Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 200440
AFDGJT

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
1040 PM MDT Sat Aug 19 2017

.SHORT TERM...(Tonight through Sunday night)
Issued at 348 PM MDT Sat Aug 19 2017

Afternoon convection has developed as expected today as a weak
wave passes through the region bringing typical summertime
scattered showers and storms over the higher terrain. Available
moisture remains low for this time of year as evident by the below
normal precipitable water value of 0.58 inches seen on this
morning`s upper air sounding. With that said, the primary threat
from storms this afternoon and continuing into the evening hours
will be gusty outflow winds. Synoptically the overall weather
pattern remains the same as early this morning with a Pacific
upper level low situated well to our west over southern California
and a broad area of high pressure extending across portions of
the southwestern CONUS and further east into southeast
Texas/Louisiana.

A shift in the upper levels will begin during the latter half of
Sunday as the aforementioned Pacific low deepens, ushering the high
pressure east. This will allow for moisture to be advected into our
forecast area on the backside of the high, increasing precipitable
water values to around 0.75 inches by Sunday afternoon and to around
an inch Sunday night in the Four Corners region and portions of
southwest Colorado. The Euro does now appear to be more on board
with the moisture increase so the onset and coverage of showers and
thunderstorms should be earlier and more scattered on Sunday
compared to previous days.

As the previous forecaster mentioned, after a warmer day today
daytime highs should cool a few degrees on Sunday in response to
clouds and showers, especially for areas south of I-70 where
convection will be favored throughout the day.

.LONG TERM...(Monday through Saturday)
Issued at 348 PM MDT Sat Aug 19 2017

Of course, the big headline and forecast concern for the beginning
of the extended period is in regards to expected cloud cover during
the total solar eclipse on Monday, August 21. Early Monday will see
the closed low continuing to spin over southern California but the
area of high pressure looks to gradually shift back to the southwest
throughout the day. This will leave our region in a messy
transitional zone so while we will not have a more direct stream of
moisture from down south as seen on Sunday, some moisture will
still be allowed to trickle into the region. Additionally, there
will be plenty of residual moisture which will allow for another
round of scattered afternoon convection over the higher terrain,
especially across the San Juans. Given the presence of this
residual moisture confidence is high that the majority of the
forecast area will see at least some mid and high based clouds
throughout the day. One promising sign is the presence of
relatively high dewpoint depressions at the 700mb level for Monday
which would suggest the introduction of some drier air into the
region and trend away from heavier cloud cover. Definitely
encourage everyone to check in with the forecast over the next 24
to 36 hours as we get closer to Monday!

Slightly better coverage of convection is expected for Tuesday
with showers and storms firing off around midmorning and
continuing through the evening hours as the low keeps spinning
over southern California and the high remains centered to our
southeast. A deep upper level low will begin to drop south off
the coast of the far Pacific Northwest on Wednesday and will be
the driving force to finally usher the SoCal low into the Great
Basin for late Wednesday and through the end of the week. The
arrival of the SoCal low will draw substantial monsoonal moisture
into our region, bringing better coverage of showers and storms as
well as cooler temperatures. The GFS is the more progressive and
wetter solution at the moment but either way, both the GFS and the
Euro are in agreement for a cooler and unsettled end to the work
week.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1040 PM MDT Sat Aug 19 2017

Some mid level cloudiness will be drifting out of the Great Basin
to the Rockies overnight. Any cigs from these clouds should stay
above ILS break points so VFR will rule over the next 24 hours.
Moisture is on the increase however and a better coverage of
storms is anticipated by peak heating on Sunday. Gusty winds from
these showers will be the main threat with this initial push of
moisture.

&&

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

$$

SHORT TERM...MMS
LONG TERM...MMS
AVIATION...TGJT


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