Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 211146
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
446 AM PDT Mon Sep 21 2020

.DISCUSSION...In grouping periods of similar weather, the
forecast for the next week can be separated into four parts.
1) Breezy and dry into early Wednesday, 2) windy and wet (for
most of our area, though not a sizeable chunk of the east side)
late Wednesday into Thursday, 3) cooler with the potential for a
few mainly light showers mainly west and north of Jackson County
Thursday night into Saturday morning, and 4) warming and drying
with periods of west side easterly winds late Saturday through
early next week.

First, a series of very weak shortwave disturbances in a
southwest flow aloft are expected now into Wednesday morning. This
will create a modest cooling trend, breezy afternoon into evening
westerly winds (strongest east of the Cascades and trending
stronger each day), periods of increased (mainly high level/
cirrus) clouds, and a continued dry to very dry air mass away from
the coast and Douglas County. The strongest of these very weak
disturbances will produce a stronger push of moist low level
marine air on Monday night into Tuesday morning. As such, the
amount of coastal and Umpqua Valley sky cover during that period
has been adjusted higher to overcast. The marine layer does not
look to become sufficiently deep to make coastal drizzle a
certainty. But, patchy late tonight/early Tuesday morning drizzle
cannot be ruled for the coast north of Cape Blanco. Another
noteworthy aspect of the forecast will be a trend of increased
smoke moving from northern California across eastern Siskiyou,
eastern Klamath, Modoc, and Lake counties. There will also likely
be a more local increase due to the Slater Fire, over and near the
Siskiyou Mountains into the Shasta Valley. Thickness of smoke is
not forecast to reach the levels that were experienced last week.

The second period, from Wednesday afternoon through Thursday
morning, looks to be the most impactful. As such, it has been
receiving an extra bit of scrutiny over the past several shifts.
It covers the arrival and subsequent weakening of a cold front.
Gusty east side and Shasta Valley winds are expected ahead of the
front on Wednesday afternoon into the evening. Southwest winds
(south in the Shasta Valley) continue to be forecast to reach 20
to 30 mph with gusts mainly 25 to 35 mph, but locally up to 50 mph
near Summer Lake in Lake County and over ridge tops from the
Cascades eastward. Although about 10 to 20% of ensemble members
have noteworthy differences, confidence in the probability of and
timing of rainfall associated with the front has increased and I
think be called highly confident. The amount of rain to expect
shows more variation and will likely come into clearer focus as
time draws nearer.

A blended model solution shows storm total amounts of: a half
inch to an inch at the coast and also in northern and eastern
Douglas County; a quarter of an inch to a half inch for the Umpqua
Valley, Josephine County, and far western Siskiyou County; a
tenth to a quarter of an inch for the remainder of western
Siskiyou County, Jackson County, and far northern Klamath County;
and elsewhere quickly tapering southward and eastward with
sizeable portions of Lake and Modoc Counties most likely to remain
dry.

The third period, covering Thursday afternoon into Saturday
morning, has the highest uncertainty. While GEFS and ECMWF
ensemble model solutions do have a minority of members that show
sizeable variations in timing and strength. There is some higher
confidence that any precipitation from a series of shortwave
disturbances will be very light to light and mainly occurring
west and north of Jackson County. Amounts would be highest for
Coos and northern Douglas counties, most likely during Thursday
night into Friday morning and Friday night into early Saturday
morning.

Lastly, in a sign to suggest that anyone should be very cautious
in being too hasty to proclaim the preceding weather as putting
anything more than a dent/temporary reprieve in the southern
Oregon and northern California fire season, there is growing model
consensus for a strong warming and drying trend Saturday into
Wednesday. This would include easterly winds and moderate to
locally poor ridge top humidity recovery early next week.

&&

.AVIATION...For the 21/12Z TAFs...Areas of IFR cigs/vsbys in low
clouds and fog will continue along the coats and over the coastal
waters through this morning. Conditions should improve by late
morning, but fog and low clouds are expected to return late this
evening and tonight. Elsewhere, VFR conditions will prevail through
Monday night, with two exceptions. The first is patchy IFR
cigs/vsbys in low clouds and fog over the lower Umpqua Basin early
this morning and again late tonight. The second is local MVFR vsbys
in smoke near and downwind of active wildfires. -BPN

&&

.MARINE...Updated 200 AM PDT Monday 21 September 2020...A weak front
will move onshore today, but winds and seas will remain below small
craft advisory criteria through Tuesday. A much stronger front will
approach the region late Tuesday into Wednesday, then move onshore
Wednesday night. South winds will increase through this time, with
Gales possible Wednesday along with very steep wind-driven seas.
Winds will diminish after the front moves onshore, but seas will
remain steep due to heavy west swell which will develop rapidly
early Thursday, peak Thursday afternoon at around 16 feet at 15
seconds, then slowly subside through Friday night. -BPN

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...None.

CA...None.

Pacific Coastal Waters...Gale Watch from late Tuesday night through
     late Wednesday night for PZZ350-356-370-376.
     Small Craft Advisory from 5 PM Tuesday to 2 AM PDT Wednesday for
     PZZ350-356-370-376.

$$

DW/BPN


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