Gale Warning 18, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
907 AM PDT Mon Aug 10 2020

.UPDATE...An upper level ridge remains in place, centered
offshore. Also a surface thermal trough is along the coast. The
thermal trough is not quite as strong as yesterday. However, still
expect strong gusty northerly winds at the coast and over the
coastal waters today. The Brookings area is not expected to be as
hot as yesterday but still expect a Chetco warming effect along
the southern Curry coast with highs in the lower to mid 80s.
Inland, temperatures will warm further with highs in the 90s to
lower 100s (except warmer in the lower Klamath River Valley area
near Happy Camp and Somes Bar).

The main change is with thunderstorm chances. Models are
indicating that for late this afternoon and this evening the best
chance for thunderstorms will be just south of the area. However,
there remains a slight chance for storms into the south- central
and southeast Sikiyou area near Mount Shasta and far southeast
Modoc County. Have updated the forecast to reflect this change,
reducing the area of coverage for thunderstorms today.

Tomorrow as an upper trough approaches from the northwest expect
strong gusty winds east of the Cascades in Oregon. This combined
with dry humidities will bring increased fire weather concerns.
Additionally, there is higher confidence that isolated
thunderstorms will develop in southeast Siskiyou, Modoc and far
southeast Lake Counties Tuesday afternoon and evening. For
details, please see the previous discussion below.

&&

.AVIATION...For the 10/12Z TAF Cycle...At the coast and just
offshore. The main concern will be gusty winds are expected again
late this morning into this evening, especially north of Cape
Blanco, including North Bend. Meanwhile, the latest satellite images
used for detecting low clouds and fog is showing marine stratus
moving from the north offshore, except near the coast north of
Newport. This will move south into the morning hours and could
result in IFR ceilings just off the coast between 15-17z. There's a
chance this could continue to move south towards Cape Blanco late
this morning. North Bend could experience patchy areas of fog for a
few hours this morning with the difference between air and dewpoint
temperatures currently at 1 degree F and this was added to the TAF.
We'll have to keep a watch for brief LIFR ceilings between 14-15z
but confidence was not high enough to include it in the TAF. Any
patchy fog/low clouds will clear out with VFR conditions into this
evening when LIFR to IFR fog and stratus is expected to begin to
affect the Brookings area and coastal areas north of Cape Blanco.

Elsewhere, VFR conditions will continue through the TAF period with
gusty breezes at Roseburg this afternoon into this evening. The one
exception is visibility restrictions due to smoke from the Red
Salmon Fire in western Siskiyou County through the TAF period.
-Petrucelli

&&

.MARINE...Updated 800 AM PDT Monday 10 August 2020...The thermal
trough will gradually weaken and move onshore this evening. North
winds will slowly diminish tonight into Tuesday, with winds dropping
below gale force this evening. However, moderate to strong winds and
steep seas will persist. The thermal trough will rebuild again
Thursday into Friday, so north winds and seas will increase at that
time. Marine headlines are likely to affect most of the waters from
Thursday through Saturday afternoon. The thermal trough will build
north along the coast this weekend, which will result in diminishing
winds and seas.

&&

.PREV DISCUSSION... /Issued 459 AM PDT Mon Aug 10 2020/

DISCUSSION...Over the past few days, the most noteworthy aspects
of the forecast for this week have been consistent. Warmer and
drier than normal conditions with periods of stronger winds, and a
slight chance of thunderstorms in portions of northern California
late today and again on Tuesday (with coverage expanding slightly
into southern Lake County). Also, a shorter duration and less
coverage of coastal stratus compared to normal.

The model trends over the past couple of days have been to
slightly strengthen the gusty late day winds inland on both
Tuesday and Wednesday (ahead of the passage of a dry trough on
Wednesday), slightly lessen the extent of instability and amount
of available mid- level moisture over southern portions of
Siskiyou and Modoc Counties this evening, and (as mentioned)
slightly expand the extent of weak instability on Tuesday evening.
Originally, much of the instability was expected to shift east of
our area on Tuesday. But, Tuesday now looks to be the more
favorable for development of isolated thunderstorms than today.
The highest probability in our area today will be in southern
Siskiyou County from the vicinity of Mount Shasta due eastward,
then on Tuesday over both the northern and extreme southeast
portions of Modoc County. The modest probability of thunderstorms
on both days will be offset by the fact that very dry lower levels
of the atmosphere will result in little accompanying rainfall.
The outlook is for a stable air mass Wednesday through at least
Saturday. A minority of GEFS ensemble members, and the operational
GFS do indicate weak instability over the northern portion of the
southern Oregon Cascades into central Oregon on Sunday. We have
continued with the drier, stable majority solution.

There remains little change from previous forecast issuances in
the trend or extent of heating. Today will be the hottest day of
the week for most of our area, but a modest cooling trend Tuesday
into Wednesday will be followed by high temperatures trending
hotter again into Saturday. In fact, highs on Saturday look to be
quite similar to those of today. The exceptions will be at the
coast with highs near to slightly below normal through the week,
but another hot day at Brookings today and downslope heating
from northeast winds likely again at Brookings on Friday.

.FIRE WEATHER...Updated 300 AM PDT Monday 10 August 2020...A thermal
trough along the coast will move inland this afternoon. A weaker
thermal trough will then set up at the coast again tonight into
Tuesday morning.  This will result in the poorest humidity recovery
this morning along the Coast Range and ridges of western Siskiyou
County, including zones 618, 619, western 620, and western 280. But,
mainly moderate recoveries tonight will still be noticeably drier
than normal.

There is a slight chance of afternoon and evening thunderstorms in
southern portions of Siskiyou and Modoc counties today, then in much
the same area on Tuesday but also edging into far southern Lake
County. Any thunderstorms that do develop will not produce much
rainfall.

The warming trend will peak this afternoon. Temperatures will then
trend cooler into Wednesday, but remain above normal through the
week. The brief, modest cooling will be accompanied by the passage
of a dry cold front on Wednesday. Thus, gusty winds and low relative
humidities are expected Tuesday and Wednesday east of the Cascades.
A Fire Weather Watch is in effect for Oregon fire weather zones
624/625 for Tuesday afternoon and evening, and may become necessary
for Wednesday also. Compared to Tuesday, conditions on Wednesday
look to be slightly cooler, but similarly dry and breezy to windy.
Minimum relative humidity east of the Cascades on both days will
generally be in the lower teens with 15 to 25 mph winds gusting as
high as 40 mph.

A warming trend with dry weather will follow during the second half
of the week. By Saturday, temperature and humidity levels look to be
similar to those of today. -DW

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Fire Weather Watch from Tuesday afternoon through Tuesday
     evening for ORZ624-625.

CA...None.

Pacific Coastal Waters...Small Craft Advisory until 11 PM PDT Tuesday for PZZ350-356-370.
     Hazardous Seas Warning until 11 PM PDT this evening for PZZ350.
     Gale Warning until 11 PM PDT this evening for PZZ356-376.
     Hazardous Seas Warning until 11 PM PDT Tuesday for
     PZZ356-370-376.

CC