Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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000
FXUS65 KSLC 211033
AFDSLC

Area Forecast Discussion
National Weather Service Salt Lake City UT
433 AM MDT Wed Jun 21 2017

.SYNOPSIS...High pressure will start to weaken and shift south
today as a drier northwest flow develops, and continues for the
remainder of the week. A cooler airmass will spread into the north
starting Thursday while the south stays hot.

&&

.SHORT TERM (Through 12z Friday)...Early morning water vapor and
H5 analysis indicate a net encroachment of the progressive
northern branch with short wave energy sliding east along the
US/Canadian border. As such, have noted the core of the upper high
suppress south some with center now over central AZ. This said,
influence of this high remains the primary talking point today
with excessive heat again expected across the southern tier, and
well above average temps elsewhere (+15F above climo on average)
with H7 temps progged to remain near +17 C (north) to +21 C
(south). Heat headlines remain in place across the south/east as
such, and forecasting 101 F for KSLC during peak heating this
afternoon.

A mid level moisture axis extends from the Tavaputs NW into north
central Nevada this morning, with average PWAT ranging between
.7-.8 inches within this pooling environment. SPC SREF ensemble
continues to indicate sufficient instability this afternoon/eve
driven by diurnal thermal input within this zone, and NCAR
simulated reflectivity points toward another round of convective
initiation over both the eastern Nevada mtns and along/downstream
of the Uinta/Wasatch Plateau spine. Timing of the NV convective
evolution into the west/north looks to hold off until after 00z in
large, so opted to trim PoPs slightly across the west this
afternoon, but left isolated potential along the UT mtn spines and
downstream. Areal extent of isolated convection does look to
increase this evening across northern/central portions aided by
more focused convergence from the aforementioned northern branch,
and downstream maintenance of some of the cells out of Nevada
along and ahead of a shallow surface boundary moving into the
north. With sufficient DCAPE, gusty wind threats remain a
potential through about the middle of the night.

The aforementioned boundary will surge south into central Utah come
Thursday, cooling the north some 10 degrees, but shallow nature and
loss of upper support will limit any notable sensible weather
changes across the south. Convective potential is expected to wane
as well Thursday, as mid level moisture/instability will dissipate
in wake of passage to the north, and ahead of it in the south.
Rolling with a dry forecast with only some buildups over the
southern UT mtns Thu afternoon.


.LONG TERM (After 12z Friday)...A drier and slightly cooler
northwest flow will be in place across all but the southern CWA
at the start of the long term period with possibly a few lingering
mid clouds across the south.

The med range models continue to differ on the extent of the
cooling with the EC about 3-4C warmer than the GFS at H7 over nrn
UT by 00z Sat. Also, the EC doesn`t indicate much if any cooling
for the south and keeps a small chance of convection over the srn
mtns Fri aftn. Temps Fri remain a compromise between these
solutions altho do lean a little towards the GFS up north and
towards the EC over the south.

These differences persist Sat with the GFS drier and cooler over
the south. Have leaned towards the GFS on the convection threat
and kept the CWA dry Sat but leaned towards the EC on temps. There
is a possibility that the srn UT Heat highlights may need to be
extended into the weekend if the EC is correct.

Model differences begin to resolve themselves Sun into next week
as the ridge builds back over the wrn Great Basin then shifts
overhead Mon into Tue. Expect warmer temps to return to nrn UT in
this pattern but the southwest may finally cool a little as the H7
thermal ridge axis shifts north into the CWA with the high. Expect
an increase in convection Mon and Tue as well altho it will still
be isolated and mainly limited to the higher terrain.

&&

.AVIATION...Southeast winds are expected to continue at the SLC
terminal through 18-19Z, then switch to the northwest for the
afternoon. There is a 30 percent chance that winds do not shift
until 20-21z. Gusty erratic outflow winds from nearby
thunderstorms could impact the airfield between about 23z-07z.

&&

.FIRE WEATHER...Strong high pressure will maintain excessive heat,
bone dry RH, and a Haines index of 6 across the south, with temps
across the north hovering near 15 degrees above climo. Isolated
critical winds remain forecast for portions of the SW this
afternoon, and though speeds are borderline, an RFW remains in
place for gusts over 30mph and low RH coupled with the high
Haines. Rapid growth potential remains high for ongoing fires and
potential new starts there.

Isolated showers/thunderstorms remain a potential across the
Uinta/Wasatch Plateau this afternoon, and much of northern/central
Utah this evening ahead of an approaching surface boundary. Any
organized cells will continue to have a gusty microburst
potential, with wetting rain potential remaining low due to the
high based nature of the convection.

The surface boundary will work into northern Utah late this
evening then track south along the Wasatch Front overnight
supporting this convective potential through about midnight, with
winds trending northwesterly across the northern half by Thursday
morning in wake of passage. This boundary is expected to continue
south Thursday and does look to create critical fire weather
conditions across cured areas in the east and south Thursday
afternoon, with NW gusts approaching 35mph coupled with the bone
dry RH. Have upgraded the going Fire Weather Watch to an RFW this
morning. See FWF/RFW for details.

Wind trends should decrease come Friday, but isolated critical
conditions may once again become established during the afternoon
hours across Castle Country and some southern valleys. Will
continue to assess future guidance regarding this potential.

&&
.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Excessive Heat Warning until 10 PM MDT Thursday for UTZ019-021.

     Red Flag Warning from noon to 10 PM MDT Thursday for UTZ482-484-
     488-489-493-494-496-498.

     Red Flag Warning from noon today to 10 PM MDT this evening for
     UTZ495-496.

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

WY...None.
&&

$$

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

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http://weather.gov/saltlakecity

For frequently asked questions about the Area Forecast Discussion
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