Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 281610

Area Forecast Discussion
National Weather Service Medford OR
910 AM PDT Sun May 28 2017

An update was sent a short time ago to refine low cloud and fog
coverage and forecast for this morning through this evening. The
possibility of thunderstorms was also increased north of the
Crater Lake Rim to chance (33%) and slight chance wording was
expanded to Chemult and areas along and west of Highway 97 north
of Chemult. BTL


The marine layer has infiltrated the Roseburg area this morning as
well as many of the river canyons, to include past Agness in the
Rogue River Valley. Fog and low clouds will recede coastward this
morning in most of the narrower river valleys, but will be slower
to leave the Roseburg area, and is likely to remain in place along
and near the coast through the day today.

Model guidance indicates plenty of instability developing over
the Cascades and Siskiyous, with 850mb computed LIs of -4C on the
GFS and -5C to -7C on the NAM12 late this afternoon into the early
evening. However, moisture appears marginal in the mid levels and
it is debatable whether there is any trigger aside from surface
heating. Our Medford morning sounding does indicate decent 0.79"
PWAT, with plenty of moisture below 600mb. Anyway, the bottom line
is that there are mixed signals for thunderstorms today and,
while the previous forecast did a good job of depicting areas of
most concern, there is an increasing possibility of showers and
thunderstorms forming just north of or near Crater Lake later this
afternoon into early this evening, drifting north to
northeastward this evening.

The possibility of thunderstorms increases in both area and
probability across the forecast area the afternoon and evening of
Memorial Day. BTL


.AVIATION...28/12Z TAF CYCLE...Along the coast and over the coastal
waters, MVFR/IFR cigs will prevail over the next 24 hours. Patchy
drizzle is possible as well. Over the Umpqua Basin, MVFR/IFR
cigs/vsbys in have spread to KRBG, where there will be MVFR ceilings
between 1300 and 1800 UTC. The lower conditions will burn back to
the coast after 18 UTC. Over the remainder of the area...VFR will
prevail through Sunday evening... but some cumulus will develop over
the Cascades and Siskiyous with isolated afternoon and evening
thunderstorms possible. -BPN


.MARINE...Updated 200 AM Sunday 28 May 2017. The marine layer
will remain quite deep over the coastal waters today, resulting in
widespread stratus and fog that could reduce visibility to less than
a half of a mile at times. Drizzle is also possible. Winds and seas,
however, will remain light.

North winds will increase with the redevelopment of a thermal trough
late tonight. They will peak (likely below small craft advisory
levels) on Monday, before weakening as a disturbance moves onshore
Tuesday. Additional weak disturbances will keep generally light,
variable winds and low seas over the area Wednesday/Thursday and
perhaps even into Friday. At this point it looks like the thermal
trough will eventually return late in the week or next weekend.


.PREV DISCUSSION... /Issued 224 AM PDT Sun May 28 2017/

DISCUSSION...A ridge of high pressure remains over the PacNW
today and will produce another mostly sunny, hot, dry day similar
to yesterday. One exception is along the coast where extensive
and stubborn stratus will keep rather cloudy, cool weather in
place. Another possible exception will be over the mountains (the
Cascades and Siskiyous, in particular) where there is at least
some chance of an afternoon thunderstorm. It will certainly be
unstable enough, but a general lack of moisture and forcing will
probably limit any convection to cumulus buildups. However, a
thunderstorm could poke through in the mountains, so the slight
chance was left in.

Conditions for thunderstorm development improve tomorrow. It will
remain hot and unstable, but moisture will increase, and
shortwaves ahead of an approaching trough will provide some
forcing to get things going. Guidance all continues to point at
the Cascades and Siskiyous as the most likely places to see
storms, but steering flow that is from the S-SE at 20-30 knots
could push a storm out over the west side valleys late in the day.
This was already well covered in the going forecast, so few
changes were made.

Changes were made to Tuesday, however, as the upper trough comes
ashore. There is a little better agreement among the guidance
that Tuesday will be noticeably cooler and more stable west of the
Cascades, so thunderstorms were pulled in favor of regular old
showers. The mountains and east side still stand a good chance for
showers and thunderstorms Tuesday, and pops were increased,
especially in the mountains. The GFS does remain a wet outlier,
but it is not quite as bullish as it was, and other models have
trended toward it as well.

From Wednesday on into next weekend, the models are in decent
agreement in showing general zonal flow with little chance for
precipitation and no obvious hot or cool spells. -Wright




Pacific Coastal Waters...None.


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