Dense Fog Advisory 4, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
248 PM PST Tue Jan 12 2021

.DISCUSSION...Moderate to strong winds and flooding are the main
concerns through tonight.

Heavy rain with flooding is possible, especially for the coast and
coastal mountains in southwest Oregon and a Flood watch and flood
advisory are in effect. Please see FAAMFR and FLSMFR for more
details.

Moderate to strong winds are being observed at the coast, coastal
mountains, portions of the east side and Shasta Valley with high
wind criteria being met at the coast, and higher terrain east of the
Cascades. High wind warning and a wind advisory remain in effect.
Please see NPWMFR for more details.

Satellite image shows a long plume of moisture courtesy of an
atmospheric river (AR) from about the Hawaiian Islands with strong
upper jet support already in our area. This in addition to a
favorable upslope flow will squeeze out even more moisture over the
coastal mountains through this evening. However there are some
indications rainfall rates could diminish some on the overnight
period before increasing again for about a 6 hour period late
tonight. This is being supported by the operational GFS and GEFS
ensemble mean which suggest the Water Vapor Transport will actually
decrease later this evening and tonight. Typically these events can
be tricky in that rainfall rates can fluctuate up and down, so
confidence on decreasing rainfall rates is low. For example, Red
Mound reported 0.28, 0.50, 0.57, 0.33 and 0.59 of an inch in the
past 5 hours.

With that said, expect periods of moderate to heavy rain at the
coast into early Wednesday morning as moisture transport is aided by
strong 850-700mb flow within the AR. Storm total rainfall from this
evening into Wednesday afternoon are expected to be 1-2 inches north
of Cape Blanco and 2-4 inches in Curry County (highest totals in the
coast ranges due to orographic enhancement). The southwest component
will result in less rainfall in the Rogue Valley However will be
higher along the Cascades, where another area of more enhanced
precipitation is possible north of Highway 140 in the mountains.
Rain amounts for the Cascades are expected to be 1-3 inches, and
0.30-0.50 in the Rogue valley with the best chance for 0.50 in the
northern and eastern part of the valley. The rain will benefit from
some absorption by the snow pack in the Cascades/Siskiyous. Despite
that, some melting of the snow is expected with snow levels between
8000 and 9000 feet, therefore expect some added inflows into the
drainages. While significant rises are expected on most basins from
the Cascades westward, most main stem rivers (at least at this time)
are expected to peak below flood stage. The one exception could be
the Coquille River at Coquille where the rise could peak out on the
cusp of minor flooding, hence the flood watch. East of the Cascades,
rainfall are expected to be less, especially for portions of eastern
Lake County and Modoc County.

Moderate to strong winds will continue at the coast and over the
east side where high wind warnings remain in effect. Strong KOTH-
KACV gradients of -8 to -11 mb continue to support wind gusts of 60-
70 mph at the coast with up to 80 mph at the headlands. The 700 mb
winds east of the Cascades during the warnings peak at 65-70 kt, so
gusts of 65 mph should also occur along Highway 31 near Summer Lake.
-8 to -10 mb KMFR-KRDD pressure gradient also supports advisory
level winds in the Shasta Valley. Due to more of a southwest or
westerly component to the mid-level flow. If the flow were valley-
aligned, then winds would be stronger, but that's not the case here.
Breezy conditions are expected elsewhere at times and especially at
the southern portion of Rogue Valley.

With snow levels as high as they are (between 8000-9000 feet) winter
impacts will be minimal. However as the cold front moves onshore
Wednesday, snow levels will drop to around 5000 feet, but
precipitation will be diminishing, then ending by that time.

Wednesday, precipitation will be winding down pretty quickly during
the morning with mainly dry conditions in the afternoon. It will
still be windy in the morning, but they too should be diminishing
during the course of the day. Wednesday night will be dry and the
atmosphere will stabilize as upper ridging builds over the area. Due
to ample moisture, patchy to areas of fog is likely for most if not
all interior valley locations Wednesday night into Thursday morning.
As a result max temperatures on Thursday were adjusted down due to
the clouds and fog lingering through at least the morning hours,
thus slowing down the amount of warming. -Petrucelli

Long-term...Friday through Tuesday...This period can be summed up as
relatively uneventful compared to the very active pattern we're
currently experiencing. A couple weak fronts will pass through the
forecast area, late Friday and Sunday. Otherwise, an upper level
ridge of high pressure will dominate the weather, and precipitation
through Tuesday will be minimal to nil.

