076
FXUS66 KMFR 220500
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
900 PM PST Thu Nov 21 2024

.DISCUSSION...Low pressure with multiple low centers is
intensifying offshore this evening around 41-43N and 130-132W.
This is expected to consolidate into one surface low overnight as
it moves northward along and just inside 130W, deepening to around
980mb. It will move to a position SW of Vancouver Island by
Friday morning. Mid-level flow ahead of this system will continue
to bring copious moisture transport along a strong atmospheric
river that has been aimed at NorCal during the past 24 hours. As
the low heads north, expect moisture to continue to bring
southerly upslope flow into the Mt Shasta region in NorCal
resulting in a continuation of moderate to, at times, heavy
rainfall. We expect another 3-5 inches of rain in these areas. It
pretty much goes without saying that flooding continues to be a
huge concern given all the snow that fell yesterday and the deluge
that followed last night and today. The areal Flood Warning
remains in effect through early Saturday morning, at least. Snow
load is also an issue. We`ve gotten reports that the saturated
snow is bearing a lot of weight on top of buildings. This has
resulted in at least one instance of roof damage at a local
library in Dunsmuir. Structural damage and even collapse is
possible, so please be aware of this danger if you still have a
lot of snow on your roof.

Meanwhile, the focus area of moderate to heavy rainfall is also
expected to move back toward the north, impacting coastal areas
and just inland. Expect another round of rain in Douglas, Coos and
Curry Counties as well as western Josephine County. Radar is
showing this nicely with a solid area of rain moving in from the
coastal waters. Models show pressure gradients tightening tonight
as the low moves north and this will also bring strong winds to
the coast, the Shasta Valley and over the east side. These areas
are under High Wind Warnings for wind gusts in excess of 60 mph.
Other areas are under Wind Advisories including portions of the
Rogue Valley. It appears the flow will back just enough to bring
SE winds to at least the southern portion of the Rogue Valley with
gusts to 45 mph. The winds may also briefly reach Medford, but the
latest guidance suggests best winds will be to the south. While
we still expect some rain, what this will do is likely keep the
steadiest rain to our west and south overnight due to some
downslope off the Siskiyous.

Since the mid-level air mass has warmed since yesterday and will
continue to do so overnight, we are not concerned about snow this
time around, except over the highest mountains. Snow levels near
or around Crater Lake this evening will even rise above there for
a time overnight (near or even above 8000 feet).

After tonight, the cold front will move onshore Friday, pushing
east of the Cascades during the afternoon. The atmospheric river
that has been impacting the region will finally get a shove to the
south and east with more of a focus into the Lassen/Tahoe
region/northern Nevada Friday night into Saturday. Snow levels
will lower back down to around 4000 feet during this time with
some (mostly) light accumulations expected in the mountains
(mountain passes) and over portions of the east side. Amounts of 3
to 8 inches appear to be most likely above 5000 feet (highest
amounts near Crater Lake NP). -Spilde

&&

.AVIATION...22/00Z TAFS...Strong low pressure off the south
Oregon Coast will move north along 130W tonight. This will allow
for continued abundant clouds and precipitation across the region
for the next 24 hours with widespread mountain obscuration. South
winds will increase with developing wind shear at all terminal
this evening into Friday morning. Winds will decrease after 18z
Friday but widespread MVFR conditions with local IFR and
widespread mountain obscuration will continue. -CC

&&

.MARINE...Updated 800 PM PST Thursday, November 21, 2024...
Conditions worsen tonight as another deep low pressure develops
near 40N 130W and moves north along 130W through early Friday.
Though this system will move closer to shore than the Tuesday
system, it will be weaker than previous storm. Strong, south gales
develop this evening and persist into Friday afternoon. There
will be a brief period tonight (around and just after midnight and
again Friday morning around sunrise) when isolated areas could
experience storm force wind gusts of 55 to 60 kt. Seas will
rapidly build again, reaching up to 20 ft late tonight through
Friday. Conditions will begin improving overall as winds ease late
Friday into Saturday. -Stavish/CC

&&

.PREV DISCUSSION... /Issued 504 PM PST Thu Nov 21 2024/

Updated...Aviation Discussion.

AVIATION...22/00Z TAFS...Strong low pressure off the south Oregon
Coast will move north along 130W tonight. This will allow for
continued abundant clouds and precipitation across the region for
the next 24 hours with widespread mountain obscuration. South
winds will increase with developing wind shear at all terminal
this evening into Friday morning. Winds will decrease after 18z
Friday but widespread MVFR conditions with local IFR and
widespread mountain obscuration will continue. -CC

SHORT TERM (Today through Friday night)...

