Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 251618

Area Forecast Discussion
National Weather Service Medford OR
918 AM PDT Wed Apr 25 2018

.SHORT TERM...A strong ridge aloft is over the region with a
closed low positioned under the ridge off the California coast
near 39N 135W. At the surface a thermal trough has moved inland.
This pattern is resulting in very warm temperatures across the
region today for inland areas. Record or near record warm
temperatures are expected for some areas including at Roseburg,
Klamath Falls and Montague. Along the coast, an extensive marine
layer with low clouds and fog has moved up the coast from
California to the Oregon-Washington border. This marine layer will
persist today with cool and cloudy conditions along the immediate

Models continue to show the upper low moving towards the northern
California and Southern Oregon coast Thursday. Although not quite
as warm as today, expect another day of warm temperatures  on

Of concern, is the risk for thunderstorms on Thursday.
Mid level moisture will spread into the area from the south and
models show some decent instability, CAPE and steep lapse rates
developing over inland areas late Thursday. With an upper level
disturbance moving into the area ahead of the low, this will bring
a chance for thunderstorms as well as showers to the area. A cap
is expected to inhibit growth of storms until later in the
afternoon when temperatures will be warmest. So the chance for
thunderstorms will be greatest in the late afternoon and evening.
The best chance of thunderstorms is expected over the western
slopes of the Southern Oregon Cascades eastward into Klamath
County and over the Siskiyous. Also expect a slight chance to
chance across eastern Siskiyou, Modoc and southwest Lake counties.
For areas west of the Cascades, given the southeast flow aloft,
expect that thunderstorms may move into eastern Douglas, eastern
Josephine, and Jackson counties as well. Given the dry subcloud
layer with steep lapse rates, some the thunderstorms that develop
Thursday afternoon and evening may be strong and produce strong
winds, especially for storms in or near the Cascades.

Thursday night, expect continued chances for showers over much of
the area as well as a slight risk for thunderstorms from the
Cascades into Klamath county.

On Friday, the upper low will begin to move inland and expect
scattered to numerous showers over the area. A slight risk for
thunderstorms is also expected for eastern portions of the area,
mainly from the Cascades eastward and west of the Cascades in the
Shasta Valley. Instability and CAPE is weaker than on Thursday so
do not expect strong thunderstorms to develop.


.AVIATION...25/12Z TAF CYCLE...Over the coastal waters and along the
coast...A mix of IFR/LIFR conditions persist in low clouds and fog.
While low conditions will affect coastal communities, inland
progress will be minimal. There may be a brief period of improvement
in cigs along the coast this afternoon, but confidence is low in
this occurring and have maintained IFR conditions in the KOTH TAF.

Over the remainder of the area, VFR conditions will persist through
Wednesday evening. /BR-y


.MARINE...Updated 130 AM PDT Wednesday 25 April 2018...Conditions
will be relatively calm through Friday with weak low pressure over
the waters. High pressure will build in from the west Saturday into
Sunday, and small craft conditions are possible Sunday into Sunday
night as northwest swell builds. -FB/BR-y


.PREV DISCUSSION... /Issued 446 AM PDT Wed Apr 25 2018/

Near record high temperatures around 20 degrees above normal will
occur again today into Thursday. A cold core, upper level low
pressure system will then bring much cooler Pacific Ocean air
Friday through Monday. This will drop highs by 20 to 30 degrees.
Showers and isolated thunderstorms are expected across much of the
area Thursday into early next week.

A daily record high temperature was broken at Roseburg yesterday.
The new record is 88 degrees, and the OLD record was 82 degrees,
set in 1999.

For today we`re forecasting records to broken at Roseburg, Klamath
Falls, and Montague. For Thursday we`re forecasting another record
high to be broken at Klamath Falls.

A mostly closed low centered off of the West Coast between 130
West and 140 West longitude this morning is expected to be bumped
eastward tonight through Thursday. This will put the forecast
area in the unstable northeastern quadrant of the upper level low
Thursday afternoon through Friday. Statistical model guidance is
indicating high probabilities of measurable rainfall across much
of the forecast area Friday through Sunday. While statistical
probabilities are high, the deterministic models still vary quite
a bit on the magnitude of precipitation amounts and the location
where the precipitation is likely to focus. The consensus of the
guidance indicates the greatest precipitation probabilities to be
along and near the Cascades, across Coos and Douglas counties, and
across the mountains. 4 day precipitation totals are expected to
be over an inch for portions of the Siskiyous, Umpqua Divide, and
Douglas County. Most inland areas are forecast to receive a
quarter of an inch or more of rain from this weather system.
Confidence regarding precipitation amounts is currently low also
due to the hit and miss nature of showers and thunderstorms.

The greatest threat for thunderstorms is Thursday evening along
and near the Cascades eastward, bumping south and east Friday
through Friday evening, and then lingering on the east side over
the weekend.

Snow levels are expected to lower below 9kft Friday morning,
lowering to between 4kft and 5kft over the weekend and into early
next week. Current new snow amounts where we still have snowpack
are expected to be 3-6 inches, with 3 inches or less down to about
3500 feet. Snow ratios have been kept on the low side, more
consistent with climatology than model guidance. The current warm
period and high water content in the existing snowpack is likely
to limit accumulations from what they otherwise would be due to
energy and moisture transfer between the old and new snow. Travel
problems are generally not expected on area roadways due to warm
road surface temperatures. However, there is some slushy road
potential for brief periods of time during and immediately
following heavier snow showers.

One other minor concern will be the potential of frost across
some West Side areas Monday and Tuesday mornings. The coldest
morning currently appears to be Tuesday morning due to partial
clearing expected Monday night. Still, forecast minimum
temperatures are within about 5 degrees of daily normals, so the
only real concern is for those trying to get an early start on
summer gardens. BT




Pacific Coastal Waters...None.


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