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afdmfr

Area forecast discussion
National Weather Service Medford or
1035 PM PDT Thu Apr 27 2017

Updated aviation discussion

Discussion...the main update this evening was to lower
temperatures west of the Cascades. After assessing cloud cover
trends and latest MOS guidance compared to observations, we think
there is a good chance that temperatures will dip to freezing in
colder portions of Josephine and Jackson County valleys tonight.
Since we think that readings will bottom out at 32f and not lower
and thus likely only for a few hours at most, we are not issuing a
freeze warning. Still, the goal of the forecast update and
addition of graphics to our website is to bring attention to the
potential of brief freezing conditions tonight, and agricultural
interests can take necessary precautions.

Please see the previous discussion below for details on the
transition to drier and warmer conditions early this weekend and
again next week.

&&

Aviation...28/06z taf cycle...moisture in northwest low level flow
tonight is banking up against higher terrain, especially northwest-
facing slopes. Ceilings have scattered out at the coast and should
remain generally VFR through the night. MVFR conditions are expected
in and around Roseburg early Friday morning as moisture pools there
overnight. Elsewhere, mostly VFR conditions are expected. Again,
partial mountain obscuration is expected especially in the Cascades,
Umpqua Divide, and Siskiyous. By tomorrow afternoon, all locations
should be VFR under a drying air mass. Sk

&&

Marine...updated 200 PM PDT Thursday, 27 April 2017...weak high
pressure will persist over the waters into early Friday with
slowly diminishing moderate seas. The models are in good agreement
showing small craft conditions primarily for the southern waters.
However it's possible portions of the northern waters south of
Bandon could experience small craft conditions. Confidence was not
high enough to include the southern portion of the northern
waters, but later shifts will need to take another look at this.
Gusty north winds will increase Friday afternoon as high pressure
strengthens offshore while a thermal trough develops along the
coast. Winds will be strongest from 8 to 30 nm from shore in the
waters south of Gold Beach. The pattern will weaken slightly on
Saturday but strengthen again early next week with advisory
strength northerly winds likely to develop. -Petrucelli

&&

Previous discussion... /issued 230 PM PDT Thu Apr 27 2017/

Discussion...

Short term...tonight through Sunday night...although satellite
imagery doesn't show it all too well, satellite and surface
observations show numerous light showers moving across the
forecast area this afternoon. These will diminish through the
evening as the jet stream axis and upper level trough slide to the
east. High pressure and upper level ridging will then build in
over the eastern Pacific, and this will be the primary driver of
our weather for the next several days. Also of note, the clear
skies and residual cool air in the area may produce some isolated
areas of frost in the rogue, Applegate, and Illinois valleys
overnight tonight into early tomorrow morning.

With the ridge placed to our west, and a general trough over the
central United States, we will reside under swift northwest flow
aloft for the duration of the term. The ridge will deflect most
energy and precipitation producing systems to our north, leaving
the area dry and warmer. However, the trailing edge of a front
passing to our north on Saturday night into Sunday will skirt the
northern half of the forecast area, producing showers and
slightly cooler temperatures, and also disrupt the thermal trough
pattern that will periodically produce much warmer temperatures
over the ridges and along the southwest Oregon coast near
Brookings. The models have not been consistent on the strength of
the front as they pass to our north, nor have they agreed on where
the best chances for showers will be. So, have kept a slight
chance to a chance of showers in some areas and expanded coverage
to others in an effort to more closely align with the latest
guidance, but suspect this may change in later forecast cycles.
Otherwise, expect gradually warming temperatures through the week.
-Bpn

Long term...Monday through Thursday...the area will be under the
influence of an upper ridge Monday through Wednesday with mainly
warm and dry conditions. A weak disturbance is expected to move over
the ridge on Tuesday, and will bring a slight chance for light
precipitation to northern portions of the area.

Wednesday, models and ensembles continue to show the upper ridge
will build inland over the area and bring dry and very warm
conditions to the area. Highs in the upper 70s to low 80s are
expected in western valleys with in the low 70s possible for eastern
valleys.

Thursday, the upper ridge is forecast to shift eastward while a
shortwave trough moves into the region. This trough combined with
southerly moisture and instablity will bring a slight chance of
showers and thunderstorms to inland locations. -Cc

&&

Mfr watches/warnings/advisories...
or...none.

California...none.

Pacific coastal waters...Small Craft Advisory from 11 am Friday to 5

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