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Area forecast discussion 
National Weather Service Medford or
901 PM PST sun Dec 28 2014

Discussion...an Arctic front is draped across Washington state
this evening with weak low pressure over the olympic peninsula.
The front and area of low pressure will move steadily southward
overnight into Monday crossing the entire forecast area by Monday
evening. Upper level support for the front is strong, but since
the bulk of this system will be taking a track over land, it will
not have a significant supply of moisture. Even so, latest
satellite and radar imagery are showing plenty of cellular
showers over both Portland and Seattle cwas this evening and
expect this to be the case later tonight and especially Monday as
the front moves southward through Medford County Warning Area. The 00z NAM and GFS
haven't given ME much reason to deviate from the going forecast,
but there is the potential for low elevation snow tomorrow and
this could be problematic should it occur. Right now, this is
well covered by the Special Weather Statement (spsmfr) and will
let it up to the middle shift to see if any advisories are necessary.

After the front moves through, a very cold air mass will move in
at midweek (tue-thu). In Medford, highs will probably be no
better than the 30s with lows in the teens. Coastal areas will
have nights down near and below freezing...see freeze watch at
npwmfr...and...in some cases...east side areas will have low
temperatures below zero along with bitter wind chills. Spilde

&&

Aviation...for the 29/00z taf cycle...showers will continue across
the area through the morning. Late this evening through the
overnight MVFR ceilings are expected to return to the region, lowering
to IFR and isolated LIFR in showers and patchy fog overnight into
early Monday morning. Widespread mountain obscurations late tonight
into Monday morning as a secondary cold front pushes into the area.
Sven

&&

Marine...updated 212 PM PST December 28, 2014... northwest 10 to
12 foot combined seas are present primarily areas north of Cape
Blanco. Another cold front will move in from the north on Monday
bringing showers and gusty northwest through northeast winds.
Freezing temperatures are expected near the shore north of Cape
Blanco Monday...Tuesday and Wednesday nights...and near the shore
from Cape Blanco south on Tuesday night. Moderate north to
northeast winds and steep seas are expected Monday night through
Wednesday. Sven

&&

Previous discussion... /issued 244 PM PST sun Dec 28 2014/

Short term...things dried out right on schedule today, and it's
turned out to be a fairly sunny and seasonable afternoon for many
areas. The exception is in the upslope mountain areas where
clouds and isolated showers are hanging in.

The upper trough and Arctic front that we've been talking about
are currently chugging south through southern British Columbia.
Interestingly, we can see the cold air blowing out toward the
southern b.C. Coast on infrared satellite and winds shifting to the east
in the observation as the front passes.

This system will move through our area tonight and Monday. Since
it is almost entirely over the continent right now and it won't
move out over water much at all, it isn't picking up much
moisture. The system will be fairly dynamic and it will produce
strong cold advection, but it likely won't produce much
precipitation. Haven't changed the going forecast for rain/snow
accumulations at all today, and none are expected to reach
advisory criteria.

That said, it is still possible to see some snowflakes down into
the higher west side valleys as cold advection really kicks in
Monday afternoon and snow levels fall. By that time, though,
precipitation will mostly be done and the airmass drying out. Some
accumulation is possible in the usual spots like Hayes Hill and
Sexton Summit, but should be spotty elsewhere. Confidence in the
location of any low elevation snow is low, so we opted to expand
the Special Weather Statement rather than issue any advisories for tomorrow.

The more certain aspect of this storm is the very cold air coming
behind it. Cold air will overspread our entire area on Tuesday and
remain in place through early Thursday. Temperatures will be well
below normal through middle to late week, and with some gusty winds,
we will also see cold wind chills, especially over the mountains
and east side. We are not quite expected to meet wind chill
criteria either, so an Special Weather Statement seems sufficient to cover it at this
point. See pdxspsmfr for details. Did issue a freeze watch for the
entire coast for Tuesday through Thursday morning. Things should
begin to moderate on Thursday. -Wright

Long term...Thu, Jan 1st through Sun night, January 4th...
yesterday's numerical model signals toward a wetter pattern
developing over the area next weekend have become less clear
primarily in terms of timing in the guidance that has arrived during
the last 24 hours. In short, it appears that the transition from
cold air over our area to a wetter pattern is likely to be delayed
as a frontal system expected to develop between Roseburg and
Portland now appears as if it will end up in the northern portion of
that range area, most likely putting our area under high pressure
as temperatures moderate and southwest flow develops. While the
cfsv2 has flip-flopped on its week 2 forecast for wet weather, the
GFS and the naefs continue to indicate a wetter pattern developing
along the West Coast in the the 6-10 day period, centered on the
Pacific northwest for week 2, which is January 4th through the
10th.

On New Year's Day we expect a cold and dry air mass with lows near
zero over the cold spots of the east side to 30s along and near the
coast. Due to the dry air mass, even with an inversion, the lack of
any significant fog will mean high temperatures about 5 degrees
below normal, which will put them about 20 degrees above morning
lows for the valleys.

Friday through Sunday the operational gfs40 still indicates a front
moving southward across the forecast area with light to moderate
rain and snow, but the higher resolution gfs40 looks much like the
European model (ecmwf) in keeping this boundary weaker and well to our north. Thus, I
have cut back on precipitation probabilities during this period. If
current trends continue we may very well end up with a dry weekend
as temperatures moderate. If the European model (ecmwf) solution is correct, it is
also possible that air stagnation could become an issue during this
period. Btl

&&

Mfr watches/warnings/advisories...
or...freeze watch from late Monday night through Thursday morning for
orz021-022-615-616-618-619.

California...none.

Pacific coastal waters...Small Craft Advisory for hazardous seas until
4 PM PST Tuesday for pzz350-356-370-376.

$$

Mas/trw/sbn/btl

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