Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 190432

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
1032 PM MDT Tue Jul 18 2017

.SHORT TERM...(Tonight through Wednesday night)
Issued at 234 PM MDT Tue Jul 18 2017

Thunderstorms have developed on schedule again this afternoon over
the usual high terrain locations. Precipitable water on the KGJT
sounding this morning was 1.06 inches or approximately 1.5
standard deviations above normal. Not expecting anything too
intense today, but brief locally heavy rain possible.

An upper level ridge of high pressure extends from the Four
Corners east across the central Plains to the Mississippi River
Valley. An easterly wave on the southern side of the ridge is
moving across New Mexico, enhancing the convection in those areas
and perhaps as far north as the San Juan and Four Corners region
of our area this afternoon and evening. However, this feature will
round the western periphery of the upper level ridge Wednesday and
tap into the higher moisture pooled over AZ. As it move northward
over Utah Wednesday precipitable water will increase over our
region to between 1.2 and 1.3 inches, or about 2 standard
deviations above normal. Combine this with deep southerly flow and
quite a bit of potential instability, and concern is heightened
for heavy rain from thunderstorms which could lead to flash
flooding. Rainfall rates of around one inch per hour will be
possible. At this time, it appears the richest moisture will exist
from the Four Corners up through eastern UT and western CO to
about the Tavaputs/Bookcliffs. Will issue a Flash Flood Watch for
those areas for Wednesday, and discuss impacts in slot canyons,
slick rock canyons, and normally dry washes. This agrees well with
the slight risk of excessive rainfall issued for the general
region by the Weather Prediction Center. With the shortwave moving
through the region Wed night, would expect convection continue
overnight Wed night.

.LONG TERM...(Thursday through Tuesday)
Issued at 234 PM MDT Tue Jul 18 2017

Thursday looks to be a repeat of Wednesday but with an expansion
of the rich moisture farther east and northeast. Will likely need
to issue another Flash Flood Watch for most of the area Thursday.
By Friday there is good agreement with the upper level anti-
cyclone shifting east and heights lowering in response to an upper
level trough moving over the Pac NW. Will still have plenty of
moisture in place Friday, but after that drier air pushes south.
Will see reduced coverage of aftn/eve showers/thunderstorms Sat
thru Mon. Thereafter, confidence is low since extended range
models are not in agreement.

Temperatures will be slightly below normal Thursday and Friday due
to the increase in cloud cover and expected rainfall, but will
notch back up near normal for the weekend.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1032 PM MDT Tue Jul 18 2017

VFR conditions will prevail at all TAF sites until around 18Z
Wednesday when scattered to numerous showers and thunderstorms
will develop across the region. With storms, heavy rain and gusty
winds as well as local ILS CIGS in the vicinity of storms will all
be possible. Mountains will frequently be obscured. Coverage of
showers and storms will diminish after 03Z but some nocturnal
convection will persist, especially across eastern Utah.


CO...Flash Flood Watch from 10 AM MDT Wednesday through Wednesday
     evening for COZ003-006-007-009-011-017-020>022.

UT...Flash Flood Watch from 10 AM MDT Wednesday through Wednesday
     evening for UTZ022-025-027>029.



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