Flash Flood Warning 9, NWS Grand Junction, CO

Area Forecast Discussion
National Weather Service Grand Junction CO
541 PM MDT Thu Aug 17 2023

.SHORT TERM...(Tonight through Friday night)
Issued at 351 PM MDT Thu Aug 17 2023

Moisture levels remained anomalously high across the forecast area
today. Dew points were running in the mid 40s to lower 50s across
the region and the 12Z/GJT sounding held 0.96" precipitable water
(PW). High pressure continued to dominate the southern Plains
while a quasi-stationary closed low was spinning over the eastern
Pacific off the southern California coast. With few clouds to
limit solar insolation this morning, surface warming resulted in
thunderstorm formation over the Abajo mountains by 1730Z. Since
then, storms continued to develop over the La Sal and San Juan
Mountains, the eastern Tavaputs and Roan Plateaus, the Flat Tops
and portions of the central Colorado mountains. Unlike yesterday
at this time, today's storms were moving northward as the axis of
high pressure ridge over the area shifted east over the past 24
hours. Slow storm movement combined with deep moisture raises the
threat for excessive rainfall and the potential for localized
flash flooding.

Models indicated a weak short wave trough will push across
northeast Utah and northwest Colorado this evening which is likely
to keep shower/thunderstorm activity going well into the evening.
Otherwise, diurnal cooling will bring a slow diminishment of moist
convection during the night though isolated showers may linger
through sunrise Friday.

Southerly flow aloft increases a bit on Friday as the eastern
Pacific low moves toward the coast sharpening the pressure
gradient overhead. Meanwhile, PW levels will remain high with
values in the 1 to 1.2" range for much of the area which is close
to 150% of normal. As mentioned in the previous discussion, jet
divergence will add dynamic lift to diurnally driven instability
which should enhance storm coverage and intensity. Though storm
motion should increase slightly compared to today, the potential
for excessive rainfall and localized flooding will continue.

Increased clouds and showers are expected to bring slightly
cooler temperatures tonight through Friday morning, though
readings will continue to run just a little above normal.

.LONG TERM...(Saturday through Thursday)
Issued at 351 PM MDT Thu Aug 17 2023

Saturday's forecast and beyond will be keeping an eye to Hilary and
its impact on moisture volume here on the West Slope. Presently,
deterministic models are keeping the richest moisture too far west
for our benefit. However, a bit of recycled moisture will likely get
tapped for some afternoon convection on Saturday, mostly on terrain
and short lived beyond sunset. A few rich PWATS are showing up in
ensemble forecasts over northwest CO on Saturday afternoon, this
could make for a few stronger showers there. A formidable high
pressure center baking the Plains pushes back westward enough to cut
things off on Sunday and likely allow those summer temperatures to
return. Mountain areas under the stronger high pressure will see
temperatures climb 5-10 degrees above August averages Sunday and
Monday. Meanwhile desert valleys will still be 5 degrees above
average for the weekend and starting the work week.

Height falls from Hilary making its way ashore out west will shift
some moisture east enough for a stray shower or two along the Divide
and on the San Juans Tuesday. This will begin another uptick for
monsoonal moisture. Wednesday and Thursday both look active with
afternoon convection. As previously discussed here, some attention
to an easterly wave will be addressed in coming packages thanks to
the potential for a strong push of moisture. Models seem to be
struggling with the next wave coming ashore out west and the impacts
on CONUS pressure heights. Suffice to say, the persistent high
pressure pattern we've endured over the last several weeks will
continue to deliver warm conditions under sunny skies, punctuated
with a few cool spots under storm activity. Recyclable moisture is
not under threat from an extended dry spell either. So, stray
afternoon convection will stick around in the extended period, even
without a good monsoonal push.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 533 PM MDT Thu Aug 17 2023

Winds will remain largely terrain driven outside of storms this
overnight. Convection is expected to taper off by midnight with a
few clouds hanging around the overnight hours and widespread VFR
conditions returning by daybreak. On Friday, widespread afternoon
/ evening showers and storms should be anticipated.

&&

.HYDROLOGY...
Issued at 351 PM MDT Thu Aug 17 2023

A stream of subtropical moisture will continue to move northward
across the region through the end of the week. Elevated moisture
levels combined with slow storm movement pose a threat for
localized excessive rainfall each afternoon/evening through
Friday. Areas with unstable soils in steep terrain, or where soils
are already saturated from rain earlier in the week are at
greatest risk.

Models have evolved since earlier in the week, with the consensus
now indicating that the subtropical moisture plume will shift west
of the area as high pressure expands westward this weekend.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...None.
UT...None.
&&

SHORT TERM...NL
LONG TERM...LTB
AVIATION...TGJT
HYDROLOGY...NL/TGJT

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Special Weather Statement
National Weather Service Grand Junction CO
519 PM MDT Thu Aug 17 2023

COZ001-UTZ024-025-172345-
Lower Yampa River Basin CO-Eastern Uinta Basin UT-
Tavaputs Plateau UT-
519 PM MDT Thu Aug 17 2023

...A strong thunderstorm will impact portions of west central Rio
Blanco County through 545 PM MDT...

At 519 PM MDT, Doppler radar was tracking a strong thunderstorm 10
miles southwest of Rangely, or 44 miles southeast of Vernal, moving
northeast at 20 mph.

HAZARD...Winds in excess of 40 mph and half inch hail.

SOURCE...Radar indicated.

IMPACT...Gusty winds could knock down tree limbs and blow around
         unsecured objects. Minor damage to outdoor objects is
         possible.

Locations impacted include...
  Rangely.

This includes the following highways...
 Colorado 64 between mile markers 10 and 20.
 Colorado 139 between mile markers 69 and 72.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If outdoors, consider seeking shelter inside a building.

&&

LAT...LON 3993 10894 3999 10906 4017 10893 4007 10873
TIME...MOT...LOC 2319Z 219DEG 20KT 4000 10895

MAX HAIL SIZE...0.50 IN
MAX WIND GUST...40 MPH

15

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