Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 221753
AFDGJT

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
1153 AM MDT Sat Jul 22 2017

.SHORT TERM...(Today through Sunday)
Issued at 454 AM MDT Sat Jul 22 2017

Current water vapor satellite indicates mid and high level
moisture still streaming across the area from the south. The low
levels are drying out in some areas but some patches of low level
moisture remain which may be a result of the rain-cooled air from
last night`s precipitation leaving higher dew point values in
some areas. No fog to speak of yet, but there may be some patchy
fog due to the recent rainfall that occurred yesterday evening
along the leading edge of a frontal boundary and upper level
shortwave trough that progressed slowly across the area from north
to south, with precipitation yesterday evening focused along the
central areas of the CWA.

This mid and high level moisture draws some concerns for the
going forecast for the rest of today. Short term models indicate
that drier air has progressed from north to south behind this
boundary/shortwave trough in a northerly flow with forecasted
precipitable water (PW) values less than 0.7 inches along and
north of I-70 with some as low as 0.5 across the northern valleys.
Radar trends yesterday evening indicated subsidence and
dissipation of shower activity behind this boundary in a northerly
flow as shower and storm activity moved from north to south,
becoming more stratiform as the clock approached midnight. The
models do tend to want to dry the atmosphere out too quickly after
a monsoonal moisture surge, but thinking this trough and boundary
was robust enough to push some of this deeper moisture out of the
northern parts of the area and focus it further south. Is this
the case this time? We shall see.

Overall, expecting a decreasing trend in thunderstorm activity
today, especially for areas along and north of I-70. Enough
moisture remains for storms to fire over the higher terrain this
afternoon, so introduced mention of isolated storms over the
northern higher terrain, including the bookcliffs. The focus for
thunderstorm activity will be over the central and southern
mountains with perhaps some drift into the southern and central
adjacent valleys as storms drift off the terrain to the south.
The shear profile shows more backing for northern and central
areas which will make it difficult to sustain updrafts resulting
in some pop and drop variety storms. Down south however across
southeast Utah and southwest Colorado, the shear profile shows
some slight veering, where we could see some stronger storms
capable of small hail, gusty outflow winds and heavy rain. While
storms will still be capable of heavy rain due to still present
moisture down south, expect the flash flooding threat to be more
localized with a few stronger storms having this potential.
Decided to not issue any Flash Flood Watch today as the coverage
is not as widespread as previous days. Expect most activity to
wind down this evening with no distinct shortwave or disturbance
to keep activity going overnight once surface heating is lost.

High pressure over the Great Basin will continue to push drier air
across the region on Sunday in a northerly flow, suppressing the
best moisture to the southern mountains and valleys. Expecting
some afternoon storms over mainly the higher terrain with
temperatures moderating towards near or slightly above normal
levels.

.LONG TERM...(Sunday night through Friday)
Issued at 454 AM MDT Sat Jul 22 2017

High pressure over the Great Basin slides east on Monday over our
CWA with some moisture being recycled under the high, resulting in
afternoon storms mainly over the higher terrain but otherwise
drier conditions. The ridge continues to slide east of the area by
Tuesday into Wednesday, resulting in another increase in monsoonal
moisture from the south. A Pacific trough moves onto the west
coast at this time as well, with the upper level jet positioned
favorably over Utah and Wyoming, leaving our CWA in the left exit
region. The forecast models seem to be honing in on early to
middle of next week for the next moisture surge even though there
are some discrepancies on exact timing. Embedded disturbances move
through the flow on Tuesday and Wednesday resulting in an
increase in thunderstorm coverage. This trend should continue
through the rest of the week as moisture remains over the area, so
began trending this direction with PoPs showing a better increase
in chances and coverage from Tuesday onwards.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1153 AM MDT Sat Jul 22 2017

Moisture was being driven to the south today confining showers and
thunderstorms to the central and southern mountains of Colorado
primarily. Consequently, expect all TAF sites with the exception
of KTEX and KDRO will have CIGS above ILS breakpoints and VFR
conditions. Thunderstorms already starting over the San Juan
Mountains and it`s likely the KTEX will experience thunderstorms
with heavy rain resulting in brief MVFR visibility. There is a
good chance that northerly flow will carry cells from the San
Juans over KDRO later this afternoon into early evening. Quiet
weather will return after 03Z and will continue through noon
Sunday.


&&

.GJT WATCHES/WARNINGS/ADVISORIES...

CO...Flash Flood Watch until 9 PM MDT this evening for COZ019.

UT...None.

&&

$$

SHORT TERM...MDA
LONG TERM...MDA
AVIATION...NL



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