Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 120540
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
1140 PM MDT Tue Aug 11 2020

.UPDATE...
Issued at 811 PM MDT Tue Aug 11 2020

Wind speeds have generally dropped below Red Flag criteria early
this evening. Therefore, the Red Flag Warning has been allowed to
expire for today over the western slope. A few isolated gusts
above 25 mph will still be possible on higher ridgetops through
early evening before winds fully diminish overnight.

&&

.SHORT TERM...(Tonight through Wednesday night)
Issued at 256 PM MDT Tue Aug 11 2020

Very dry conditions are in place with single digit relative
humidity in many lower elevations and low teens in some of the
higher spots. Deep mixing is occurring as the 12Z morning sounding
from GJT showed an elevated mixed layer to above 500 mb. This is
quite evident in the surface winds as winds have been gusting to
30 mph or even higher in the areas under a Red Flag Warning for
today. Satellite imagery on the Pine Gulch fire is showing three
distinct hot spots and it is quite evident that these conditions
are resulting in some extreme fire behavior, noticeable from the
massive smoke plume when looking north of the office outside. Some
accas are present across the higher terrain and valleys to the
north, with a few isolated virga showers but nothing significant
to speak of on radar as the atmosphere is incredibly dry. A
shortwave and 60 kt jet streak moving through the area is helping
keep winds elevated, gusting above the 90th percentile in terms of
model guidance once again. This has been a common trend last
several days as the models are not reflecting this deep mixing
to the surface very well at all.

Another shortwave will move through the area Wednesday as a deeper
upper low responsible for the stronger winds aloft moves further
inland across the Pacific Northwest and into the Intermountain
West. High pressure will slide westward a bit but remain south of
our area, keeping us in a drier and breezy westerly flow.
Expecting deep mixing once again in combination with hot and very
dry conditions. This will result in critical fire weather
conditions for east-central to southeast Utah as well as northwest
and west-central Colorado, areas closer to the tighter gradient
and stronger winds aloft. Very isolated virga showers are
possible, but this remains very limited...so anticipating another
dry, hot and breezy day.

.LONG TERM...(Thursday through Tuesday)
Issued at 256 PM MDT Tue Aug 11 2020

Ensemble and deterministic guidance remains in good agreement
through the long term period with a hot and dry pattern likely to
continue. On Thursday, a broad trough will traverse across the
Pacific Northwest and into the northern Rockies. Meanwhile, a stout
ridge of high pressure will remain centered over the southern Four
Corners. Here in between, the pressure gradient will tighten up and
deep PBL mixing will result in another breezy afternoon. While the
strongest belt of mid-level winds will be confined to Idaho, Montana
and Wyoming, winds here in eastern Utah and western Colorado will
still be enough to lead to potentially critical fire weather
conditions. Held off on issuing any additional highlights for this
forecast package and will let the next shift take another look with
the latest data. Aside from an isolated shower or storm in the high
terrain near the Continental Divide, a dry sub-cloud layer will keep
most of the CWA rain-free.

On Friday, the aforementioned trough lifts into the northern
Plains while the ridge remains in place to our south. A weak,
closed-low located a few hundred miles off the California
coastline will then help steer a weak stream of sub-tropical
moisture into the forecast area, particularly in southeast Utah
and southwest Colorado. This will lift PWATS into the 0.5-0.8"
range (which isn`t overly impressive, but is certainly higher than
anything we`ve seen over the past week or two). Under normal
circumstances, this would likely be enough to produce scattered
showers and storms. Unfortunately, as was mentioned in the
previous discussion, the residence time of this moisture will be
short-lived and likely won`t be enough to erode the very dry sub-
cloud layer (as evidenced by deep inverted V forecast soundings.
So once again, showers and storms will be most favored in the
highest terrain, and even there virga, gusty outflow winds and
lightning will be more likely than wetting rains. Winds will
overall be less of a concern compared to Thursday, but still can`t
rule out localized critical fire weather conditions.

Looking ahead to this weekend and into early next week, the strong
ridge to our south will become even stronger and build northward
over the Great Basin and Four Corners. In fact, ensemble guidance is
advertising 500MB heights rising to 2-3 standard deviations above
the mean. Combined with 700MB temps making a run at +20C, another
extended stretch of hot weather is on the table. Grand Junction has
reached 100 degrees or higher 13 times so far this year, and at this
point it`s looking likely we`ll be adding to that total a few more
times as we head into the middle of August.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1140 PM MDT Tue Aug 11 2020

Smoke carried to the northwest from the Grizzly Creek and Pine
Gulch fires were being detected by the sky sensors at KHDN and
KEGE in the early morning hours. It`s unclear whether the opacity
of the smoke layer constitutes a ceiling per se, but without
evidence otherwise have no choice but to add this detection as a
cloud deck for those sites. In the case of KEGE, the smoke has
resulted in a ceiling below the ILS breakpoint and it will likely
remain there through much of the night. Otherwise, VFR conditions
prevail. Strong winds aloft will mix to the surface around midday
resulting in breezy west or southwest winds during the afternoon
Wednesday. Showers and/or thunderstorms will be isolated and
limited to the Continental Divide and eastern Uinta Mountains and
are unlikely to impact TAF sites in the region.

&&

.FIRE WEATHER...
Issued at 256 PM MDT Tue Aug 11 2020

Strong flow aloft resulting from a disturbance moving across the
northwestern U.S. over the next few days is expected to bring
breezy conditions to the region through Thursday. Gusty southwest
and west winds combined with above normal heat and drought
stressed vegetation is expected to bring critical fire weather
conditions to southeast Utah and portions of western Colorado
today. Therefore, a Red Flag Warning remains in effect until 8 PM
this evening. Similar conditions are likely Wednesday for
northwest to west-central Colorado and east-central to southeast
Utah, so the Fire Weather Watch has been upgraded to a Red Flag
Warning with a few zones added that weren`t in the watch.
Additional highlights may be needed Thursday as conditions show
little change from the current pattern.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Red Flag Warning from 11 AM to 8 PM MDT Wednesday for COZ200-202-
     203-205.

UT...Red Flag Warning from 11 AM to 8 PM MDT Wednesday for UTZ487-490-
     491.

&&

$$

UPDATE...MAC
SHORT TERM...MDA
LONG TERM...MDM/MDA
AVIATION...NL
FIRE WEATHER...TGJT


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