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690
FXUS65 KPUB 170946
AFDPUB

Area Forecast Discussion
National Weather Service Pueblo CO
346 AM MDT Tue Sep 17 2024

.KEY MESSAGES...

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- Strong to severe storms possible today across most of southern CO,
  mainly for a damaging wind potential.

- Patchy frost/light freeze possible for central San Luis Valley
  tonight though coverage looks too limited for highlights at
  this time.

- Warm, dry and breezy days and seasonally cool overnight lows
  into the end of the work week with spotty critical fire
  weather conditions across the plains.

- Trending cooler and wetter for the weekend.
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&&

.SHORT TERM /THROUGH TONIGHT/...
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Issued at 342 AM MDT Tue Sep 17 2024

Potent closed upper low will lift northeastward from the Great Basin
into the northern U.S. Rockies with upper jet energy and a upper
forcing crossing southern CO during the late morning and afternoon
hours.  Rounds of showers and thunderstorms will continue this
morning out west before a more consolidated broken line of
convection develops by late morning across the western mountains and
spreads eastward across the valleys and southeast plains during the
afternoon and evening hours.  Strong south to southwest flow will
advect some modest low level moisture northward, with afternoon dew
points in the 40s to mid 50s across the valleys and southeast
plains.  Combined with some cooling aloft with the approach of the
upper trough, mean sfc based CAPE off HREF ranges from up to 1000
J/kg across the interior valleys to up to 1250 J/kg across the
southeast plains by peak heating. Deep layer shears will range from
30 to 50+ kts, with greatest values across western and northern
portions of the forecast area where strongest upper jet level winds
are maximized. Hodographs exhibit more straight line structure,
though some slight curvature in the low levels exists across the
plains. This would bode for splitting supercell potential if
sufficient instability is present, with a slight favor towards the
right movers tough deep layer shears orthogonal to frontal boundary
will favor merging outflows/linear structures.  Storm motions will
be a swift north northeast component around 50 mph with the greatest
severe risk being damaging winds due to momentum transport of
stronger winds aloft to the surface.  There could also be some
severe hail, especially where CAPE and low level moisture can stay
up.  Low level drying along/behind the trough axis/front chases the
higher CAPE eastward during the afternoon which may keep greatest
hail threat in and near the mountains where shear is best and there
is less atmosphere for melting to occur.  Can`t rule out some severe
hail even out east though if surface dew points stay in the higher
range of HREF probabilities (upper 50s.) This occurrence looks is less
than a 20-30% chance for the far eastern counties based on 00z
runs/probabilities.

Thunderstorms should push east of the I-25 corridor by 5-6 PM and
into KS by midnight, though the best severe threat may be done by 8
PM across the eastern most counties.  Dry air spreads in overnight
which should keep which will allow for good radiational cooling
across the mountains and valleys where winds decouple.  We could see
a patchy frost or a light freeze for the San Luis Valley tonight
though coverage looks too limited for a Frost Advisory right now.
Will let later shifts reassess.  Otherwise, a crisp night is in
store for most of southern CO as lows even across the plains drop
into the upper 40s to lower 50s for most areas which is closer to
or slightly below climo. -KT
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&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
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Issued at 342 AM MDT Tue Sep 17 2024

Wednesday-Thursday...Moderating westerly flow across the region on
Wednesday becomes modestly southwest through the day on Thursday, as
upper level ridging builds across the Southern Rockies and into the
Southern High Plains ahead of another eastern Pacific system digging
across the West Coast. Model data remains in good agreement of keeping
much drier air in place across the region through Thursday. The dry air
and generally weak downslope flow will help warm temperatures through
the day, with highs at to slightly above seasonal levels in the 80s
to lower 90s expected across the plains, and mainly in the 60s and 70s
across the higher terrain, save for 40s and 50s at the peaks. The dry
air will also lead to the potential for good radiational cooling and
seasonally cool overnight lows in the 40s to lower 50s across the plains
and mainly in the 30s across the higher terrain, with the potential for
the high mountain valleys to see temperatures around freezing. The dry
air and expected good mixing, will also lead to breezy afternoon`s and
some spotty critical fire weather conditions across the plains, though
there does not look like enough coverage for any fire weather highlights
at this time.

Friday-Monday...Latest model data has trended slower and further south
with the West Coast system, which now digs into southern California
through day Thursday before slowly ejecting out across the Great Basin
on Friday. Models do indicate increasing low level moisture within
increasing southerly flow aloft across the Rockies on Friday. This will
lead to breezy southerly winds and the potential for continued spotty
critical fire weather conditions across western portions of the plains,
while increasing low level moisture may keep rhs up enough to preclude
any fire weather highlights. The increasing southerly flow will also bring
the potential for some waa showers across the region Friday afternoon
and evening. Friday highs will remain at and above seasonal levels before
and expected cool down into the weekend.

While there still remains differences in model timing and strength of the
system, a slower and more southern trend has emerged, with the GFS and
ECMWF solutions indicating a closed upper low lifting out across the Four
Corners region into south central Colorado early Saturday morning, which
then continues to track north and east into northeastern Colorado through
early Sunday morning before lifting out into the Northern High Plains
Sunday night. This slower and more southern track would bring better
chances of precipitation to south central and southeast Colorado, along
with much cooler temperatures for Saturday and Sunday behind a passing
front. The operational Canadian model does indicate a slightly faster and
more open wave moving across the Rockies through this timeframe, with all
model suites indicating cool northwest flow developing behind the passing
system for Sunday into early next week. Time will tell how this pattern
will evolve, however, a cooler and wetter pattern looks to be in the offing
for this weekend.
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&&

.AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/...
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Issued at 342 AM MDT Tue Sep 17 2024

Southerly flow has produced some advection fog early this morning
across portions of the San Luis valley with MVFR to IFR conditions
possible through 12-13z at KALS. Thunderstorms will develop over the
western mountains by mid to late morning and push eastward through
the afternoon and evening hours. South to southwest winds will
increase at all three terminals by late morning with gusts up to 30-
35 kts possible.  Thunderstorms will be possible at all three
terminals, with a high likelihood for at least thunderstorm outflow
enhanced wind gusts even if thunderstorms do not directly impact the
terminals.  Have introduced a Prob30 for this as gusts could hit 40-
50 kts from TSRA. There is the possibility for some hail up to 1
inch in diameter and brief MVFR conditions due to decreased vis from
heavy rain if a stronger storm impacts the TAF sites, though this
probability looks much lower. Thunderstorms should clear to the east
of all three terminals by 00z with clearing skies and diminishing
winds overnight. -KT
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&&

.PUB WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

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