Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 241048

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
448 AM MDT Mon Jul 24 2017

Issued at 445 AM MDT Mon Jul 24 2017

Flash Flood Watch issued for central and southern regions of
eastern Utah and Western Colorado from noon today through midnight
Wednesday morning. A renewed surge of monsoonal moisture will work
northward through the Four Corners region today and tonight,
increasing the coverage of showers and thunderstorms over the
watch area. Another round of thunderstorms is expected on Tuesday
afternoon and evening.


.SHORT TERM...(Today through Tuesday)
Issued at 445 AM MDT Mon Jul 24 2017

Primary forecast updates early this morning in the short term have
been to increase both PoP and QPF from noon today through
Wednesday morning area- wide. As high pressure sits just east of
the Continental Divide, southwesterly flow aided by a trough of
low pressure along the west coast will allow for a rapid return of
available moisture over the central and southern portions of the CWA.
Forecast PWAT values steadily increase through the day today and
into Tuesday, peaking at near- record values (1.3 - 1.4") by
Tuesday morning per GJT sounding climatology. While some dry air
to the north of the Tavaputs/I-70 corridor will remain, confidence
is high that any slower-moving storms in central and southern
zones will have the potential to produce flash flooding. CAM
guidance hits the southeast Utah canyon- country and the San Juan
mountains in Colorado particularly hard, with three distinct waves
of precipitation through the period. The first comes this
afternoon in a typical daytime heating regime. By the 06z to 12z
timeframe, a shortwave/vort max lifts northward from Arizona, with
overnight convection spreading through southeast Utah and western
Colorado. A final round of diurnally-driven convection on Tuesday
afternoon will bring additional rainfall to much of the region
before a drier air trickles in from the west after midnight.
Therefore, have issued FFA products accordingly...beginning at
noon today and lasting through midnight Wednesday morning. See
FFAGJT for further details.

.LONG TERM...(Tuesday night through Sunday)
Issued at 445 AM MDT Mon Jul 24 2017

Flow of subtropical moisture into our area is expected to be
suppressed beginning Wednesday as the upper high circulation
migrates westward. However, still a very moist airmass likely to
remain over our area on Wednesday and Thursday as flow aloft
should be light and residual moisture likely to remain. In fact
GFS keeps one inch plus PWATS over most of our area through
Thursday. Drier air then expected to make deep inroads into our
CWA from the north Friday and Saturday, though significant
moisture likely to remain over southeast UT and southwest CO, much
like what occurred this past weekend. Then the GFS has the upper
high drifting eastward on Sunday in response to a trough
approaching the Pacific NW, resulting in a southerly flow which
will draw more subtropical moisture back into our area. The latest
EC model is similar to the GFS during the long term period,
except for late next weekend as it keeps the upper high center
well to our west on sunday and continues to keep subtropical
moisture suppressed to our south. Therefore, not a lot of
confidence at this time in the forecast for next Sunday.


.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 1042 PM MDT Sun Jul 23 2017

Subtropical moisture will be moving back northward into the area
today under a light southerly flow aloft that will lead to
increasing mid and high level clouds. Then expect scattered
showers and thunderstorms to begin developing from the south after
18Z and spreading northward, a few with heavy rain obscuring
mountain tops mainly above 12K feet. Otherwise, VFR conditions
will prevail across eastern UT and western CO through Monday


CO...Flash Flood Watch from noon MDT today through Tuesday evening
     for COZ003-006>014-017>023.

UT...Flash Flood Watch from noon MDT today through Tuesday evening
     for UTZ022-025-027>029.



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