Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Medford or
245 PM PST Sat Dec 20 2014
Short term...heavy rain is impacting the coast today with
moderate rainfall extending inland. Satellite imagery shows a
large area of deep moisture with 1.5 inches of precipitable water
or more aimed at the extreme southern Oregon and northern
California coast. Rainfall totals over the last 24 hours along
the coast have ranged from 2 to 3 inches with isolated amounts of
4 to 6 inches in the coastal Curry County mountains. Similar
amounts are forecast for the next 24 hours on the coast.
Across the eastern Pacific, satellite imagery shows a large area
of moisture with 1.0 to 1.5 inches precipitable water extending
from the Oregon coast westward. As strong onshore flow continues
and an upper level jet provides continued support aloft, expect
very high rainfall rates along the coast today through Sunday. Through
Sunday, heavy rainfall is also forecast across the southern Oregon
Cascades with moderate rain over the inland west side valleys and
light to moderate rain east of the Cascades.
Models have trended higher with forecast precipitation from the
Cascades west. Integrated vertical moisture transport forecast by
the models supports very high rainfall rates with ivt values
forecast to exceed 600 kg/MS over the area through Sunday evening
with peak values around 800 kg/MS this evening through early
Sunday. So have raised the forecast rainfall amounts with around 4
to 6 inches of rain forecast in the southern Oregon Cascades north
of Highway 140 and in the coastal mountains. Along the coast
expect an additional 2.5 to 4 inches of rainfall this evening
through Sunday. These amounts combined with rainfall that has
already fallen are expected to bring significant impacts to
coastal areas and for areas just inland into Josephine and Douglas
counties and into the south central Oregon Cascades. Small stream
flooding and urban flooding is possible in these areas. Also heavy
rain will bring the potential for mudslides, landslides or other
debris flow on area roadways. Gusty winds in the mountains today
and Sunday and along the coast through this evening, may allow
additional hazards as winds combine with heavy rain and bring
lowered visibilities at times and the potential for trees down.
Strong south to southwest winds are also expected in the Shasta
Valley and for areas east of the Cascades, especially in the
Summer Lake area and over the mountains. As 700 mb winds increase
to 60 miles per hour or more this evening and tonight, expect gusts to
around 70 miles per hour in the Summer Lake area. A High Wind Warning is out
for Summer Lake and higher mountains east of the Cascades with a
Wind Advisory for the Shasta Valley and areas east of the
Cascades. Winds are expected to lower in the Shasta Valley late
tonight but continue east of the Cascades through Sunday.
Late Sunday into Monday, models are continuing to show a ridge
building into the region from the southwest with moisture values
decreasing and 700 mb flow becoming northwest early Monday then
northerly during the day Monday. This will bring a drier air mass
into the area. Rainfall is expected to decrease in intensity and
coverage Sunday night then diminish over the area Monday
afternoon and evening with the change to northerly flow.
On Tuesday, there is a chance for light rain for extreme
northwest portions of the County Warning Area as some models indicate a front to
the north may extend into northwest portions of the area.
Otherwise expect dry weather over the remainder of the area.
Long term...Wednesday through Saturday night... the ridge in place
over the West Coast Tuesday will slide east as an upper level trough
pushes into the region from the Pacific. The air mass preceding the
front will be quite warm, with snow levels well above most area
peaks, but there will be a significant influx of cold air as the
front passes Wednesday night. Model solutions are suggesting a more
seasonally typical moisture content with this system, so rainfall
amount should not approach values seen with the current weekend
event, but modest amounts are expected.
For those thinking of the approaching Holiday, yes there will be
moisture and cold air moving into the region Christmas evening. However,
most of the precipitation will fall as rain, with snow levels not
dropping far enough for significant area coverage until most of the
moisture has left the area. The end result will depend greatly upon
one's location, with the coast and the west side valleys seeing only
rain, and waking up to a wet Christmas morning. Now the good news,
for those above 2500 to 3000 feet, there is a good chance of freshly
fallen snow Christmas morning, although amounts will be light.
Unfortunately, this elevation also includes many area passes,
including Siskiyou Summit, snowman's Summit, Black Butte Summit, and
all of the Cascade passes. Last minute travel in those areas could
be impacted, so those expecting to travel during this time should
stay updated with the latest forecasts and keep an eye on dot Road
Beyond Christmas, there is fairly good confidence in the development
of ridging overhead. This should keep most of the area dry, although
some precipitation is possible in the northern half of the forecast
area as swift midlevel flow is likely to continue pushing moisture
onshore. This time of year, high pressure usually results in
significant valley fog. With that in mind, the next front expected
to approach and possibly clear the area would be on Saturday,
however, models are not in clear agreement, nor do they show a
strong front. Confidence is therefore low, so have not added more
than a slight chance for rain heading into next weekend. -Bpn
Aviation...based on the 20/18z taf cycle...expect VFR
with areas of MVFR ceilings/visible early today to deteriorate to widespread
MVFR with terrain obscured by this afternoon. Local IFR is also
possible. Lowered flight conditions will persist through at least
Sunday morning. Precipitation will be light to moderate...except at
the coast and in the mountains where it will be heavy at times. Low-
level wind shear will be ending at the coast as winds begin to
surface, but may continue at krbg through 15z.
Marine...updated 230 PM PST Sat Dec 20 2014...winds have decreased
below gales and will continue to diminish through Sunday then remain
relatively light through Tuesday. Seas will also fall through
Sunday as both swell and wind wave gradually diminish. The wind
wave component will be much less Sunday night through Tuesday, but
moderate west swell will continue and thus a small craft for
hazardous seas has been issued through Wednesday. A strong cold
may bring moderate north winds and wind waves to the area
Wednesday through Thursday. -Wright
or...High Wind Warning until 10 PM PST Sunday for orz030-031-624-625.
Flood Watch through Sunday evening for orz025-027-617-623-624.
Flood Watch through Sunday evening for orz024-618-621.
High surf advisory until 10 PM PST this evening for
Flood Watch through Sunday evening for orz616-619-620.
Flood Watch through Sunday evening for orz021-022-615.
Flood Watch through Sunday evening for orz023.
California...High Wind Warning until 10 PM PST Sunday for caz085-285.
Wind Advisory until 1 am PST Sunday for caz081-281.
Pacific coastal waters...Small Craft Advisory for winds until 4 am PST Sunday for
Small Craft Advisory for hazardous seas from 10 PM this evening
to 10 PM PST Wednesday for pzz350-356-370-376.
Hazardous seas warning until 10 PM PST this evening for