Scientific Forecaster Discussion
fxus65 kslc 210956
Area forecast discussion
National Weather Service Salt Lake City Utah
356 am MDT Tue Aug 21 2018
Synopsis...monsoon moisture will support widespread thunderstorm
coverage today and tomorrow, potentially bringing locally heavy
rainfall. Drier and a little warmer late week.
Short term (through 12z friday)...
* Flash Flood Watch this afternoon and evening for portions of
southern and east central Utah
* strong gusty/erratic microburst winds possible eastern valleys
* copious lightning
* smoke across northern Utah
exceptional drought is occurring centered across Sanpete County
and near The Four Corners. Extreme drought is affecting most areas
south of Highway 40 and east of I-15. Drought conditions scale
down to moderate drought along the border of Nevada, Idaho and
an elongated trough is over the Pacific northwest and northern
rockies, while a ridge is centered over New Mexico.
Local observations and trends...
00z kslc radiosonde observation shows a dry boundary layer and steep lapse rates to
near 550mb, with better moisture and moderately strong southwesterly
winds in the mid and upper levels.
Precipitable water values range from 0.30"-0.50" northern valleys
and mountains to 1.00"-1.40" most valleys south of I-80.
A trough extends from north of Las Vegas into north central Utah.
Smoke has expanded southward across northern Utah, while southerly
winds are breezy across portions of central and southern Utah.
24 hour trends:
* precipitable water 0.50"-0.75" higher central and southern
* temperatures are 5-15f warmer
* dewpoint depression 10-25f lower (moister)
have included mention of smoke across northern Utah through this
evening given favorable flow from the north as advertised by the
hrrr smoke model.
Experimental CIRA advective precipitable water product indicates
moistening continues from the southwest particularly in the
700-500mb and 850-700mb layers.
Expect an active convective day, with much of the activity along
and east of The Spine of the mountains based on the monsoon
moisture trajectory. Combination of instability, shear and
dynamics (associated with upstream trough and upper level jet)
should support widespread deep convection. This is supported by
simulated reflectivity on respective convective allowing models.
With widespread deep convection in the forecast, thermodynamic
considerations point to heavy rainfall and a flash flood
potential. These include a normalized cape around 0.1, local around
675mb, dewpoints well into the 40s and 50s, and a warm cloud
layer of about 5kft. While storm motion is swift at 30kts from the
southwest, training storms could bring multiple rounds of heavy
rainfall. Flash flooding is a significant concern near burn scars,
Slick Rock, slot canyons and normally dry washes. Flooding and
debris flows may impact travel.
Strong/gusty microbursts are also a concern, particularly across
the eastern valleys where forecast soundings show a bit more
dcape due to drier lower levels.
Tomorrow looks similarly active. The elongated trough transitions
to a closed low over southern Idaho tomorrow. This should draw
additional moisture a bit further west to the Nevada border. Best
height falls are to our northeast, but the upper level jet axis is
overhead and may lead to organized storms once again. The heavy
rain threat will continue through Wednesday evening.
With the aforementioned closed low crossing the northern rockies
Thursday, drier air is advected into Utah from the west. Best
chance of convection appears to be south of I-70 but should be
widely scattered at best.
Thursday will also begin a warming and drying trend.
Long term (after 12z friday)...
the global models are indicating a pattern shift will be taking
place towards the end of this extended period. The ridge which has
dominated the western Continental U.S. Over the past month is expected to be
replaced by a trough. The exact position and depth of this trough
is yet to be determined. Although the models are in agreement
with a trough over the Pacific northwest into western Montana the
farther south the less confidence there is as to the sensible
weather expected. The gefs ensemble mean is broader and not as
deep with this trough as the deterministic GFS. The 12z ec was
also similar in this regard. Unfortunately the 00z ec did not get
sent tonight due to computer issues, but even without this run
caution should be exercised due to the fact that there are two
typhoons in the western Pacific which will influence the jet over
the western Pacific in some manner (most likely stronger). Bottom
line, have left the grids untouched beyond Saturday in fear that
there will be multiple iterations of where the trough will set up
and what its strength will be. Would not be surprised that the
ridge in the Gulf of Alaska will be stronger allowing the trough
to dig farther southwest and cutoff west of northern California.
If this was to happen then more moisture would be brought into
south to southeast winds will prevail at the slc terminal through
about 16z or so then shift to the northwest. There is a 30%
chance that winds will not shift to northwest until 18z. Showers
and thunderstorms are expected to be more numerous to the south of
the terminal and this could cause southerly outflow winds to
impact the terminal from mid afternoon through this evening.
erc values are running below the 50th percentile southwest Utah,
near the 80th percentile elsewhere. Ercs have been trending higher
most locations during the past several days.
Monsoon moisture remains in place. Very active especially along
The Spine of the mountains and eastern valleys today. These areas
could see locally heavy rainfall and gusty/erratic microburst
winds in addition to copious lightning. Wednesday looks similarly
Drier air invades from the west Thursday limiting convection to
isolated coverage across central and southern Utah. Little to no
convection is expected Friday into the weekend. A warming and
drying trend takes place Thursday through the upcoming weekend.
Utah...Flash Flood Watch from noon MDT today through this evening for
Short term/fire weather...10
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