Gale Watch 33, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
217 PM PDT FRI AUG 12 2016

.DISCUSSION...The forecast looks on track this afternoon. Under
sunny skies, we are running a solid 4-7 degrees warmer than this
time yesterday across the board. We are going to top out between
100F and 105F in many west side valley locations with at least low
90s across most of the east side. Stratus continues to hug the
coast very closely, and will likely surge back onto the coast
quickly this evening as daytime heating wanes. If you wish to
escape the heat, the coast is a very good option. The mountains
are an option, too, They will, of course, not be as hot as the
valleys, but they will be much warmer than the coast.

The features responsible for the heat are an upper ridge and
surface thermal trough both of which are currently situated right
over us. Both features will edge further east tonight and
tomorrow, and the changes this induces will be subtle but
important. First, it will not cool off as much as usual tonight,
then tomorrow will be even hotter than today by at least a few
degrees. Due to the combination of two days of very hot day time
temperatures and little chance to recover at night, the heat
advisory continues for the interior west side valleys. Next, as
the thermal trough moves east tomorrow and the marine layer builds
offshore, pressure gradients will increase, and gusty afternoon
northwest winds will be the result. The combination of hot
temperatures, low relative humidities, and gusty winds has
prompted a red flag warning for the Rogue and Illinois valleys
tomorrow. See the fire weather discussion below for details.

The next feature of interest is an upper trough that is currently
located near 40N/140W. This trough is forecast to move in as the
ridge departs Sunday. Model consistency is fairly high with this,
and they show the main energy moving across northern California
during the time of peak heating. Moisture and instability look
sufficient for thunderstorms over the Cascades and Siskiyous, so
the limiting factor may be trigger. While the main vorticity max
goes in south of us, there should be enough dynamics in the
trough axis to spark a couple of thunderstorms. The previous shift
expanded our slight chance area to include portions of western
Siskiyou County, and this looks good. In fact, it may actually be
the most likely location at the moment considering the track of
the low.

Thunderstorms don't look sufficient to warrant any headlines for
fire weather on Sunday, but winds are expected to be even
stronger as the upper system goes by. This is especially true on
the east side, and a fire weather watch for winds and humidity has
been issued for Modoc County (details below). West side locations
won't be as hot Sunday, and humidities will be a touch higher, so
we aren't expecting red flag conditions. That said, it will
probably be a pretty breezy evening on the west side Sunday.

The forecast for next week wasn't changed much. The upper low
drops south and east of us Monday, but some broad troughing
remains over us into mid week. At the same time, additional
energy moves through and feeds into a developing closed low off
Southern California. This induces some south-southeast flow over
our area by the end of the week with monsoonal moisture starting
to play a factor over time. The signals were not clear enough to
put any thunderstorms in the forecast yet, but the models are in
decent agreement in the overall pattern, so storms certainly are
possible late next week into the weekend. We'll keep an eye on
this and add them in as confidence grows. -Wright

&&

.AVIATION...12/18Z TAF cycle...A persistent weather pattern is
expected through tomorrow. Clouds have cleared away from the coast
this morning. The marine layer LIFR conditions will return to the
coast this afternoon and continue overnight...without much inland
push because the marine layer is so shallow. Otherwise, VFR
conditions will prevail for the next 24 hours. SK

&&

.MARINE...Updated 200 PM PDT, Friday 12 Aug 2016...Winds and seas
have calmed down some since this morning. The best conditions in
terms of winds and seas for the next 7 days are pretty much from now
until Saturday morning. Another thermal trough will set up Saturday
with north winds reaching small craft conditions late Saturday
afternoon into Saturday night mainly for the southern outer waters,
but it can also affect the southern portion of the northern waters
(zone 370) and close to 10 nm from shore in zone 356. Models suggest
winds could reach gales as early as Sunday morning mainly south of
Cape Blanco and around 5 nm from shore and this could end up being a
prolonged event with gales possibly lasting into most of next week.
Keep in mind the exact location of where gales, hazardous seas and
small craft conditions could change, so watch for updates on this.
-Petrucelli

&&

.FIRE WEATHER...Updated 2 PM PDT Friday 12 August 2016...Hot and
dry conditions will continue through Saturday and will result in
some critical fire weather conditions for portions of the forecast
area. First, northwest to southeast pressure gradients will increase
some tomorrow across the forecast area, and this will result in
winds trending higher than today. Gusty afternoon north to northwest
winds in portions of the Rogue and Illinois Valleys will combine
with humidities in the low teens, likely reaching Red Flag Warning
criteria for a few hours. Additionally, even drier and gustier
conditions are expected in Modoc County. Therefore, a Red Flag
Warning has been issued for Saturday for Fire Weather Zones 620,
622, and 285.

On Sunday humidities will increase some in the West Side, so we have
not issued a Fire Weather Watch for gusty winds/low humidity there.
That said, future shifts should keep an eye on the latest guidance
to determine whether dry air will linger Sunday afternoon, leading
to the potential for another round of critical fire weather
conditions.  Across the Cascades into Modoc County afternoon
humidities will basically be the same as Saturday, and guidance
suggests winds gusting to 20 mph, so we have issued a Fire Weather
Watch for Sunday afternoon and evening for Fire Weather Zone 285.

It is unlikely that scattered thunderstorms will form on Sunday,
mainly due to upper energy splitting to the north and south of the
area, so we don't have any fire weather products for abundant
lightning for Sunday.

Looking further out into late next week and next weekend, the 12Z
GFS suggests some monsoonal moisture driving northward around an
upper low off the California coast, signifying the potential for
abundant lightning.  This is still far out in the forecast, so
confidence is low on any thunderstorms at this time, and we'll be
monitoring model trends and communicate these changes to land
management agencies and the public. SK

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from 3 PM to 8 PM PDT Saturday for ORZ620-622.
     Heat Advisory until 7 PM PDT Saturday for ORZ024-026.

CA...Fire Weather Watch from Sunday afternoon through Sunday evening
     for CAZ285.
     Red Flag Warning from 1 PM to 9 PM PDT Saturday for CAZ285.

Pacific Coastal Waters...Gale Watch from late Saturday night through
     Thursday evening for PZZ356-376.
     Small Craft Advisory from 4 PM Saturday to 11 PM PDT Thursday
     for PZZ350-356-370-376.
     Small Craft Advisory for hazardous seas until 5 PM PDT this
     afternoon for PZZ350-356-370-376.