Area Forecast Discussion
Issued by NWS Salt Lake City, UT

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FXUS65 KSLC 191037

Area Forecast Discussion
National Weather Service Salt Lake City UT
437 AM MDT Mon Jun 19 2017

.SYNOPSIS...High pressure will remain in place across the Great
Basin into Wednesday, bringing very hot temperatures to the
region. A series of dry cold fronts will start a gradual cooling
trend for northern Utah after Wednesday while the south stays hot.


.SHORT TERM Through 12Z Wednesday)...Upper level ridge axis will
be moving overhead today through Tuesday. The 700mb temperatures
will increase by about 3-4 degrees C across northern Utah, about 3
degrees C near Cedar City and west central Utah and about 1-2
degrees C elsewhere. As a result, surface temperatures will
increase about 6-8 degrees across the north, 5-6 degrees over west
central areas and 2-4 degrees elsewhere. Temperatures are
expected to increase a tad more on Tuesday.

Temperatures will be near or above record values today and Tuesday
across many locations across the area, with temperatures becoming
dangerously warm over southern Utah`s lower elevations. Going Heat
Advisories and Excessive Heat Warnings cover the threat well and
did not make any changes.

Some moisture made its way under the west side of this ridge
across western Nevada and the Sierras as seen on the cloud
development that went on there Sunday. As the ridge shifts
eastward today some of the lingering high clouds will filter in
across northwest Utah today. By late afternoon some mid to high
level clouds will develop across southwest Utah as well, but not
expecting any convection as the LIs remain positive in all areas
except one minor area of negative LIs near Boulder Mountain at
00Z. The PWs increase across the northwest late today and moves
across northern portions of the CWA overnight while southern areas
trend drier through Tuesday. Have removed the threat of high based
thunderstorms over southern mountains while left the slight chance
farther north, although trimmed them back a little.

.LONG TERM (After 12z Wednesday)...The strong upper ridge in
place over UT at the start of the long term period is forecast to
flatten Wed as the first of several low amplitude trofs slides
east along the Canadian border and into the nrn Plains. Med range
models continue to differ on the strength of these waves and just
how much cooling occurs across the forecast area, as well as the
timing of this cooling.

The EC is slower initially to push cooler air into the nrn CWA
with the dry front Wed and has been consistent on this for several
runs. The GFS has been less consistent and the latest 06z run is
now very close to the EC. This would result in temps Wed across
the north very close to Tue highs. Both still bring in additional
cooling on Thu with a second short wave but the GFS is about 3C
cooler at 700mb by 00z Fri. Models then get out of phase with the
GFS dropping additional cooling into the CWA on Fri while the EC
is now stronger with this next short wave trof but it does not
arrive until Fri night. (and temps warm back up Fri ahead of it)

Bottom line is confidence is low on the timing and extent of the
cooling and the forecast is a rough compromise showing a gradual
cooling trend for the north and eventually the entire CWA heading
into the weekend. Both models do keep the south hot thru Thu so
the existing Heat highlights remain on track. If the EC is
correct, then the south will stay hot again on Fri too.

Isolated convection along and south of the dry fronts will be over
the nrn CWA Wed then shifts south Thu into Fri, then out of the
CWA Sat.

Models do build heights back over the CWA on Sun but diverge again
on the evolution of this ridge so forecast confidence is low Sun
and Mon on temps and potential return of convection.


.AVIATION...Southeasterly winds at the KSLC terminal will shift
to northwesterly around 18z.


.FIRE WEATHER...The two main concerns over the next several days
will be the excessive heat and a Haines index of 6 across the
southern half of Utah. The excessive heat and low min RHs in the
single digits and lower teens in most valleys will accelerate the
curing of not only the fine fuels but impact the larger fuels as
well. This ridge of high temperatures will also create a Haines
Index of 6 which will prevail nearly continuously across the
southern half of the CWA from this afternoon through Friday.
Ongoing fires and any new starts of reasonable size have the
potential to become plume dominated under this weather pattern.

Another concern, albeit a little less, is moisture that has moved
up the west side of the ridge which was evident by the cloud
development over the Sierras Sunday afternoon. This moisture is
forecast to slide over Utah late today through midweek. While the
GFS model shows convection developing over the higher terrain, the
instability based on the LIs does not look very supportive of
thunderstorms. The EC and NAM models are less bullish with the
threat of thunderstorms, although they both maintain some threat
across the north. Have kept a minor threat of high-based
thunderstorms across mainly some of the higher elevations of the
north, but confidence is rather low that anything much more than
build ups will occur.

Depending on the timing of this ridge flattening due to a short wave
trough moving southeast across the northern Rockies late in the
week, westerly winds could approach or exceed RFW thresholds across
the eastern and southern FWX zones either Thursday or Friday. Sub
RFW threshold winds are in the forecast at this time due to model
inconsistencies, but worthy of mentioning.


UT...Heat Advisory from noon today to 10 PM MDT Thursday for UTZ013-

     Excessive Heat Warning from noon today to 10 PM MDT Thursday for





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