Area Forecast Discussion
Issued by NWS Medford, OR

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
000
FXUS66 KMFR 171233
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
433 AM PST Tue Jan 17 2017

.DISCUSSION...A strong storm system will begin to impact the area
today. Southerly winds will increase out ahead of this system
and there are myriad headlines out for this at NPWMFR. Model
guidance hasn`t wavered much here with the OTH-ACV gradient still
peaking around -10mb, which is a good rule of thumb for strong,
warning- level winds (>=58 mph) along the coast. Local guidance
for the coast shows wind gusts potentially to 70 mph. The gradient
from MFR-RDD is also quite strong, peaking near -10 mb Wednesday
morning. This will almost certainly bring warning level winds to
the Shasta Valley and portions of the east side (usual areas like
Summer Lake). It`s a little more tricky here in the Rogue Valley,
since the flow isn`t quite as well-aligned. Even so, we have a
wind advisory out for the valley, from about Medford southward
with gusts of 40 mph expected.

Heavy precipitation will get going this evening and continue,
unabated from the coast southward into western Siskiyou County
through Wednesday. Right now, we expect 3 to 6 inches of rain in
these areas with up to 8 inches in the Coast Ranges. We have
issued an Areal Flood Watch (FFAMFR) given the potential for
rainfall rates in excess of 0.50"/hour at times late tonight
through Wednesday. Snowmelt will also add to inflows on area
waterways. Farther inland, the winds aloft are quite strong, and
we expect some downslope in the valleys west of the Cascades for a
while. So, the risk of flooding there isn`t as high anymore.
Even so, just about everyone will get in on the rain with
0.50-1.50" of rain expected. Amounts east of the Cascades will be
much lighter, generally 0.25-0.50".

By far, the toughest part of the forecast is snow potential near
Mount Shasta late tonight through Wednesday night. Model guidance
is coming in line showing a significant snowstorm down there
during this time period. Snow levels will initially be high, but
with very strong, upslope flow (50-60kt at 700mb) arriving into
the Mount Shasta region, heavy precipitation is expected to lower
snow levels significantly by Wednesday morning. So much so, they
may reach all the way down to Dunsmuir. The NAM12 is coldest and
would suggest heavy snow is a slam dunk for Mount Shasta City.
Other models are slightly warmer. Local studies have shown that
850 temps near 2C and 1000/700mb thicknesses of 2884m almost
always (94% of the time) result in heavy snow impacting the
interstate with cooling due to heavy precip rates overcoming any
warming. All models show this, so we have upgraded the Winter
Storm Watch to a Winter Storm Warning (WSWMFR).

It should be noted that profiles along and south and east of the
Cascades show an elevated warm layer (>0 degrees C) tonight into
Wednesday morning. Temperatures may be below freezing at the
surface at the onset of precipitation, so there is a slight chance
of freezing rain. Confidence in this scenario wasn`t high enough
to warrant an advisory at this time, however. We did issue a
winter weather advisory for the Cascades north of Highway 140
above 5000 feet. Expect 6-12 inches of snow from late Wednesday
morning through Wednesday night.

Thursday through Sunday.

The EC/GFS/DGEX operational solutions are in fairly good agreement
through most of the extended period, with some differences showing
up toward the end of the period. A fairly fast progressive pattern
will prevail throughout. It will remain relatively wet over the
Medford CWA with periods of gusty winds.

A long wave trough will move onshore along the west coast Thursday
morning. This will be followed by a brief period of short wave
ridging into Thursday night, which will then be followed by a long
wave trough moving onshore Friday.

Post-frontal showers will gradually diminish Thursday, then the
upstream upper trough will push a surface cold front onshore
Thursday night into Friday morning.  This front will not be
particularly wet and windy, but the south coast, south Coast Range,
and western Siskiyou county will get a significant amount of
precipitation out of this system.

Post-frontal showers will continue into Friday night, when a weaker
secondary front will move onshore. The bigger concern during this
time frame will be heavy surf.  Current guidance suggests that wave
fronts may approach 40 feet as very heavy long period swell moves
into the waters. Waves like this will run way up beaches and some
coastal infrastructures may be threatened.

A broad flat ridge will build into the west coast Saturday, followed
by another trough on Sunday. The next front will move onshore
Saturday night, and once again the heaviest precipitation will occur
along the south coast, south Coast Range, and western Siskiyou
county. Post frontal showers will then persist through Sunday night.

