Wildfire in Oregon, August 10 to 31, 2017

Area Forecast Discussion
National Weather Service Medford OR
757 AM PDT Thu Aug 10 2017

.DISCUSSION...An update is not necessary. The 12Z NAM and 06Z GFS
data support the short term forecast with moderate to strong
instability developing this afternoon and evening to support
scattered to numerous thunderstorms. As with yesterday's activity,
storm motion will be a little slower than normal. Typically,
southeast at 5 to 15 MPH. The relatively slow storm motion and
increase in available moisture will produce downpours with the
potential for large hail.

The pattern will begin to change on Friday, but a slight chance of
thunderstorms will persist in the afternoon and evening from the
Siskiyou and Cascade Mountains northeastward.

&&

.AVIATION...10/12Z TAF CYCLE...Along the coast and over the coastal
waters...IFR may thin/lift to MVFR for a while this afternoon, but
IFR will return this evening.

For inland areas, areas of MVFR visibilities due to smoke
will persist. Smoke is mainly in portions of Douglas, Jackson,
Siskiyou, and Modoc Counties. Thunderstorms will develop again this
afternoon into this evening. This includes the possibility of gusty
winds, hail and lightning, primarily near MFR and LMT. /FB

&&

.MARINE...Updated 230 AM PDT Thursday 10 August 2017...Winds
will remain highest south of Cape Blanco through early next week.
High pressure offshore and low pressure inland will persist through
Saturday. North winds will increase modestly into Saturday as the
high moves closer to shore. The approach of a cold front will likely
bring shifting winds Saturday night into Sunday. Much stronger north
winds and rapidly rising seas are likely to follow early next week
with gales possible south of Cape Blanco. /FB

&&

.PREV DISCUSSION... /Issued 225 AM PDT Thu Aug 10 2017/

DISCUSSION...An outflow boundary from collapsing thunderstorms
just to the west of the Cascades from highway 140 northward caused
a swath of thunderstorms to develop rapidly along I-5 this evening
that moved west before weakening in about an hour. The wobbling
low that has kept an active thunderstorm pattern over the area for
what seems like an endless stretch moves inland this afternoon
directly over the forecast area. With PW values rising above 1.25
over the west side this afternoon and weak steering flow,
thunderstorms may put down significant rainfall in places with
slow moving thunderstorms that may latch to higher terrain early
afternoon. Steering flow is expected to increase later this
afternoon as the upper low moves over the extreme southern
Cascades according to the GFS. Confidence is not high with the
exact location of the upper low as the NAM, only 30 hours into the
run, shows a more westerly location of the upper low with steering
winds much weaker than the GFS. However, both are showing a string
vorticity axis swinging around through the area around max
heating and into the evening. With all this in mind this afternoon
into this evening looks to be another very active thunderstorm
event to add to the already long stretch of thunderstorms. The
storms that do develop should be very wet and many will contain
hail, some possible reaching an inch in diameter or larger.

On Friday models have the upper low opening and being absorbed
into an approaching upper level trough that approaches the west
coast this weekend. Instability remains in roughly the same place
that it has been over the past several days on Friday, but with
an upper trough offshore instead of a closed low over the region
the steering flow is southerly across the are instead of easterly
and short wave move from south to north instead of east to west,
and this should decrease the amount of thunderstorms west of the
Cascades. Saturday will see the upper level trough move into the
Pacific Northwest with the instability moving into the usual
western Siskiyou county and along the Cascades and across the
east side pattern. The upper trough continues to shift east,
pushing the instability east of the region. Sven

FIRE WEATHER...Updated 230 AM Thursday 10 August, 2017...
Thunderstorm threat remains high again today as the low moves inland
over the area with plenty of shortwaves through the area. Moisture,
instability will be similar to yesterday, so we can expect another
round of thunderstorms this afternoon into this evening. Steering
flow is expected to be from the southeast, so there's a chance for
some storms to migrate off the Cascades and move northwest into
portions of the west side, including Fire Weather Zones 616, 620 and
622. There's also a chance a few storms that develop over the
Siskiyous could make their way into Fire Weather Zone 619, but
confidence is not as high for this.

Model consensus continues with a cooling trend and a period of more
stable conditions from Saturday through early next week. However
instability will be marginal along the Cascades and Siskiyous
Saturday afternoon, so could not rule out a slight chance of
thunderstorms in these areas.

Sunday could be problematic east of the Cascades, especially for any
new/existing fires, as windy conditions are possible with low
humidity ahead of the approaching trough. /FB

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from noon today to 10 PM PDT this evening for
     ORZ624-625.
     Red Flag Warning from 2 PM this afternoon to 10 PM PDT this
     evening for ORZ616.
     Red Flag Warning from 2 PM this afternoon to 10 PM PDT this
     evening for ORZ617-620>623.

CA...     Red Flag Warning from noon today to 10 PM PDT this evening for
     CAZ280>282-284.
     Red Flag Warning from noon today to 10 PM PDT this evening for
     CAZ285.

Pacific Coastal Waters...None.

DW/MAP/FJB