FXUS66 KMFR 250414

Area Forecast Discussion
National Weather Service Medford OR
914 PM PDT Thu Sep 24 2020

.DISCUSSION...A few days ago, the forecast for Friday into
Saturday looked to be a nondescript, relatively eventless finish
to our half-week of cool, moist, noticeably fall-like conditions.
Since then, there has been a gradual but consistent trend in the
bulk of our model data sources toward a stronger, wetter solution
for a cold front and following shortwave disturbance that will
track across the area from early Friday morning through early
Saturday afternoon.

In fact, this strengthening has been such that it now bears a
resemblance to the previous front that passed last night into this
morning. Similarities are the inland amounts and coverage of
rainfall, also that the highest amounts will be near the coast and
north of our area. Rain will likely begin near North Bend around 6
AM PDT, near Brookings and Roseburg around noon, in Grants Pass
around 4 or 5 PM (with a few sprinkles as early as noon), then
likely for Happy Camp and much of the Rogue Valley Friday evening
(with at least a chance for the entire valley). Representative
rainfall amounts are about a half inch to an inch near the coast,
around a quarter of an inch near Roseburg, up to a half inch in
eastern Douglas County to the southern Oregon Cascades, around a
tenth for far western Siskiyou, Josephine, and much of Jackson
counties, and east side rain of mainly a tenth north of Chiloquin
to two tenths near Chemult.

It will likely remain dry in Modoc County and in significant
portions of eastern Siskiyou, southern Klamath, and southern Lake
counties. But, the Klamath Basin, Shasta Valley near Yreka, and
northern Lake County all have a slight chance to chance of a
measureable amount, particularly during Friday evening.

A difference with this front is that winds ahead of it will not be
as strong, though still westerly and still with the strongest
breezy to windy conditions during the afternoon east of the
Cascades at 10 to 25 mph with gusts of 15 to 40 mph (strongest
over ridge tops).

Clouds across the area will diminish Saturday afternoon and allow
temperatures to climb to around normal. This will start a
previously advertised lengthy return of summer-like weather.
A Brookings effect of very warm downslope temperatures is
expected Saturday into Tuesday. Meantime, Inland locations will
already be warm to hot on Sunday afternoon with hot temperatures
likely to follow for the whole week.


.AVIATION...For the 25/00Z TAFs...VFR will likely continue
through this evening, although some areas of low to mid level clouds
will remain in the area. Some MVFR ceilings are expected along the
coast tonight as warm moist air pushes into Oregon. The next front
will pass through the coastal waters and move onshore Friday
afternoon and evening, producing MVFR ceilings and visibilities with
rain and terrain obscurations along and west of the coastal ranges
and into portions of the Umpqua basin. -BPN/Smith


.MARINE...Updated 800 PM PDT Thursday 24 September 2020...Steep,
long period west swell will persist through tonight. South winds
will increase again late tonight and Friday as another front passes
through, keeping conditions hazardous to small craft through Friday
night. Conditions should improve by Saturday afternoon. Then, after
a brief break, a thermal trough is expected to produce gusty north
winds and steep seas Sunday into Monday. -Smith/BPN


.PREV DISCUSSION... /Issued 230 PM PDT Thu Sep 24 2020/

DISCUSSION...Satellite observations showing a lot cumulus clouds
over the forecast area with some high level cirrus streaming
overhead. Isolated to scattered showers continue this afternoon,
but amounts being recorded are light with only a few hundreths
falling in the last few hours. Showers will continue this
diminishing trend as the afternoon wears on and we'll see a brief
break in the precipitation for most of the nighttime hours. High
temperatures will be around 10 degrees cooler than yesterday's
highs thanks to cloud cover and broad upper level troughing over
the PacNW.

This upper troughing will remain over the area through Saturday.
Another, weaker, front will move through the area early Friday morning.
This will bring another round of beneficial rain and gusty winds to the
area Friday. Models have trended farther south with precipitation
potential. Now it looks like many of the same places that received
wetting rains over the last 24 hours, could receive another round of
precipitation, albeit with about half as much expected. Even a few
hundredths are expected to make it east of Highway 97 with this front,
maybe even reaching the Brattain Fire. In addition to beneficial
rainfall, another round of gusty winds is expected Friday with this
front. Winds with this event will be more westerly than southwesterly
with 700 mb flow up to 45 kts. This will enhance precipitation along
north/south oriented terrain and bring gusty winds to the higher
terrain and east of the Cascades. While wind speeds won't quite reach
advisory criteria across the East Side, it will be close in some
locations, especially along high terrain of the Warners and Winter Rim.
Expect high temperatures Friday to be similar today's.

Showers will continue Friday night into Saturday, tapering off by
Saturday afternoon. Conditions on Saturday will be much like those of
today, with scattered light showers in the morning, seasonable
temperatures and a drying trend as the day comes to an end. However,
instead of expecting another front early Sunday morning with continued
fall-like conditions, we'll take a U-turn back Summer as upper level
ridging quickly builds over the Western CONUS and a long duration
summer like pattern returns to the area.

Sunday will be a transition day. As the upper ridge builds,
temperatures will warm by about 10 degrees and the thermal trough will
return in earnest. This will result in moderate east to northeasterly
flow across the area, especially over the higher terrain and the
Coastal Range of Curry County. This will also lead to the Chetco Effect
developing, and temperatures in the Brookings area will climb into the
mid to upper 80s Sunday. High temperatures will remain that warm for
Brookings while inland locations continue a warming trend Monday into
Tuesday. From Tuesday onward, the upper ridge amplifies and the thermal
trough expands northward. This will weaken the east to northeasterly
flow, but it will remain hot and dry across all areas through much of
next week, if not longer. /BR-y

FIRE WEATHER...Updated 1 PM PDT Thursday 24 September 2020...

Cooler and breezy conditions are expected today and Friday with
another round of rainfall late Friday into Saturday morning, focused
primarily over the coast, Southern Oregon Cascades, and Umpqua
Basin. Areas will be similar to yesterdays precipitation, but
amounts will generally be half or less than the amounts in the past
36 hours. West winds will trend higher on Friday compared to today.
Strong high pressure is likely to follow with a warming and drying
trend from the weekend into the foreseeable future, with
temperatures back into the mid 90s in the Rogue valley and near 100
in western Siskiyou county Sunday/Monday. With the upper ridge
building over the region starting Sunday, a thermal trough will
deepen on the southern Oregon coast, with a tight pressure gradient
Sunday bringing strong northeast winds to the higher elevations on
the southern coastal ranges. The thermal trough spreads north on
Monday, decreasing the pressure gradient and bringing lighter winds,
but with the continued subsidence and prolonged offshore flow
humidities will be lower. Sven




Pacific Coastal Waters...Small Craft Advisory until 5 AM PDT
Saturday for PZZ350-356-370-376.


Back to Main Forecast Discussion Page