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Area forecast discussion 
National Weather Service Medford or
943 PM PDT Thursday Aug 27 2015

Discussion...the 00z GFS data has arrived and it is largely
consistent with its previous run as well as the 00z NAM and 12z
European model (ecmwf). The biggest change in this run of the GFS may be a
slightly farther south and stronger depiction of the main shortwave
that is expected to ripple along the cold front late Friday night.
Thus, precipitation amounts may need to be bumped up to an
expectation of around a half inch in the valleys of western
Siskiyou County and southwest Josephine County. The bulk of
rainfall will occur between midnight and 9 am on Saturday on the
west side then lighter precipitation on the east side between 5 am
and noon. The southwest low level flow will have a downslope
influence in the rogue and Shasta valleys with amounts of a tenth
to a quarter of an inch.

Besides the rain, the fast moving front will bring strengthening
winds late Friday night. The strongest coastal winds look to occur
between 3 am and 9 am on Saturday morning with gusts to 65 miles per hour
expected at Cape Blanco and widespread gusts to 40 miles per hour. Southwest
winds will also be strong inland...strongest on the east side with
gusts of 35 to 50 miles per hour between 8 am and 4 PM on Saturday.

Post-frontal showers will be very light and mainly at the coast
Saturday afternoon and evening then the next front will follow
late Saturday night and dissipate as it moves inland Sunday.
Measureable rain is likely to be limited to the west side with
this system with the bulk of rainfall occurring west of Interstate
5. The outlook is for another front late Tuesday or Tuesday night
with similar characteristics to the Sunday system. Otherwise, the
upcoming week looks to be dry but cool with a persistent maritime
influence keeping temperatures below normal.

&&

Aviation...for the 28/00z taf cycle...along the coast...VFR
conditions will prevail initially...then areas of MVFR ceilings and MVFR
visibilities in rain will develop Thursday evening with areas of higher
terrain becoming obscured. Conditions will improve to VFR Friday
morning and remain VFR through Friday afternoon. Inland...VFR
conditions will prevail...except for areas of MVFR visibilities in smoke
and haze through Friday afternoon. Some higher terrain may become
obscured in the Umpqua basin. -Jrs

&&

Marine...updated 315 PM PDT Thursday 27 August 2015...light winds
and seas are expected through early Friday. Confidence is increasing
regarding the details of a frontal system which will enter the
waters on Friday and move through them over the weekend. A period of
moderate to strong southerly winds is expected to begin on Friday
evening into Saturday. This will create periods of small craft winds
and seas across all of our waters with the possibility of gales and
very steep wind driven seas occurring north of Gold Beach. In
addition, occasional heavy rain and low visibilities will be
possible. Multiple long period swells are also expected this weekend
with possible increased sneaker wave activity. Conditions will
temporarily calm down Monday, but moderate long period northwest
swell will develop over the waters next week. -Schaaf

&&

Previous discussion... /issued 325 PM PDT Thursday Aug 27 2015/

Discussion...the latest visible image shows a few high clouds moving
in from the southwest and cumulus in southeast Modoc County. Elsewhere
skies are clear with patchy to areas of smoke moving north from the
fires in del Norte County. Dry and warm conditions will continue for
most locations one more day, then significant changes are on the way
starting Friday night and lasting through the weekend. Details will
follow below.

A large upper low near 43n/139w will move southwest over the next 24
to 36 hours and an upper trough rotating around the upper low will
move into southwest Oregon and become negatively tilted and a
surface low deepening inside of 130w late Friday night into Saturday
morning. Normally this type of pattern will lead to moderate to
strong winds along the coast, especially over The Headlands and near
Cape Blanco and at least gale force winds over the northern marine
waters. At this time, we think winds at the coast and headlands will
remain below warning criteria. However, its possible in a few spots,
say around Cape Blanco and headlands to the north could briefly
reach warning criteria Saturday morning. Later shifts will need to
reevaluate. Winds should gradually decrease Saturday afternoon as
the low moves north. Additionally, moderate to strong winds are
possible over the higher terrain Friday night and Saturday along and
east of of the Cascades. A few spots around Summer rim possibly
reaching high wind criteria but there is a better chance for greater
portion of the east side reaching Wind Advisory criteria.

