Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 281809

Area Forecast Discussion
National Weather Service Grand Junction CO
1209 PM MDT SUN AUG 28 2016

Issued at 1208 PM MDT Sun Aug 28 2016

A band of showers and thicker clouds on the north side of the Four
Corners low has set up roughly along/south of the I-70 corridor
this morning. Radar returns through the morning filled in along
this band. And with daytime heating adding its support, a few
thunderstorms have developed by around noon with cells taking on a
more convective nature. This trend will continue as
showers/thunderstorms move into the Flat Tops and develop over
the Tavaputs Plateau. As a result, I have upped pops/clouds for
much of the area into the evening. Expect that a few stronger
storms, especially along the edges of the cloud cover, could
produce brief heavy rain.


.SHORT TERM...(Today through Monday)
Issued at 243 AM MDT Sun Aug 28 2016

The radar mosaic this morning is clearly showing cyclonic rotation
around the closed low that looks to be over central Arizona. Both
the NAM and GFS initialized well and just about nailing this
location. A few showers and storms have popped up over Arizona at
this hour which is to be expected thanks to the lift supplied by
the low pressure itself. Models agree that this low will shift
eastward through the day straddling the New Mexico and Colorado
border as it does so. For today then, models continue to put
showers and storms over the usual areas: San Juans, Continental
Divide and Bookcliffs. The forecast follows suit. IR satellite is
showing some cloud cover over the forecast area but not as much as
yesterday. If this remains the case, convection will get going
around noon unlike Saturday where stratified clouds really
hindered any significant convective development. Heavy rain and
gusty winds will be the main concerns.

By Monday, the low pressure will still be over northern New Mexico
but it will have shifted far enough to the east that our upper
level winds will also be northeasterly to easterly. When this
happens, afternoon convection usually starts to diminish as
moisture stops advecting into the area while favorable orographics
will stay over the Front Range. Even so, the location of the low
pressure will provide some lift so some showers and storms remain
possible. Interestingly, models are favoring San Juans, which
makes sense due to that low pressure, but they are also favoring
the central mountains and even the Bookcliffs to a lesser extent.
Models are not showing any upper level support while QG forcing is
nonexistent though some midlevel instability remains and looks to be
enough for convection to get going.

Temperatures on both days will be right about where they should be
for this time of year.

.LONG TERM...(Monday night through Saturday)
Issued at 243 PM MDT Sat Aug 27 2016

A low pressure trough will move inland over the west coast
starting Monday night with the first piece of energy ejecting
through the Intermountain West. This will cause the ridge of high
pressure to move slightly eastward late Tuesday, allowing for
moisture to increase from the southwest. A slight increase in
thunderstorm coverage is expected Tuesday, with the western
Colorado mountains being favored. The trough will move further
inland with the flow shifting more to a southwest to northeast
oriented pattern on Wednesday into Thursday. Precipitable Water
values increase to 0.75 to an inch plus by Thursday with a
stronger shortwave moving through the flow. This trough is not as
amplified as model runs 24 hours ago depicted, so we end up seeing
a southwest flow instead of a more direct southerly tap which
would bring in more moisture. However, sufficient jet support with
our CWA in the right entrance region as well as forcing associated
with the shortwave will lead to a more active day on Thursday with
more scattered to numerous shower and thunderstorm coverage.
Therefore, increased pops in coordination with neighboring
offices. A third impulse is seen tracking across on Saturday.
While details could vary on timing of these shortwaves, it looks
like a more active and unsettled pattern will be in place from the
middle to the end of next week.


.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1208 PM MDT Sun Aug 28 2016

Moisture wrapping around a low pressure system that will move into
northwest New Mexico this evening, will keep the chance of
showers and thunderstorms across our forecast area this afternoon
and into this evening. Thunderstorms will become more prevalent by
around 20z. Then both showers and thunderstorms will diminish from
02-03z. Heavy rain may be possible with the stronger storms and
may briefly drop cigs/vsby into the MVFR category at some TAF
sites or other airports in the CWA. Mountains may be obscured at
times in showers. Most of the precipitation will occur south of a
line from the Flat Tops Mountains to Canyonlands airport. North of
that line conditions will be drier. After 03z VFR is expected


.GJT Watches/Warnings/Advisories...


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