Special Weather Statement, NWS Salt Lake City, UT

Area Forecast Discussion
National Weather Service Salt Lake City UT
1053 AM MDT Wed Jul 5 2017

.SYNOPSIS...A ridge of high pressure will remain the dominant
weather feature across the region through the upcoming weekend.

&&

.DISCUSSION...
Water Vapor Satellite shows a ridge overhead. 400-200mb MDCARS
wind observations place a 60-90kt anticyclonic jet over the
Canadian border. GOES/SLC 12Z RAOB/HRRR indicate that
precipitable water values vary from 0.20"-0.33" southwest and
south central Utah to 0.50"-0.65" northern and eastern Utah.

Added a Heat Advisory today and tomorrow for the Salt Lake Valley,
given the warm days/nights. No changes for the Excessive Heat
Warnings near the Arizona border tomorrow and Friday.

Temperatures today will be 8 to 13F above normal which puts us at
Record territory today for several places including Salt Lake City
which has a record high of 104F in 1973.

Other story today is convection. Given instability profiles,
majority of convection will be north of Interstate 70 and along
and east of the Wasatch Front. Mountain sites have PoPs since they
show a greater chance for measuring precipitation. In the valleys,
went with dry thunderstorms with gusty winds mentioned due to the
1000J/kg of DCAPE.

&&

.PREVIOUS DISCUSSION...
Mid level ridging centered across the forecast area will continue
to strengthen today through Thursday resulting in a continued
warming trend. Heat levels for the lowest elevations of southern
Utah reach warning criteria by Thursday with the St George area
once again reaching the 110F mark for daytime highs, thus have
issued an Excessive Heat Warning for Utah's Dixie as well as the
Lake Powell area for Thursday/Friday. Across the north, max
temperatures will be near or above daily records along the Wasatch
Front through the short term period.

Given the degree of daytime heating a few high based
showers/thunderstorms will again be possible along the Uintas and
Wasatch Plateau this afternoon, however drier air is expected to
spread into southern Utah, and have removed a mention of
convection across the central and southern mountains for this
afternoon and evening. Mid level moisture is expected to slowly
increase through the short term period, which may result in
slightly better coverage of high based showers and dry
thunderstorms as the week progresses.

Model to model and run to run consensus continues to favor mainly
a mid-level moisture increase Friday into early next week in
association with the development of a relatively weak Southwest
Monsoonal pattern. Current model trends would indicate the main
threat would be limited to the higher terrain and adjacent valleys
Saturday.

Mid-level moisture may begin to deepen across southern Utah Sunday
though coverage is expected to remain somewhat limited through early
next week. Current forecast remains similar to the previous package.

Heat threat index may remain high enough into Saturday for an
extension of the excessive heat warning but with the possibility of
a moisture increase, decided against extending the excessive heat
warning for St. George and Lake Powell into Saturday afternoon.

&&

.AVIATION...
The SLC terminal will see VFR conditions throughout the
day with mostly clear skies. Southeast winds are expected to shift
to the northwest between 20Z and 22Z. There is a 20 percent chance
that this shift will occur later than expected or winds will stay
southerly throughout the day. There is also a 20 percent chance of
gusty and erratic winds between 21-00z from possible isolated
vicinity thunderstorm activity.

&&

.FIRE WEATHER...
ERC values have risen above their 90th percentile across southern
and central Utah, with more than several reporting stations above
their 97th percentile. North of Interstate 80 ERC values have
risen to between their 50th-79th percentiles.

With a ridge overhead winds will mainly be typical slope driven.
Mid-level moisture increases during the next several days
expanding the areal coverage of isolated to scattered high-based
thunderstorms from just the mountains into the valleys as well through
the weekend. Valley locations will see dry activity through the
weekend. Dry microbursts will cause locally gusty and erratic
winds near showers. Showers and storms may become more hybrid
dry/wet next week.

Lightning is a big concern for new starts.

Given the trend in showers and storms, expect the excessive heat
to gradually trend lower into next week as bone dry single digit
RH gradually rises into the 10-15% range next week.

&&

.SLC WATCHES/WARNINGS/ADVISORIES...
UT...Excessive Heat Warning from noon Thursday to 10 PM MDT Friday
     for UTZ019-021.

     Heat Advisory until 10 PM MDT Thursday for UTZ003.

WY...None.
&&

PUBLIC...Rogowski/Seaman/Kruse
AVIATION...Traphagan/Verzella
FIRE WEATHER...Rogowski

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

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