Scientific Forecaster Discussion
fxus65 kslc 212231
Area forecast discussion
National Weather Service Salt Lake City Utah
431 PM MDT Thu Sep 21 2017
Synopsis...a cold core upper low developing over Nevada will
drift slowly east across the Great Basin through the weekend.
Much colder temperatures are expected across the region, with
periodic valley rain and mountain snow concentrating on the
northern half of Utah and southwest Wyoming throughout the
Short term (until 06z monday)...animation of water vapor imagery
shows the upper low in the early stages of development over
northwest Nevada/extreme southeast Oregon later this afternoon.
This upper low will track slowly east across the Great Basin
beginning Friday, and continuing through the end of the weekend.
The near 700mb baroclinic zone over northwest Utah will begin to
work east across the area late this afternoon through the
overnight hours. Solid convergence into this low-level boundary
with cooler mid-level air across northeast Nevada/extreme
northwest Utah has produced a steady band of valley rain/high
elevation snow today. Ahead of the baroclinic zone scattered weak
convection developed early this afternoon. This convection along
with the precip associated with the baroclinic zone has been on
the increase in intensity this afternoon as the exit region of the
upper jet settles into the western half of the state.
Looking at a fairly active band of precip with embedded convection
associated with the low-level baroclinic zone this evening. The
strongest lift should focus on west-central through northern Utah
in the area of best low-level forcing supported by mid-level
dynamic and strong upper divergence associated with the jet.
Precip areal coverage and intensity should taper off later tonight
as the baroclinic structure begins to weaken after midnight.
The balance of the short term forecast period will be much cooler
statewide with periods of valley Rain/Mountain snow expected. The
heaviest and best organized precip will likely form on the north
and northeast flanks of the now formed upper low as it tracks
slowly east across the Great Basin. The current forecast track of
the low would put northern Utah and southwest Wyoming in the best
position to receive the heaviest storm total rain/snow.
Southern through east-central Utah will likely see breezy
conditions and somewhat less cooling ahead of the upper low this
weekend. Any precip will likely be convection that forms along
vorticity lobes rotating around the advancing low. Anticipating
this convection to be short-lived with minimal accumulating
precip from these events.
Long term (after 06z monday)...the center of the upper low
continues to track slowly east of the forecast area on Sunday night,
with wrap-around precipitation potentially continuing across
northeast Utah and far southwest Wyoming. The greatest chance of
precipitation with this feature is expected to push east of the area
through the day Monday.
There is still a fair amount of uncertainty on synoptic pattern
details for the beginning of next work week, but the general idea is
that a longwave trough will persist over the western Continental U.S.. a couple
of shortwaves will likely drop into Utah from the north Monday
through Wednesday to help maintain a cool airmass over the area, but
models indicate that little if any precipitation will be generated
by these waves.
After this period of below normal temperatures, a further drying and
warming trend looks likely late Wednesday through the end of the
week, as high pressure builds over the Great Basin. This period
capping off September currently looks to be the closest Utah has
experienced to "normal" temperatures (i.E.- Not a fair amount above
normal or below normal temperatures for the time of year) in a while.
Aviation...ceilings at the slc terminal should drop to MVFR levels
between 22z and 02z, as rain fills in across northern Utah.
Northwesterly winds may become gusty at times through 03z, with a 30
percent chance of wind gusts greater than 30 mph. Timing the
improvement of ceilings is less certain; some improvement is
possible between 06z and 10z, but MVFR conditions may simply persist
through at least 15z.
Fire weather...the cold upper level trough moving into the Great
Basin Friday will bring fall-like temperatures and precipitation
to Utah through the weekend. Temperatures, outside of southeast
Utah, will be below to much below normal through Sunday. Periods
of widespread valley rain and high elevation snow will exist,
mainly across the northern half of the state.
The only area of any real fire weather concerns remains across
southern through east-central Utah. The surface cold front
extending northeast to southwest through central Utah along with
fairly strong southwest flow aloft will maintain strong southerly
winds into Friday. The cold front will moves slowly east into
southeast Utah during the day Friday, then exit the state Friday
night. Winds will drop off behind the front as the winds aloft
drop off significantly for Saturday.
Utah...Wind Advisory until 9 PM MDT this evening for utz013-015-016-
Winter Weather Advisory until noon MDT Friday for utz007>009.
Short term/fire weather...Conger
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