Scientific Forecaster Discussion
fxus65 kslc 231130
Area forecast discussion
National Weather Service Salt Lake City Utah
430 am MST Tue Jan 23 2018
Synopsis...high pressure aloft will return midweek before a fast
moving storm system impacts the area Thursday into Friday
morning. High pressure is expected to return for the upcoming
Short term (through 12z friday)...
water vapor satellite shows a wave sliding down The Rockies as
ridging amplifies from the southern Pacific into the West Coast.
400-200mb mdcars wind observations place a 100-120kt anticyclonic
jet from off the Pacific northwest coast over the Pacific
northwest. GOES/slc 00z radiosonde observation/hrrr indicate that precipitable water
values vary from 0.05"-0.15" mountains to 0.20"-0.30" most
valleys. A weak atmospheric river stretches from near Hawaii into
the Pacific northwest.
Snow showers have dissipated as cold advection wanes. A strong
warming trend and height rises aloft will ensue with the ridge
building into the interior west today and tomorrow. Southerly
winds build tomorrow and tomorrow night with height falls aloft
occurring over the Pacific northwest.
Still some disagreement in the models in the details and timing of
the next trough, affecting the region primarily Thursday and
Thursday night. The GFS/NAM are faster with the system than the
European, and continued to favor a slower solution given recent
storm trends. Even if the faster GFS is correct, pre-frontal
environment will be quite dry and will take a lot of lift and
cold advection before saturating. Pops Thursday morning were
thusly scaled back, increasing for the Wasatch front as the
afternoon progresses and maximizing most areas during the evening,
with a bit of orographic on northwest flow, which quickly shifts
southwesterly later Thursday night. Kept pops most areas late
Thursday night with the cold pool aloft still overhead, offering
the best lapse rates during the storm system.
Next few days should allow many Road surfaces to trend warmer, so
hopefully the combination of warmer roads and lower snow rates
with this system should lower the threat of a long duration of
Road snow. Though the Thursday evening commute will have to
continued to be monitored.
Went on high side of guidance for Wednesday night lows and low
side for overnight relative humidity ahead of the cold front.
Long term (after 12z friday)...
progressive nature of the low amplitude trough continues to limit
areal extent of snow showers (largely orographically driven)
Friday across the north, even with passage of the final trailing
short wave which is looking less impressive with each run. Warm air advection
precip remains more expansive across the far north in the latest
GFS vs. The ECMWF, with the latter representing more amplified
ridging faster Saturday. Regardless, even the more bullish GFS quantitative precipitation forecast
projections depict an extremely minor event relegated to the
extreme north. Have left mention for showery precip Saturday
(largely snow ) for that area, but that's more semantics than
Mean ridging remains the dominant weather feature late weekend into
early next week. Temps are expected to trend some 5-10 degrees above
climo during that timeframe with tranquil weather dominating. Do
need to monitor the potential of another progressive trough towards
mid week of next week, but we'll see how the pattern evolves leading
cigs in place (6000ft agl) at the kslc terminal are expected to
thin through mid morning, while light southeasterly winds will
remain in place through the morning. A switch to light
northwesterly winds is expected to occur between 18-20z.
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