Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 220545
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
1045 PM PDT Sat Jul 21 2018

Updated AVIATION Section

.DISCUSSION...Current observations are showing smoke south and
east of the Umpqua Divide. This is currently bringing air quality
concerns to most of southern Oregon and northern California. This
is generally well forecast and the air quality alert will
continue for the foreseeable future.

The current forecast appears to be mainly on track and will not
be making any changes. The forecast is tricky, and the day shift
has done a great job at outlining the complexities. Thus, I
encourage you to read the previous discussion (below the marine
update) for more details of the forecast. -Schaaf

&&

.MARINE...Updated 830 PM PDT Saturday 21 July 2018...The strong
thermal trough of the past couple of days is expected to weaken
and shift inland over the next 12-24 hours. North winds will
diminish during this time frame and steep seas will gradually
subside. However, residual wind waves and a moderate north to
northwest fresh swell will persist and this will make for choppy
seas with a dominant period of only 7 to 8 seconds. Areas in close
to the coast, perhaps within about 3 nm from shore, should see
seas lower to around 4-5 feet by Sunday afternoon, but again, we
expect a fairly short period, so smaller boats will probably still
get tossed around a bit.

From Monday onward, we aren`t expecting any strong thermal troughs
to develop, but the general theme will be to maintain northerly
winds averaging 10-20 kt beyond about 3 nm from shore with waves
mostly in the 5-7 foot range. There could be a southerly eddy at
times, especially nearshore around Brookings where winds and seas
could be a little calmer. -Spilde

&&

.AVIATION...For the 22/06Z TAFs...Wildfire smoke continues over most
of the inland valleys west of the Cascades and south of the Umpqua
Divide. Lower visibilities along with MVFR conditions will exist
near these fires tonight into Sunday. There`s a chance visibilities
could be reduced further tonight (IFR) as smoke settles into the
valleys.

At the coast north of Cape Blanco, onshore flow is expected to
increase enough there late tonight for marine stratus or some fog to
form. Have lowered ceilings to IFR at 12z. Conditions should improve
around or after 16Z.

Inland north of the fires and east of the Cascades, VFR will prevail
for the TAF period, though some light MVFR smoke is possible closer
to the Cascades.

Isolated thunderstorms will develop in portions of Northern
California and over the East Side late Sunday afternoon/evening.
Have included VCTS in the Klamath Falls TAF between 00 and 04Z.
-Spilde

&&

.PREV DISCUSSION... /Issued 534 PM PDT Sat Jul 21 2018/

DISCUSSION...Smoke from area wildfires continues to impact mainly
the the southwestern portion of the forecast area this afternoon,
with similar coverage and generally worse air quality than
yesterday at this time. We maintain an Air Quality Alert through
early next week, and this may very well be extended further,
pending fire behavior. Winds are northerly and gusty again this
afternoon, especially in portions of the Coast Range and in the
Illinois Valley. Here we have a Red Flag Warning in effect through
the afternoon due to gusty winds and low humidity. Days of
moderate to strong north winds west of the Cascades (especially
towards the coast) will come to an end tonight and Sunday as the
thermal trough weakens and moves inland. Winds will switch at the
immediate coast near Brookings to southerly and allow for some
marine stratus and fog to affect the coast. Further inland, winds
will still be northerly Sunday afternoon but not as strong.

Thunderstorm chances enter the picture on Sunday, thanks to energy
moving up from the south and increasing moisture and instability.
Models have ebbed and flowed in terms of how favorable the
environment will be on Sunday for thunderstorms. The environment is
not as unstable, and we don`t have the strong trigger like we did
last Sunday, so it`s a very different situation than the event
that brought thousands of lightning strikes to the forecast area.
Add the fact that we`ll have some smoke that will likely limit
surface instability...and the picture becomes pretty fuzzy. We`ve
taken the consensus of guidance, and what we`re expecting tomorrow
are mostly isolated thunderstorms, with a few areas that could
bump up to scattered coverage. We are aware that fuels
(vegetation) in the area are in record territory for dryness, so
it won`t take many lightning strikes to cause fires. So we`ve
issued a Red Flag Warning for tomorrow for mainly the Siskiyous
southward and Cascades eastward. Confidence is this Warning is
lower than normal because of the aforementioned added variables.
Please see the Fire Weather Discussion below for more details on
that aspect of the forecast.

