Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
925 am MDT Monday Mar 10 2014
Synopsis...the next cold Pacific storm system will reach
northern Utah late today...then move south through the state
tonight through Tuesday. A remnant of this storm will impact
extreme southern Utah late in the week.
Discussion...water vapor loop shows ridging flattening over the
eastern Great Basin ahead of the next system making its way into
the Pacific northwest coast. Amdar 400-250mb wind observations
reveal a southwesterly 100-140kt jet from northern California into
the northern rockies. GOES/GPS/rap/12z radiosonde observation indicate the
precipitable water value ranges between 0.20" and 0.48".
Breezy and warm ahead of the storm system today. Bumped highs up a
few degrees owing to +3c 700mb temperatures on south flow from the
rap this afternoon. Though plenty of cloud cover streaming in as the
ridge breaks down.
Models continue to back off on precipitation chances. Really thinned
probability of precipitation this morning to just slight chance or low chance northwest of
Ogden. Hrrr and local 4km WRF models show little precipitation
affecting valleys this afternoon...mainly Ogden northward. Thinned
probability of precipitation slc and south to slight chance and Ogden area to low likely.
Most significant change was to add a Wind Advisory for the west
desert Post frontal late this afternoon into tonight. Rap indicates
pressure rises on the order of 8-14mb/6hr this evening into early
tonight with a nice northwesterly pressure gradient behind the cold
front. While most persistent strong winds expected along sunrise-
30...there will likely be a period of strong Crosswinds across I-80
this evening with the cold frontal passage...which is also shown by
the hrrr-3km and local 4km WRF models.
Previous discussion follows...
the upper trough currently positioned along the Pacific northwest
coast will continue east- southeast into northern Utah by this
evening. The associated surface cold front will arrive over
northwest Utah early this afternoon...then sweep south across the
state as a fairly strong cold front overnight. Much colder air
will follow the front...leaving temperatures below normal across most of
the state through at least midweek.
Looking at strong thermal and dynamic support for deep lift along
and behind the cold front across northern Utah late this
afternoon through early Tuesday. The first area likely to see
heavy precipitation will be the far northern portion of the state in
vicinity of the middle-level trough and cold pool. Deep instability
will result in moderate to heavy convective precipitation...snow levels
dropping quite rapidly due to the intense precipitation.
A secondary vorticity lobe swinging south behind the main trough
will bring enhanced precipitation further south over northern and western
Utah later tonight through Tuesday. Much colder air already in
place behind the surface cold front should keep precipitation as
rain/snow or all snow with this second feature late tonight
Model guidance appears to have a reasonably good handle on the
evolution of this storm. One concern is the quantitative precipitation forecast guidance...
specifically the low numbers in the guidance. Considering the
subtropical moisture tap ahead of this cold trough...and the good
baroclinic structure of this feature...one would think that the
amounts would be higher with this feature. Have pushed the qpfs
higher in the forecast grids...though with the caveat that if
convection dominates the precipitation then the higher numbers will end
up more localized.
Heading into midweek this second feature mentioned above continues
south and evolves into a closed low over southern Nevada late
Wednesday. Guidance takes this feature east across northern
Arizona/southern Utah late in the week...with an increasing threat
of precipitation with time. Available moisture again is an issue with
this feature...with the various model guidance slow to develop
moisture and precipitation. Have kept probability of precipitation on the low side for
now...though with decent lift near the upper low would think that
the threat of precipitation will end up greater across southern Utah than
currently indicated in the guidance.
The upper low will move to the east of the forecast area early
Friday...with another disturbance forecast to cross north of the
forecast area Friday night into Saturday. The bulk of the precipitation
associated with this second system will remain to the north of
Utah. The ec shows a slightly deeper system...and suggests that
northeastern Utah could see a period of light precipitation
Saturday morning. However...due to uncertainty between
models...decided to hold of on increasing probability of precipitation for that time
Temperatures will be slow to rebound due to the unsettled northerly
flow over the forecast area. Lowered maximum temperatures a few degrees
next weekend and early next week as both the GFS and ec show a weak
cold front moving through the state Saturday...with a second weak
front for Monday.
Aviation...winds have started to pick up at the slc terminal this
morning from the south. These will increase in strength this
afternoon ahead of the front. Winds will shift to the northwest and
ceilings will drop to around 6000ft between 21z and 23z. MVFR ceilings with
light rain will develop between 23z and 01z...with rain changing to
snow between 3z and 6z.
Utah...Winter Weather Advisory from 4 PM this afternoon to 6 PM MDT
Tuesday for utz007>009.
Wind Advisory from 4 PM this afternoon to 4 am MDT Tuesday for
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