927
FXUS65 KPUB 202031
AFDPUB

Area Forecast Discussion
National Weather Service Pueblo CO
231 PM MDT Thu Jun 20 2024

.KEY MESSAGES...

- Strong storms possible through this evening across the
  mountains and adjacent northern I-25 corridor.

- Flash flood concerns increase tonight through Friday night for
  the Continental Divide and burn scars in the southeast
  mountains as anomalously moist airmass spreads up from the
  south ahead of an approaching trough.

- Isolated afternoon thunderstorms will be possible on Sunday,
  with an uptick in thunderstorm activity thereafter through
  the mid part of next week.

- Continued warming trend this weekend through the mid part of
  next week.

&&

.SHORT TERM /THROUGH FRIDAY NIGHT/...
Issued at 209 PM MDT Thu Jun 20 2024

A strong southerly moisture fetch ahead of the approaching trough
out west has brought precipitable waters of 100-125% of normal
across southern CO according to satellite measurements this
afternoon. Moisture advection continues tonight with Precipitable
Waters increasing to up to 250% of normal funneling into the
southwest mountains. Dew points in the 40s across the mountains
and valleys are already yielding CAPE values of up to 1000 J/kg.
Combined with deep layer shears of 20-30 kts (greatest values
to the north) a few strong to near severe thunderstorms will be
possible across the mountains and valleys where hail up to 1
inch in diameter and wind gusts up to 60 mph will be possible.
Locally heavy rainfall capable of producing rock slides and
flash flooding will be the primary risk however and burn scars
and areas with susceptible soils will be monitored closely.

There will likely be one or more rounds of showers and embedded
thunderstorms continuing over the mountains through tonight with the
focus for sustained precipitation across the Eastern San Juan
mountains given southwesterly orographic forcing. Question will be
how intense thunderstorms and rainfall rates will be overnight with
the risk shifting more towards flash flooding as soils become
saturated. Model blends show mean QPF values of up to 1.5 inches
across the eastern San Juans through tonight with most areas
elsewhere in the 0.25 to 0.5 inch range across the mountains.  90th
percentile shows some areas of up to 3 inches of rain across the
eastern San Juans. There is likely some convective feedback
leading to some of these higher values within the model spread
(as we saw last night with convection forecast to spread into El
Paso county), but there is an upslope signal that could lead to
higher precipitation amounts from the model blends which can`t
be completely discounted. Given the dry antecedent soil
moistures will hold off on any Flash Flood Watches at this
point, but tonight`s rainfall may prime the pump for a greater
flash flood risk on Friday.

Another round of showers and thunderstorms will redevelop early on
Friday as forcing increases ahead of the next upper trough. Once
again, flash flooding will be the primary concern across the
mountains with high precipitable waters (250%+ of normal) still
over the region. CAPE values look a little lower but still
ranging up to 750 J/kg if we can get enough afternoon heating
with the residual convective cloud cover. So another round of
strong to marginally severe thunderstorms will be possible
across the mountains. Deep layer shears will be stronger
tomorrow given increasing flow aloft so if we can realize
sufficient instability there could be a better chance for severe
thunderstorms. Once again convection will linger well into the
night as the upper trough slowly moves across CO with focus
shifting southward with time.

The southeast plains will feel little impact from this system as the
majority of the forcing and instability stays over the mountains. An
isolated storm or two will be possible as they drift off into the
adjacent plains with gusty winds and brief heavy rainfall the
primary risks for Friday.

Meanwhile, temperatures for eastern areas will continue the warming
trend with highs back in the mid 90s across the lower elevations.
The valleys and mountains will stay cooler given the early onset to
convection with highs in the 60s and 70s. -KT

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
Issued at 209 PM MDT Thu Jun 20 2024

Friday night through Saturday...

An upper level trough is going to be transitioning over the region
by later in the evening on Friday, and the axis will move overhead
right around midnight and through the early morning hours over the
CWA. This will translate to the possibility of showers and
thunderstorms continuing over much of the region during the
overnight hours. With an abundance of moisture in the mid-levels and
even low levels with upsloping, PWAT values have been between 200-
300% of normal and the risk for excessive rainfall and possible
flash flooding will still be in place as a result. Heavy rains
possible with some of these storms could also exacerbate snow
melt and also lead to water levels potentially reaching action
stage for rivers within the San Luis Valley. The other areas of
concern will be burn scar areas, as well as Monarch Pass and
the Chalk Cliffs/Cottonwood Pass. After the trough moves
through, it will be relatively stable behind the wave with
subsidence occuring, and therefore mostly dry by later in the
day on Saturday. There could still be a couple an isolated
shower or storm developing later in the afternoon over the
higher terrain with daytime heating and some residual moisture
in place, otherwise it will be predominantly dry by later in the
day for most of southeast Colorado. It will be slightly warmer
as well since upper level ridging will move over behind the
exiting trough.

Sunday through Thursday...

The longwave ridge over the inner-mountain west will slowly
propagate eastward with the axis over the region on Sunday.
Meanwhile, there will be some moisture advected up in the mid-levels
from the southwest. This will allow for the development of afternoon
and evening showers and thunderstorms, mainly over the mountains,
with westerly flow pushing some of the storms out over the plains by
later in the day. With the ridge axis moving over, it will also
cause temperatures to continue to warm with temperatures approaching
the 100 degree mark for locations within the lower Arkansas River
Valley.

The ridge will flatten and move east with another larger longwave
ridge moving over in its wake towards the middle of the week. This
will result in a continuing warming trend with temperatures over 100
degrees becoming more likely during the day over much of the plains
from Monday through Wednesday. There will also be some subtropical
moisture advected up in the ridge over the Four Corners area which
will help increase chances of afternoon and evening thunderstorms
over the mountains, with some of these moving out into the plains by
later in the day. In addition, some of the deterministic models are
also cueing in on a potential tropical system moving out over the
Pacific Ocean south of Baja California, which might get tapped into
the southwesterly flow and increase mid-level moisture over the
region towards the middle of next week which may also help to
increase thunderstorm chances, although this is still quite far out
in the forecast and therefore subject to change. By Thursday, an
approaching trough from the northwest will help break down the ridge
and also increase southwesterly flow ahead of it. This may help to
pull up more mid-level moisture form the southwest but also cause
gusty surface winds to present fire weather concerns over portions
of the CWA during the afternoon through early evening on Thursday.
-Stewey

&&

.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
Issued at 209 PM MDT Thu Jun 20 2024

While conditions will remain VFR, thunderstorms developing over
the mountains may move off into the adjacent lower elevations
affecting the TAF sites this afternoon and overnight. Will carry
VCTS in all three TAFS beginning around 20-21z with the best
chance through 01z. Gusty erratic outflow winds, heavy rainfall
with brief MVFR conditions and small hail will be possible with
the stronger storms, though these should be outliers with a
low probability of impacting the TAF sites. Gusty outflow winds
will likely affect afternoon and evening wind directions with
gusts up to 40 kts possible near -TSRA. Winds will become more
steady southerly overnight with another round of -SHRA/-TSRA
after 06z. Odds are greatest at KALS. KCOS could see some VCSH
overnight though confidence is lower as it is less likely to be
persistent. Cigs at KALS could drop into the MVFR category
overnight as waves of rain move up from the south. Some clearing
expected tomorrow then another round of showers and
thunderstorms will develop over the mountains which could affect
the terminals in the afternoon and evening. -KT

&&

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

&&

$$

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