Scientific Forecaster Discussion
fxus65 kslc 221606
Area forecast discussion
National Weather Service Salt Lake City Utah
1006 am MDT sun Sep 22 2019
Synopsis...high pressure aloft building into Utah will bring
moderating temperatures across the area today. The next storm
system will arrive early in the week, and will impact mainly
southern Utah Monday into Monday night. Dry and seasonal weather
conditions will prevail midweek.
Discussion...morning water vapor and 500 mb analysis depict a short
wave ridge extending from the eastern Great Basin north into the
northern rockies region, with a splitting trough upstream along
the pacnw coast. Beneath the aforementioned ridge today, dry and
nearly cloud free skies will persist within a light northwesterly
12z kslc radiosonde observation indicates a mid level subsidence inversion that
continues to lower, dropping from roughly h6 24 hours ago, to
almost 700 mb this morning. Coupled with a relatively cool airmass in
place, light flow and lowering sun angle, not expecting valley
areas will realize full mixing potential of the warming airmass
aloft, but do expect a net temp increase of 5-7 degrees vs.
Yesterdays highs (especially western valleys). Current forecast
captures this well and have made no updates this morning.
Moving forward will continue to monitor the approach and evolution of
the splitting trough as it translates into the western Great Basin
later today. Guidance remains locked into the trough closing over
Nevada tomorrow, prior to settling into the lower Colorado River
valley region Tuesday. In large this trough will bring limited
impact to the forecast area, but areas along the Arizona strip
region from roughly Lake Powell east into Utah's Dixie will see an
increasing potential for showers and/or isolated storms later
tomorrow afternoon on into the evening hours, this coincident
with a mid level deformation axis that remains likely to develop
along the northeastern periphery of the low. Forecast retains
slight chance/chance pops for those areas, but will assess latest
guidance and update as needed for the afternoon package.
Previous discussion...dry Tuesday with strong height rises overhead
and warming aloft, along with anti-cyclonic curvature courtesy of
the building ridge axis from the northwest. Northerly downslope
winds may become gusty across outlying Dixie. Benign weather
Wednesday with the flow aloft orienting northwesterly.
Utah will be located between the opening closed low near the 4
corners and a short wave ejecting eastward across southern Idaho
ahead of the main trough developing along the Pacific northwest coast.
The ec creates some quantitative precipitation forecast over southern Utah Thursday afternoon
despite lack of any dynamics not to mention anticyclonic flow at
500mb, have removed mention of pops there but increased pops in
the north where the cold front and favorable jet dynamics combine.
Pops continue to increase across the north as the trough develops
along the West Coast and the winds back over Utah creating an up
Glide type of environment over the cooler air north of the surface
boundary that the short wave pushed into the area on Thursday. After
this point the forecast becomes even more challenging as to how
embedded shortwaves within the longwave trough carve out the trough.
As mentioned in previous long term discussion subtle movement of the
trough axis 100-200 miles inland or farther off the coast will have
significant sensible weather impacts on Utah. If the ec/GFS ensemble
mean comes to pass then a warm dry southerly flow will persist over
southern and eastern Utah through Saturday, which could elevate fire
weather concerns. Unfortunately, not knowing the individual timing
and effects of the embedded shortwaves this forecast loses
confidence in details beyond Friday night.
Aviation...southerly winds at the slc terminal will shift to the
northwest between 19-20z. Otherwise, a quiet day is expected.
Fire weather...a ridge builds in today, support a warming and
drying trend today, continuing tomorrow across southern Utah.
Critical relative humidity values will develop today across central and southern
Utah, as the north continues to dry out. Tomorrow with a cold
front crossing northern Utah, relative humidity values rise, while it remains
critically dry across central and southern Utah. Winds tomorrow
will likely be locally gusty ahead of the cold front, which may
lead to local critical fire weather due to wind and relative humidity.
The next next smaller but impactful storm crosses southern Nevada
Monday and moves into central Arizona Tuesday. Areas across the
south stand the best chance for showers and thunderstorms Monday,
with all areas dry Tuesday.
The cold front clears the Arizona border by mid evening Monday.
Tuesday locally gusty downslope winds across Dixie and surrounding
areas may support critical fire weather conditions with
borderline relative humidity values.
previous discussion/fire weather...rogowski/struthwolf
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