Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 201751

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
1151 AM MDT Sun Aug 20 2017

.SHORT TERM...(Today through Monday)
Issued at 350 AM MDT Sun Aug 20 2017

As models had predicted, low pressure along the West Coast
encroaching on high pressure to the south has brought a
southerly flow to areas south of the I-70 corridor. This pattern
shift is expected to bring a modest increase in moisture across
the southern half of the forecast area. This, in turn, should lead
to an uptick in diurnally driven shower/thunderstorm activity
over the southern and central mountains this afternoon and
evening. Moisture remains relatively high based so main threat
will be from outflow winds and lightning. Pockets of moderate to
heavy rain become possible as subcloud moisture increases later in
the afternoon and into the evening.

Diurnal cooling is expected to bring an end to moist convection by
midnight, though its likely areas of virga will persist as has
been the case last night and again early this morning. The pattern
appears largely unchanged over the area on Monday, so expect more
of the same with a modest increase in shower/thunderstorm coverage,
mainly over the higher terrain south of the I-70 corridor.

Forecast highs are expected to respond inversely to the increased
clouds/showers, most noticeably across southeast Utah and
southwest Colorado today and Monday. In contrast, overnight lows
are expected to be fairly persistent.

.LONG TERM...(Monday night through Saturday)
Issued at 350 AM MDT Sun Aug 20 2017

Ensemble and operational models suggest the upper ridge will
remain a dominant feature over the Southern/Central Rockies
through mid week. The ridge axis is generally over our CWA and
this doesn`t seem to be the best location to fully tap the monsoon
region. In fact PWAT anomalies early this week suggest drier air
being forced into the Gulf of Cali region with the subtropical tap
being entrained into the low off the left coast and pooling well
to our west. By late Wed there is stronger northern stream low
dropping out of the Gulf of AK with the attendant trof beginning
to force the Cali Low inland by Thursday morning. This pattern
shift in the West will be enough to force the ridge axis east and
we should get a better push of moisture in the region. Forcing
from a piece of the weakening southern low to our west will
combine with the moisture for probably the best precipitation
chances Thursday into early Friday. Otherwise the typical
diurnally driven storms favoring the terrain can be expected
Tuesday into Wednesday and again Friday. Northwest flow setting up
over the Rockies by next weekend due to the ridge rebounding back
to the west suggest a drier forecast. Temperatures will follow a
similar pattern with near to above normal going into Thursday then
cooler late week before popping back up for the weekend.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1151 AM MDT Sun Aug 20 2017

Scattered thunderstorms will occur over the higher terrain this
afternoon and continue through this evening with most activity
winding down after sunset. Expect gusty outflow winds and
lightning to be the primary concerns with storms with higher
elevation TAF sites being most at risk for impacts, including
KTEX, KASE, and KEGE where VCTS was introduced. Mid and high level
cloud cover will increase late this afternoon and evening from the
south and spread northward overnight with some partial clearing
possible across the north by Monday morning. VFR conditions should
prevail at all sites except brief reduction in CIGS and VSBY due
to showers over the higher terrain, but not expected to reach ILS




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