Special Weather Statement, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
403 PM MDT Thu Jul 20 2017

.SYNOPSIS...Monsoon moisture will persist over most of Utah
through today. Drier air will gradually spread back into the area
from the northwest Friday into Saturday. Moisture start to spread
back into Utah Sunday night and Monday.

&&

.SHORT TERM (Through 12Z Sunday)...Deep moisture remains over the
forecast area this afternoon, with satellite derived PWs in the
1.0 to 1.2 inch range, near the maximum observed this time of
year. Despite ample moisture, instability is on the low side
without a well defined forcing mechanism, so afternoon convection
is primarily confined to the higher terrain so far. Still,
expecting coverage of convection to continue to increase into the
early evening, and with enhanced southerly flow, would think at
least some of the storms would come off the terrain. Given the
very moist airmass and the possibility of heavy rain with any
storms that develop, a Flash Flood Watch continues through the
evening over central and southern Utah. Under high pressure, maxes
are running near seasonal normals over northern Utah and a little
below over southern Utah.

The ridge over the area is expected to flatten by tomorrow as a
trough moves over the top of it, shifting the flow aloft to a more
westerly direction. This will allow a drying trend over the forecast
area, primarily over northern Utah and southwest Wyoming. GFS
indicates PWs over northern and central Utah dropping to the 0.5 to
0.7 inch range by Saturday afternoon as PWs in excess of 1.0 inch
persist over southern Utah. As a result, coverage of convection will
decline significantly over primarily the northern half of the
forecast area while the threat of thunderstorms with heavy rain
continues for southern Utah.

.LONG TERM (After 12Z Sunday)...The ridge over the region remains
centered to the west of UT at the start of the long term period
with the drier air that had spread into the CWA still in place
across the north and east. Deep moisture will remain pooled along
the AZ border extending northwest thru NV. Expect most convection
Sun will remain limited to the south but isolated storms will be
possible up the mountain spine into the Uintas, and northward
along the NV border.

The high is forecast to shift east thru Mon allowing the moisture
to our south and west to spread back into the CWA with a
corresponding increase in convection. The deepest surge looks to
arrive Tue altho guidance begins to diverge at this point. The GFS
has a short wave that moves into the northwest states and nrn
Rockies on Tue, then spreads drying over the north for Wed while
the EC keeps the deep moisture in place thru Wed and lifts the
wave weakly past on Wed night with only a little drying Thu.
Raised pops to scattered for the entire CWA for Tue and kept a
mention area wide thru Thu, but confidence in the forecast drops
off quickly after Tue.

&&

.AVIATION...Southerly winds will prevail at the SLC terminal
through the rest of today and this evening, gusty at times, but
there is a 20 percent chance of a shift to the northwest between
22Z and 00Z. There is a 30 percent chance that showers and
thunderstorms in the vicinity of the terminal will result in
periods of erratic winds between about 23-04z.

&&

.FIRE WEATHER...Monsoonal moisture remains over the fire district
this afternoon on the back side of an upper ridge. Showers and wet
thunderstorms have developed, primarily over the higher terrain,
and will continue into the evening. Heavy rain will be the primary
threat with any thunderstorms. Given the deep moisture, relative
humidities will remain high and southerly flow will remain breezy.

Drier air will start to move into the district from the northwest
beginning tonight, and this drying trend will continue into the
weekend. The most noticeable drying will occur over northern Utah,
with deeper moisture remaining pooled over southern Utah through
the weekend. Moisture is expected to return to the district early
next week.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Flash Flood Watch until midnight MDT tonight for UTZ010-012>016-
     019>021-517-518.

WY...None.
&&

SHORT TERM/FIRE WEATHER...Traphagan
LONG TERM/AVIATION...Wilensky

For more information from NOAA's National Weather Service visit...
http://weather.gov/saltlakecity

For frequently asked questions about the Area Forecast Discussion
visit...
http://weather.gov/saltlakecity/general/afd_faqs.php

Hazardous Weather Outlook
National Weather Service Salt Lake City UT
316 AM MDT Thu Jul 20 2017

UTZ001>016-019>021-517-518-WYZ021-211100-
Cache Valley/Utah Portion-Northern Wasatch Front-
Salt Lake and Tooele Valleys-Southern Wasatch Front-
Great Salt Lake Desert and Mountains-Wasatch Mountain Valleys-
Wasatch Mountains I-80 North-Wasatch Mountains South of I-80-
Western Uinta Mountains-Wasatch Plateau/Book Cliffs-
Western Uinta Basin-Castle Country-San Rafael Swell-
Sanpete/Sevier Valleys-West Central Utah-Southwest Utah-
Utahs Dixie and Zion National Park-South Central Utah-
Glen Canyon Recreation Area/Lake Powell-Central Mountains-
Southern Mountains-Southwest Wyoming-
316 AM MDT Thu Jul 20 2017

This Hazardous Weather Outlook is for the western two thirds of
Utah and southwest Wyoming.

.DAY ONE...Today and Tonight

Showers and thunderstorms will form across much of the outlook
area again today. Heavy rain will be a possibility, leading to an
increased threat of flash flooding, especially along prone
drainages in southern and central Utah.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday

Drier air will spread into northern Utah Friday into Saturday,
but moisture will remain in place across southern through east-
central Utah where showers and thunderstorms will continue. Sunday
will likely be drier across most of the area.

More significant moisture will return to the entire outlook area
starting Monday, bringing an increased chance of showers and
thunderstorms for the first half of next week. Some of these
storms may produce heavy rainfall.

.SPOTTER INFORMATION STATEMENT...

Weather spotters are encouraged to report significant weather
conditions according to standard operating procedures.

For more information from NOAA/s National Weather Service visit...
http://weather.gov/saltlakecity

For information on potential road travel impacts visit...
http://www.udottraffic.utah.gov/roadweatherforecast.aspx

Special Weather Statement
National Weather Service Salt Lake City UT
456 PM MDT THU JUL 20 2017

UTZ010-011-202330-
Wasatch Plateau/Book Cliffs UT-Western Uinta Basin UT-
456 PM MDT THU JUL 20 2017

...SIGNIFICANT WEATHER ADVISORY FOR NORTHEASTERN CARBON AND
SOUTHEASTERN DUCHESNE COUNTIES UNTIL 530 PM MDT...

At 455 PM MDT, National Weather Service doppler radar indicated a
strong thunderstorm 25 miles south of Myton, or 31 miles south of
Fort Duchesne...moving east at 20 mph.

Another storm is developing to the south of the main storm and may
become strong in the next 10 minutes.

Penny to nickel size hail and winds in excess of 40 mph will be
possible.

This storm will remain over mainly rural areas of northeastern Carbon
and southeastern Duchesne Counties.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

If threatening weather approaches, take shelter in a sturdy building.

&&

LAT...LON 3978 10992 3972 10995 3974 10999 3971 10997
      3969 11000 3967 10998 3961 11005 3988 11009
      3992 10998 3981 10997 3979 10993 3980 10992
      3979 10988
TIME...MOT...LOC 2255Z 251DEG 17KT 3983 10997

CRK

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SALT LAKE CITY UT
627 PM MDT THU JUL 20 2017

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0415 PM     FLASH FLOOD      6 N REDMOND             39.08N 111.88W
07/20/2017                   SANPETE            UT   BROADCAST MEDIA

            FLASH FLOODING OVER CLARION COUNTY ROAD SW OF
            CENTERFIELD. RADAR ESTIMATES OF 1.0-1.5 INCHES OF
            RAINFALL IN A ONE HOUR PERIOD.

&&

ALUKIN

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