The next concern is rain. The models have delayed the onset of
rainfall for the coast and inland areas until Friday night, but
still bring the most of the rainfall to the coastal mountains and
Cascades. Also the models have trended lower with precipitation amounts,
but confidence is low that will be the case, therefore amounts
remain close to what was forecast last night. However I did reduce
values in the Umpqua and Rogue Valley due to downsloping effects. In
short, it still looks like enough rain to make an appreciable
difference across the area, with amounts upward to to an inch along
for the coastal mountains in southwest Oregon.

Saturday we expect showers, mainly for the Cascades westward.
Amounts will be less than a quarter of inch along and near the coast
and less than a tenth of an inch elsewhere. Winds are expected to be
strongest Saturday morning, then will gradually diminish Saturday
afternoon as the surface low moves north and weakens. Temperatures
will remain cool with readings mostly in the 70s for the valleys
along with higher relative humidity, burning conditions will be much
less favorable for area wildfires.

Weather will be calmer Sunday through Monday with a weak troughiness
over the area and a couple of weak front brushing by the northwest
part of the area. The next significant front will arrive Tuesday
afternoon bringing another round of rain, but it won't be as strong
as the one that will move in Friday night. -Petrucelli

Fire weather...updated 200 PM PDT Thursday 27 August 2015...a
large upper trough off the coast this afternoon and its associated
frontal system will move inland over the weekend. This will bring
a wetting rain to much of the area from the Cascades westward. On
the east side, less rain is expected with the front, but many
areas will get some. The bigger concern east of the Cascades will
be the dry conditions and gusty southwest winds that we'll get
this afternoon and again Friday afternoon. Humidities will be
coming up, but conditions will be near critical in spots both
days. Essentially, the humidities will be lowest today and the
winds stronger tomorrow, but widespread red flag conditions are
not expected, so warnings are not anticipated. We will continue to
headline for these conditions in the fire weather forecast,
however.

A strong surface low is expected to form along the front offshore
Friday night into Saturday morning. This will bring strong winds to
the coast, coastal mountains, and inland ridges Saturday morning.
While humidities will be increasing (especially where it rains), a
negatively tilted upper trough is a potential high wind maker on the
east side as well. We have increased winds on the east side, but
we'll have to watch this closely as it could be quite windy Saturday
morning.

While we will get rain over many of our ongoing fires, there are
specific concerns about this storm with respect to those fires and
the personnel Manning them. There is the potential for heavy rain
over all fire areas west of the Cascades. This is expected to be a
gentle soaking rain rather than convective, high intensity rain, so
burn scar flooding and debris flows are not a huge concern. But it
is something to think about. Also, snags, burned trees, and other
material that have been weakened over the Summer have the potential
to come down from wind, the added weight of getting wet, or a
combination of both. Firefighters should remain very aware of their
surroundings this weekend, especially on or around existing fires
and burn scars. -Wright

&&

Mfr watches/warnings/advisories...
or...none.

California...none.

Pacific coastal waters...gale watch from late Friday night through
Saturday afternoon for pzz350-356-370-376.
Small Craft Advisory for winds from 11 PM Friday to 5 PM PDT
Saturday for pzz356-370-376.
Hazardous seas watch from late Friday night through Saturday
afternoon for pzz350-356-370-376.
Small Craft Advisory for hazardous seas from 11 PM Friday to 5
PM PDT Saturday for pzz356-370-376.
Small Craft Advisory for winds from 11 PM Friday to 5 am PDT
Saturday for pzz350.
Small Craft Advisory for hazardous seas from 11 PM Friday to 5
am PDT Saturday for pzz350.

$$

Dw/map/jrs

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