Scientific Forecaster Discussion
fxus65 kslc 212257
Area forecast discussion
National Weather Service Salt Lake City Utah
357 PM MST Tue Feb 21 2017
Synopsis...a cold front tonight will bring a period of cold
unsettled weather lasting through the end of the week.
Short term (through 00z saturday)...the upper level jet
positioned across the sierras and into the Great Basin this
afternoon is shearing the moisture tap from the Pacific as the
trough swings inland. A tightening baroclinic zone draped across
northwest Utah associated with the approaching cold front is
evident per satellite and area observations. Early afternoon
thunderstorms are developing across the northern portion of Utah
and into southern Idaho ahead of the front expected to approach
the Wasatch front early this evening. Gusty southerly winds
continue to peak this afternoon and should diminish after sunset.
With the frontal passage, potential exists for convective
precipitation along the Wasatch front and dynamics are in place
for moderate to heavy snow in the mountains with the sharp drop in
temperatures aloft. As the front pushes southeastward overnight
and stalls out, a brief lull in precip is forecast for Wednesday.
The upper level low and right entrance region of the jet sag
southward with the open wave of the trough, thus re-enforcing the
frontal band that previously stalled out across central Utah
Wednesday evening into Thursday.
The flow will more prominently shift to the west- northwest
Wednesday evening, as another round of mountain snow is expected
to light up. The evening commute Wednesday could have light valley
snow, with the more troublesome commute being Thursday morning.
Model guidance suggests there could be lake enhancement from snow
showers early Thursday morning, but confidence is low at this time
to put mention in the forecast. Worth keeping an eye on if models
continue to hint at this. Have extended the Winter Weather
Advisory through early Friday morning for the northern and central
mountains and included the Wasatch Plateau.
Per the 12z slc raob, 700mb temps leveling out around 0c should
continue to decline throughout the week as cold Arctic air fills
in with the longwave trough moving through the Great Basin.
Moisture begins to shut off Friday with the departing trough to
the east and the cold air mass sinking southward.
Long term (after 00z saturday)...fairly active long term forecast
period will continue. Later Friday evening, sufficient moisture may
remain for a few showers across the higher terrain of northern Utah.
An upper level low is expected to dig down the Pacific coast
Saturday. In response to this low, expect rising heights and mid-
level warm air advection through the day Saturday. As the trough
opens up and crosses the Great Basin Sunday, the associated cold
front will likely cross Utah. The 12z ec and 12z GFS had an overall
similar picture, though the movement and location of the opening
wave were a bit different in timing and orientation. The 12z GFS
also established at least a limited tropical moisture tap.
By early next week, another upper level low digs down the coast and
then opens and ejects across the southern Great Basin/Desert
Southwest in some form.
Adjusted pops a bit higher on Friday evening/Saturday morning given
current model trends, after which kept much of the previous package
in tact. Given the difference in the global models with how far west
the upper level low veers off the coast before crossing the area and
whether a tropical moisture tap can be established, do not have the
confidence to go with higher pops at this time.
Aviation...VFR conditions are expected at the slc terminal through
the afternoon, with clouds increasing ahead of an approaching cold
front. Cigs are expected to drop to or below 6000 feet above ground level in rain
by 02z. Gusty southerly winds will continue through the afternoon,
with gusts in the 30-40 mph range. Winds are expected to shift to
the northwest by 00-01z with the passage of a cold front.
Utah...Winter Weather Advisory until 4 am MST Friday for utz007>010-517.
Wind Advisory until 9 PM MST this evening for utz003-005-015-016.
For more information from noaa's National Weather Service visit...