The thunderstorm threat does diminish but won`t go away Monday and
Tuesday. There will still be weak shortwave energy moving up from
the south and some instability and moisture. We`ve kept slight
chances mainly from the Siskiyous south and Cascades east.
Regarding nighttime thunderstorms, we haven`t identified any
periods that are favorable enough to add them to the forecast.
Sunday night there is a touch of upper level instability and some
shortwave energy in the vicinity, so this will need to be looked
at.

Next...heat. We`re in the climatologically hottest time of the year
now, and an upper ridge building from the southeast will bump our
clear air temperatures up to 10 degrees above normal for this time
of year. I use the term "clear air" because we know that smoke in
areas will probably keep the temperatures lower than the potential
warmest readings that models are predicting. In areas where we
expect the heaviest smoke, we`ve kept forecast temperatures below
guidance for now. Other areas, like much of the East Side, Mount
Shasta, and the Umpqua Basin, we have kept in line with guidance. We
decided to issue an Excessive Heat Watch today because we`re
expecting a long-duration heat wave, and the impacts from the heat
could be exacerbated by smoke. Subsequent shifts can examine another
day of observations and how much smoke has impacted high
temperatures, and they can help determine which areas will or won`t
need to be upgraded to an Excessive Heat Warning.

Models suggest some cooling by late next week, but it doesn`t look
significant at this time.

AVIATION...For the 22/00Z TAFs...Wildfire smoke continues over
most of the inland valleys west of the Cascades and south of the
Umpqua Divide. Lower visibilities along with MVFR conditions will
exist near these fires tonight into Sunday. There`s a chance
visibilities could be reduced further tonight (IFR) as smoke settles
into the valleys.

At the coast north of Cape Blanco, north winds will be gusty this
evening at North Bend with gusts to 35 kts and VFR conditions.
Onshore flow is expected to increase enough there later tonight for
marine stratus to form. Have lowered ceilings to MVFR around 08z,
then IFR around 13Z. Conditions should improve around or after 16Z.

East of the Cascades and inland north of the fires, VFR will prevail
for the TAF period.

Isolated thunderstorms may develop in portions of Northern
California and over the East Side late Sunday afternoon/evening.
-Spilde

FIRE WEATHER...Updated 200 PM PDT Saturday 21 July 2018...Red
Flag conditions over north-south oriented valleys on the west
side will moderate later today. After that, the primary concern
is thunderstorms.  The best day for thunderstorm coverage will
be Sunday, when available moisture, surface heating, and an
upper level trigger will combine for the most optimal conditions.
High level Haines indices will be 5 over the west side, but mid-
level Haines may be 6 in some locations. Ongoing fires may become
plume dominated at times. Moisture will gradually diminish Monday
and Tuesday, and there isn`t a trigger Monday and a weak one
Tuesday. Therefore, expect isolated thunderstorm coverage at best on
those days. By Wednesday, the southwest U.S. high will be far enough
west to cut off the monsoonal moisture, so thunderstorms are not in
the forecast from Wednesday on. It will be hot and dry inland Monday
through Thursday, especially in smoke-free areas.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from 11 AM to 11 PM PDT Sunday for ORZ621>625.
     Red Flag Warning until 9 PM PDT this evening for ORZ619-620.
     Excessive Heat Watch from Monday morning through Wednesday
     evening for ORZ023-025.
     Excessive Heat Watch from Monday morning through Wednesday
     evening for ORZ024-026.

CA...Red Flag Warning from 11 AM to 11 PM PDT Sunday for
     CAZ280>282-284-285.
     Excessive Heat Watch from Monday morning through Wednesday
     evening for CAZ080>082.

Pacific Coastal Waters...Small Craft Advisory until 5 PM PDT Sunday
     for PZZ356-370-376.
     Small Craft Advisory for hazardous seas from 2 AM to 5 PM PDT
     Sunday for PZZ350-356-370-376.
     Hazardous Seas Warning until 2 AM PDT Sunday for PZZ356-376.
     Small Craft Advisory until 2 AM PDT Sunday for PZZ350.
     Gale Warning until 2 AM PDT Sunday for PZZ376.

$$

BMS/MAS/NSK



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