Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 202140

Area Forecast Discussion
National Weather Service Salt Lake City UT
340 PM MDT Tue Jun 20 2017

.SYNOPSIS...High pressure will remain in place across the eastern
Great Basin into Wednesday. A series of dry cold fronts will
start a gradual cooling trend after Wednesday, primarily for
northern portions of the forecast area.


Water Vapor Satellite shows a strong ridge centered over Arizona,
with westerlies along the Canadian border. 400-200mb MDCARS wind
observations place an anticyclonic subtropical 40-70kt jet from
central California into northern Utah and across the Central
Rockies. Meanwhile a cyclonic 110-160kt jet resides over the
Canadian border. GOES/SLC 00Z RAOB/HRRR indicate that
precipitable water values vary from 0.33"-0.50" Idaho border to
0.75"-0.90" valleys.

Concern today is lightning/dry microburst potential. Based on
instability the thought is for isolated development in the
mountains from the Uintas southwest to Boulder Mountain. A better
area of coverage late today should be off the higher terrain of
Nevada, propagating across Dugway and into the Wastach Front by
evening. SREF indicates that seeing 30kts of effective shear in
this area is not out of the question late day. More importantly,
despite strong DCAPE on the order of 1500j/kg, it appears that
CAPE will be strong enough to balance the cold pool with new storm
development on the outflow. This would support maintenance of the
activity across the aforementioned area. Despite weak flow with
storm motion around 24kts, the added winds due to the strong DCAPE
could push wind gusts above severe criteria should organization
develop. SPC continues to have a marginal risk for severe winds
through the evening across northern and central Utah.

Otherwise continued hot under the ridge. Convective development
appears more isolated tomorrow across the western valleys and
along and east of the Uintas to Boulder Summit line. If they can
hold together, isolated convection may reach the Wasatch Front
tomorrow evening.

Convective chances remain low Thursday, took out all mention as
the cold front moves in from the north. Westerly winds become
somewhat gusty. Highs only near 90 for SLC, but continued hot and
toasty for the south. Heat Warnings/Advisories may need to be
extended further out in time.

A shortwave trough will dig southward across the Northern Tier
Friday afternoon flattening out the upper level ridge across the
Southwest. Meanwhile, a weak cold front located across south-
central Utah, will continue to push through southern Utah into
Arizona Friday afternoon into Saturday morning, but given the lack
of moisture, little to any shower activity is expected.

H7 temperatures will cool a few degrees Friday afternoon
across northern Utah, with highs returning to near-normal readings.
Meanwhile, southern Utah will remain unseasonably warm, with high
temperatures 10-15F above average, with a few locations potentially
recording record highs.

Model guidance begins to diverge Saturday as the shortwave trough
continues to dig into Wyoming. The EC and GFS bring the shortwave
trough across central Wyoming, while the CMC pushes the trough
further to the east. Thus, H7 temperatures among the EC and GFS are
about 2C cooler than in the CMC.

The upper level ridge is expected to slowly amplify once again
across the Southwest Sunday through early next week as a shortwave
trough approaches the Pacific Northwest. A few weak vort. maxes
rotating within the upper level ridge along with an increase in mid-
upper level moisture will allow for isolated thunderstorms to
develop across the higher terrain Monday and Tuesday afternoons.
High temperatures will continue  to run about 10F above average
across southern Utah Sunday into early next week, while northern
Utah sees high temperatures about 5F above average.


A line of the convection across the UT/NV border will continue to
move eastward towards the SLC terminal with the line potentially
reaching the terminal as early as 0000 UTC. There is about a
40-50% chance of this line holding together as it approaches the
terminal. For now have decided to add VCSH to the TAF and will
monitor closely for trends in convective activity. Gusty and/or
erratic outflow winds up to 50 knots along with lightning will be
the primary storm threats with thunderstorm activity.


ERC values across southern Utah in the 70th-89th percentile have
been steady for the past 3 days, while northern Utah remains below
the 50th percentile most locations, though ERCs are trending

A ridge is dominate across the region during the next couple of
days. Excessive heat, single digit RH, and a Haines of 6 will be
common across southern and even parts of central Utah. Across the
North it will remain quite warm and dry, but lesser so.

Southwest winds increase enough to Warren critical fire weather
conditions Wednesday. On Thursday gusty westerly winds will impact
central and eastern Utah providing another day of critical fire
weather conditions.

A slight cooling trend, especially north, will occur at the end
of the work week with the arrival of several cold fronts. These
boundaries will struggle to reach south central Utah. Low RH
however will carry through the work week across the region.

Isolated to scattered convection is expected today into the
evening, with the best converge from the west dessert into the
Wasatch Front. Lightning starts and strong gusty/erratic winds are
the primary threats, especially considering some of these
thunderstorms will be dry. Activity will be more isolated after
today and work from north to south as the aforementioned cold
front makes better progress into south central Utah.


UT...Excessive Heat Warning until 10 PM MDT Thursday for UTZ019-021.

     Fire Weather Watch from Thursday afternoon through Thursday
     evening for UTZ482-484-488-489-493-494-496-498.

     Red Flag Warning from noon to 10 PM MDT Wednesday for UTZ495-496.

     Heat Advisory until 10 PM MDT Thursday for UTZ013-020.




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