Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 201819
AFDGJT

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
1219 PM MDT Thu Jul 20 2017

.SHORT TERM...(Today through Friday)
Issued at 448 AM MDT Thu Jul 20 2017

A fairly active night yesterday and morning as shower and
thunderstorm activity continued through the early morning hours.
The best focus was concentrated over the Four Corners region just
before and after midnight as an impressive MCS (Mesoscale
Convective System) or thunderstorm developed near Shiprock, NM
and tracked over the Four Corners region with GOES-16 satellite
data indicating an expansive cold cloud top as cold as -66 degrees
C with frequent lightning. Likely some impressive heavy rainfall
out of that system that impacted extreme southwest Colorado and
most of southeast Utah south of Blanding. Storm coverage yesterday
was more than the short term forecast models indicated as
expected with model QPF values not very representative of what
occurred. Rainfall amounts across the region ranged as high as
0.75 inches in some areas with some locally higher amounts that
may have occurred, with high rainfall rates and some minor
flooding across the watch area. Activity has largely diminished
after 3 am with clearing skies across southwest Colorado and
lingering showers with isolated thunderstorms over southeast Utah
from that previous complex, and some light showers up north
associated with the passing shortwave.

As far as today is concerned, moisture remains in place with over
an inch of precipitable water (PW), mainly along and south of the
bookcliffs. Suspect the models are underplaying the QPF once again
and extent of the storm activity. Looking at H5 dewpoint
depressions, this seemed to capture the activity last night fairly
well, so using this as a basis for storm coverage today instead of
model PoPs. Higher PW values are shown in eastern Utah with values
approaching 1.4 inches towards the Four Corners. Looking at
streamlines from the low to mid levels, it appears like several
embedded disturbances and wiggles rotate through the southerly
flow this afternoon and evening. Leftover boundaries from
yesterday`s convection will also lend themselves to convective
initiation and storm interactions. Due to recent rainfall, the
ground is already saturated in many areas. Therefore, expect once
surface heating gets going this morning, this will help act on the
deep moisture already in place to generate scattered to numerous
showers and thunderstorms all capable of heavy rainfall rates.
Areas already saturated will be more susceptible to flash
flooding. Therefore, in coordination with SLC, decided to issue a
Flash Flood Watch for this afternoon and evening as well until
midnight tonight for this potential. The area included is along
and south of the bookcliffs (not including the central and
southern divide mountains). Decided to also include COZ008 which
includes Carbondale and Glenwood Springs, which tend to be
somewhat flash flood prone areas since that area is likely
saturated from recent rains yesterday.

Storm activity will mostly wind down after midnight tonight, but
expect some to linger through the overnight due to embedded
disturbances and interacting outflow boundaries with deep moisture
in place. The moisture shifts slightly eastward on Friday but
still remains in place across much of the area. The focus for
heavy rain and potential flash flooding shifts towards the central
and southern divide mountains as well, as this will be the third
day of saturated soils from previous rainfall. The mountain areas
tend to be more prone after a few days of rain. This will be
something to watch and consider for future shifts.

.LONG TERM...(Friday night through Wednesday)
Issued at 448 AM MDT Thu Jul 20 2017

Some nocturnal showers and thunderstorms will more than likely
continue through Friday night, especially along the Continental
Divide, as plenty of monsoonal moisture remains. Additionally, the
GFS tracks an embedded disturbance across the forecast area which
would help enhance precipitation overnight, though the Euro is not
on board with this solution just yet. By Saturday, extended models
have high pressure building over the Great Basin. This will usher
drier air into the region and stunt the advance of monsoonal
moisture. As a result, there will be a trend toward decreasing
clouds and thunderstorm activity for the weekend. However, some
moisture from down south may ride along the eastern side of the high
which will keep scattered convection in the forecast for the
southern and central Colorado mountains.

The area of high pressure will gradually slide east through the
first half of the work week, though models continue to be at odds as
to when the next surge of monsoonal moisture will be able to move
into the area. The Euro persists in reintroducing moisture into the
region as early as Monday while the GFS holds off until closer to
midweek. This is much different than 24 hours ago when the GFS was
more in line with the Euro, resulting in lowered confidence on the
timing of the next monsoonal moisture surge. Given the continued
model differences, PoPs for the new work week remain on the
conservative side with just scattered convection favoring the
higher terrain with an increase in coverage towards mid-week.
Ultimately, the precipitation forecast for next week will all ride
on how the ridge of high pressure progresses across the
Intermountain West.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1216 PM MDT Thu Jul 20 2017

Scattered to numerous showers and thunderstorms expected through
this evening, some with heavy rainfall and small hail that will
obscure mountain tops and reduce ceilings and visibilities to MVFR
category, including reaching ILS breakpoints. Showers and
thunderstorms will largely diminish after 06Z tonight, though some
will continue overnight and into the morning hours at a lesser
intensity.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Flash Flood Watch until midnight MDT tonight for COZ003-006>009-
     011-017-020>022.

UT...Flash Flood Watch until midnight MDT tonight for UTZ022-025-
     027>029.

&&

$$

SHORT TERM...MDA
LONG TERM...MMS/MDA
AVIATION...JRP



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