325
FXUS65 KCYS 131135
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
535 AM MDT Mon Oct 13 2025

.KEY MESSAGES...

- Frost Advisories are in effect until 9AM for much of the High
  Plains.

- Gusty southerly winds are expected today along with a return
  of scattered showers during the afternoon and evening.

- Areas of locally dense fog are possible near the I-80 summit
  and over the High Plains both tonight into Tuesday morning and
  Tuesday night into Wednesday morning.

- The next storm system will slowly move through the area late
  Wednesday through early Friday, bringing first showers and
  thunderstorms, following by a chance for mountain snow and
  strong winds.

&&

.SHORT TERM /THROUGH TUESDAY NIGHT/...
Issued at 400 AM MDT Mon Oct 13 2025

Skies are mostly clear over the area this morning, with the
exception of a little bit more cloud cover than originally forecast
over far southeast Wyoming and the southern Nebraska panhandle. This
is associated with a weak vort-max traversing through this morning.
Temperatures are running a little warmer under this cloud deck,
which may prevent frost formation here depending on how long it can
linger over the area. This should clear out in the next 2-3 hours,
so there still could be a short window before sunrise to develop
some frost. Elsewhere, chilly temperatures are on track with
widespread lower 30s across the High Plains. The clouds should be
gone by sunrise, so frost should burn off quickly as temperatures
warm.

Today will be a somewhat unusual meteorological setup. A surface
high is currently extending into our area, but is expected to begin
shifting off to the east shortly. We`ll have a strong reverse
pressure gradient over the area, leading to fairly strong southerly
to southeasterly winds. This is not too atypical in the Laramie
Valley, which can expect the strongest winds, possibly seeing a few
gusts to near 50 mph, but a little less common to see southerlies
this strong further east. Widespread gusts between 30 and 45 mph are
expected for the entire day, and these may weaken slightly after
dark but are not expected to drop off. Adding to today`s oddities
will be a return of the pseudo-monsoonal pattern that dominated the
Rocky mountain region last Friday and Saturday. This will be caused
by a powerful upper level closed low currently diving southward
along the West Coast. As this digs in, the ridge over Texas will
also amplify, turning the flow aloft back to southerly at 700-mb and
southwesterly aloft. Deep moisture will be pulled northward once
again along with a few vort-maxes meandering over New Mexico and
Arizona this morning. Precipitable water will increase rapidly today
with cloud cover coming in quickly around midday. Mesoscale model
guidance shows fairly potent positive theta-e advection over the
area between noon and about 9pm as the first vort-max moves through.
This should provide sufficient forcing to kick off scattered showers
over the area. PoPs were increased quite a bit with this update to
account for decent shower coverage this afternoon and early evening.
Forecast soundings show little to no instability, so thunder is not
likely but a few rumbles can`t be ruled out. The best chance for
thunder will be in Carbon County.

While the forcing associated with the first-vort max dries up around
9pm, a second region of increased moisture and modest overrunning
aloft will move into Carbon County, which may not see much of a
break from the scattered shower activity. Expect shower coverage to
expand once again overnight through about midday Tuesday before
moisture aloft and forcing for ascent dissipate during the afternoon
and evening hours. Meanwhile, the persistent southeast flow over the
High Plains will return dewpoints to exceptionally high levels for
mid October by this evening. We may see some patchy fog develop near
the I-80 summit and over portions of the High Plains overnight and
Tuesday morning. As the middle/upper atmosphere dry out, we should
see fog come in once again Tuesday night into Wednesday morning.
There is potential for Wednesday morning`s fog to be fairly
widespread.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Issued at 400 AM MDT Mon Oct 13 2025

Wednesday will be the warmest day of the week. The area will remain
under southerly 700-mb flow and southwesterly flow aloft as the
powerful closed low encroaches inland. The ridge amplitude aloft
will peak Wednesday pushing 700-mb temperatures to the +9 to +11C.
We probably won`t fully realize those potential temperatures at the
surface due to continued southerly upslope-ish flow, cloud cover,
and  extremely high boundary layer moisture, but we`ll still be
looking at highs generally 10F above seasonal norms. The closed
low`s journey over the area will occur in two phases this week. The
first will be a vort-max ejecting out of the parent trough Wednesday
afternoon and evening, while the second is the primary trough axis
sliding through the area Thursday. The first phase will feature a
deepening lee cyclone over north central Colorado ejecting
northeastward through our area Wednesday into Wednesday night.
Cyclogenesis, vorticity advection aloft, and weak instability
present should help produce scattered showers and thunderstorms
during the afternoon and evening hours. Shower activity will
probably be fairly disorganized, but continuing more or less through
the day on Thursday. Models are fairly consistent in showing the
parent trough axis accompanied by strong frontogenesis as it sweeps
through the area sometime between Thursday morning and Thursday
evening. Southerly flow aloft will also overrun the southwest to
northeast oriented frontal boundary and aid in forcing for ascent.
We should see coverage of showers tick up as this passes through.
Most of the precipitation will probably fall along or just ahead of
the front rather than behind. Snow levels may come down quickly
enough to cause rain to mix with snow along and west of the Laramie
Range, but those to the east will probably remain mostly rain.

Once the sprawling upper level low ejects out to the east, expect to
find drier air moving in, descending motion, and cooler temperatures
over the area. There is some potential for another round of strong
winds for the wind prone areas Thursday night into Friday, similar
to this last event. Deterministic models show 700-mb winds exceeding
50-knots during this period. This is supported by about 25% of
ensemble members too, so this is not a lock yet, but worth watching
in the coming days. Another shortwave trough is likely to drop in
from the northwest sometime Friday or Saturday and get absorbed by
the strong departing upper level low. Some models have this
occurring late enough that the parent trough is already out of our
area, while others result in this new shortwave reinforcing the cool
temperatures over the area. There is quite a bit of uncertainty in
Saturday`s high temperatures as a result of this discrepancy. The
colder/slower solutions would also bring a chance for some light
snowfall with the secondary shortwave, even into the High Plains.
There is actually better agreement on the forecast beyond Saturday,
with good model consensus showing ridging building back into the
area and sending temperatures back above seasonal norms for Sunday
into Monday.

&&

.AVIATION /12Z TAFS THROUGH 12Z TUESDAY/...
Issued at 530 AM MDT Mon Oct 13 2025

Gusty southeast winds have already begun at KLAR and will spread
to remaining terminals by mid morning. Look for gusts of 25 to
35 knots through most of the TAF period. Gusts exceeding 40
knots are possible at KLAR. Shower activity will return to the
area between about 18z and 03z. Most will be light, but we could
see some brief dips into MVFR VIS/CIG as these pass overhead.
Confidence was a little too low in exact timing and location of
these showers to add them as prevailing, so this is still
handled with PROB30 groups at this time. Lastly, areas of low
clouds and fog are likely to produce MVFR to IFR conditions at
High Plains terminals heading into the end of the TAF period
Tuesday morning.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Frost Advisory until 9 AM MDT this morning for WYZ106>108-
     117>119.
NE...Frost Advisory until 9 AM MDT this morning for NEZ002-003-
     019>021-054-055-095-096.

&&

$$

SHORT TERM...MN
LONG TERM...MN
AVIATION...MN