Flash Flood Watch 2, NWS Grand Junction, CO

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
1203 AM MDT Thu Jul 20 2017

.UPDATE...
Issued at 1202 AM MDT Thu Jul 20 2017

Showers and thunderstorms have decreased in coverage over the past
few hours with the loss of daytime heating and influence of
diurnal cooling. Therefore, the Flash Flood Watch will be allowed
to expire at midnight. However, the passage of an embedded trough
across the CWA will allow isolated to scattered showers and
thunderstorms to persist overnight. As a result, stronger storms
will be capable of producing heavy rain and localized flooding.

&&

.SHORT TERM...(Tonight through Thursday night)
Issued at 334 PM MDT Wed Jul 19 2017

Classic monsoonal pattern in place featuring a low pressure trough
over the West Coast and high pressure centered over the southern
Plains. Circulation between these two features continues to carry
subtropical moisture northward over the forecast area which has
raised precipitable water values (PW) to over an inch in the 12Z
sounding at KGJT. Deep subtropical moisture combined with daytime
heating will continue to fuel scattered to numerous showers and
thunderstorms late this afternoon and into the evening with
localized flash flooding possible. Consequently, will leave the
Flash Flood Watch in place through midnight.

Diurnal cooling will bring a downturn in shower/thunderstorm
coverage and intensity during the night and into Thursday
morning. However, with very little change in the weather pattern
described above indicated by latest models, expect conditions
Thursday afternoon/evening will be similar to today's. However,
models indicated a decrease in PW which will translate into fewer
storms and a reduction in heavy rain producing storms. In the
absence of dynamic forcing, expect showers and thunderstorms will
largely diminish late Thursday night as the atmosphere stabilizes
with diurnal cooling.

Night time lows will remain above normal tonight and Thursday
night as clouds and moisture limit radiational cooling.
Conversely, highs on Thursday will remain a bit cooler than normal
due partly to mostly cloudy conditions and scattered to numerous
showers.

.LONG TERM...(Friday through Wednesday)
Issued at 334 PM MDT Wed Jul 19 2017

Monsoon moisture will continue to stream across the region from
the southwest on Friday. Both Nam and ECMWF indicated moisture
levels will rise with NAM suggesting PW values near 1.2 inches.
GFS was the outlier with near steady state moisture in the
atmospheric column. Regardless, it seems likely that
afternoon/evening showers and thunderstorms will remain capable of
putting down heavy rain with localized flooding possible. Should
models settle on a solution close to the latest NAM ab additional
Flash Flood Watch may be warranted.

NAM/GFS, and to a lesser extent the EC, indicated high pressure
will again migrate west bringing drier air down from the north
this weekend and into early next week. As a result, there will be
a trend toward decreasing clouds and thunderstorm activity from
Saturday into Sunday. EC and GFS were at odds as to when the next
monsoon surge arrives next week. In the EC solution moisture
returns Monday while the GFS only allows modest moisture back into
the area beginning Tuesday. Given model differences left low end
chance PoPs for Monday with increased chances Tuesday and
Wednesday. Temperatures this weekend through early next week will
be near or a little above normal.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1202 AM MDT Thu Jul 20 2017

Nocturnal showers and thunderstorms will continue through the
overnight hours as an embedded wave in the flow moves through the
region. Heavy rain, small hail and local MVFR cigs/vsbys will be
the primary threats with storms. Similar conditions are expected
for Thursday as moisture continues to steadily stream in from the
south. Convection will first fire off over the higher terrain after
18Z before coverage becomes scattered to numerous after 20Z. This
will increase the likelihood of more areas with mountain
obscuration and local MVFR conditions.

&&

.GJT WATCHES/WARNINGS/ADVISORIES...
CO...None.
UT...None.

&&

UPDATE...MMS
SHORT TERM...NL
LONG TERM...NL
AVIATION...TGJT

Special Weather Statement
National Weather Service Grand Junction CO
1046 PM MDT WED JUL 19 2017

COZ003-006-007-009-011-017-200530-
Roan and Tavaputs Plateaus CO-Grand and Battlement Mesas CO-
Uncompahgre Plateau/Dallas Divide CO-Debeque to Silt Corridor CO-
Grand Valley CO-Central Gunnison and Uncompahgre River Basin CO-
1046 PM MDT WED JUL 19 2017

...SIGNIFICANT WEATHER ADVISORY FOR WEST CENTRAL DELTA AND CENTRAL
MESA COUNTIES UNTIL 1130 PM MDT...

