Area Forecast Discussion
Issued by NWS Medford, OR

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690
FXUS66 KMFR 140400
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
800 PM PST Fri Dec 13 2019

.DISCUSSION...14/00Z NAM in. 14/00Z GFS in through 48hr.

Thursday was a wet one across the Medford CWA with just about
every station reporting significant precipitation. The heaviest
precipitation overall fell along the south coast, the adjacent
Coast Range, arcing southeast into western Siskiyou county. While
the precipitation was significant, it was not excessive. Only two
stations reported 24 hour precipitation exceeding 2.0 inches; one
was near Brookings and the other was at the Illinois Valley
airport.

The Medford CWA remains under brisk zonal flow with low amplitude
troughs and ridges moving rapidly through the area. A trough is
now moving through and the upstream ridge will move through
tonight into Saturday. Showers will diminish into Saturday. Snow
levels will drop from around 4000 feet to 2500-3500 feet by
Saturday morning, but most of the precipitation will have ended
by then, so snow impacts to passes will be minimal. The main
concern after the snow diminishes will be ice on any wet or slushy
roads as temperatures drop below freezing. With the clearing, fog
may develop in the valleys late tonight into Saturday.

The next upstream short wave will move onshore Saturday afternoon,
as well as the surface front it will be supporting. This will
bring another round of mostly light precipitation to the area,
mostly to the south coast, adjacent Coast Range, and western
Siskiyou county. Snow levels will be around 3500 feet during this
event.

That system will move out Saturday night, and a strong long wave
ridge will then build into the area. Most of the precipitation
will end by Sunday morning, but short waves sliding down the
backside of the upstream trough may support some shower activity
through Sunday.

A warm front will push north through the area Sunday night and it
may bring some light precipitation to the area at that time,
mostly from the Cascades west.

There will then be a dry break from Monday into Tuesday as the
ridge traverses the area. Stable conditions will support plenty of
overnight low clouds and fog in the valleys. The upper ridge axis
will break to the east of the area Monday night into Tuesday as
an upper trough approaches from the west. After that the forecast
gets murkier as the medium-range models are showing diverging
solutions.

Extended forecast discussion from the Friday afternoon AFD...
Wed 18 Dec through Sat 21 Dec 2019. The flow across the Pacific
will remain mostly zonal until the 21st. A front moving onshore
Wednesday into Thursday (18th to 19th) is associated with a
surface low that splits to the north and south of the forecast
area. This will break up some of the uniform lift, but will not
eliminate the chance for precipitation. The coast and coastal
mountain ranges should still receive rain, with amounts around
0.75 to 1.25 inches. While the northern Cascades may see a light
rain, snow mix, as snow levels rise to 4500+ feet across the area,
amounts should be between 0.5 to 1.5 inches of snow and at most
0.3 inches of liquid precipitation. The exception being the east
side (Modoc, Lake, and eastern Klamath Counties) where no
precipitation is expected at this time until the weekend.

Chances for rain will persist, and increase for east side locations,
into the weekend as the upper and mid-level jets shift from west to
southwest and remain stationary due to the upper level low
weakening. The jet will shift when another disturbance moves in
north of Washington. The flow should become more meridional, from
the south, and weakens some at the mid-levels. The challenge with
this system going forward will be the intensity of the low. The GFS
deterministic and some GEFS members show a stronger, cut off low
moving south near the Bay area. This would shift the bulk of the
precipitation south and precipitation chances focused mainly along
the coast. Given the variability in the remaining GEFS members, many
ECMWF ensemble members, and the less predictable nature of a
potential cutoff low, the low could remain weaker and more north.
This would result in greater precipitation chances for southern
Oregon. The fickle nature of cut off lows will result in the need
for patience and allow the models to gain a consensus on the the
character of the low forming offshore saturday the 21st. -miles

&&

.AVIATION...For the 14/00Z TAFs...Drier air is beginning to move into
the area. Precipitation will be focused away from the coast and
along inland mountains, Cascades and Siskiyous. Expecting a
transition to snow for Klamath falls by late this evening, where
conditions will shift between MVFR/IFR. Freezing levels in northern
california should remain above 4500 ft. and above 3000 ft. from Coos
and Douglas County east, in Oregon. Inland valleys may see fog form,
dropping conditions to IFR/LIFR by late morning should the area
stabilize some. Guidance shows some fair agreement in this, but
confidence is low/moderate considering the mid-level cloud cover
still moving onshore. Given that the afternoons sounding shows a
deep, dry air column and the guidance`s uniformity with fog, have
included it in the TAFs. -Miles

&&

.MARINE...Updated 700 PM PST Friday, 13 December 2019...Heavy
and steep long period swell will remain high tonight into Sunday,
with winds remaining below advisory criteria through at least
Wednesday. Seas will gradually diminish this weekend, subsiding
below advisory criteria late Sunday into Monday. This will be short-
lived, however, as another swell train moves into the waters Tuesday
ahead of the next frontal system. This front will bring increasing
southerly winds Wednesday, and with building seas, conditions will
likely become hazardous to small craft. In the meantime, periods of
moderate to heavy rain will continue with a relative break expected
Monday into Tuesday. /BR-y

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 10 PM PST this evening above 4500
        feet in the for ORZ029-030.
     Winter Weather Advisory until 10 PM PST this evening above 5000
        feet in the for ORZ031.
     Winter Weather Advisory until 10 PM PST this evening for ORZ028.
     Winter Weather Advisory until 8 PM PST this evening above 4500
        feet in the for ORZ027.

CA...Winter Weather Advisory until 4 AM PST Saturday above 5500 feet
        in the for CAZ085.

Pacific Coastal Waters...Small Craft Advisory until 10 PM PST Sunday
        for PZZ350-356-370-376.

$$

15/15/03



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