Scientific Forecaster Discussion
fxus65 kslc 211046
Area forecast discussion
National Weather Service Salt Lake City Utah
446 am MDT Sat Sep 21 2019
the storm system that brought precipitation and cooler
temperatures to the area will exit to our east today. Cool
temperatures will moderate Sunday ahead of the next storm system,
which will bring scattered showers and thunderstorms Monday. Dry
and seasonal weather continues through mid next week.
Short term (through 00z wednesday)...
infrared satellite shows some mid and high clouds across central Utah,
with lower cloud bases relegated to the Wasatch front eastward.
These clouds likely will aid in restricting closed valley fog
potential early this morning.
Radar is currently quiet. We are monitoring for the potential of
isolated showers this morning due to the mid-level cool pocket
overhead coincident with the trough axis. Secondary disturbances
are expected to move into the area from northern Nevada and
southern Idaho today. BUFKIT forecast soundings modified for a 60f
Great Salt Lake surface water temperature (derived from GOES east
surface temperature product) shows only 110 j/kg cape (ncape
0.04) and equilibrium level near 10kft. With infrared satellite
indicating some low clouds building over the lake and many cams
including the hrrr/href and NCEP WRF-arw/nmm/NAM-nest showing a
slug of decent to unbelievable quantitative precipitation forecast from the Great Salt Lake
southeast have included isolated pops from Salt Lake northward
this morning. It's interesting that our local WRF-arw is not
indicating any quantitative precipitation forecast until late morning west of Ogden. Snow level
generally above 8kft this morning increasing to above 9kft this
afternoon. Snow may reach 1-1.5kft lower locally and briefly
during any stronger showers that may develop.
Trough axis pulls east of the forecast area this evening. Left
some isolated showers in the northeast part of the state this
evening mainly for any leftover shower after 00z. Strong height
rises and warming aloft will end the threat of showers my mid-
evening at the latest.
Drying and warming Sunday with ridging building in. Did go toward
the lowest end of the (bias-corrected and nbm) guidance envelope
across most valleys to account for the lower sun angle taking
some efficiency off of daytime mixing, and moving out of the the
very warm earlier portion of the month.
Next trough along Pacific northwest coast splits Sunday as a
strong northerly jet takes the majority of the energy southward.
This evolves to a closing compact storm system which crosses over
southern Nevada Monday and progresses into central Arizona
Tuesday. This results in southwesterly breezes while relative humidity values
remain quite low across southern Utah Monday. As the day
progresses, so does the chance of showers, especially southern
Utah in closer proximity to the height falls and tropopause fold
associated with this storm system.
Dry Tuesday with strong height rises overhead and warming aloft,
along with anti-cyclonic curvature courtesy of the building ridge
axis from the northwest.
Long term (after 00z wednesday)...
a closed low will be located over southwest Arizona while a ridge
moves across the northern rockies during the Wednesday resulting
in a dry day across the County Warning Area. However, the global models diverge of
their solution after this time with the GFS opening the closed
low up as it moves it east-northeast across northwest New Mexico on Thursday with
little if any impact on the southern portion of the County Warning Area other than
some clouds. Meanwhile, the ec drifts the low north northeast
into south central Utah by 00z Friday. Not sure which model has
the best handle on the forecast track so used a blend of the two
for the forecast which results in low end pops for southern
mountains and the uintas for Thursday afternoon.
The models all develop a huge deep trough either along the West
Coast (ec) or just inland (gfs) Friday into Saturday. The GFS
ensemble mean is closer to the ec operational and ensemble mean so
have leaned in this direction. The ec has a weak short wave kicking
east northeast out ahead of this developing trough on Friday which
will drape a cold front across northern Utah late Thursday night
into Friday. Have cooled temps across the north Friday and increased
pops accordingly. This front should washout as the strong trough
continues to develop and southwest winds aloft strengthen. However,
the ec depicts pops all across the County Warning Area late Friday night through
midday Saturday which does not look reasonable considering the lack
of dynamics. Have trimmed pops over eastern and southeastern
portions of the County Warning Area during this period but confidence is low in
lowering the pops across the rest of the cwa, but would not be
surprised when push comes to shove that the entire County Warning Area is dry
Saturday into Sunday as well as windy due to the trough remaining
well west of County Warning Area.
southerly winds are expected at the slc terminal through
about 17-18z before shifting to the northwest. There is a 15% chance
of a brief shower with cigs near 5-6k ft above ground level through about 18z.
Otherwise, clearing skies and light winds for the remainder of the
weak instability triggered by the passing storm system and Warm
Lake temperatures may support isolated showers, with isolated snow
showers above 7kft. This evening clears off ahead of a ridge
building in. This will result in a warming and drying trend into
Monday. Winds will be a little breezy from the northwest today and
south tomorrow. Critical relative humidity values restricted to near Arizona and
the swell today (along with a drying west desert), and most areas
south of I-80 tomorrow (as the north significantly drys).
Warming toward climatology Monday and Tuesday as the next smaller
but impactful storm crosses southern Nevada Monday and moves into
central Arizona Tuesday. Areas across the south stand the best
chance for showers and thunderstorms Monday, with all areas dry
Tuesday. Locally critical fire weather conditions are possible
Monday due to gusty winds and low humidity across southern Utah.
Winds at this time appear marginal for headlines.
Short term/fire weather...10
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