Area Forecast Discussion
Issued by NWS Grand Junction, CO

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712
FXUS65 KGJT 210545
AFDGJT

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
1145 PM MDT Tue Jun 20 2017

.UPDATE...
Issued at 1001 PM MDT Tue Jun 20 2017

Added mention and increased coverage of showers and isolated
storms for areas north of I-70 as a shortwave continues to track
across northern portions of the CWA keeping shower and storm
activity ongoing through the evening. HRRR is picking up on this
better than most models and shows showers continuing to track
eastward through the early morning hours before dissipating. Not
getting much precip out of these showers with mainly gusty outflow
winds the primary concern with a few lightning strikes here and
there.

&&

.SHORT TERM...(Tonight through Wednesday night)
Issued at 200 PM MDT Tue Jun 20 2017

A large upper-level ridge analyzed at 596 DM over central Arizona remains
firmly in control of the weather over the western CONUS this
Tuesday afternoon. Along the northern fringes of this ridge, some
mid-level moisture and a bit of faster flow aloft has resulted in
convective development over the higher terrain of the Great Basin
and middle Colorado River valley this afternoon. In northeastern
Utah, around 500 j/kg of mixed-layer CAPE will fuel a few
convective showers through the afternoon. Dry air near the surface as
evidenced in model and observed soundings does not bode well for
precipitation making it to the ground, however some locations in
the Roan, Tavaputs, and eastern Uintas may see a hundredth or two
out of any stronger cells that develop. Dry lightning will be a
low, but not non-existent, threat this afternoon as one or two of
the strongest cells may grow high enough above the freezing
level to form graupel. Any convection will quickly wane after
sunset this evening as daytime heating is lost. Temperatures this
afternoon look on track as of 2pm, and will run near or just
slightly below record values for the day.

Despite 500mb heights lowering by 2-4 DM over the region
tomorrow...an uptick in winds aloft, some slight WAA from the
southwest, and a bit better mixing will likely make Wednesday the
warmest day of the year across the Four Corners. Slightly faster
flow aloft will aid any convection that develops in the higher
terrain north of I-70, and PoPs have been adjusted slightly upward
in this package.

.LONG TERM...(Thursday through Tuesday)
Issued at 200 PM MDT Tue Jun 20 2017

The main forecast challenge in the long term will be with winds on
Thursday. Forecast soundings from the NAM/GFS indicate the
potential for mixing down winds of 20mph with some 30+ mph gusts
across eastern Utah and especially northwest Colorado. The ECMWF
is much less bullish regarding wind speeds at the surface, as it
models the 300mb jet streak weaker and faster than its American
counterparts. Due to the continued low confidence in this
forecast, have opted to refrain from any Fire Weather headlines on
Thursday for now. Future shifts will continue to monitor the
forecast for any changes.

After the passage of a weak frontal boundary on Friday morning,
the heat wave will ease a bit as we head into the weekend. Some
afternoon cloud cover will help curtail the warmth on Friday
afternoon. Heights will lower notably by Saturday and Sunday.
While temperatures are still expected to remain about 5 degrees
above average, they will be 10 or so degrees cooler than the
midweek-heat currently gripping the region.

Late in the weekend, guidance is in fairly good agreement
developing a new ridge of high pressure over the desert southwest.
Eastern Utah and western Colorado will be on the eastern fringes
of this ridge, and forecast models indicate a bit of lingering
mid-level moisture lingering around the Continental Divide early
next week. This would result in a better chance for showers and
thunderstorms in the mountains than we have seen in recent days,
before the ridge kicks this moisture east by midweek. Despite a
relatively benign weather pattern upon first glance, there are
certainly plenty of subtle forecast challenges to monitor through
next week.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1145 PM MDT Tue Jun 20 2017

Scattered showers with some isolated lightning strikes will
continue across areas north of I-70 through the early morning
hours before dissipating after 09Z. Gusty outflow winds will be
the primary concerns with little in the way of rain with
occasional breezes from time to time. Few to scattered clouds will
hang around through the morning with mostly sunny skies and light
terrain-driven winds. More shower and thunderstorm development is
possible Wednesday afternoon across mainly the high terrain with
mixing during peak heating allowing for occasional breezy
conditions in the afternoon to affect some TAF sites. Gusty winds
and lightning once again the primary concerns with any storms that
form, but overall another hot and mostly dry day.

&&

.FIRE WEATHER...
Issued at 200 PM MDT Tue Jun 20 2017

Dry weather will continue over much of the forecast domain with
only a small chance of convective activity in the Roan and
Tavaputs as well as the eastern Uintas. Only trace amounts of
precipitation are expected out of any development in these
locations over the next two days. A low chance of some lightning
does exist with any shower that develops. Winds will increase
slightly during the afternoon on Thursday as a weak mid-level jet
moves overhead. Under full mixing, some of these winds may
translate to the surface resulting in near-critical Fire Weather
conditions on Thursday. Confidence remains low on this borderline
event and will not issue headlines as of this afternoon forecast
package.

&&

.HYDROLOGY...
Issued at 200 PM MDT Tue Jun 20 2017

A flood advisory has been issued for the Roaring Fork River in
Aspen as snowmelt and tunnel closures upstream have caused the
river to reach bankfull. This advisory will run through Friday
evening.

A number of streams and creeks also continue to run strong, cold,
and high and will continue to do so through the week. Where water
is running high, river banks may become saturated and unstable.
Lowlands along the rivers may also become inundated with water.
Remember if you are going to raft, kayak or tube on the rivers be
sure to always wear a life jacket.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...Heat Advisory until 10 PM MDT Thursday for COZ006.

UT...Heat Advisory until 10 PM MDT Thursday for UTZ022-027-029.

&&

$$

UPDATE...MDA
SHORT TERM...MAC
LONG TERM...MAC
AVIATION...MDA
FIRE WEATHER...TGJT
HYDROLOGY...TGJT



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