Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 231534

Area Forecast Discussion
National Weather Service Medford OR
834 AM PDT Mon Apr 23 2018

.DISCUSSION...Other than a few high clouds east of the Cascades,
skies are clear and will remain that way through this evening.
Other than making adjustments to the sky cover today, the rest of
the forecast is on track and no other changes are needed. Today
we`ll look at the potential for showers later in the week as the
upper low moves closer to our forecast area. Please see previous
discussion below for details on the forecast thinking through
Wednesday. -Petrucelli


.AVIATION...23/12Z TAF CYCLE...VFR conditions under clear skies will
prevail today through tonight. The only exception may be the
Coquille Valley, where there may be patch IFR cigs/vsbys in low
clouds and fog early this morning. Areas of IFR/MVFR cigs/vsbys are
possible along the coast tonight, mainly from Cape Blanco southward.
Expect continued VFR at KOTH through tonight due to breezy northeast
flow continuing over Coos and western Douglas counties. -CC


.MARINE...Updated 830 AM PDT Monday 23 April 2018...Winds and seas
will gradually lower today as the thermal trough gradient weakens
along the southern Oregon coast. There will be steep seas across
much of the waters this morning along with areas of small craft
advisory level winds, mainly from Cape Blanco south beyond 10 nm
from shore. Steep seas will linger into the early afternoon for
areas south of Cape Blanco before subsiding.

Seas will increase Tuesday morning and afternoon as west swell
builds into the waters. Seas are expected to remain just below small
craft advisory levels with seas around 8 to 9 feet at 15 seconds on
Tuesday. 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 some.


.PREV DISCUSSION... /Issued 229 AM PDT Mon Apr 23 2018/

DISCUSSION...Today through Wednesday night...The short term will
be dominated by  a building ridge in the upper levels, a surface
thermal trough and a cut-off low that develops offshore Tuesday
that lingers offshore through at least Thursday. This results in
the main story for the short term being very warm temperatures
that are more typical for late June into July than late April.

With the exception of some thin high level clouds, skies are clear
for across the forecast area this morning thanks to offshore flow.
The thermal trough will remain along the coast through late this
morning and east to northeast flow will bring another round of warm
temperatures to the coast today. The thermal trough will expand
north and push inland this afternoon and this will turn the flow
along the coast to onshore by this evening. Because of this, today
should be last relatively warm day along the coast and temperatures
will moderate Tuesday onward. A coastal wind reversal still looks
likely late tonight. This will lead to a surge of marine stratus
moving south to north along the coast. Timing of these reversals is
usually difficult to pin down, and have left the previous thinking
as is with stratus reaching Brookings some time around midnight and
possibly reaching Port Orford by daybreak Tuesday.

Away from the coast, temperatures will begin a rapid warming trend
with highs today about 10 to 15 degrees warmer than they were Sunday
west of the Cascades. East of the Cascades, the warm-up will be
slightly less dramatic with highs of 5 to 10 degrees warmer today
than Sunday. The thermal trough will continue to shift inland
Tuesday and Wednesday and the warming trend will continue. Wednesday
looks to be the warmest day this week, with some inland valley
locations flirting with the 90 degrees mark.

Let`s not forget the cut-off low that`s offshore Tuesday into
Wednesday. There are still positional differences between the models
with the GFS continuing to have the low closer to shore Wednesday
than the EC. Although the GFS still indicates some instability
Wednesday afternoon, there doesn`t appear to be sufficient moisture
for showers/thunderstorms to develop. As such, have removed the
chance for showers for Curry and western Siskiyou counties Wednesday
afternoon and evening. The low will begin moving towards the coast
Thursday and this should begin a cooling temperature trend. The
uncertainties of the progression of this low are covered well in the
extended discussion from Sunday afternoon and follows.

LONG TERM...Thursday through Sunday...An upper low approaching from
the west will probably begin to cool us down and bring a chance for
precipitation Thursday. I say probably, because while the models all
agree on the existence of this low, there is a lot of difference in
the position and movement the low in the deterministic models. For
example, by Thursday afternoon, the GFS has the center of the low
approximately 300 miles closer to us than the European model.

While there are also differences between the various model
ensembles, they are not as extreme. That said, ensembles from both
the GFS and European do show a fair amount of spread, and most of
this spread is on the continental side of the low, and this seems to
indicate that the deviant members tend to be closer to us than the
mean (although this is very hard to see in the spaghetti plot). In
other words, there is some support for the closer operational GFS
solution. Needless to say, confidence in the evolution of this low
is not high. As a result, I haven`t made any extreme changes to the
extended forecast, but rather just simply nudged everything toward
the general model consensus.

While confidence in the details isn`t very high early on in the
extended, I think we can say with high confidence that the low will
move in Friday and over the weekend, and that this will result in
much lower temperatures than in the short term as well as lower snow
levels and a good chance for showers. We may also see some
thunderstorm activity, although this is complicated by snow cover in
the mountains (which dramatically lowers surface-based instability).
The most promising day for thunderstorms looks like Friday, since
upper dynamics are the greatest. For the remainder of the weekend,
thunderstorm chances are focused on the east side where there is
little to no snow cover remaining. -Wright




Pacific Coastal Waters...Small Craft Advisory until 11 AM PDT this morning for

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