Flash Flood Watch 16, NWS Grand Junction, CO

Area Forecast Discussion
National Weather Service Grand Junction CO
1130 PM MDT Tue Jul 20 2021

.SHORT TERM...(Tonight through Wednesday night)
Issued at 109 PM MDT Tue Jul 20 2021

Water vapor imagery pretty much sums up the story for our
anticipated weather pattern over the next several days. Deep push of
north to northeast marching monsoonal moisture will bring daily
showers and thunderstorms. Afternoon focus will be over the higher
terrain with valley drift occurring in the late afternoon and
evening hours. Weak steering flow will result in slow moving cells,
with heavy rain likely from the stronger storms. Precipitable water
values were around an inch on the 12Z soundings from Grand Junction
and Flagstaff with over two inches down in Phoenix. As we start to
see more widespread convection and heavier rainfall, soils will
start to saturate in some areas (hard to believe after the months of
dry weather) with potential for flooding/flash flooding increasing.

Looking for triggers in these deeper monsoon pushes can be
challenging, especially if we stick to the standard pressure levels.
However, cross sections can help find a few hidden triggers that may
aid in convective development and there's some weak support
anticipated to drag across the northern zones Wednesday morning.
This should be enough to trigger a few showers and possibly a
thunderstorm or two tomorrow morning somewhere between Meeker, Craig
and Steamboat Springs (at least that's the plan right now). Slight
exhale from mid-morning into the early afternoon, before we start
building another round of diurnally driven convection Wednesday
afternoon and evening. Since most of the forecast area will just be
getting started with more robust rainfall potential on Wednesday,
we're still holding off on any type of hydro watches, however. our
burn scars will continue to be monitored closely for short duration
watches and/or warnings as needed.

Temperatures will continue to moderate somewhat for afternoon highs
with values dropping into the "near normal" range, but mild to
somewhat warm nights will stick around with sfc dew points in the
50s and generally muggy conditions (muggy for us) sticking around.

.LONG TERM...(Thursday through Tuesday)
Issued at 109 PM MDT Tue Jul 20 2021

Not much change from previous model runs as increased moisture
remains across the forecast area. Afternoon and evening showers and
thunderstorms will be common, more so down south, with some heavy
rain mixed in thanks to the monsoonal push.  Forecast PWATs continue
to run in the 1+ inch range over much of the area with some spots
seeing values running up to 1.3 inches. This is a lot of available
moisture for daytime heating, favorable orographics, and any little
wiggles that will create increased instability. With PWAT values
that high, and remaining high day after day, flash flooding risks
will increase especially over areas that have seen continued
rainfall or recent burn scars. For now, areas along and south of I-
70 are favored for the precip, sorry northern valleys, with the San
Juans really looking to see the brunt of heavier precip.

Through the long-term period, there will be days with more
widespread convection and some days with a bit less but the guidance
does show precip for every day. The high pressure system will wobble
and move around as disturbances to the north deflect the ridging and
force the center of the high to shift the east. When this occurs,
the high will tap into moisture from the Gulf of Mexico and stream
it up into the CWA for the weekend. Some drying does look to occur
Monday and Tuesday according to the GFS while the EC keeps precip
going. Suffice to say, we'll see plenty of precip through the period
which is good news for the drought, bad news for possible flash
flooding. If travelling along the I-70 corridor through this period,
please stay up-to-date on the Grizzly Creek burn scar as heavy rain
over the scar may force occasional road closures from time to time.

Temperatures will range from near average to a bit below depending
on cloud cover and precipitation.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1130 PM MDT Tue Jul 20 2021

Scattered storms will continue overnight and into Wednesday
morning, focused on the eastern tier of the forecast area. Skies
will remain scattered to broken through the night with generally
light terrain driven winds, with occasionally breezy conditions
due to lingering outflow winds. Storms will redevelop over high
terrain after 18Z Wednesday with drift into valleys after 21Z with
potential for gusty outflow winds and heavy rain. Conditions will
generally remain VFR with brief reductions to MVFR in heavier
showers.

&&

.HYDROLOGY...
Issued at 109 PM MDT Tue Jul 20 2021

Deep monsoonal moisture is expected to bring heavy rain producing
showers and thunderstorms to much of Eastern Utah and Western
Colorado over the next 7 to 10 days. Storms will be slow moving with
some training of echoes anticipated. Burn scars will be especially
susceptible to heavy rainfall. We will continue to monitor things
closely as we work through the week and as grounds become more
saturated each day.

&&

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

SHORT TERM...JDC
LONG TERM...TGR
AVIATION...TGJT
HYDROLOGY...JDC

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GRAND JUNCTION CO
1152 PM MDT TUE JUL 20 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0925 PM     DEBRIS FLOW      3 NE SHOSHONE           39.61N 107.15W
07/20/2021                   GARFIELD           CO   DEPT OF HIGHWAYS

            NUMEROUS PHOTOS AND WEBCAM IMAGES OF DEBRIS
            FLOW OVER I-70. WESTBOUND LANES BLOCKED AND
            REQUIRE DEBRIS REMOVAL CLEANUP.

&&

MAC

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GRAND JUNCTION CO
1158 PM MDT TUE JUL 20 2021

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0530 PM     FLASH FLOOD      37 NNE CASTLE VALLEY    39.13N 109.17W
07/13/2021                   GRAND              UT   NWS STORM SURVEY

            NWS STORM SURVEY FOUND EVIDENCE OF A MAJOR
            FLASH FLOODING EVENT IN THE WESTWATER
            DRAINAGE VALLEY. WATER WAS ESTIMATED TO BE 6
            TO UP TO 12 FEET DEEP BASED ON EROSION OF
            THE ARROYO WALLS. NUMEROUS INSTANCES OF DRY
            ARROYO CROSSINGS WASHED OUT WITH MAJOR
            TRANSPORT OF SEDIMENT AND DEBRIS DOWNVALLEY.

&&

MAC