Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 171626
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
926 AM PDT Thu Aug 17 2017

.DISCUSSION...The latest visible image shows marine stratus in
portions of the Coquille Basin. This will burn off later this
morning with clear skies the rest of the day. The main concern
will be patchy to areas of smoke. Some areas will have more
extensive smoke resulting in air quality concerns. Please see
AQAMFR for more details on the Air Quality Alert Message.

The latest surface analysis has the thermal trough just off the
coast and Brookings is already at 84 degrees F and they will
continue to heat up as the day progresses.

Made a few adjustments to the sky cover to reflect the latest
visible image and adjusted max temperatures up for Brookings given
they hit 93 degrees F yesterday and were expecting similar
conditions today.

The main focus will be thunderstorms later this weekend (Sunday
afternoon) into early next week. A preliminary look at the models
suggest the threat for storms could be an issue as early as Sunday
afternoon. -Petrucelli


&&

.AVIATION...17/12Z TAF CYCLE...VFR will prevail across the area
today and tonight. However, stratus will cause IFR along the coast
north of Cape Blanco until around 16Z this morning. Wildfire smoke
will also degrade visibility. This smoke is affecting most areas
except for the coast and Lake County. At the coast and offshore,
gusty north winds will be strongest south of Cape Blanco this
afternoon and evening. Stratus will return north of Cape Blanco by
07Z tonight. -Spilde/SK


&&

.MARINE...Updated 230 AM PDT Thursday 17 August 2017...High
pressure centered offshore and a thermal trough along the coast
will continue moderate to strong north winds and steep to very steep
seas through the weekend. Conditions will at least be hazardous to
small craft, but warning level winds/seas will occur beyond 5 NM
from shore and south of Gold Beach through the period. Winds and
seas will be highest during the afternoons and evenings each day
through Sunday, easing a bit during the overnights and early
mornings. Model guidance shows peak boundary layer winds of ~50kt on
both Saturday and Sunday afternoons, then the pressure gradient
weakens with winds gradually easing early next week. -Spilde/DW


&&

.PREV DISCUSSION... /Issued 401 AM PDT Thu Aug 17 2017/

SHORT TERM...A high pressure ridge over the region with a surface
thermal trough along the coast will persist into Friday. This will
bring dry and warm conditions with periods of gusty winds along
the coast and into the coastal mountains. On Saturday, a
shortwave disturbance will move towards the Northern California
coast then shift southward along the California coast on Sunday.
This will result in a slight chance for thunderstorms over
southeast portions of the area Sunday and Monday. Smoke will
remain a concern into next week. Little change is expected in the
areas getting smoke impacts today into this weekend and will see
similar smoke levels with some fluctuations each day.

Today, expect warm and dry conditions across the area. High
temperatures will rise into the upper 90s in western valleys and
in the upper 80s in eastern valleys. Dry offshore, easterly winds
are bringing breezy to gusty conditions over the coastal
mountains. Also the easterly winds are bringing a Chetco effect to
the Brookings area with highs expected in the 90s today. Along the
coast, gusty north winds will develop in the afternoon into the
evening.

On Friday, the upper ridge remains in place but flattens due to a
trough moving inland well to north. Expect little change in the
weather pattern compared to today. So the area will see another
day of very warm temperatures across inland areas and dry weather.

For the weekend, models continue to show a shortwave trough
approaching the coast on Saturday then shifting southward along
the California coast Saturday night into Sunday and developing
into a closed low. Southerly moisture is expected to move up from
the Sierras around into southeast portions of the area. This
combined with weak instability and and weak disturbances will
bring a slight chance for thunderstorms to Modoc and southeast
Lake counties late Sunday afternoon and evening. Otherwise expect
continued warm temperatures over the area. The surface thermal
trough will linger along the coast and bring periods of gusty
winds to coastal areas.

Monday into Tuesday, the upper low is forecast to remain off the
south-central California coast and mid level moisture will move
up into the area from the south. Models are continuing to show a
slight chance of thunderstorms over portions of Siskiyou, Modoc
and southeast Lake counties on Monday. The forcing and instability
remain weak and there is variability in the track and strength of
the low. So confidence is low in the details for MOnday. By
Tuesday, there is increasing confidence for isolated thunderstorms
developing a over portions of Northern California and from the
Cascades east.

&&

FIRE WEATHER...Updated 330 AM Thursday 17 August 2017...
A thermal trough along the coast will maintain moderate northeast
offshore winds in the SW Oregon Coast Range and portions of western
Siskiyou County this morning. Humidity recoveries in these areas
have been poor to moderate, generally 35-45 percent at mid
slope/ridge level, with wind speeds peaking at 10-15 mph. Occasional
gusts of 25-30 mph have been reported at a couple of the more
exposed RAWS like Red Mound and Slater Butte overnight.

It will remain very dry over much of the area today through the
weekend with daily minimum relative humidity in the 10-20% range
south of the Umpqua Divide and also east of the Cascades. Nighttime
and early morning marine layer clouds will be confined to the
immediate coast north of Cape Blanco. Today will be very warm and
dry even on the southwest coast - a Brookings effect is in full
force with the current temperature as of 3 am sitting at 81F.
Guidance indicates a high temperature there similar to yesterday, in
the low 90s with corresponding MinRH in the upper 20s to near 30
percent. Expect some marine influence to raise MinRH values a bit
there over the coming days, but it will remain lower than usual.

High pressure aloft will flatten across the area today through
Friday allowing the atmosphere to become more unstable. High level
Haines of 5 will be common west of the Cascades today, then across
the entire area on Friday. Haines 6 is possible in portions
of fire zones 624/western 625 on Friday. Temperatures inland
will be back to 5-10 degrees above normal.

An offshore upper level disturbance will dig southeastward this
weekend and close off near the central/southern California coast
early next week. It`s still uncertain, but this may be able to
bring some moisture back to the north up the Sierra crest and
eventually into portions of our forecast area. We are continuing to
indicate a slight chance of thunderstorms in Modoc on Sunday
afternoon, then near the Trinity Alps/Medicine Lake area into Modoc
and perhaps Goosenest area Monday afternoon. Models have come into
better alignment showing a better chance for lightning from the
Cascades/Siskiyous eastward on Tuesday afternoon/evening. -Spilde

&&

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

CA...None.

Pacific Coastal Waters...Small Craft Advisory until 11 PM PDT Sunday for
     PZZ350-356-370-376.
     Gale Warning until 11 PM PDT Sunday for PZZ356-376.

$$



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