Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 211734
AFDGJT

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
1134 AM MDT Mon Aug 21 2017

.UPDATE...
Issued at 902 AM MDT Mon Aug 21 2017

Updated sky cover grids this morning through the afternoon based
on current observations and recent trends. Most areas are clearing
out nicely for a good viewing of the eclipse while more cloud
cover is moving into southwest Colorado and portions of eastern
Utah.

&&

.SHORT TERM...(Today through Tuesday)
Issued at 409 AM MDT Mon Aug 21 2017

A complex weather pattern was in place over the southwestern U.S.
early this morning. Low pressure pressure remained over southern
California with a separate low circulating over northwest New
Mexico. Meanwhile, a mid-level high was overhead with a shallow
trough over the northern Rockies. Stretching deformation between
these systems was responsible for increased cloud cover across
much of the forecast area and virga/sprinkles over northwest
Colorado and northeast Utah.

The trough to the north shifts eastward during the day while the
low over New Mexico drifts slowly to the northeast and fills in.
Meanwhile, the high overhead remains quasi-stationary and mid-
level flow will be weak and north/northwesterly in direction which
will limit moisture advection into the region. However, residual
moisture undoubtedly sufficient for another round of afternoon
and evening showers and thunderstorms, mainly over the higher
terrain. Unfortunately, clouds associated both with deformation
and instability will create some challenges for eclipse viewing
during the late morning and early afternoon.

Models indicated a disturbance ejected from the southern California
low will get showers going Tuesday morning south of I-70 corridor.
Thereafter, rising temperatures destabilize the airmass and
showers and thunderstorms will break out over the higher terrain
to the north during the afternoon.

Temperature guidance has been a bit cool over the past couple of
days so went a bit warmer than previously forecast for today, but
still a degree or two cooler than yesterday. The cooling trend
should continue into Tuesday in response to increased clouds
early in the day.

.LONG TERM...(Tuesday night through Sunday)
Issued at 409 AM MDT Mon Aug 21 2017

So the general trend of extended models remains the same. The
upper ridge laying across the length of the Rockies on Wednesday
will become sheared off by a fairly strong northern stream
system, retreat southward through weeks end, only to rebuild
stronger back to the west by early next week. With this ridge the
driver of the weather this time of year the two low pressure
systems in the picture really only have bit parts. The stronger
system mentioned above remains well north of the CONUS border and
aside from suppressing the ridge, is too far north to even push a
front into our region. The second low, off the southern left
coast, has trended a bit southwest from 24 hours ago. It also
appears to be too far south to be much affected by the low to the
north. So Thursday remains the best shot at precipitation coverage
for our CWA. This is when some energy is released from the
southern low and a brief surge of moisture mixes over the Eastern
Great Basin/4 Corners. Not impressive but the best signal of the
extended forecast. The westerlies are a bit stronger on Friday
which could lead to some stronger storms forming in the lingering
moisture. Going into the weekend, drier northwest flow sets up
over the Rockies as the high builds over the Intermountain West
and storm coverage should decrease significantly. As for
temperatures, seasonal should cover it.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1134 AM MDT Mon Aug 21 2017

VFR conditions expected to prevail at most TAF sites through the
next 24 hours. Mostly to partly sunny skies will become partly to
mostly cloudy through late afternoon into evening as more moisture
works in from the south and west. Expect thunderstorms to favor
higher terrain this afternoon and evening with brief reductions in
vsby/cigs due to showers, mainly for mountain TAF sites of KASE
and KTEX. KDRO and KGUC could see some isolated chances as storms
drift off higher terrain.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...None.
UT...None.
&&

$$

UPDATE...MDA
SHORT TERM...NL
LONG TERM...15
AVIATION...MDA



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