Overview:

Snow levels continue to rise today with most areas above 5K feet as
of writing this, and this has resulted in widespread rainfall across
the region. Siskiyou County in northern California in particular has
received a lot of rainfall (upwards of 10 inches) on top of snowfall
(2-3 feet), and there is the concern for flooding across the area. A
Flood Warning is in place across Siskiyou County. In addition, we
have a Flood Warning across southwest Oregon, and we also have a
Flood Watch in place for parts of the South Fork Coquille River at
Myrtle Point. We are expecting upwards of 3-8 inches of additional
rainfall now through Saturday for these areas mention in northern
California and southwest Oregon. Highest amounts expected over
southern Siskiyou County in and around the Mount Shasta City and
Dunsmuir areas but as far east as Pondosa. Additionally, expecting
impacts from wind speeds through tomorrow afternoon, and we do have
wind hazards out for this threat.

Further Details:

A broad area of cyclonic flow is observed on mid-level water vapor
imagery from GOES-18. Analysis shows a couple distinct areas of
circulation within this broader area. Another closed surface low
will develop in this area, and will start to approach 130W by
midnight tonight. The pressure gradient is progged to maximize
around 1 AM to 2 AM tonight, and will remain strong overnight
through mid to late afternoon on Friday. Please see the latest high
wind warning and wind advisory for further information.

Within the aforementioned area of cyclonic flow, we will have a
steady strong upper level jet, and this will lead to divergence
aloft--with minor orientation changes--through the end of the short
term (into Saturday actually). Also during this time, a steady fetch
of moisture from the Pacific will be present. Given the strong
dynamics and forcing in place, we are looking at the potential for
impacts from flooding, especially low lying flood prone areas and
small creeks/rivers. We are looking at upwards of 6-8 inches of
additional rainfall for some areas in Siskiyou County and upwards of
3-5 inches in southwest Oregon. Please see the latest Flood Warning
and Flood watch for further details.

-Guerrero

LONG TERM (Saturday Morning through Thursday)...

Unsettled weather will continue over the weekend into at least
early next week as an upper low off the British Columbia coastline
slowly moves south along the Pacific Northwest coastline into
Tuesday. Snow levels will drop Saturday morning to around 4000
feet...dropping to around 3000 feet Sunday morning. We were
generally be in a showery pattern...with not much accumulation
forecast...however we will dive deeper into this over the next 24
to 48 hours once we get past our current busy weather situation.

It does look like the weather pattern will become less active...
with periods of dry weather beginning mid next week. Looking through
the clusters and ensemble data...only about 10% of members are
showing an active weather pattern by Wednesday of next week. In
fact...both the 6 to 10 day and the 8 to 14 day outlooks from the
Climate Prediction Center show between a 40 to 60% chance of below
normal temperatures and a 30 to 40% chance of below normal
precipitation.

-Riley

MARINE...230 PM PST Thursday, November 21, 2024...Conditions
worsen this evening as another deep low pressure develops near 40N
130W this evening and moves north along 130W through early Friday.
Though this system will move closer to shore than the Tuesday
system, it will still be weaker than previous storm. Strong south
gales develop this evening and persist into Friday afternoon. There
will be a brief period tonight (around midnight) when isolated areas
could experience storm force wind gusts of 55 to 60 kt. Since this
will only be during about a 4 hour window and for isolated areas,
have decided to just stick with a Gale Warning for this storm since
it will be the main threat. Seas will rapidly build again, reaching
up to 20 ft late tonight through Friday. Conditions will begin
improving overall as winds ease late Friday into Saturday.

-Stavish

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning until 4 PM PST Friday for ORZ021-022-029>031.

     Wind Advisory until 4 PM PST Friday for ORZ026-029>031.

CA...High Wind Warning until 4 PM PST Friday for CAZ081-084-085.

     Wind Advisory until 4 PM PST Friday for CAZ084-085.

PACIFIC COASTAL WATERS...Gale Warning until 4 PM PST Friday for
     PZZ350-356-370-376.

     Hazardous Seas Warning from 4 PM Friday to 4 AM PST Saturday for
     PZZ350-356-370-376.

&&

$$

MAS/MAS/MAS