FXUS66 KMFR 240359

Area Forecast Discussion
National Weather Service Medford OR
859 PM PDT Mon Apr 23 2018

.DISCUSSION...The going forecast is on track and no updates were
made this evening. See previous discussion below.


.AVIATION...24/00Z TAF CYCLE...VFR conditions under clear skies will
prevail into tonight. Areas of IFR cigs/vsbys will likely develop
tonight over the coastal waters and along the coast as the surface
trough has moved north of Cape Blanco. The latest guidance is quite
aggressive on IF conditions at OTH and we are leaning toward the
worsening condition there. Confidence is not yet high on OTH going
down to IFR but they could be IFR conditions in the area. Have
introduced very low cloud base but kept it as a scattered deck with
MVFR vis in fog around 10-16Z. The remainder of the area will remain
VFR through Tuesday morning. -FB


.MARINE...Updated 830 PM PDT Monday 23 April 2018...Winds and seas
will be relatively light tonight as a thermal trough continues to
weaken near the coast.

Seas will increase Tuesday morning and afternoon to around 8 feet at
15 seconds as west swell builds into the waters, but this will
likely remain below small craft advisory levels. Seas will then
subside Tuesday evening through Wednesday. Meanwhile winds will be
light Tuesday and Wednesday with weak low pressure over the waters.

Winds and seas will remain relatively light Thursday, but winds will
become southerly. Northwest swell will move in late Friday, raising
seas a bit. Heavier swell will arrive next weekend and seas could
build to small craft level Sunday. -FB


.PREV DISCUSSION... /Issued 458 PM PDT Mon Apr 23 2018/

DISCUSSION...The story over the next few days will be a rapid warm
up starting today and lasting through Wednesday with the warmest max
temperatures so far this season. In fact we could see max
temperatures ranging between 15-20 degrees above normal Tuesday and

Skies will remain clear through this evening over most of not the
entire forecast area. Temperatures compared to this time yesterday
are much warmer over most locations, including the coast. Currently,
Brookings is 78 degrees courtesy of an offshore flow which is
expected to into this evening, so 80 degrees or higher is not out of
the question by late this afternoon.

The latest surface analysis shows the thermal trough situated just
off the coast with the trough axis running north all the way to the
Washington coast. The thermal trough will begin to weaken tonight
and there's good agreement we'll have a coastal reversal setting up
tonight through Tuesday. However there is increasing evidence the
marine stratus could be delayed along the south coast until early
Tuesday morning. This is being supported by lower boundary layer
relative humidity among the models. Also model soundings at
Brookings are dry at the low levels this evening, then show a very
shallow moist layer towards daybreak Tuesday. Given the above
mentioned reasoning, have adjusted the cloud cover along the coast
tonight into Tuesday and removed the slight chance of drizzle. The
marine stratus is expected to slowly march northward Tuesday through
Tuesday night.

Meanwhile dry and very warm weather will continue through Wednesday
for many inland locations as the thermal trough shifts inland over
the west side valleys. Wednesday should be the warmest day of the
forecast period with a few spots touching 90 degrees in the Rogue
Valley, mid to upper 80s in the Umpqua Basin, and upper 70s to near
80 degrees east of the Cascades.

The next area of concern is the location of the upper low that will
be lurking offshore Wednesday night through Thursday. There's better
agreement among the models with the position and track of the cutoff
low, but still some differences. The low will slowly inch eastward
towards our forecast area Wednesday night through Thursday. None of
the models are showing any obvious signs of instability ahead of the
cutoff low and the best dynamics will still be offshore, so we'll
keep it dry Wednesday night into Thursday morning.

Things could interesting on Thursday, especially in the afternoon
and evening. The general consensus is for increasing instability
instability along with a trigger mainly east of the Cascades. The
NAM may be too bullish with the amount of instability along the
Cascades from Crater Lake north, especially given the fact there is
still some snow pack there. Suspect the instability depicted by the
NAM is mainly elevated. Therefore we'll keep the slight chance of
thunderstorms Thursday afternoon in Klamath and Lake County.

The coverage area of showers are expected to increase Thursday night
as the upper low moves closer to the Oregon Coast. -Petrucelli

LONG TERM...Friday through Monday...Model guidance looks a lot
better in the long range than it did yesterday. There is general
agreement in an upper low approaching our coast on Friday, and
deterministic solutions look similar to each other as well as their
respective ensembles. As a result, confidence in the beginning of
the long term forecast has increased.

What this means in terms of weather on Friday is a marked increase
in the potential for showers, noticeably cooler weather, and
lowering snow levels. There are, of course, some difference in the
details, but regardless of the model choice, southerly flow ahead of
the upper low should target the mountains over central Siskiyou
County on up into the Oregon Cascades for the most shower activity.
In addition, instability and dynamics will be favorable for
thunderstorms on Friday afternoon and evening, especially in the
mountains and east side. Under a southeast flow pattern aloft, it's
not inconceivable that a storms drifts out over the west side, but
the chance of that is low enough that we'll keep it out of the
forecast for now. Just keep in mind that it's possible on Friday.

Like I mentioned yesterday: model differences notwithstanding, the
low will move in over the weekend and result in cool, showery
weather area-wide. It's really only a question of whether it comes
right in and produces a lot of valley rain and mountain snow showers
or, as upper lows often do, drifts in south of us and deals us only
a glancing blow. I tend to believe in more of a direct hit from the
upper low, and I have raised pops a little and lowered temperatures
a lot. There will still be a chance for thunderstorms over the
weekend, but it doesn't look overly exciting due to instability-
limiting factors such as cloud cover and mountain snow pack. Early
next week we get into drier northwest flow behind the departing
upper low which will help us warm up and dry out. -Wright




Pacific Coastal Waters...None.


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