Finally, the long wave trough will move onshore Monday. Post-frontal
showers will continue through Monday. -JRS

&&

.AVIATION...17/06Z TAF CYCLE... There will a big change in
conditions across the area on Tuesday as stagnant weather conditions
give way to the first in a series of frontal systems. Thin mid level
clouds this evening will become thicker Tuesday morning. An increase
in winds aloft and these clouds will disrupt fog and low cloud
formation yielding only patchy IFR CIG/VIS restrictions on the West
Side late tonight into early Tuesday morning. Wind shear is expected
beginning around 12Z at KOTH then persisting through the day. Light
rain, patchy MVFR CIG/VIS, and partial terrain obscurations are
expected from the Coast Range and Umpqua Basin beginning around 18z
Tuesday then conditions deteriorate in the late afternoon into the
evening with areas of moderate to heavy rain and widespread MVFR.
Other areas are expected to remain VFR into Tuesday evening, though
the Cascades will experience increasing terrain obscurations late in
the day on Tuesday and low level southerly winds will be increasing.
-DW

&&

.MARINE...Updated 400 AM PST Tuesday 17 January 2017...
South winds will increase today into tonight as the first in a
series of strong fronts approaches. Gales and rapidly building very
steep seas will develop today. Storm force winds will then develop
late this afternoon into tonight with these strong winds continuing
into Wednesday. Peak winds are expected tonight and Wednesday morning
with peak seas late tonight through Wednesday afternoon. Combined
southerly wind waves and west swell are likely to become very high
and very steep tonight then peak in the 18 to 27 foot range on
Wednesday. Winds will diminish by Wednesday evenings. Seas will
gradually decrease Wednesday evening but will remain very steep
hazardous...lowering to steep seas Wednesday night. Another
strong front is expected Thursday night and may bring a period of
gales. There is moderate confidence that very long period west swell
will build into the waters behind this front Friday into Saturday,
peaking in the 25 to 30 foot range at a period of 17 to 18 seconds.
This will bring extremely dangerous conditions to the waters as
these very high and very steep seas build into the waters.
-CC

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning from 4 AM Wednesday to 4 AM PST Thursday for
     ORZ030-031.
     Wind Advisory from 4 AM Wednesday to 4 AM PST Thursday for
     ORZ029>031.
     Air Stagnation Advisory until 10 AM PST this morning for ORZ029.
     Winter Weather Advisory from 10 AM Wednesday to 4 AM PST
     Thursday for ORZ027-028.
     High Wind Warning from 4 PM this afternoon to 4 PM PST Wednesday
     for ORZ021-022.
     Flood Watch from this evening through late Wednesday night for
     ORZ022-024.
     High Surf Advisory from 10 PM this evening to 4 PM PST Wednesday
     for ORZ021-022.
     Air Stagnation Advisory until 10 AM PST this morning for ORZ023.
     Air Stagnation Advisory until 10 AM PST this morning for
     ORZ024-026.
     Wind Advisory from 10 PM this evening to 10 PM PST Wednesday for
     ORZ026.

CA...High Wind Warning from 4 AM Wednesday to 4 AM PST Thursday for
     CAZ085.
     Wind Advisory from 4 AM Wednesday to 4 AM PST Thursday for
     CAZ084-085.
     Flood Watch from this evening through late Wednesday night for
     CAZ080.
     Winter Storm Warning from 4 AM Wednesday to 4 AM PST Thursday
     for CAZ080.
     Winter Storm Warning from 4 AM Wednesday to 4 AM PST Thursday
     for CAZ082-083.
     Winter Weather Advisory from 4 AM Wednesday to 4 AM PST Thursday
     for CAZ080.
     High Wind Warning from 10 PM this evening to 10 PM PST Wednesday
     for CAZ081.
     Wind Advisory from 10 PM this evening to 10 PM PST Wednesday for
     CAZ081.
     Winter Weather Advisory from 4 AM Wednesday to 4 AM PST Thursday
     for CAZ083.

Pacific Coastal Waters...Small Craft Advisory until noon PST today for
     PZZ350-356-376.
     Storm Warning from 10 PM this evening to 4 PM PST Wednesday for
     PZZ356-376.
     Gale Warning from noon today to 4 PM PST Wednesday for
     PZZ350-356-376.
     Hazardous Seas Warning from 4 PM to 10 PM PST Wednesday for
     PZZ350-356-370-376.
     Storm Warning from 4 PM this afternoon to 4 PM PST Wednesday for
     PZZ350-370.
     Gale Warning until 4 PM PST Wednesday for PZZ370.

$$

MAS/CC/DW



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.