Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 191715
AFDGJT

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
1115 AM MDT Wed Jul 19 2017

Updated aviation section

.SHORT TERM...(Today through Thursday)
Issued at 430 AM MDT Wed Jul 19 2017

Some showers with isolated lightning strikes are still lingering
across southeast Utah and southwest Colorado early this morning
with IR and water vapor satellite imagery indicating clouds and
moisture increasing across Utah and far western Colorado. Further
west, thunderstorms are blossoming near and north of Page, Arizona
and west of Lake Powell in south-central Utah. The focus of this
activity appears to be associated with an upper level shortwave
that will move through central and eastern Utah this morning and
afternoon, continuing to track northward along the far western
Colorado border and into northwest Colorado this evening. This
shortwave has a mid level cyclonic circulation and potential
vorticity maximum associated with it, which could mean the
presence of a MCV (Mesoscale Convective Vortex) which would lead
to thunderstorm activity lasting through the evening where this
tracks. Along with this forcing, precipitable water (PW) values
increase across eastern Utah and far western Colorado with values
exceeding an inch and as high as 1.3 inches over eastern Utah.
This axis of deeper moisture extends north towards the Wyoming
border. Therefore, expanded the Flash Flood Watch to include the
Uintas as well as the northeast Utah and northwest Colorado valley
zones for today. This shortwave will act as a focus for stronger
storms as this forcing acts upon the deeper moisture, with
potential for heavy rain and flash flooding. Areas more vulnerable
to flash flooding are slot canyons, slick rock canyons, normally
dry washes and any recent or previous burn scar areas. The Flash
Flood Watch now includes all of eastern Utah and far western
Colorado where the deepest moisture and better forcing exists.
Storm motion appears to be fairly light, which heightens flash
flooding concerns. Expect storm activity to continue through the
overnight hours as well due to the passing of this shortwave.

Moisture will shift slightly eastward on Thursday leading to the
development of scattered to numerous showers and thunderstorms
once again. However, there is no discernible shortwave moving
through to act on the deep moisture at this time and confidence on
flash flood threat areas is low for Thursday. Will want to wait
and see how today plays out before deciding whether or not to
issue a Flash Flood Watch for Thursday, as while the heavy rain
threat will be there, the flooding threat may be more localized.

.LONG TERM...(Thursday night through Tuesday)
Issued at 430 AM MDT Wed Jul 19 2017

An upper level trough pushes across the northern Rockies on
Friday, with moisture still in place to generate another day of
thunderstorm activity. This added forcing could lead to some
stronger storms as well with heavy rain. The focus looks to shift
towards the central and southern portions of western Colorado on
Saturday though as drier airmass advects in from the northwest
and pushes this moisture further south and east, with models
showing some higher QPF amounts along the divide and towards the
Front Range. This appears to be result of a shortwave moving
through. Eastern Utah and northwest Colorado look to see somewhat
of a downturn as this drier air advects in thanks to a ridge of
high pressure trying to build back over the Great Basin.

The ridge quickly shifts eastward again by Sunday into next week,
allowing for southerly flow to tap into the monsoonal moisture
again as PW values rise above an inch for the Four Corners into
western Colorado. Much better upper level support looks to be in
place by next week as an upper level jet sets up over Utah,
putting much of Colorado and southeast Utah in the left exit
region. Embedded disturbances move through this flow as well,
leading to an increase in storm activity and potential for heavy
rain.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1115 AM MDT Wed Jul 19 2017

Anticipating a stormy day today. Beginning to see cloud tops
increase in the San Juans where clear air has led to increasing
lapse rates. Thunderstorm activity should be fairly widespread
through 03z before shifting interest to NE Utah and NW Colorado
and will bring sites below ILS breakpoints at times. MVFR will be
possible if a TAF site is hit by stronger storms, but is expected
to be short lived. A similar pattern is expected Thursday.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Flash Flood Watch through this evening for COZ001>003-006-007-
     009-011-017-020>022.

UT...Flash Flood Watch through this evening for UTZ022>025-027>029.

&&

$$

SHORT TERM...MDA
LONG TERM...MDA
AVIATION...JAM


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