Scientific Forecaster Discussion
fxus65 kslc 242139
Area forecast discussion
National Weather Service Salt Lake City Utah
339 PM MDT Wed may 24 2017
Synopsis...a cold front will sag south through northern Utah
tonight prior to stalling over central Utah Thursday as an
upper trough noses into the area. A secondary trough will
move across the area Friday maintaining unsettled conditions
into the first half of the Holiday weekend.
Short term (through 12z saturday)...msas mslp analysis places a
cold front stretching from extreme northwest Wyoming southeast across central
Idaho. Afternoon water vapor and 500 mb analysis indicates the core of
the upper trough its tied to continues to deepen over southern
Alberta. A short wave lobe developing along the western periphery
of this trough will drop southeast into northern Utah driving this
front south overnight prior to stalling across central portions
of the area during the day tomorrow.
Primary concern moving forward relates to pre-frontal convection
attempting to take form attm. Mid level moisture continues to
increase while advecting in from the SW coincident with the
southern branch jet. Deep mixing combined with a very dry boundary
layer continues to enhance an inverted-v signature in vertical
profiles, and coupled with weak instability and modest shear (both
greatest over the far north), do expect a continued uptick in
cellular development into this evening. Hrrr-3km continues to
develop convection over eastern Nevada over the next few hours, then
translates it east-northeast across the Wasatch front periodically between
00-06z. Gusty microburst winds will be the primary threat, with
some isolated severe speeds possible (especially west desert).
Areal coverage of showers/convection is expected to increase
through the middle of the night tied closely to the area of best
baroclinicity shifting northwest-southeast across northern Utah/SW WY, prior to
slowly phasing away towards dawn over central portions as the
front stalls and trends more diffuse. With loss of upper support
for the time being, any convection Thursday will likely remain
isolated and once again tied to the remnant boundary, or sparked
by diurnal influences over the mountains during the peak heating
Temps this afternoon have been the warmest yet thus far this warm
season (kslc just hit 89 f), but cold air advection in wake of frontal passage
will cool the northern half of the area down to near normal
levels over the next 24 hours. Gusty SW winds will remain south
of the frontal boundary as well across the south tomorrow, but are
expected to remain sub-advisory.
The aforementioned northern rockies trough will continue to spin
up through Friday, with another short wave lobe dropping into the
area from the northwest during the day. Baroclinicity will once
again tighten over the area, and coupled with modest upper forcing
and diurnal trends do expect scattered convection to develop
across the northern half of the area once again. Made minimal
changes to pops as forecast trends remain on track.
Long term (after 12z saturday)...a mid level ridge is forecast to
be the dominant weather feature through the majority of the forecast
period. Initially this ridge will remain centered west of the
forecast area along the West Coast, as the last in a series of
shortwave troughs exits the forecast area during the day Saturday.
After this time, as the ridge axis slowly builds eastward, expect a
warming trend through the latter half of the weekend into the middle
part of next week. Lingering moisture will provide a small chance of
afternoon and evening convection primarily along the terrain east of
I-15. The long range models diverge a bit during the
Wednesday/Thursday timeframe, with the GFS breaking down the
ridge, while the ec holds onto a stronger ridging solution over
the Great Basin. As such have held off on making any significant
changes to the latter portion of the forecast period.
Aviation...main concerns at kslc this afternoon/evening will be the
potential for strong (>30 kts.) Erratic microburst winds...with
these winds most likely between 23-03z. Some lightning is also
possible with these afternoon/evening storms but precipitation
should be minimal. Away from storms VFR conditions and northwest
winds should prevail through the period.
Fire weather...upper level storm system with attendant cold front
continues to encroach on northern Utah this afternoon, with
frontal zone passage anticipated to enter the far north later this
evening. Isolated showers and storms are beginning to fire ahead
of this front attm, and expect areal coverage to increase and peak
by mid evening. High based nature of storms and very dry low
levels will promote a gusty microburst threat across the northern
third of the district this evening, and the middle third overnight
coincident with the front.
The frontal boundary will stall across central Utah Thursday
maintaining in general an isolated shower/storm threat, but will
then push south Friday across the remainder of the district with
widely scattered showers/storms expected. Trends in relative humidity will climb
Gusty southwesterly winds are expected to continue south of the
front tomorrow, with some sites along the Utah/Arizona border region
gusting at times around 30 mph in the afternoon.
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