Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 250600
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
1200 AM MDT Fri Sep 25 2020

.UPDATE...
Issued at 802 PM MDT Thu Sep 24 2020

Winds have decreased while relative humidities recovered above 15
percent across northwest Colorado early this evening. Therefore,
the Red Flag Warning which was in effect until 8 PM has been
allowed to expire. Hot, dry and breezy conditions redevelop Friday
afternoon and a Red Flag Warning remains in effect from 1 PM to 8
PM Friday evening for northeast and east-central Utah as well as
northwest and west-central Colorado.

&&

.SHORT TERM...(Tonight through Friday night)
Issued at 340 PM MDT Thu Sep 24 2020

The ridge of high pressure will continue to break down over the
next 6 to 12 hours as a strong Pacific trough treks across the
northwest CONUS. As has been the trend over the last several model
runs, eastern Utah and western Colorado will not be receiving any
precipitation from this system. Instead, high clouds will pass
overhead with gusty southwest winds developing at the surface as
the gradient tightens. The focus for these strong winds will
remain across northwest Colorado through this evening before
coverage spreads south centered near I-70 tomorrow afternoon and
early evening. Localized conditions will be possible elsewhere,
particularly across southeast Utah.

No other weather concerns to speak of in the short term apart from
the Red Flag conditions in the aforementioned areas. Temperatures
will continue to run above normal for late September...by around
6 to 12 degrees during the day and around 5 to 10 degrees
overnight.

.LONG TERM...(Saturday through Thursday)
Issued at 340 PM MDT Thu Sep 24 2020

Critical fire weather conditions may be a concern on Saturday as the
northern half of the forecast area will be on the southern fringe of
a broad trough and strong 40-80 knot 500mb jet. Forecast soundings
are advertising deep inverted V signatures which should lead to
efficient boundary layer mixing. Fuels will still be dry and
relative humidities still low, so localized Red Flag conditions are
possible. We opted not to issue any highlights with this package but
future shifts may need to consider. Otherwise, expect a dry and
unseasonably warm Saturday with temperatures 5-10 degrees above
normal.

The latest 12Z guidance has introduced a fair amount of uncertainty
with the forecast for Sunday into early next week. Prior
deterministic and probabilistic models were insistent on a deep
trough of low pressure descending out of Canada and digging into the
northern Plains and upper Midwest through the early/middle part of
next week. By the end of the week, the base of the trough was
progged to reach as far south as the northern Gulf Coast before
spreading eastward. To our west, a large amplitude ridge was
forecast to build over the western CONUS. Our CWA was largely stuck
in between these two major features, which essentially resulted in a
dry and unseasonably warm forecast. Fast-forward to this afternoon
and the models are now showing a vastly different solution. Instead
of keeping the vort maxes embedded within the trough more
consolidated, one particularly potent shortwave now digs from the
Pacific Northwest into the central Rockies and passes directly over
eastern Utah and western Colorado Sunday into Monday. Associated
with this shortwave is also a strong cold front that would be
capable of pushing through the entire forecast area. This is a big
shift west which obviously has multiple implications to the
forecast. These implications include winds, temperatures
(significant cool down) and, yes, even precipitation. Given ensemble
means are also beginning to catch on to this idea, my gut is that
this isn`t just a one off solution and is likely the start of a
trend. It`s also possible that the adjustment westward isn`t
completely finished, with more shifts to come. With so much
uncertainty introduced with this package, will opt to go with the
blended solutions for now. However, it should be noted that if this
trend continues, large scale changes to the forecast will be
necessary.

The trough lifts eastward out of the central Rockies and into the
Plains late Monday into Tuesday, with high pressure, dry weather and
moderating temperatures taking control through the end of the long
term period.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1158 PM MDT Thu Sep 24 2020

VFR under SKC conditions can be expected over the next 24 hours.
Winds will be increasing Friday afternoon with frequent gusts in
the 25 mph range expected at many of the TAF sites.

&&

.FIRE WEATHER...
Issued at 802 PM MDT Thu Sep 24 2020

The storm track will remain across the Northern Rockies the next
few days where precipitation is possible. In portions of Utah and
Colorado only wind will result from this storm track. Afternoon
temperatures running 5 to 10 degrees above normal will allow
afternoon humidity to again drop into the lower teens across the
northern and central zones on Friday where Red Flag Warnings are
in effect. Localized conditions are possible farther south,
particularly in southeast Utah. Winds decrease this weekend as the
dry and warm weather continues.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Red Flag Warning from 1 PM to 8 PM MDT Friday for COZ200>203-205.

UT...Red Flag Warning from 1 PM to 8 PM MDT Friday for UTZ486-487-490.

&&

$$

UPDATE...NL
SHORT TERM...MMS
LONG TERM...MDM
AVIATION...TGJT
FIRE WEATHER...TGJT


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