Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
350 PM MDT Friday Sep 4 2015
Synopsis...an upper level trough of low pressure will move across
the northern rockies and Great Basin through Saturday...with an
associated cold front dropping through southwest Wyoming and
Utah. A general drying and warming trend is expected next week as
a ridge of high pressure builds over the region.
Short term (through 06z monday)...
water vapor loop shows a deep closed low over the Pacific
northwest...with Tropical Storm Kevin well southwest of Baja California.
Amdar 400-250mb wind observations show a 80-120kt cyclonic jet off
the California coast into the northern rockies. There is also a
southwest 60-80kt jet from Southern California into The Four
Corners. GOES/GPS/rap/12z slc radiosonde observation indicate the precipitable water
value ranges between 0.25" mountains...to 0.75"-1.00" across the
Msas analysis shows a surface cyclone over northwest Utah with a 6mb
southerly pressure gradient across western Utah. Rap indicates 700mb
flow is on the order of 40kts. These factors will continue to
contribute to southerly wind gusts up to 50mph across the western
valleys through this evening. While some locations decouple
tonight...expect winds to remain elevated on the ridgelines along
with southwesterly oriented canyons across this portion of the state.
Rap 850-700mb saturated equivalent potential temperature/streamlines
and thickness fields suggest the baroclinic zone catches up to the
surface trough overnight across the west desert...with possible
advancement of the baroclinic feature into the Great Salt Lake by
morning. Expect southerly barrier flow to be enough to keep the
boundary to the west of the Wasatch front through the morning
hours...with building baroclinicity due to diurnal heating in the
warm sector. This should strengthen the boundary allowing it to cross
into central Utah late evening eventually settling across south
central Utah overnight as it losses support aloft. This feature
lifts northward as a weakening warm front Sunday.
With the storm track a bit further north than originally
forecast...gusts should largely remain in advisory criteria. In
addition...further proximity to synoptic scale dynamics and cold
core aloft will result in less convective coverage. Most locations
will see isolated convection...with scattered convection mentioned
closer to the Idaho border.
Long term (after 06z monday)...GFS and European model (ecmwf) models in fairly
good agreement with indicating a very broad cyclonic to nearly
zonal flow over region from Monday through Wednesday. The
resulting westerly flow should prohibit any significant moisture
from lifting north into forecast area...though GFS does hint at a
brief period of southwest low to middle level flow pushing moisture
to the southern reaches of Utah Tuesday night and Wednesday.
Given lack of moisture in European model (ecmwf)...will keep forecast dry
Models indicate ridge of high pressure will build to the south and
west of forecast area from Thursday into Friday...resulting in a
very dry northwest flow. With agreement in deterministic models
and most ensemble members...confidence relatively high in keeping
region dry with warming temperatures.
Aviation...strong and gusty southerly winds will continue at the
kslc terminal through this evening before gradually diminishing
overnight. Gusts in excess of 30 miles per hour are expected through 02z...with
a 30 percent chance that intermittent gusts of that magnitude will
occur overnight tonight. South winds will ramp back up tomorrow
morning ahead of a strong cold front expected to move through the
terminal between 19-20z. Brief periods of blowing dust are
possible...most likely through 02z tonight...and 17-20z tomorrow.
Fire weather...red flag warning continues through Saturday evening
for the western valleys. Relative humidity values between 10 and 15 percent...along
with very strong winds will contribute to critical fire weather
conditions. Southerly gusts up to 50 miles per hour for the rest of this
afternoon...potentially locally stronger tomorrow afternoon into the
early evening. A significant wind switch to the northwest is
forecast with the cold front Saturday afternoon and evening. Winds
diminish Saturday night. Cooler conditions can be expected Sunday.
Convection appears isolated most locations...but lightning and
enhanced/erratic winds will be a threat concurrent with the larger
scale wind system for the remainder of today and tomorrow.
Utah...red flag warning until 10 PM MDT Saturday for utz478-492-495-497.
Wind Advisory until 10 PM MDT Saturday for utz003-005-015-016.
Wind Advisory from 8 am to 10 PM MDT Saturday for
Wyoming...red flag warning until 9 PM MDT this evening for wyz277
For more information from National oceanic and atmospheric administration/S National Weather Service visit...
http://weather.Gov/saltlakecity (all lower case)
For frequently asked questions about the area forecast discussion
http://weather.Gov/saltlakecity/general/afd_faqs.Php (all lower case)