Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 180555
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
1055 PM PDT Thu Aug 17 2017

.DISCUSSION...24 hour change data shows a warming and drying trend
for most of the forecast area. This trend will continue one more day
on Friday before a weak upper trough brings some cooling and higher
humidity to most places on Saturday. Fires were yet again very
active today in the hot and dry air mass. Smoke is generally moving
north to south across the forecast area, and this "smoke steering
flow" will be similar Friday afternoon. No changes were needed to
the forecast for now and please see the previous area forecast
discussion and fire weather discussion for more details on the
weekend weather.

&&

.AVIATION...18/06Z TAF CYCLE...VFR will prevail across inland area
tonight with the exception of smoke-reduced visibilities. Wildfire
smoke will degrade visibility and could create partial terrain
obstruction, affecting a broad section of the forecast area. At the
immediate coast and offshore, gusty north winds will be strongest
south of Cape Blanco this evening. IFR ceilings and visibility will
return to areas north of Cape Blanco by around 09Z tonight and last
through mid morning before clearing. FB/SK

&&

.MARINE...Updated 230 PM 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. /FB

&&

.FIRE WEATHER...High pressure centered over the eastern Pacific
and low pressure over the Northern Rockies will continue to
maintain NW flow aloft through Friday. At the surface, this will
lead to NE winds and moderate to poor relative humidity recoveries
each night and morning through Sunday morning along and near the
coastal ranges, specifically in FWZ`s 618, 619, 620, 621 and 280.
This morning a couple of locations, specifically, Red Mound and
Slater Butte, were very close to Red Flag criteria. The model
NAM12 indicates slightly stronger winds Friday morning than this
past morning, except possibly in isolated locations in the coastal
range of FWZ 619; however, other guidance shows lighter winds.
Therefore, we decided to continue with a headline instead of an
RFW.

Otherwise, it will be dry and hot each afternoon, driest and
hottest Friday afternoon. Locally derived guidance (FWG)
indicates that Friday`s relative humidities will be lower than the
national guidance, and we have nudged our forecast for Friday
afternoon and night toward our local guidance. Dryness and
instability on Friday will lead to a moderate to high probability
of active wildfires becoming plume dominated as the Haines Index
reaches 5 at most places. Haines 6 is possible over the Oak and
Prescott Fire in far western Siskiyou County. It should be noted
that the difference between a Haines 5 and 6 is not much once a
fire starts putting up a column of sufficient size - that is
pushing up to around 10,000 feet off the ground. Thus, large
uncontrolled fires stand the greatest chance of being problematic
Friday afternoon and evening.

This weekend winds aloft will become more southerly leading to a
slight possibility of thunderstorms over northern California and
the east side, mainly in and near Modoc County by Sunday and
Monday. There is a growing confidence in a more organized
possibility of thunderstorms on or about Tuesday as both the ECMWF
and GFS models indicate a short wave trough moving through,
bringing showers and thunderstorms, roughly from the Cascades
and Siskiyous eastward. This possibility could linger into
Wednesday, but confidence is currently low due to wavering model
guidance/ BTL



&&

.PREV DISCUSSION... /Issued 613 PM PDT Thu Aug 17 2017/

DISCUSSION...An upper ridge centered in the eastern Pacific will
shift slightly east towards our area in the next 24-36 hours. This
will bring a continuation of hot and dry conditions over our area
through at least Friday. The latest visible image shows no cloud
cover over our area, but patchy to areas of smoke is evident at the
Eclipse Complex in Western Siskiyou County and Chetco Fire in Curry
County. The upper winds carrying the smoke from northeast to
southwest. Of note it looks like most of the above mentioned fires
are becoming active as evidenced by the smoke plumes on the visible
image.

The thermal trough remains just off the coast with offshore flow.
The Chetco effect is in full force with Brookings toping out at 94
degrees. It`s has dropped down about 12 degrees in the past hour,
but were still expecting offshore flow to continue tonight with
gusty northeast winds and milder temperatures near and at the
ridges.

Not much is going to change in the next couple of days with
continued offshore flow. The Chetco Effect will remain in place,
therefore expect mild temperatures at Brookings tonight followed
by another warm day Friday. Skies should remain clear tonight,
including the coast, but we`ll still have to deal with patchy to
areas of smoke.

A weak shortwave trough will ride over the ridge Saturday and could
bring slight cooling to most inland locations, but only by a few
degrees on average. The models show marginal instability at best,
but mid level moisture is lacking, so were not expecting any
thunderstorms. However, cumulus build ups are possible over the
mountains in Northern California and east of the Cascades.

The models show a weak upper trough developing in central California
Saturday night into Sunday and could tap into some monsoonal
moisture. The northward extent of the moisture should be confined to
Northern California, specifically Modoc County due in part to a
westerly flow setting up in the northern Part of the forecast area.
There may be enough instability and moisture and weak trigger to
generate isolated thunderstorms. So kept in a slight chance in for
southeast Modoc and Lake County late Sunday afternoon/evening.

The upper trough will close off and remain in central Californaia
Monday and it remains to be seen how much moisture and instability
will move into our area. The GFS shows more instability, moisture
and slightly stronger trigger. The ECMWF is weaker with all of these
elements and has weak or almost nonexistent flow over us. Given the
model differences this far out, no changes will be made to the
coverage of slight chance of thunderstorms.

Tuesday afternoon and evening could turn out to be the most active
day in terms of thunderstorms. The GFS continues to be the most
bullish with plenty of moisture, instability and trigger. Right now,
most of the action should be in northern California and Cascades
eastward. However steering winds could push some of these storms
into portions of the westside valleys, including the Rogue and
Illinois Valley. Keep in mind this is still several days out and the
details on this could change from one day to the next. The bottom
line is confidence is becoming higher we could have an active day
Tuesday afternoon/evening, but less confident on the exact location
of thunderstorms. Also could not rule nocturnal storms Tuesday night
east of the Cascades.

Beyond Tuesday, we`ll start to feel the influence of an upper low
moving southeast from the Gulf of Alaska towards Vancouver Island.
The models differ with position of the upper low. The ECMWF shows a
deeper trough which if correct would draw in moisture and
instability resulting in at least a slight chance for thunderstorms
east of the Cascades. The GFS shows a southwest flow, keeping the
best chance for thunderstorms east of our area. Despite the
differences, both models suggest some cooling possible by next week
at this time. It`s a ways out and lot`s could change, so stay tuned.
-Petrucelli

&&

.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.

$$

NSK/MAP/FJB


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