Excessive Heat Warning 3, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
900 AM PDT Thu Aug 31 2017

.UPDATE...A few updates were needed this morning to bring clouds
into line with satellite observations. Otherwise, the forecast
appears to be on track. Heat will begin to build under high
pressure today, and this will only intensify into the weekend, and
although there has been some improvement for portions of the area,
smoke will return to most areas, becoming a nuisance yet again.

For more information on this and the rest of the forecast, see the
previous discussion below. -BPN

&&

.AVIATION...31/12Z TAF CYCLE...Along the coast and coastal
waters...IFR cigs/vsbys are mainly beyond 5 NM offshore early this
morning but with areas at the coast north of Cape Blanco. This
stratus will gradually erode near shore during the morning. With the
thermal trough expected over Curry County coast, strong north winds
are expected at OTH and sustained winds could reach 20 knots with
gust to around 30 knots this afternoon. As a result of the offshore
winds, conditions should remain VFR tonight. Over the remainder of
the area...IFR visibilities with the thickest smoke will persist in
southern Curry County and western Siskiyou County. Areas of MVFR
visibilities from wildfire smoke will persist through
Thursday...mainly elsewhere in northern California and into south
central Oregon including KLMT. There will be a gradual worsening
trend into early afternoon then improvement late in the day into the
evening. -DW

&&

.MARINE...Updated 900 AM PDT Thursday 31 August 2017...The latest
observations at buoy 27 shows breezy north winds. So far they are
below small craft, but we expect winds there to increase to small
craft late this morning through this afternoon. Made some
adjustments to the winds today, mainly to decrease them some near
the shore, otherwise the forecast is on track with respect to the
coverage area of the small craft conditions and start time of gales.
High pressure offshore and a thermal trough along the coast will
bring strong north winds and steep to very steep short period seas
into Saturday morning. Winds are expected to reach gale force south
of Port Orford beyond 5 nm from shore around midday. Gales and very
steep wind driven seas are likely to peak this evening but linger
into Friday morning. The thermal trough is expected to drift
westward over the coastal waters Saturday and Sunday leading to
lighter winds and diminished seas. North Winds will briefly increase
on Monday. Winds and seas are likely to decrease on Tuesday with
seas becoming swell dominated on Wednesday for the first time in
quite a while. -DW

&&

.FIRE WEATHER...Updated 500 AM PDT Thursday 31 August 2017...

Stronger low level northwest winds brought temporary improvement in
air quality into southwest Oregon, but improvement was less
noticeable in south central Oregon and northern California. Smoke
will gradually thicken south and east of the Umpqua Divide this
morning then northerly flow should bring improvement in southwest
Oregon and north of Chiloquin this evening.

There are a number of fire weather concerns:  Gusty easterly winds
with low humidity this afternoon roughly in the area west of
Interstate 5, stronger easterly winds with poor to moderate humidity
recovery tonight, very hot temperatures Friday through Monday with
hot temperatures today and Tuesday/Wednesday, the potential for high
level Haines 6 conditions near the Chetco Bar Fire/Kalmiopsis
Wilderness on Friday and then much of the area east of the Cascades
on Saturday, and finally...the potential for a daily
risk of thunderstorms next week on Sunday through Thursday.

Easterly winds developed overnight and become gusty over the Coast
Range and Western Siskiyou County. A dry air mass will gradually
descend over those areas, with low humidity on ridges early today.
Northeast winds will increase this afternoon and humidity will be
very low, so a Red Flag Warning remains in effect for the solid
moderate-strength Chetco Effect event from noon continuing through
early Friday morning. The Cascades, Siskiyous, and Western Siskiyou
County will also see gusty overnight winds and poor humidity
recoveries tonight, and they are included in the Red Flag Warning.
Northeast/east winds do diminish toward the coast during the day on
Friday, but we are not expecting moist onshore winds to affect much
of the Chetco Bar fire. Instead, conditions on Friday become hot,
dry, and unstable with generally lighter winds for much of the
forecast area as the thermal trough begins to nudge inland. This
continues into Saturday with High Haines Index values expected to be
5 to 6 in southwest Oregon on Friday and east of the Cascades on
Saturday.

