FXUS66 KMFR 300300

Area Forecast Discussion
National Weather Service Medford OR
800 PM PDT Mon May 29 2017

.DISCUSSION...30/00Z NAM in.

The Medford CWA is under southwest flow aloft between a long wave
ridge over the intermountain states and a trough offshore between
140W and 150W. Several factors came together to promote
thunderstorm development this afternoon and evening.

The day shift summed it up nicely, but basically increased PWs
combined with instability and surface heating provided the basic
ingredients while an upper level short wave enhanced the
convective environment upstairs. At this time moisture is still in
place...but the surface heating is waning and the air mass is
stabilizing at lower levels. There are still some thunderstorms in
progress...mostly over southern Douglas County...but those should
end soon.

A short wave ejected from an offshore long wave trough and an
associated cold front will move through the area Tuesday. This
will bring in much cooler air and a slight chance to chance of
showers and thunderstorms. Unlike Monday, a most of the
precipitation will come from showers rather than thunderstorms.
An marine layer intrusion will stabilize the west side...and
precipitation there will be in the form of light rain and drizzle
tonight into Tuesday...then showers. The best instability will
be east of the Cascades where the maximum PVA will be...moving
farther east during the day. That's where the thunderstorms will
develop. Precipitation amounts will be light for the most
part...0.10 inches or less. It will also be breezy across the
area with gusts up to around 30 mph over portions of the East

Tuesday highs will be quite a bit cooler that the Monday highs
over the Oregon west side. Highs will be around 5 degrees below
normal along the coast and in the Umpqua Basin...near normal over
the rest of the Oregon west side. Meanwhile it will still be warm
over the east side and northern California...with highs 5 to 15
degrees above normal.

The showery regime will continue into Wednesday with more short
waves moving through. The air mass will continue to cool and
stabilize with almost all of the precipitation in the form of
showers...but an afternoon/evening thunderstorm or two may still
get going east of the Cascades. A number of the models have been
depicting an enhanced area of rainfall, with amounts around a
tenth of an inch or more, over portions of the East Side Wednesday
morning into the afternoon. Eastern Siskiyou, Modoc, southeastern
Klamath, and Lake County all stand at least a chance of wetting
rainfall from this feature.

Highs will be near normal along the coast and over the west
side...and 5-10 degrees below normal over the east side.

Flat ridging will nudge into the area from the south Thursday...
bringing more stable and warmer conditions to the area. There will
be some lingering showers Thursday...but those will come to an
end Thursday night. Highs will be near normal values over most
if not all of the Medford CWA.

Long term discussion from the Monday afternoon AFD...Friday
through Monday. The GFS and the EC are in good agreement Friday
morning with a weak ridge over the coast washing a trough out just
offshore. On Saturday both operational models are in agreement
with the ensemble means, with an upper level trough beginning to
deepen offshore. However, as we move forward into Sunday the
models flip positions on what they were showing yesterday, with
the GFS showing a somewhat deep upper trough and the EC showing a
transition from zonal flow to a building ridge later Sunday. Both
models ensemble means are indicating a weak upper trough with the
GFS's deepening of the upper trough into a closed low in far
eastern OR being a far outlier from the ensemble means, but by
this time the ensemble plots are very noisy and confidence is low
for any solution as we start early next week. Sven


.AVIATION...30/00Z TAF CYCLE...A mix of MVFR/IFR ceilings will
persist over the coastal waters and along the coast tonight with
some fog and drizzle. Scattered thunderstorms inland this evening,
some accompanied by gusty winds and hail, will mostly end overnight,
though a few showers may linger. Overall, expect VFR inland, except
in the vicinity of thunderstorms where conditions may lower
temporarily. MVFR ceilings will fill back into the Umpqua Valley
overnight. Expect MVFR ceilings along the coast and into the Umpqua
most of Tuesday with some showers west of the Cascades. Southwest
winds will become gusty over the East Side Tuesday afternoon with a
slight chance of thunderstorms. -Spilde


.MARINE...Updated 800 PM Monday 29 May 2017...North winds
associated with a weak thermal trough are now peaking...and will
subside overnight. Several weak disturbances will move through
Tuesday through Thursday resulting in relatively light, variable
winds and slight seas. The thermal trough will return Friday night
and persist into the weekend with increasing north winds and
building seas, highest south of Cape Blanco.




Pacific Coastal Waters...None.


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