Area Forecast Discussion
Issued by NWS Medford, OR

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
FXUS66 KMFR 141702

Area Forecast Discussion
National Weather Service Medford OR
900 AM PST Thu Dec 14 2017

No updates are necessary this morning since the going forecast looks
to be well on track. East winds have increased across the upper
slopes and ridges and have found there way down into the 2.7kft-4kft
elevation range along and near the Cascades. Observations indicate
gusts in the 25 to 40 mph range in some areas, so expect we`ll see
warmer air this mix down to lower elevations near 2kft this

The front for tomorrow remains on track to clear out the
inversions across the forecast area, bring some light
precipitation, and return us to a more normal temperature regime
for tomorrow afternoon through this weekend. BTL


.AVIATION...14/12Z TAF CYCLE...Going with a persistence forecast,
current LIFR conditions are expected to continue through the
morning for portions of the Umpqua Basin, including KRBG, and
the lower portions of the Rogue Valley around Grants Pass. LIFR
conditions at KMFR are expected to return around sunrise, then clear
by late morning.

VFR conditions will prevail elsewhere, including the East Side,
coast, and coastal waters. Similar fog coverage and duration is
expected tonight into Thursday morning with a similar large-scale
pattern persisting. Fog coverage and duration is expected to be less
Friday night and Saturday morning. Sven/SK


.MARINE...Updated 200 AM PST Thursday 14 Dec 2017...Building high
pressure offshore will result in northerly winds increasing this
afternoon, primarily from Cape Blanco southward. Moderate west
swell will build this afternoon, combining with northerly wind waves
beyond 10NM from shore south of Cape Arago, resulting in small craft
advisory conditions. On Friday the swell will subside, and a weak
front will move onshore. Offshore high pressure builds in again
Saturday with increasing north winds that could reach small craft
advisory levels mainly over the southern waters. Confidence is
growing in this second round of small craft advisory conditions
over the weekend. North winds should subside some on Monday ahead of
a stronger cold front due to arrive Tuesday. Confidence, however,
is much lower on this part of the forecast. SK


.PREV DISCUSSION... /Issued 410 AM PST Thu Dec 14 2017/

Updated aviation discussion.


A strong upper level ridge continues to dominate the weather
pattern over the west coast states. Over the Medford CWA, this has
resulted in very dry conditions with warm temperatures and breezy
winds over the ridges. Over the west side valleys, inversions are
making for cooler conditions with little or no winds, along with
areas of low clouds and fog/freezing fog.

The ridge will weaken Friday as a short wave moves over the top of
it. This will push a weak front onshore at that time. Given the
persistence of the ridge event, it will be moving into a very dry
air mass. However, the models are trending stronger with this
system, and it now looks like the associated precipitation will be
more widespread. It will still be relatively light though.
Amounts will be less than 0.10 inches for the most part, but the
northern Cascades may see as much as 0.25 inches. There will
likely be enough cooling aloft to break the valley inversion so
there should be some decent mixing in the valleys Friday.

However, that break won`t last long. The ridge will return this
weekend as a rex block sets up offshore. Post-frontal showers will
end by daybreak Saturday, then the conditions currently over the
area will return.

Long term discussion from the Wednesday afternoon AFD...Sun
(12/17) through Wed (12/20). It looks like we chose wisely
yesterday as the 13/12Z European Center (EC) model has trended
toward the more consistent GFS solutions in the long range. While
there are still significant differences in the details, the
overall pattern evolution looks similar between the two models
through the current forecast period. Not only that, but both
deterministic solutions are becoming more similar to their
respective ensemble solutions, which is good news.

There is the potential for a significant pattern change late next
week as ensembles show the long wave ridge that has kept us dry
seemingly forever retrogrades offshore and opens the door to more
northerly flow aloft over the PacNW. This pattern would lend itself
to cold air intrusions into the Great Basin which would affect our
east side locations. This pattern would tend to be dry and keep cold
air bottled up east of the Cascades except if and when shortwaves
buckle the flow enough to drive cold air westward and produce any
over water trajectory to moisten things up. Ensembles give us
confidence in the long wave pattern (e.g. cold air on the east
side), but they average out important details like shortwaves (e.g.
cold air moving westward and snow potential). Deterministic models
can often help us with the important details, and that gets better
as we get closer in time.

Bottom line: confidence is growing in a westward shift in the upper
ridge axis and a trend toward colder weather, especially on the east
side toward the latter half of next week. The details on whether any
cold air gets over to the west side or shortwaves that could produce
snow are uncertain, but should get better with time. Prior to that,
weak upper ridging will likely keep our area dry from Sunday through
early Wednesday. Valley inversions are likely to produce stagnant
air in the valleys again during that timeframe. The forecast has
been trended in this direction with the removal of pops, adjustments
to temperatures, and the addition of night time fog. Guidance all
shows a frontal system bringing some chance for precipitation into
the area later Wednesday, and pops have been raised. Although it`s
beyond the current forecast period, after Wednesday, the ridge
likely retrogrades bringing colder air to the east side. We`ll have
to wait on any more specific details than that. -Wright


OR...Air Stagnation Advisory until 10 AM PST Friday for
     Freezing Fog Advisory until 10 AM PST this morning for


Pacific Coastal Waters...
- Small Craft Advisory for hazardous seas from 4 PM this
  afternoon to 10 AM PST Friday for PZZ370-376.


BTL/NSK/JRS is the U.S. government's official web portal to all federal, state and local government web resources and services.