Scientific Forecaster Discussion
fxus66 kmfr 210336
Area forecast discussion
National Weather Service Medford or
820 PM PDT Tue Mar 20 2018
Discussion...evening update. Very weak radar returns are showing on
the radar, yet the ASOS at the Medford Airport is indicating rain.
With this one must conclude that the showers are very low topped.
The better action is slowly moving in from the south and heading to
the Mt Shasta region, but scattered showers are moving over the
region and current forecast looks on track. Sven
Short term...the radar is still showing returns indicating virga
over parts of the County Warning Area. Upper clouds are streaming into the area
from the southwest, ahead of a strong low just west of 130 west.
This low will pivot toward the coast and phase with a trough off
the coast of British Columbia, bringing heavy precipitation
Wednesday into Thursday. Expect the heaviest precipitation along
the coast, with southeast winds causing a slower start to
precipitation in the rogue and Shasta valleys. Snow levels will be
around 6000 to 7000 feet Wednesday afternoon, due to the
southwest origins of the moisture. Expect up to around 3 inches of
precipitation in the southern coastal mountains from early
Wednesday through Thursday afternoon. A cold front associated with
this system will approach the coast Wednesday night, bringing
strong winds Wednesday night into Thursday morning to the rogue
and Shasta valleys, the Cascades, Siskiyous, and higher terrain
east of the Cascades including Summer Lake.
As this front moves inland Thursday, snow levels will quickly drop
from west to east, and precipitation will transition to a showery
pattern. Models show lifted indices around 0 degrees over the
east side as the front approaches Thursday afternoon, which could
help support isolated thunderstorms on the east side. Much will
depend on the timing of the front.
Long term...Thursday night through Tuesday...deterministic and
ensemble model guidance are in good agreement in an unusually cold
system for late March moving in Thursday night and Friday. Projected
temperatures during the period, especially in the mid levels, are
currently in the 0.5 percentile. Put another way: at this time of
the year, it is warmer than what models are showing for late this
week 99.5% of the time. Put a third way: this would be a very
unusual cold event for late March. Furthermore, ensemble guidance
shows very little spread in temperatures, and thus confidence is
unusually high for such an anomalous event.
A strong cold front moves through late Thursday, then cold air pours
in Thursday night into Friday. Snow levels will lower rapidly
Thursday night and may drop as low as 1000 feet by Friday morning.
At the same time, an atmospheric river (ar) sets up and pumps
copious moisture onshore. The combination of cold air and moisture
sets the stage for heavy snow accumulations. The focus for heavy
snow is likely to be over the Kalmiopsis Wilderness and into western
Siskiyou County as the Arkansas slides through the area Thursday night and
Friday. Snow in these locations will probably be measured in feet by
the time it's done. However, other mountainous areas are likely to
receive heavy snow as well, including the Cascades, Mt. Shasta area,
and even portions of the east side. It's not out of the question to
see valley snow accumulations, as well, including all the passes
between Grants Pass and Roseburg. It should be noted that at this
time of year the days are long and the sun is strong, so the best
chance for accumulations on roadways will be at night. During the
day, Road surfaces will warm and likely be wet. The exception during
the day would be during periods of intense snowfall.
Last but not least, we will see some very heavy rainfall totals
along the Curry coast. This comes on top of the heavy rain that
falls during the short term. Places like Brookings and adjacent
mountains could easily see a storm total rainfall of 6 inches by the
time the bulk of the Arkansas moves south of US Saturday.
The main upper low moves by to our west Saturday and Saturday night.
At this time, models agree that the bulk of this system will stay
offshore. This will bear watching, because cold air remains in
place, and if this storm moves any closer, we could see another
round of heavy mountain snow. The system moves away Sunday, then
Monday and Tuesday look relatively dry with moderating temperatures
as a weak ridge builds offshore. -Wright
Aviation...21/00z taf cycle...VFR conditions will continue
into tonight with mid to high level clouds streaming over the area
ahead of a warm front. Cigs will gradually lower late tonight with
higher terrain becoming partly obscured. Cigs are likely to remain
VFR, but areas of MVFR are expected to develop as rain moves in from
south to north between 10z-15z tonight.
A strong front will approach the area Wednesday and winds will
increase ahead of it late in the afternoon. Winds will be strongest
over the higher terrain, in the Shasta Valley, and east of the
Cascades. Low level wind shear does not look to be a concern at this time, but
mountain wave turbulence is likely into Thursday. Br-y/petrucelli
Marine...updated 800 PM PDT Tuesday 20 Mar 2018....relatively calm
conditions are expected through Wednesday night. A weak warm front
will move north through the waters tonight, followed by a cold front
Wednesday night. South winds will increase ahead of the cold front,
but winds and seas will remain below Small Craft Advisory criteria
A stronger front will approach the waters Thursday afternoon and
bring increasing south winds and building choppy seas. We'll at
least get craft conditions late Thursday afternoon into Thursday
evening. There's also a chance we could approach gale force winds,
especially over the northern waters with steep wind driven seas. The
possible gales is a new development, so we'll have to keep a watch
on this and see if the models continue to show this.
Cold air aloft will move over the waters Thursday night into Friday.
Showers will be widespread, and there may be some isolated
thunderstorms as well.
There's growing confidence we'll have a heavy west swell in the 11
to 13 foot range moving into the waters Thursday night into Friday.
Seas will subside a bit Friday night, then increase again Saturday
afternoon into Saturday night. Right now the models vary with the
swell height, but it looks like the highest swell may be confined to
the southern waters. During this interval the weather pattern will
remain active, but winds are expected to remain below small craft.
or...Wind Advisory from 5 PM Wednesday to 5 PM PDT Thursday for
California...Wind Advisory from 5 PM Wednesday to 5 PM PDT Thursday for
Wind Advisory from 2 PM Wednesday to 11 am PDT Thursday for