Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Medford or
446 am PDT Sat Aug 29 2015
Discussion...unusually strong winds will be the main impact today as
a very deep low pressure trough offshore from Oregon moves to the
Washington coast. Its associated cold front is fast-moving, and has
already moved inland. With such a strong front, its position is
readily apparent from either the satellite imagery or the narrow band
of intense rainfall. Winds will reach peak strength this morning as
the front passes, but the low level pressure gradient is expected
to produce strong winds through the afternoon. Mountain and east
side wind gusts are already at 25 to 40 miles per hour and will likely reach
35 to 55 miles per hour except locally as high as 70 miles per hour at east side ridges
and along Highway 31 between Paisley and Silver Lake in Lake
County. West side valley wind gusts will not be as strong at 20
to 30 miles per hour...except to 40 miles per hour near Weed, but there will likely be
downed branches with drought stressed trees still full of leaves.
Rainfall will occur mainly early this morning with impressive
amounts in excess of half an inch limited to Curry County, and
the western portions of Josephine and Siskiyou counties. An inch
to an inch and a quarter has been measured during the past few
hours in the Curry County mountains...and that will be the bulk of
the coastal rainfall as the front has moved east of Coos and Curry
counties. Rain will be mainly light, locally not just moderate but
heavy on the west side through around 8 am. The west side valleys
along and east of Interstate 5 are expected to receive around one
to two tenths of an inch of rain...mainly toward the low end of
that range. The front will weaken as it reaches the east side
with the highest probability of measureable rain in northern
Klamath County through this morning. Post-frontal showers will be
sparse, light, and mainly over the coastal range.
Though this is a fast moving system, this change to an active, cooler,
onshore weather pattern will persist through the next week. The
upper low will track northeast into Washington late today then
another broad low will move from the Gulf of Alaska into the
Pacific northwest. Winds with this next system will not be nearly
as strong as the center of this low with the coldest air/strongest
pressure gradient will remain well north of our area. But, it will
bring another period of mainly west side rainfall late tonight
into Sunday morning. The GFS and European model (ecmwf) have trended wetter with
this system. The NAM remains the weakest of the models with the
amount of moisture but cannot be discounted as it has initialized
very well with today's storm. As a result, I raised the west side
rainfall amounts but shaved a bit off of the GFS indicated
amounts. Rainfall of more than a tenth of an inch still looks to
be limited to the coast and Douglas County. Showers will gradually
taper off Sunday afternoon into Sunday evening.
Monday will be cool while our area remains in a longwave trough
pattern. But no rain is expected as the storm track remains aimed
at Canada and Washington. The next front will keep its focus well
north of US on Tuesday but the southern end of the front has a
chance of bringing very light rain to southwest Oregon...
especially the coast and Douglas County late Tuesday into Tuesday
It will be the same story but with a bit more uncertainty in the
details of timing and strength with another few additional
disturbances possible in the westerly flow aloft Wednesday into
Friday. By next weekend and into Labor Day, the GFS may be too
progressive and strong with ridging but there looks to be a modest
drying and warming trend.
Aviation...for the 29/12z taf cycle...an unseasonably strong low
pressure system will move northward along the coast this morning,
shifting north of the area during the afternoon. The main aviation
impact will be strong winds along the Coast, Mountains, Shasta
Valley and east side this morning into this afternoon with breezy
winds elsewhere. Prior to the onset of the stronger surface winds
low level wind shear is expected early this morning, especially
along the coast and into the Umpqua basin. The most impacting
conditions at each location for wind shear, wind, and rainfall is
likely to last only 3 to 6 hours, greatest along and near the coast,
as this weather system is a fast mover. The front will also bring
rain with widespread MVFR ceilings/visible along the coast and areas of MVFR
visible/ceilings from the Cascades west this morning. MVFR ceilings are expected
to persist along the coast through the afternoon and tonight. /Cc
Marine...updated 400 am PDT Saturday 29 August 2015... a strong low
will bring very windy conditions to the waters this morning.
Conditions will rapidly deteriorate early this morning as low
pressure rapidly deepens as it moves inside of 130 west longitude
near 42 north latitude and then moves northeastward toward the
Pacific northwest coast. The latest model runs are indicating a
slightly more offshore track of the low and the 925 mb winds more
parallel to the coast...but still in excess of 50kt north of Cape
Blanco. With good vertical motion indicated in the heavier rain
these winds should reach the surface as gusts and have kept the
Storm Warning...mainly for gusts...for areas north of Cape Blanco.
Southerly seas will reach warning conditions for most of the area
with the expected winds...with very steep wind driven seas likely
peaking in the 14 to 17 foot range from Gold Beach northward. South
winds and elevated seas will begin to subside this afternoon into
this evening. Showers and lighter winds are expected Sunday. /Cc
Fire weather...updated 400 am PDT Saturday 29 August 2015...a
frontal system will move inland over the area this morning while a
strong surface low pressure will track north off the southern Oregon
coast this morning. This will will shift north during the afternoon.
This low will bring strong winds to the Coast, Mountains, Shasta
Valley and east side today, with breezy conditions elsewhere. Light
to moderate rain is expected this morning. Wetting rain likely
along the coast and in the Cascades with a chance of wetting rain
for inland areas, mainly west of the Cascades.
While we will get rain over many of our ongoing fires, there remain
concerns about this storm with respect to those fires and the
personnel Manning them. There is the potential for moderate to
locally heavy rain over all fire areas in the Cascades and coastal
areas. This is expected to be a gentle soaking rain rather than
convective, high intensity rain, so burn scar flooding and debris
flows are not a huge concern. But it is something to think about.
Also, snags, burned trees, and other material that have been
weakened over the Summer have the potential to come down from wind,
the added weight of rainfall, or a combination of both. Firefighters
should remain very aware of their surroundings this weekend,
especially on or around existing fires and burn scars.
Conditions will dry and winds decrease by this evening. A secondary
front is then expected to bring additional rain showers from the
Cascades west late tonight and Sunday. General troughiness will
remain over the pacnw through the upcoming week, so cooler and more
humid than normal weather is expected. Rain will be possible again
middle week as another front moves through. /Cc
or...High Wind Warning until 2 PM PDT this afternoon for orz021-022.
High surf advisory until 5 PM PDT this afternoon for orz021-022.
High Wind Warning until 5 PM PDT this afternoon for orz030-031.
Wind Advisory until 5 PM PDT this afternoon for orz029.
California...High Wind Warning until 5 PM PDT this afternoon for caz085.
Pacific coastal waters...Small Craft Advisory for winds from 2 PM
this afternoon to 11 PM PDT this evening for pzz350-370.
Storm Warning until noon PDT today for pzz350-370.
Gale Warning until 2 PM PDT this afternoon for
Hazardous seas warning until 11 PM PDT this evening for
Small Craft Advisory for winds until 5 PM PDT this afternoon for
Small Craft Advisory for hazardous seas from 5 PM this afternoon
to 11 PM PDT this evening for pzz356-376.
Hazardous seas warning until 5 PM PDT this afternoon for