Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 182037

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
237 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 heigtened 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 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1043 AM MDT Tue Jul 18 2017

Still looks like scattered showers and thunderstorms mainly this
afternoon and evening south of a KEGE to KMTJ to KBDG line, some
with heavy rain, small hail and local MVFR cigs/vsbys. N of the
line expect some showers and thunderstorms, though less than to
the south of the line. Otherwise VFR conditions can be expected to
prevail through 18Z Wednesday, but an increase in showers and
thunderstorms thereafter from the south will increase the
likelihood of more areas with mountain obscuration and local MVFR


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

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



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