FXUS65 KSLC 212119
Area Forecast Discussion
National Weather Service Salt Lake City UT
319 PM MDT Mon Aug 21 2017
.SYNOPSIS...Southerly flow aloft will draw moisture north across
the forecast area early in the week. Somewhat drier air will
return to the south midweek.
.SHORT TERM (THROUGH 00Z THURSDAY)...
Water Vapor Satellite shows a closed low off the southern
California coast, with ridging over Utah. 400mb-200mb MDCARS
wind observations place a 40kt-55kt southwesterly jet from
southern California into the Central Rockies, along with a 90kt-
125kt anticyclonic jet over western Canada. GOES/SLC 12Z
RAOB/HRRR indicate that precipitable water values vary from
0.25"-0.50" mountains, 0.75"-1.15" across most valleys.
With dewpoints in mid 40s to lower 50s Southern Utah there could
be some heavy rainers into the evening hours. BUFKIT model
forecast profiles suggest modest warm cloud depth. During peak
heating dewpoint depressions will rise through the 40s and near
50F in vicinity of storm development. This will pose the threat of
locally 35-45mph winds from dry microbursts.
The wave that drove the southwest Utah overcast rainy conditions
earlier today will slowly lift toward the northeast overnight into
tomorrow as the ridge slides to our east. Despite high RH in model
fields, SREF indicates enough destabilization for thunder. Should
cloud up pretty good by evening across the far North. Dewpoint
depressions and DCAPE still adequate for locally gusty winds
across northern and central Utah tomorrow. The combination of loss
of daytime heating along with the weakening wave axis passing to
the north should thin out precipitation across the north tomorrow
Closed low off of Southern California moves inland Wendesday,
through ridging from the southeast fights its way back into Utah.
Thus not expecting a moisture surge that often occurs ahead of
California closed lows. SREF still shows plenty of afternoon
destabilization, mainly confined PoPs to mountains where terrain
circulations trigger showers and storms.
Did undercut tomorrows max temperatures a bit from blend of best
verifying guidance due to the extend of cloud cover across central
and later northern Utah.
Height falls tomorrow ahead of the passage of the upper level
.LONG TERM (AFTER 00Z THURSDAY)...
The long term period begins with a progressive trough over the
PAC NW, a decaying closed low off the California coast, and a N-S
oriented ridge over the Northern Rockies. The best moisture Wed
evening will be located in N.Utah with fcst. PWATs in the 0.8-1
in. range. This moisture combined with a mid-level disturbance
should provide the impetus for isolated- scattered
showers/thunderstorms Wed afternoon. Expect most of this activity
to be confined to far Northern Utah and the higher terrain of
central/Eastern Utah. The ridge/trough couplet will progress
eastward Thursday and the area will come under generally zonal
mid- lvl flow Thursday/Friday. Upper-lvl divergence in the left
exit region of a jet, combined with residual moisture should be
sufficient to kick off storms both afternoons, however expect
coverage to be primarily limited to the higher terrain of
northern/eastern Utah. Meanwhile building heights combined with a
general drying trend will likely squash precip chances over S.Utah
Thurs/Friday apart from the most-prone high-terrain areas.
The ridge builds overhead Saturday, ushering in an areawide
warming/drying trend for the weekend/beginning of next week with
maxes rising to 5-10 degrees above normal by Monday. Low-lvl
moisture begins to return to S.Utah on Monday resulting in increased
chances of precipitation particularly over the higher terrain.
VFR conditions will continue through the TAF period at
the KSLC terminal. Northwest winds this afternoon are expected
switch to the southeast between 0200-0400 UTC. However, there is a
20 percent chance that the switch to southeast winds will hold off
until after 0400 UTC.
ERC values are below the 50th percentile across southern and
portions of eastern Utah. Elsewhere values are higher, varying
from 50th through 79th percentile most locations. Near Dugway ERC
values are in the 80th through 96th percentile.
A wave propagating northward over Utah tonight and tomorrow will
bring increased cloud cover and chances for wetting rains across
central and northern Utah. Across southern Utah for the remainder
of today some may be locally heavy, while across mainly central
Utah dry microbursts in the 35-45 mph range are forecast through
Periods of clouds and cooling temperatures next couple of days
through remaining fairly dry RH-wise. Will have to watch for new
lightning starts where fuels are particularly dry, but feel areal
extent and other parameters not where I want to see them for any
headlines at this time.
The coverage of showers and storms will diminish through the
week. A ridge builds in next weekend drying out the state with
very warm temperatures.
SHORT TERM/FIRE WEATHER...Rogowski
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