Friday's front will push through with generally light precipitation
and snow levels around 7000 feet. High pressure will settle in on
Saturday, and we expect some morning fog and freezing fog in a
stable air mass. Another front will move through Sunday morning but
operational models and ensemble members are not enthusiastic with
precipitation with this front, so expect more fog and freezing fog
Sunday morning. Broad, dry north to northeasterly flow is expected
late Sunday into Monday, and it's likely that fog coverage and
intensity will decrease some Monday morning if this type of
offshore/northeast wind pattern sets up. Ensembles and the European
operational model suggest deep westerly flow could resume on
Tuesday, and a more substantial front could arrive. After looking at
the latest clustering of all 100 global model ensemble, the most
likely solution however, is continued dry conditions.

Looking further out in time, towards the clustering of ensemble
solutions for Day 8-10 (late next week), most ensemble members point
to cooler conditions/lowering snow levels and precipitation chances
returning.

&&

.AVIATION...For the 12/18Z TAFs...A strong front is pushing into the
region and is bringing significant turbulence, areas of low level
wind shear (LLWS), and a wide variety of conditions ranging from
MVFR to IFR ceilings. The worst conditions through tonight will be
at and near the coast where strong south winds will bring
turbulence, while pockets of lighter winds near terrain will create
LLWS conditions, and IFR/MVFR conditions with mountain obscuration
are expected. MVFR cigs/vis will gradually spread inland across
areas from the Cascades west this morning, then conditions will
become MVFR cigs/vis with areas of IFR this evening and tonight.
East of the Cascades, expect mainly VFR this morning with some
higher terrain obscured, then areas of MVFR cigs this afternoon
through tonight. LLWS conditions will affect many terminals through
early afternoon, then affect areas closer to the coast later today,
though some inland areas like KMFR could experience LLWS through
this evening. -CC/Keene

&&

.MARINE...Updated 240 PM PST Tuesday, 12 Jan 2021...
A very strong front will continue to bring gale force winds and very
steep seas this afternoon through tonight. Winds are likely to surge
to storm force again from Cape Blanco northward between 3NM and 25NM
of the coast between about 7 PM and 1 AM Pacific Standard Time.
Winds are then expected to lower from northwest to southeast quite
rapidly early Wednesday morning. Very high and very steep combined
seas will continue today and tonight due to a mix of wind seas and
long period west swell. Seas will then subside during the day
Wednesday with steep, west swell dominated seas expected from late
morning onward.

High pressure will bring a break in the action Wednesday afternoon
through Thursday night. A weaker frontal system is then expected to
move through Thursday night into Friday before high pressure and
northerly winds build in for a few days. Another long period heavy
west swell event is likely Friday night, peaking around 18 feet at
17 seconds. ~BTL/Keene

&&

.BEACH HAZARDS...Updated 240 PM PST Tuesday, 12 Jan 2021...
Long period swell will continue tonight into Wednesday morning
resulting in hazardous surf conditions with breaking wave heights of
27 to 33 ft. A second high tide around midnight tonight is expected
to be about 2 feet lower than the high tide that occured earlier
today. Thus, while there will be some risk of additional flooding
due to the combined effects of surf, rain, and the high tide around
midnight, the risk for flooding will gradually lower late tonight
into Wednesday as tides lower and dangerous surf conditions improve.

Use extreme caution along and near the beaches and stay away from
the ocean. Jetties, rocks, logs and bar entrances are not safe
places to be during this period of significant waves. The Pacific
Ocean is both cold and powerful. These large waves can easily sweep
people and large objects into the water. NEVER turn your back on the
ocean! ~BTL

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning until 10 AM PST Wednesday for ORZ021-022.
     Flood Watch through Wednesday morning for ORZ021-022-024.
     High Surf Warning until 6 AM PST Wednesday for ORZ021-022.
     High Wind Warning until 4 PM PST Wednesday for ORZ030-031.

CA...Wind Advisory until 10 AM PST Wednesday for CAZ081.

Pacific Coastal Waters...Flood Watch through Wednesday morning for PZZ350-356.
     Small Craft Advisory from 4 AM Wednesday to 8 AM PST Thursday
     for PZZ350-356-370-376.
     Gale Warning until 4 AM PST Wednesday for PZZ350-356-370-376.
     Storm Warning until 1 AM PST Wednesday for PZZ350-370.
     Hazardous Seas Warning from 4 AM to 10 AM PST Wednesday for
     PZZ370-376.