Scientific Forecaster Discussion
fxus65 kslc 201017
Area forecast discussion
National Weather Service Salt Lake City Utah
417 am MDT Thu Jul 20 2017
Synopsis...monsoon moisture will persist over most of Utah
through today. Drier air will spread back into the area from the
northwest Friday into Saturday.
Short term (through 12z saturday)...partly cloudy skies exist
across much of Utah and southwest Wyoming this morning, as deep
monsoonal moisture is still in place over the region, and the
forecast area remains on the western edge of a ridge centered over
the Southern Plains. Precipitable water values are generally in
the 0.9 to 1.2 inch range over the cwa, with somewhat higher
values over far southwest Utah. With a slow clearing trend
overnight and skies becoming more scattered, good surface heating
later today (potentially aided by subtle disturbances moving
across the region from the south and west) should help initiate
showers and thunderstorms across most of the area.
Given the deep moisture and decent anticipated instability across
the area, have decided to once again issue a Flash Flood Watch
focusing on the flood prone areas of central and southern Utah.
Potential limiting factors include lingering cloud cover and
increasing speed of storm motion. With cloud cover less thick than
yesterday morning, suspect that the lingering clouds will only
delay convective initiation by 1-3 hours at most. As for the
increasing storm speeds, aided by the increasing south-
southwesterly flow over the state through the day, southern Utah
often gets just as many flash floods with faster, training storms
as it does with the more traditional threat of plodding or
Other than the convective activity, today should be another warm
day across the area, with breezy south winds in many locations. A
trough moving through the Pacific northwest today will flatten the
ridge late today through tomorrow, and help to push some drier
air into Utah from the northwest. This should decrease the
thunderstorm coverage across the north on Friday, though continued
wet storms are expected across the south and east. This drying
trend will continue into Friday night and Saturday.
Long term (after 12z saturday)...
ridge this weekend suppresses to the south allowing a drying trend
to develop mainly north of I-70 Saturday. Ridge re-develops overhead
Sunday so think majority of convection will remain to our west.
This ridge propagates east into Colorado for the reminder of the
forecast cycle, setting the stage for daily showers and
thunderstorms, some which could be heavy rainfall producers. Naefs
perceptible water standardized anomaly places higher anomaly for
a larger area than either yesterday or today next week.
Expecting warmest temperatures this weekend as moisture trends to be
suppressed further south and west. High temperatures will trend down
as moisture deepens next week. Overnight lows are forecast in the
lower to middle 70s in Salt Lake the entire time due to the thermal
ridge and convective debris potential.
Aviation...there is a 30 percent chance of showers/thunderstorms
impacting the slc terminal today, with gusty winds, lightning, and
brief heavy rain all possible. Otherwise, winds at the slc
terminal are expected to remain gusty from the south through the
day, with only a slight chance of a switch to northwesterly winds
between 22z and 00z.
Fire weather...monsoonal moisture remains over most of Utah this
morning, and is expected to bring another round of showers and
thunderstorms to the state through this afternoon and evening.
Most of these storms will be wet, with a good chance of wetting
rain across most of the area, except perhaps the far northern
portions of Utah. South-southwesterly winds will increase across
the state today, with the strongest gusts expected in western
Utah, but relative humidities will remain relatively high in most
A drying trend from the northwest will begin late today and
continue Friday through Sunday, slowly but surely bringing a drier
airmass to the entire state by Sunday afternoon. Moisture will
hang on longer in southern and eastern Utah, where decent coverage
of showers and thunderstorms will continue through Friday and
Saturday. This drying will likely be short-lived, as moisture
will likely return for the beginning of next week.
Utah...Flash Flood Watch from noon MDT today through this evening for
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