Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
425 PM MDT Tuesday Jul 7 2015
Synopsis...high pressure will remain centered over the southern
rockies through much of this week. Abundant moisture along with
weak weather disturbances moving through the region will generate
showers and thunderstorms through midweek. A broad upper low
will move through the eastern Great Basin late in the week.
Short term (through 06z fri)...large scale set up across the
western Continental U.S. Places a broad closed low off the cali
coast...suppressed ridging over the Desert Southwest and cyclonic
flow over the northern rockies. Col evident over Oregon with a
multi-faceted deformation extending from SW Wyoming into central
Nevada. Within this deformation scattered to numerous showers and
thunderstorms have developed over northern Utah and southwest
Wyoming this afternoon.
Normalized cape values per Storm Prediction Center mesoscale-analysis indicate somewhat
shallow fat cape across the north...with SBCAPE values up to 1500
j/kg. A few of the storms have been quite impressive this
afternoon across the north...especially northern mountains where
diurnal circulations have aided initial spark to deeper updraft
formation. With weak steering flow aloft localized heavy rain has
been noted. Some small to medium hail signatures present as
well...especially with a cell over Liberty over the last hour.
Lack of shear aiding further organization will likely suppress
any large hail development today however.
Across the south scattered to numerous showers and isolated storms
have been common throughout the day as a weak disturbance has
spun east from central Utah into eastern Utah. Instability
profiles have decreased behind this wave and lapse rates have
dropped significantly over the last few hours indicative of a
suppressed environment in Lee of this wave. Though scattered
showers/storms will continue into the evening it appears that any
threat for locally heavy rain/flash flooding has waned south of
Aforementioned eastern Utah wave will continue to gradually
meander east tonight and expect a net uptick of cloud cover and
spotty nocturnal showers to occur after sunset through the
overnight hours across the southern half of the area.
A similar environment will occur tomorrow for much of the area
outside of far southwestern Utah where a drier southwesterly flow
aloft will begin spreading in...this as the cali low translates on
shore. This drying aloft will slowly spread northeast Thursday
cutting precipitable waters in half areawide. Low level moisture will be
challenging to scour completely and anticipate some isolated
convection across northern/central portions Thursday
afternoon...with higher based and likely gusty storms forming over
southwestern Utah late day as the low opens and becomes negatively
tilted over southern Nevada. This trough will lift north across
western Utah Thursday evening likely maintaining high based
convection across the western half of the state into the overnight
hours. The potential for strong convectively driven winds exists.
Long term (after 06z friday)...the shortwave lifting north across
the forecast area Thursday night is expected to have passed well
north into Idaho by Friday...however lingering moisture will support a
chance of additional convection Friday afternoon across the northern
third of the area.
An upper trough is forecast to linger along the coast through the
weekend maintaining a relatively cool airmass across the forecast
area. The GFS and ec are in disagreement with respect to the
evolution of the subtropical ridge centered across the Southern
Plains during the upcoming weekend into early next week. The ec
expands this ridge axis westward keeping any appreciable moisture
suppressed southward while resulting in a warming trend areawide.
Meanwhile the GFS keeps this ridge centered further east while
developing a moisture tap around the western periphery of this ridge
and into the forecast area beginning Sunday...and continuing into
early next week. Given these differences have only made minor
changes to the extended portion of the forecast.
Aviation...there is a 40 percent chance that a thunderstorm outflow
impacts the slc terminal between 23z and 02z...which could result in
an abrupt gusty wind shift. Showers should largely dissipate between
02z and 04z...with winds shifting to the southeast around that same
time. That said...a 20 percent chance of showers will persist
through the overnight hours.
Fire weather...moist airmass in place will hold over much of the
district through tomorrow before a drier southwesterly flow aloft
moves SW-NE through the eastern Great Basin. One more day of
elevated wetting rain potential can be expected as such. As the
drier air moves in a downward trend in relative humidity is expected for the
latter half of the week...but it will take some time to scour the
plentiful low level moisture in place. As this flow arrives...and
a broad upper low approaches the area Thursday...locally breezy
winds can be expected across the western valleys. At this time relative humidity
looks to remain above critical levels where fuels are cured...and
winds only borderline critical during the PM hours.
Passage of the upper low late Thursday afternoon and night will
likely spark high based convection with minimal rainfall but a
threat thunderstorms producing gusty and erratic outflow winds.
This potentially could be the most concerning fire weather issue
in the short term.
Yet another moisture surge is possible late next weekend or early
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