstorms develop here shortly (~19z) with a solid
line of thunderstorms by 21z. Strong gusty winds, hail, and very
heavy rain will be the main threats. There is a chance that this
line will form just to the east of KCYS (similar to what happened on
Saturday) and spare the terminal of the worst conditions. Confidence
with IFR VIS and strong gusty winds is low to moderate (20% to 40%)

For the western Nebraska terminals, there is high confidence that
all terminals will see a squall line push through between 21z today
to 01z this evening. Expect a period of IFR CIGS and VIS with
frequent lightning, strong gusty winds, and possibly hail.
Thunderstorm activity may linger through the late evening hours
behind the main squall line.

&&

.HYDROLOGY...
Issued at 300 PM MDT Mon Jun 10 2024

The slightly cooler temperatures the last few days have slowed the
melt just a touch, allowing flows to come down on area rivers and
streams. Therefore, the Flood Warning on the Little Laramie River
has been expired. Flows remain near flood stage on the Medicine Bow
River near Elk Mountain, so the Flood Watch was extended another
day. Temperatures will start to increase again Tuesday into
Wednesday, with more rapid melt anticipated. However, it is possible
we have passed the peak melt with a pretty significant loss in
snowpack over the last 7 days.

&&

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

&&

$$

SHORT TERM...MN
LONG TERM...TJT
AVIATION...TJT
HYDROLOGY...MN