Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 122134

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
334 PM MDT Sat Aug 12 2017

.SHORT TERM...(Tonight through Sunday night)
Issued at 334 PM MDT Sat Aug 12 2017

Some light showers lingered across the north this morning with
plenty of sun to work on the moisture in place to fire storms over
all higher terrain this afternoon. Seems a bit more active this
afternoon than yesterday, with convection blossoming across
eastern Nevada, Utah and northern Arizona. This convection to the
west is associated with a shortwave that will be tracking through
the region this evening and overnight. Pretty much all the near
term models are indicating some form of storm activity continuing
through the overnight hours tonight with the HRRR picking up on
this activity fairly well. Storms are expected to increase across
eastern Utah late this afternoon and move into western Colorado
this evening with the east-central Utah and west-central Colorado
corridor being favored for overnight convection through the early
morning hours on Sunday. Increased PoPs as a result as confidence
is a bit higher due to model consistency picking up on this
shortwave with the NAMNest picking up on this activity in
yesterday`s model runs. This shortwave disturbance and storm
activity also comes with an increase in precipitable water (PW)
values above an inch for most areas with pockets of 1.25 near the
Four Corners. Expect some lingering isolated to scattered showers
after sunrise Sunday over the eastern zones as this shortwave
progresses eastward. Overnight lows were much cooler the last
couple nights than model guidance suggested, due to the rain-
cooled air from evening and overnight shower activity. Lowered
low temperatures tonight as a result.

Convection is expected to re-fire across the higher terrain
Sunday afternoon as sufficient moisture and enough instability
remains in place with storm activity drifting into valleys late
afternoon and evening as another shortwave tracks across. Expect a
similar day on Sunday like today (Saturday). Storms both days
will be capable of heavy rainfall, frequent lightning, gusty
outflow winds of 40 to 50 mph and some small hail as decent shear
exists for some sustainable updrafts. Most storm activity should
wind down after midnight Sunday night as no distinguishable
shortwave moves through. Cannot completely rule out overnight
convection though so kept some isolated mention in the forecast
for some areas.

.LONG TERM...(Monday through Saturday)
Issued at 334 PM MDT Sat Aug 12 2017

Southwest flow will increase Monday as an upper level trough
deepens over the western states into the Desert Southwest. Good
upper level support exists with the upper level jet to the west
over Utah, so expect a fairly active day on Monday with storm
development. One concern is that some drier air tries to work into
the Four Corners region with this trough. However, plenty of
moisture will remain for storms to fire and become widespread with
strongest storms to fire along the moisture gradient. This
slightly drier air may help to increase instability in the
atmosphere for stronger storms. Activity looks to increase Monday
evening as a leading shortwave moves through the flow for
convection to continue overnight into Tuesday morning. The upper
level trough and associated cold front moves through on Tuesday,
allowing for a fairly active day. Due to the low dew point
depressions, remnant moisture and forcing in place, raised
chances and coverage of storms Monday through Tuesday. Expecting
these to be the most active days.

Cooler air moves in behind this trough with drier air as well. The
pattern overall looks to favor a drying trend from mid-week
through the end of the week for a downturn in storm activity.
Thinking that enough moisture will still hang around for daily
afternoon convection over the higher terrain with most lower
elevations drying out and slowly warming up towards the end of the
week from near normal/slightly below normal values towards
slightly above normal.


.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1150 AM MDT Sat Aug 12 2017

Afternoon thunderstorms are starting to develop over higher
terrain, and over NW Colorado. This trend will continue as storms
move west to east. An area of showers and tstorms is expected to
move from central Utah eastward this evening, bringing a
continuation of convection through the overnight hours. ILS
breakpoints will be reached at times, and potentially MVFR
temporarily in stronger storms if they target the airports.




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