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fxus66 kmfr 131748 
afdmfr

Area forecast discussion
National Weather Service Medford or
948 am PST Wed Dec 13 2017

Update...
no updates are needed this morning as the going forecast for
today through tonight is well on track. Fog is eroding a bit
faster than 24 hours ago in the Rogue Valley so expect it be a
bit sunnier and warmer today on the valley floor there.

&&

Discussion...
the blocking high pressure ridge continues, but a weak frontal
system was able to push in over Washington and British Columbia
into the northern rockies in the past 24 hours, weakening the top
of the ridge some. Models have trended a bit wetter, especially
the 12z GFS, for Friday afternoon and evening, so we're likely to
be expanding some mention of precipitation in the forecast for
that period, though we'll check out some of the other model
guidance before making that decision. The main effect of this
frontal system will be to cool temperatures aloft and to freshen
up valley air as the inversions mix out.

Beyond that, a weather system moving in from the northwest mid to
late next week is expected to weaken the ridging further. Guidance
indicates the blocking ridge is likely to shift north and west
yielding a colder period of weather for week 2 (dec 20th-26th),
but little precipitation.

That trend with the movement of the ridging northwestward toward
western Alaska is one that we'll be hoping continues as we move
into the end of the month so that we see the storm track return to
the area. Confidence is low in the forecast beyond day 7, and
even more so beyond day 14, as previous similar scenarios in the
climate record have shown a myriad of solutions. However, the
northwestern drift of the ridging appears to be the most likely
scenario and is what ensemble models are generally indicating. Btl

&&

Aviation...13/12z taf cycle...fog and freezing fog have returned to
the same areas this morning and will persist into the morning before
improving late this morning and this afternoon. Medford is currently
reporting MVFR visibility, but we expected LIFR visibility in
freezing fog to form between 13z and 15z. Confidence is moderate on
timing, and it may take until sunrise (like yesterday) before the
quarter mile visibilities occur. 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. Sk

&&

Marine...updated 200 am PST Wednesday 13 Dec 2017...relatively
light winds for this time of year will persist through the rest of
the week. North winds will persist and will be strongest south of
Cape Blanco, but they are expected to remain below Small Craft
Advisory level. Moderate west swell will build this afternoon
through Thursday and subside Friday, but seas should remain below 10
feet. A weak front will move onshore Friday, followed by offshore
high pressure Saturday. North winds will increase Saturday and could
be strong enough to reach Small Craft Advisory levels in the
southern waters. -Sven/sk

&&

Previous discussion... /issued 400 am PST Wed Dec 13 2017/

Updated aviation discussion.

Discussion...13/00z NAM/GFS/ec in.

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 and there will be little or no precipitation associated
with it. Amounts will be less than 0.05 inches and the bulk of the
precipitation will be limited to the coast and Umpqua basin.
However, a few renegade showers may develop elsewhere along the
front. There may be enough cooling aloft to break the valley
inversion so there may 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. This will bring a return
of the conditions that are currently over the area. The 13/00z ec
shows a weak front moving onshore next Tuesday and Tuesday night
but the 13/00z GFS remains dry. The models, especially the ec,
have been inconsistent with this feature, so will not change the
current forecast for Tuesday.

&&

Mfr watches/warnings/advisories...
or...air stagnation advisory until 10 am PST Friday for
orz023-024-026-029>031.

California...none.

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