610
FXUS65 KPUB 201000
AFDPUB

Area Forecast Discussion
National Weather Service Pueblo CO
400 AM MDT Fri Sep 20 2024

.KEY MESSAGES...

- One last day of above seasonal temperatures with spotty
  critical fire weather conditions across the plains.

- Potent Fall storm will spread widespread showers and
  thunderstorms with high elevation snow across southern CO
  Saturday afternoon through Sunday morning.

- Snow levels may drop down to 9000 feet with heavy wet snow
  accumulations of 2 to 9 inches for the higher peaks above
  10-11kft north of Hwy 50.

- Strong to severe thunderstorms possible Saturday afternoon and
  evening for the San Luis Valley, southern mountains and
  portions of the Raton Mesa Region.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 343 AM MDT Fri Sep 20 2024

Current water vapor imagery and upper air analysis has modest
southwest flow aloft across the region, with upper level ridging
in place across the southern Rockies into the southern High
Plains along with an upper level circulation evident off the
southern California coast.

No big changes in the forecast for today and tonight, as models
agree on slowly increasing southwest flow aloft across the Rockies
through the day, as the southern California low digs into southwestern
Arizona this evening and then continues to lift out across northeastern
Arizona by 12z Saturday. Southwest flow aloft induces lee troughing across
the plains, with breezy south to southwest winds again leading to some
spotty critical fire weather conditions, mainly along and west of the
I-25 Corridor this afternoon. The southerly low level flow will
increase low level moisture further east across the plains,
leading to the potential for a few afternoon and evening showers
and thunderstorms. Models to indicate some modest SBCAPE of
200-600 j/kg, though with DCAPE up to 1000 j/kg, could expect to
see gusty outflow winds up to 50 mph as the main threat from
any storms that do pop up. Today also remains the last day for
above normal temperatures for a while, with highs in the mid 80s
to mid 90s expected across the plains, and mainly in the 60s
and 70s across the higher terrain. Convection wanes through the
early evening, with a slow increase in mid and upper level
moisture across the Rockies through the overnight hours.
Increasing flow aloft will also help to keep lows above seasonal
levels in the 50s and 60s across the plains and mainly 30s and
40s across the higher terrain.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Issued at 343 AM MDT Fri Sep 20 2024

Models in good agreement tracking the upper low into the Four
Corners region Saturday afternoon and into NE CO/SW NE by Sunday
afternoon. There is some spread in the speed of its ejection
towards the end of the period. A slower ejection, as portrayed
by the NAM and Canadian members could support a slower drying
trend for Sunday, but overall differences are slight at this
time.

As for sensible weather, a wave of showers and thunderstorms will
lift northward late Saturday morning through the afternoon as the
first round of WAA/upglide overspreads the front boundary which sets
up across the TX Panhandle into eastern NM on Saturday. To the
north of the boundary, instability increases as CAA advection
from the approaching upper low spreads in, but CAPE seems
limited across the plains and rain showers may help to stabilize
the low levels initially. Deep layer shears are impressive
across all of southern CO with 50-70 kts and long straight
hodographs supportive of supercells capable of producing large
hail and damaging winds. The challenging part of the forecast
(and limiting factor for severe potential) will be the amount of
instability. Suspect there could be sufficient elevated CAPE
across southern portions of the southeast plains (northern
slopes of the Raton Mesa region) for elevated low topped
supercells. Where parameters come together better will be across
the San Luis Valley as the dry slot advances eastward and
elevated instability and increasing sfc dew points in southerly
flow could yield CAPE values of up to 1000 J/kg. There are some
helicity tracks across the San Luis Valley in HREF members so
will need to watch this trend closely. SPC has shifted the
Marginal risk westward as a result, which lines up well.

Otherwise, snow levels will drop Saturday night as the deformation
band takes shape across central CO, and easterly upslope flow
intensifies into the southeast mountains.  Most areas should see a
good wave of showers and thunderstorms with some elevated CAPE
spreading eastward across southern portions of the southeast plains
through the evening as the column cools aloft with the approaching
system. Could be looking at some small hail and gusty winds
along with heavy rainfall with the stronger storms into the
late evening hours across southern sections of the southeast
plains. Elsewhere, widespread rain is expected with snow levels
across the mountains dropping to around 9000 feet in the latest
runs early Sunday morning. Most accumulating snow will be
confined to above 10kft with up to around 8 inches or a little
more across the higher peaks above 12kft of the Pikes Peak and
the Sawatch and Mosquito ranges. There could be a brief rain
snow mix down to 9000 feet early Sunday morning, though
accumulations should be limited due to ground warmth.

The precipitation band will shift northeastward as the upper
low lifts into NE CO Sunday morning. Precipitation will continue
across the central mountains and Pikes Peak region through the
morning in area of wrap around, but overall should see a
downward trend as subsidence behind the system spreads in.
Temperatures will be quite a bit cooler with some scattered
showers lingering in northerly flow across the southeast plains
into the early afternoon depending on the timing of the
departing system.

Cool northwest flow remains over the region with quite a bit of
spread in the models regarding the next system dropping southward
Monday night and Tuesday. Operational EC still looks like the
outlier with the retrograding closed low solution across the
western U.S. portraying a wetter solution, even in its ensembles
which favors weaker more general troughing hanging back across
the Great Basin during this period. National Model blends keep
some isolated pops through this period and even GFS Ensembles
support some light precipitation across southern CO through the
period. Drying and warming returns for late week as another
upper ridge translates in. -KT

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 343 AM MDT Fri Sep 20 2024

VFR conditions are expected at COS, PUB and ALS over the next
24 hours. Winds will be diurnally driven through the day today,
with breezy southerly winds of 15-25 kts this afternoon, especially
at COS and ALS. Increasing southwest flow across the region tonight
will bring increasing mid and upper level moisture into the
Rockies through the overnight hours, with a cold front pushing
south across the plains bringing breezy northerly winds to COS
and PUB shortly after this taf period.

&&

.PUB WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from noon Saturday to noon MDT Sunday
for COZ058-060-082.
Frost Advisory until 8 AM MDT this morning for COZ069>071.

&&

$$

SHORT TERM...MW
LONG TERM...KT
AVIATION...MW