Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Salt Lake City Utah
739 PM MST Friday Feb 12 2016
Synopsis...high pressure over Utah will weaken as a couple of
weak weather disturbances cross northern Utah this weekend. High
pressure aloft will rebound across the region early next week.
Discussion...water vapor loop shows a ridge dominating the
interior west...with the next system nearing the Pacific northwest
coast. Amdar 400-250mb wind observations place a 90-110kt
anticyclonic jet from the Pacific northwest over the northern
rockies. GOES/hrrr/GPS/00z slc radiosonde observation indicate the precipitable
water value ranges between 0.05"-0.10" mountains...to 0.25"-0.40"
northern/west central valleys.
With an inversion in place...main concern is haze/fog/stratus and
temperatures. The base of the inversion is near 850mb with a
strength of 12... is stronger than 24 hours ago.
Stratus deck continues its hold over the west desert and into the
Wasatch front...but is again obscured due to cirrus on satellite
imagery. These high clouds may help to keep the fog from becoming so
dense in some locations.
Have adjusted fog wording to areas of dense fog along the Lakeshore
and benches where terrain intersects stratus. Did the same across
west central Utah and the Sanpete Valley where previous nights have
shown some of the poorest visibilities...adjacent to Utah valley.
Only other change was to adjust the temperature/dewpoint/rh and
apparent temperature curves with latest observational and lamp
guidance through the middle of the night.
the persistent ridge of the last few days will continue to
flatten through tonight...finally giving way to a very weak
shortwave trough brushing northern Utah around middle-day Saturday.
This trough will be quickly followed by another slightly stronger
impulse early Sunday. Each impulse will serve to gradually weaken
the stubborn inversion for central and northern Utah. We are
currently expecting much of the low clouds and fog to dissipate by
Saturday afternoon...though haze and smog will likely linger for
the Wasatch front urban corridor as complete mixing is not likely.
A few showers will move into the northern mountains and perhaps the
valleys as well by Sunday morning. With 700 mb temperatures falling to
around -6c at slc and given the persistent colder air at the lower
levels...any precipitation in the valleys is expected to begin as
mostly snow. However...modification of the temperature profile and
gradual warming as the inversion weakens should allow rain to
gradually mix in by Sunday afternoon. Precipitation is expected to
linger through Monday mainly across the northern mountains and
adjacent upslope regions in a moist northwest flow aloft.
Two primary concerns during the long term are the strength of
lingering inversions across the northern/western basins and the
resultant effect on temperatures...as well as timing of a
progressive shortwave trough during the latter portion of the
Regarding temperatures during the midweek period...with the
Assumption valley inversions are weakened but not completely
dislodged by the Sunday trough assume cold air will remain in place
to some degree across the western valleys. As middle level ridging
expands Tuesday through Wednesday warming aloft will act to further
strengthen/redevelop these inversions and as such have generally
gone below guidance Tuesday across these areas while trending closer
to guidance across the higher elevations as well as the
southern/eastern valleys. The one caveat would be if inversions do
indeed dissolve over the weekend western valleys may warm
considerably more than currently forecast.
This middle level ridge axis shifts east Wednesday allowing for a
strengthening southwesterly flow and developing southerly gradient.
It would seem most valleys should mix out with this scenario and
have trended temperatures closer to a guidance consensus...although
not out of the question we could see a few stubborn cold pools
Both the GFS and ec bring a progressive middle level trough through the
region during the latter portion of the week. The GFS is more
progressive with this wave than the 12z ec...which itself had
trended faster from its 00z predecessor. Given uncertainty in timing
have maintained probability of precipitation in the chance range for Thursday into Thursday
night...although did lower probability of precipitation just a bit Thursday giving deference
to the ec solution. This system will be mild with valley
Rain/Mountain snow as the p-type.
Aviation...prevailing IFR conditions in haze/fog and low stratus
are expected to remain in place at the kslc terminal through the
early portion of the night. LIFR in dense fog and/or ceilings near
200 feet are once again expected to develop by early Saturday
morning...but there is a 40 percent chance conditions remain IFR.
IFR ceilings likely through the day tomorrow...though visibilities
may improve into MVFR.
For more information from National oceanic and atmospheric administration/S National Weather Service visit...
http://weather.Gov/saltlakecity (all lower case)
For frequently asked questions about the area forecast discussion
http://weather.Gov/saltlakecity/general/afd_faqs.Php (all lower case)