Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 201137

Area Forecast Discussion
National Weather Service Salt Lake City UT
537 AM 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.


.SHORT TERM (Through 12z Thursday)...Early morning water vapor and
H5 analysis indicates the core of a strong upper high extends from
the lower Colorado river valley north into southern Utah.
Coincident, a mid level thermal axis exists and is evident
extending from SE Arizona NW into eastern Nevada. Progressive near
westerly flow remains in place from the PacNW across the northern
Rockies states.

The aforementioned thermal axis has been a focus of mid level
moisture pooling through the overnight hours, with remnant isolated
convective showers being maintained along the UT/NV border. This
zone will shift slightly north and east over the next 18-24 hours
maintaining the best potential for isolated high based convection,
largely diurnal in nature across the north and west aided by
convergent flow from the more progressive northern branch.

Though isolated convection remains a potential today, the excessive
heat remains a more relevant topic. Continued mid/upper level
subsidence beneath the axis of the ridge will aid to warm the
already anomalously warm airmass further. Have continued to hold
onto 100F at KSLC, and 110+F for Dixie, with most all areas across
the area 15+ degrees above climo come this afternoon. Excessive heat
products remain in place across the south.

The upper ridge will flatten Wed in response to passage of short
wave energy across the northern Rockies states. Guidance continues
to resolve a shallow boundary moving into northern Utah coincident
with this, allowing for both net cooling across the north, and mid
level convergence potentially allowing for convective initiation
across the north Wed night. Have added isolated PoPs roughly I-80
north due to this. Minimal southerly progression of this boundary
will do little for the south however, the heat wave will continue
into the long term.

.LONG TERM (After 12z Thursday)...The series of several low
amplitude trofs sliding past to our north will have begun at the
start of the long term period. Med range models are still at odds
on just how fast the airmass cools starting Thu.

The EC remains the slower of the two to push cooler air into the
nrn CWA with H7 temps about 2c warmer by 00z Fri. Convection
associated with this first boundary will be weakening in the
morning then redeveloping in the aftn across the central and srn

On Fri the GFS keeps pushing the cooler air south thru the CWA
with the with the H7 temps about 2-3C cooler across swrn UT by 00z
Sat than they were at 00z Fri, while the EC maintains the hot
temps across the south one more day. The EC also does not cool the
north off much on Fri either but does finally push cooler air into
the CWA on Sat. If the EC is correct, we may need to extend the
Heat Warnings and Advisories thru Fri.

Convection looks to remain a small threat Fri aftn across the far
south in the EC while the GFS dries the entire CWA. Have kept
mention of convection out of the forecast for now but confidence
is low.

The ridge starts to rebuild over the weekend with convection
spreading back into the south on Sun and gradually becoming more
widespread across the CWA Mon into Tue.


.AVIATION...Southeasterly winds at the SLC terminal will shift to
northwesterly between about 18-20z. Isolated showers and
thunderstorms are expected across northern Utah late today into
tonight with a chance of gusty outflow winds crossing the airfield
between about 23-06z.


.FIRE WEATHER...Strong high pressure will maintain excessive heat,
bone dry RH, and a widespread Haines index of 6 through mid/late
week, with focus extremes across the southern half of the
district. RH trends will continue to lower today through Wednesday
further drying fine fuels and also impacting the larger fuels as
well, though winds will remain relatively light through Wednesday.
As mentioned in previous forecasts, ongoing fires and any new
starts of adequate size will have the potential to become plume
dominated within this pattern, even without much in the way of
wind support.

Isolated showers/thunderstorms can not be ruled out along the
UT/NV border and NW portions of the district this afternoon, but
lightning potential remains on the lower side, with high based
cells more likely to produce modest wind gusts. This potential
will become more focused across the far north beginning Wed night.

A shallow boundary will work into northern Utah late Wednesday
into the overnight hours supporting this convective potential,
with winds trending northwesterly across the northern half by
Thursday morning. This boundary is expected to continue south
Thursday potentially creating critical fire weather conditions
across the eastern valleys/Castle Country Thursday afternoon, with
NW gusts approaching 30mph coupled with the bone dry RH.
Something to keep tabs on. Wind trends should decrease come
Friday, but isolated critical conditions may once again become
established during the afternoon hours across Castle Country.


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

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




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