Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 240553

Area Forecast Discussion
National Weather Service Medford OR
1050 PM PDT Tue May 23 2017

.Updated aviation discussion.

.DISCUSSION...Tomorrow will feature sharply cooler weather with
temperatures expected to drop by 10 to 15 degrees compared to the
heat this afternoon. There were daily record high temperatures
today at Medford (96) and Mount Shasta (92). A dry cold front
will move through late tomorrow. Ahead of the front the marine
layer is deepening per observations including the North Bend
profiler, and this is bringing low stratus clouds to coastal
valleys at this hour. These clouds will progress inland overnight,
reaching the Umpqua Basin before clearing out by mid morning
tomorrow. Elsewhere, generally sunny conditions are expected with
a slight chance of thunderstorms in the far southern portion of
the CWA, in extreme southern Siskiyou and Modoc Counties. There
may be just enough instability for a few lightning strikes there,
but emphasis is on the word "slight" chance for thunderstorms.

Please see the previous forecast discussion more details on the
break in hot weather.


.AVIATION...24/12Z TAF CYCLE...IFR cigs have been persistent along
the coast from Cape Blanco northward this evening, including at
KOTH. However, satellite imagery shows a very dry air mass moving in
from the north, and this will help to clear out the low clouds from
the coast early Wednesday morning. As the marine layer deepens, MVFR
ceilings are expected to impact Roseburg late tonight but should
clear out by early to mid morning as the dry air mass filters in.
Confidence is low on the exact evolution of low ceilings in the
Umpqua Basin tonight, but confidence is high that any low ceilings
that do form will clear out in the morning. Elsewhere, VFR
conditions with mostly clear skies will prevail.

North winds will be a concern at KOTH with gust to 30 knot possible
this afternoon and early evening and again on Wednesday. Gusty winds
and increased turbulence are also expected east of the Cascades
Wednesday afternoon. There is also a very slight chance of a
thunderstorm or two over Southern Siskiyou and Modoc Counties
Wednesday afternoon. SK


.MARINE...Updated 330pm Tuesday, May 23rd...A thermal trough will
strengthen over the waters through tomorrow and bring strong
winds and building seas. Winds are expected to reach gale force
from Cape Blanco south into this evening and then persist through
Wednesday night. Steep to very steep wind driven seas today will
become very steep across most the waters by this evening and
continue into Wednesday night. Winds will decrease on Thursday but
expect some continued areas of gales south of Gold Beach and
hazardous seas south of Cape Blanco. Small craft advisory level
conditions are expected north of Cape Blanco on Thursday. Winds
will subside late Thursday night into Friday morning. However,
steep fresh swell will continue into Friday for areas south of
Cape Blanco. /CC


.PREV DISCUSSION... /Issued 340 PM PDT Tue May 23 2017/

DISCUSSION...Dry and hot weather (at least away from the coast)
will continue through this evening. Right now Medford is 93
degrees and there is a chance we could tie or just barely break
the record high for today which is 95 set in 2001. The latest
visible image shows marine stratus working south along and away
from the coast. The marine stratus will continue to work south
into early this evening, but not much further south then Cape
Blanco. Elsewhere, skies will remain clear to mostly clear with a
few high clouds moving southwest to northeast and some patch
cumulus clouds along the Cascades, Siskiyous, western josephine
County, and northern Klamath and Lake County.

In the bigger picture, the latest water vapor image clearly shows an
upper trough moving into western B.C. just south of the Alaska
Panhandle. This will continue to move east-southeast tonight through
Wednesday. The main effect from this will be increased onshore flow
tonight through Wednesday. Guidance continues to suggest the marine
layer could deepen enough for patchy to areas of drizzle to develop,
so we`ll keep the chance in from about Cape Blanco eastward to Camas
Mountain and portions of southern Douglas County. Any drizzle or
light rain from the marine layer should be short lived and be a done
deal by mid morning Wednesday. The other issue is the extent of the
marine layer. Right now the thinking is it should remain just north
of the Umpqua Divide. The most significant change will be much
cooler afternoon temperatures compared to today for most inland
locations and will average 10-15 degrees cooler.

Gusty winds are expected as well east of the Cascades and westside
valleys, especially near and at Roseburg and Rogue Valley, including
Medford. However were not expecting them to reach advisory criteria.

Instability parameters are marginal at best, but the models hint at
surface convergence and there will be an air mass change as the
upper trough moves southeast through eastern Oregon. So that may be
enough for an isolated storm or two to pop up. However, anything
that would pop up will move southeast fairly quickly. Available
moisture is rather shallow, but should support some build-ups in
those areas in the afternoon/evening.

The upper low will continue to move trough southern Canada Wednesday
night through Thursday. At the same time an upper trough will move
into central Oregon and bring a cool and dry northerly flow.
Typically with this type of pattern will yield dry weather with the
best chance for precipitation staying north and east of us.
Therefore expect continued dry weather with cooler afternoon
temperatures compared to Wednesday east of the Cascades and similar
readings west of the Cascades. Skies will be mostly clear with some
cumulus buildups in eastern Klamath and Lake county in the afternoon
and early evening.

On Friday we`ll be sandwiched between the upper ridge offshore (near
130w) and upper trough in central-eastern Oregon. Dry weather will
continue with warmer afternoon temperatures for most inland
locations. -Petrucelli

Long Term...Saturday through Tuesday...The ridge will regain
control of the area, and this will produce another period of warm
and dry weather through the holiday weekend. There will be a slight
chance for daily afternoon thunderstorms, mainly along the Cascades
and in the mountains of Northern California through Monday, but
these will depend greatly on the presence of shortwave disturbance
within the flow aloft. Models suggest there could be a trough moving
into central California sometimes during the weekend. This is in
line with previous thinking and we made only minor changes to the
timing and placement of convection. For most areas, Memorial Day and
the weekend should be warm and mostly clear with no significant
threat for rain. /FB


OR...Hazardous Seas Warning until 5 AM PDT Thursday for ORZ022.


Pacific Coastal Waters...Hazardous Seas Warning until 5 AM PDT
Thursday for PZZ350-370.      Small Craft Advisory until 11 PM PDT
Thursday for      PZZ350-356-370.      Gale Warning until 5 AM PDT
Thursday for PZZ356-370.      Hazardous Seas Warning until 11 PM PDT
Thursday for PZZ356-376.      Small Craft Advisory from 5 AM to 11
PM PDT Thursday for PZZ376.      Gale Warning until 11 PM PDT
Thursday for PZZ376.


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