Storm Watch 5, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
800 PM PST Sun Feb 12 2017

.DISCUSSION...13/00Z NAM in. 13/00Z GFS in through 78hr.

The hemispheric pattern continues to show a wave number of three
around the globe. This usually makes for a stagnant or
retrogressive pattern. This time around it looks like it will be a
very slow progressive pattern over the next several days.

A strong long wave ridge has set up along the west coast...and
this is bringing the dry weather to the area. An inversion has
set up over the valleys...leading to air stagnation over those
areas with areas of late night through morning low clouds and fog.
The ridges...which are above the inversion...are warmer at night
and occasionally breezy as low level easterly flow continues over
the area.

This pattern will persist until the upper level ridge breaks to
the east of the area Monday night. At that point a short wave
ejected from the upstream long wave trough will move onshore. The
models have been trending weaker with this feature and now it is
hard to find any sort of surface feature with this wave. The
latest 13/00Z runs keep the land areas of the Medford CWA dry
through Tuesday. However, the easterly flow will end and mixing
will improve a bit in the valleys.

A strong front will move onshore Tuesday night into Wednesday...
and this one will bring widespread precipitation to the area along
with areas of strong winds.

Extended discussion from the Sunday afternoon AFD...Tuesday night
through Sunday. By Tuesday evening, there is a chance of rain at
the coast with a slight chance extending to the coastal range. The
probability of precipitation will increase Tuesday night through
Wednesday with highest amounts in Curry, Josephine, and western
Siskiyou County. At this time, it looks most likely that southerly
winds will reach advisory rather than warning strength Wednesday
into Wednesday night with gusts of 45 to 55 MPh possible in the
southern Shasta Valley and in Lake County with stronger gusts to
70 MPH at coastal headlands. This is close enough to warning
strength, that it would not take too much of a change in the model
scenario to produce warning level winds. We will wait for at
least one more model run before deciding upon any need for issuing
high wind watch products. Snow levels will initially be very
high...around 9000 feet. Another concern will be for additional
flooding on the rivers which reached flood stage last
week...namely the Coquille, lower Rogue, Klamath, Pit, and
Sprague. By Thursday morning the back end of the lingering front
will be moving inland. The strong north to south pressure
gradient will be loosening as the axis of the strong southerly
jet moves east. The jet will continue to shift east Thursday
night and a shortwave moving in behind the exiting front will
support continuing showers overnight. Both the GFS and the ECMWF
have migrated the upper low farther south into central California
with the forecast area now seeing wrap around precipitation as the
closed low moves east to the south of the area into Saturday.
Models are in remarkably good agreement with the the next system
in line that will move in Sunday on the left exit region of a 160
kt jet. However, the nose of the jet will be pointed at central
California and northern California...which will see the bulk of
the precipitation.

&&

.AVIATION...13/00Z TAF CYCLE...Late night low clouds and
fog/freezing fog are expected again for West Side valleys, with
improvement to VFR conditions around noon local time Monday.
Continued dry easterly low level winds should keep the coast clear
again tonight, and a very dry air mass east of the Cascades should
keep that area VFR too. SK

&&

.MARINE...Updated 300 PM PST Sunday 12 February 2017...High pressure
will build over the waters today with winds and seas remaining below
advisory level through Monday.  A frontal system will approach the
outer waters Tuesday and winds will shift south and increase. This
frontal system will move closer to the outer waters Tuesday
afternoon and basically stall offshore through Wednesday night.
Models show low pressure developing along the front west of the
waters Wednesday afternoon and moving northeast Wednesday night, but
they differ with the position and strength of the low. In short,
gales are possible late Tuesday through Wednesday night, with
highest winds and seas likely Wednesday afternoon. SK

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Air Stagnation Advisory until noon PST Monday for
        ORZ023-024-026-029>031.

CA...None.

Pacific Coastal Waters...Gale Watch from Tuesday afternoon through
        late Wednesday night for PZZ350-356-370-376.

15/15/15