At 1045 PM MDT, Doppler radar was tracking a strong thunderstorm 15
miles south of Palisade, or 16 miles southeast of Grand Junction,
moving northeast at 30 mph.

Frequent cloud to ground lightning, half inch hail and winds in
excess of 40 mph will be possible with this storm.

Locations impacted include...
Palisade, Mesa, Skyway, Cameo, Clifton, Whitewater and Orchard Mesa.

This includes the following highways...
 Interstate 70 in Colorado between mile markers 37 and 53.
 U.S. Highway 50 in Colorado between mile markers 37 and 58.
 Colorado 65 between mile markers 25 and 50, and between mile markers
52 and 61.
 Colorado 141 between mile markers 140 and 162.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Heavy rainfall is also occurring with this storm, and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

LAT...LON 3871 10834 3885 10861 3926 10835 3910 10790
TIME...MOT...LOC 0445Z 213DEG 28KT 3889 10837

MS

Special Weather Statement
National Weather Service Grand Junction CO
1125 PM MDT WED JUL 19 2017

COZ011-014-017>019-200615-
Upper Gunnison River Valley CO-Uncompahgre Plateau/Dallas Divide CO-
Southwest San Juan Mountains CO-
Central Gunnison and Uncompahgre River Basin CO-
Northwestern San Juan Mountains CO-
1125 PM MDT WED JUL 19 2017

...SIGNIFICANT WEATHER ADVISORY FOR OURAY...NORTHEASTERN SAN
MIGUEL...SOUTHEASTERN MONTROSE...NORTHWESTERN HINSDALE AND
SOUTHWESTERN GUNNISON COUNTIES UNTIL 1215 AM MDT...

At 1124 PM MDT, Doppler radar was tracking strong thunderstorms along
a line extending from near Sawpit to near Ouray. Movement was north
at 20 mph.

Dime size hail and winds in excess of 40 mph will be possible with
these storms.

Locations impacted include...
Ouray, Ridgway, Sawpit, Loghill Village, Colona, Placerville and Fall
Creek.

This includes the following highways...
 U.S. Highway 550 in Colorado between mile markers 88 and 121.
 Colorado 62 between mile markers 1 and 23.
 Colorado 145 near mile marker 80, and between mile markers 82 and
85.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Heavy rainfall is also occurring with these storms, and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is occurring with these storms.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

LAT...LON 3840 10804 3835 10733 3796 10754 3800 10808
TIME...MOT...LOC 0524Z 194DEG 16KT 3807 10797 3804 10762

MDA

Special Weather Statement
National Weather Service Grand Junction CO
1211 AM MDT THU JUL 20 2017

COZ021-UTZ022-200715-
Four Corners/Upper Dolores River CO-Southeast Utah UT-
1211 AM MDT THU JUL 20 2017

...SIGNIFICANT WEATHER ADVISORY FOR SOUTHWESTERN MONTEZUMA AND
SOUTHEASTERN SAN JUAN COUNTIES UNTIL 115 AM MDT...

At 1208 AM MDT, Doppler radar and satellite was tracking strong
thunderstorms along a line extending from near Twin Falls to 12
miles north of Shiprock.
Movement was northwest at 15 mph.

Frequent cloud to ground lightning and winds in excess of 40 mph
will be possible with these storms.  Very heavy rainfall is also
possible with this storm which could lead to localized flash
flooding.

Locations impacted include...
Aneth and the Four Corners area southwest of Towaoc in southwest
Colorado and southeast of Aneth and Montezuma Creek in southeast
Utah.

This includes the following highways...
 U.S. Highway 491 in Colorado between mile markers 1 and 6.
 U.S. Highway 160 in Colorado between mile markers 1 and 19.
 Utah 162 between mile markers 25 and 31.
 Colorado 41 between mile markers 1 and 9.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Heavy rainfall is also occurring with these storms, and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is occurring with these storms.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

Persons in campgrounds should consider seeking sturdy shelter until
these storms pass.

&&

LAT...LON 3700 10935 3711 10945 3729 10886 3700 10865
TIME...MOT...LOC 0608Z 138DEG 13KT 3685 10914 3697 10875

MDA