Note that the air mass this weekend will be one of the hottest and
driest so far this summer, and certainly in the hottest 5% for this
time of year. Smoke will no doubt limit the amount of heating we get
at the surface, but clearer areas should be dangerously hot. Fire
crews will be impacted by heat even in the higher terrain areas.

The models are showing increasing monsoonal moisture moving up on
the backside of the ridge into our area Sunday along with a weak to
moderate trigger. This brings the possibility of thunderstorms back
into the picture. Next Monday and Tuesday now look more favorable
than Sunday for thunderstorms, and an unstable air mass will  likely
persist through Thursday. -SK/DW

&&

.PREV DISCUSSION... /Issued 350 AM PDT Thu Aug 31 2017/

DISCUSSION...Its going to get hot again. However, this is not
very unusual for this time of year, and although we will approach
near record highs, we should remain below records for the stretch.
The same pattern that has kept Harvey dumping massive amounts of
rain over a broad area will also keep an upper level ridge over
the west coast through the weekend. On the bright side, the
northwesterly flow that moved in behind the exiting trough has
pushed the dense smoke out of most of the west side of Oregon and
that is expected to remain so into the weekend. A thetrmal trough
deepens again on the southern coast, with offshore flow returning
this morning, and spreading across the east side by Friday
morning. This will continue to push the smoke from the biggest
fires on the southern coastal ranges west but bring smoke from
smaller fires in the Cascades and the east side into the west side
through the weekend. The fires in the Cascades are currently
producing much less smoke than the fires near the coast however.
With this in mind most of Oregon North of the Siskiyous will see a
nice weekend except for the heat with river flows remaining high
compared to the last number of years of drought. It will seem very
peaceful on the rivers and lakes, but the flows are still strong
in places and can carry you off quickly, so always wear a lifevest
when playing on or in the water.

With the decreased smoke cover over Oregon as projected by the
HRR smoke models have adjusted the high temps closer to the
guidance. The Chetco winds will remain through the day and into
the evening tonight. The flow will remain strong overnight into
Friday and have adjusted the low temperature at the extreme
southern coast, especially Brookings, up well above guidance
Friday Morning. The thermal trough moves inland Friday, allowing
some onshore flow relief to that area Later Friday.

The thermal trough deepens along the coast again Saturday, and
with the easterly flow pushing the smoke from the biggest fires in
the southwest forecast area to the southwest have kept the
forecast highs close to guidance, with the only thing that might
keep those lower is increased fire activity on the further inland
fires that could produce more smoke.

The upper ridge moves inland just enough to combine with a
deepeningupper trough offshore to start to feed moist unstable
air and bring the possibility of more thunderstorms from Sunday
through the middle of next week. Sven

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from noon today to 9 AM PDT Friday for
     ORZ618>620.
     Excessive Heat Watch from Friday morning through Sunday evening
     for ORZ021>028.
     Red Flag Warning from 8 PM this evening to 9 AM PDT Friday for
     ORZ617-621-623.

CA...     Excessive Heat Watch from Friday morning through Sunday evening
     for CAZ080>083.
     Red Flag Warning from 8 PM this evening to 9 AM PDT Friday for
     CAZ280.

Pacific Coastal Waters...Small Craft Advisory until 11 PM PDT Friday for PZZ356-376.
     Gale Warning until 8 AM PDT Friday for PZZ356-376.
     Small Craft Advisory for hazardous seas from 8 AM to 11 PM PDT
     Friday for PZZ350-356-370-376.
     Small Craft Advisory until 11 PM PDT Friday for PZZ350-370.
     Hazardous Seas Warning from 8 AM to 11 PM PDT Friday for PZZ376.

BPN/SBN/DW/MAP/NSK