Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 181741
AFDGJT

Area Forecast Discussion
National Weather Service Grand Junction CO
1141 AM MDT Fri Jun 18 2021

.SHORT TERM...(Today through Saturday)
Issued at 404 AM MDT Fri Jun 18 2021

Little change to the ongoing forecast as abnormally strong high
pressure over the SW U.S. continues to sit in place, although it
will be getting suppressed or flattened out over the next 36 hours
or so. One last run at record heat today for several sites, before
we finally see heights fall enough on Saturday that out string of
heat should ease up a bit. Cloud cover may also help to stop the
record run today, especially across our northern zones, although
the morning lows have remained toasty with GJT dropping all the
way down to 78 so far. The record highest minimum temperatures at
KGJT currently stands at 71 set way back in 1918. With clouds
overhead, not expected a whole lot of movement, so there`s a good
chance we may set a new record warmest low, but we still have a
few more hours and then the evening. But since we`re smashing
records on a daily basis right now, what`s one more? In any case,
the current excessive heat warning will remain in effect for one
last day.

Enough with the heat as there`s another piece of weather to talk
about now and that`s the recycling of moisture working under and
through the ridge. Have been seeing a slow uptick in convective
coverage over the last two days and this will be peaking today and
Saturday. Surface dew points have climbed up in the mid 30s to
lower 40s this morning (which is decent compared to recent days)
and models aggressive with SBCAPE values this afternoon and again
on Saturday exceeding 500 J/kg over the higher ridges, especially
down south across the San Juans. With soundings still favoring the
inverted-V shape and best MUCAPE above the 0 degree line, the
potential for lightning will be on the increase, likely peaking on
Saturday. Very dry conditions below cloud bases will result in
gusty outflow winds as rain evaporates. Saw a few heat bursts last
evening across the west slope as well as showers decayed with
remnant outflow boundaries roaming around. So high-based
convection forming over the higher terrain and near boundaries
will bring increased concerns for fire weather. More on that
below. If any column saturation occurs, it will most like be on
Saturday and favored over the San Juans.

So the positives, 2 to 4 degrees cooler today and another 2 to 4
degree drop on Saturday and a few more clouds to help with that
cooling. The bads, isolated to scattered dry thunderstorms which
increases fire potential with very dry fuels across the region and
the blanket of clouds holding temps up at night.

.LONG TERM...(Saturday night through Thursday)
Issued at 404 AM MDT Fri Jun 18 2021

Long wave trough digging across the Northern U.S. and across the
Great Lakes shifts our persistent ridge southwest with ridging
amplifying over the Great Basin. Expect increasing northwesterly
flow to arrive on Sunday and continue into the fire part of the
new work week. Gradient sufficient to bring heightened fire
concerns back into the picture for wind and RH again vs the
ongoing lightning threat. This will also sweep any moisture over
the area right on out to the east with the ridge easing back
overhead by Tuesday. So after being teased with an early push of
northward advancing moisture this week (aka monsoonal push),
we`re back into the hot and dry pattern by midweek. But fear not,
this time the ridge should traverse eastward as another longwave
trough digs into the Western U.S and tries to draw up another
round of moisture into the Four Corners region by late in the new
week. Temperatures will remain will north of the normal line (or
above normal) through this extended period.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1133 AM MDT Fri Jun 18 2021

Isolated to scattered storms will materialize at the top of the
hour and increase in coverage this afternoon. Minimal rain is
expected to fall from said storms, with main concerns to TAF
sites focused on gusty outflow winds and lightning. Storms will
collapse after sunset, however, outflow boundaries persisting late
into evening hours may create strong wind gusts for a few hours.
Confidence in where these boundaries form is quite low, so gusts
were left out of TAF lines attm. Though, felt this was worth
mentioning as gusts could reach in excess of 50kts. Otherwise,
VFR conditions are expected through the night and into Saturday
morning with another round of convection developing in the
afternoon.

&&

.FIRE WEATHER...
Issued at 404 AM MDT Fri Jun 18 2021

Moisture continues to rotate under a potent high pressure system
allowing for isolated thunderstorms to pop up over mountainous
terrain this afternoon. Expect coverage of these storms to
increase into Saturday. Due to the prolonged period of hot and
dry conditions we have elected to issue Red Flags for dry
thunderstorms this afternoon and a Watch on Saturday. There are
increased concerns for fire starts ignited by lightning paired
with gusty outflow winds from said storms. Main issues for
Saturday will be coverage and potential that some storms may
become more efficient at producing wetting rains as moisture
profile peaks. So held off on an upgrade for now as we fine tune
things.

Fire weather concerns pivot to wind and relative humidity on
Sunday as the ridge breaks down and an approaching cold front from
the northwest causes winds to increase across much of the region,
but especially in the north. According to long range models, the
front will pass through the region largely dry on Sunday night, so
wind and relative humidity could continue to be a problem for the
southern zones heading into early next week.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Fire Weather Watch from Saturday afternoon through Saturday
     evening for COZ203-205-290>295.

     Excessive Heat Warning until midnight MDT tonight for COZ006-020.

     Red Flag Warning until 9 PM MDT this evening for COZ291-293-295.

UT...Fire Weather Watch from Saturday afternoon through Saturday
     evening for UTZ485-487-491.

     Excessive Heat Warning until midnight MDT tonight for UTZ024-027-
     029.

     Red Flag Warning until 9 PM MDT this evening for UTZ485-491.

&&

$$

SHORT TERM...JDC
LONG TERM...JDC
AVIATION...TGJT
FIRE WEATHER...TGJT


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