Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Medford or
849 PM PDT Tuesday Apr 22 2014

Update...made a few adjustments this evening to account for
current observations...and to adjust the forecast based on some
changes apparent in some of the latest model runs. Previous
forecast looks on track concerning the changeover from showery
to more steady precipitation as a warm front pushes across the
area tonight.

However...there are some aspects of the forecast beyond tonight.
Newest models hint that the approaching cold front may be more of a
one-two punch...rather than a single event. 18z GFS and 00z NAM
this evening both indicate two distinct waves moving into the area
in association with the cold front expected tomorrow. This
scenario would keep winds in place along the coast for much longer
than previously thought...and would also produce two separate
waves of precipitation across the region. There may also be some
related changes concerning the expected winds across the east side
tomorrow as well. Since this is the first indication in the models
of this feature...have not made any overreaching changes to the
forecast. Have opted to wait until confidence in either scenario is
reinforced by the arrival of the remainder of the 00z model runs.

For more information...see the previous discussion below.


Aviation...based on 23/00z taf cycle...showers will become less
widespread this afternoon with a decrease in the coverage of MVFR
ceilings and terrain obscuration. But, the next front will arrive
this evening ahead of a warm front on Wednesday. As a result, rain,
MVFR ceilings, and low Level Mountain obscuration return by
05z...mainly in western Oregon with a freezing level of 4500 to 5500
feet mean sea level. The lower levels of the atmosphere will remain moist behind
the front on Wednesday with lingering MVFR and light showers in
southwest Oregon while ceilings become VFR elsewhere. -Fjb


Marine...west swells of 8-10 feet will continue to produce steep
seas over much of the area through Wednesday. Winds have shifted
to the south and begun to increase this evening as a stronger
frontal system approaches. Feel confident that south winds will
increase to gale force late Wednesday morning as a cold front GOES
through...but latest model runs indicate that winds may stay strong
over the coastal waters into Thursday...peaking again at gale
strength Thursday morning as a strong front moves through. Winds
will likely be accompanied by very steep seas as wind waves and
continued west swell build into high, choppy seas. Another front
will bring strong winds and steep seas to the region next weekend
as well. -Fjb/bpn


Previous discussion... /issued 233 PM PDT Tuesday Apr 22 2014/

Discussion...22/12z NAM/GFS in.

A progressive pattern with moderate amplitude waves will persist
into the weekend...then the pattern will stall as the global
pattern becomes more blocky.

Until then...expect mostly wet and occasionally windy weather over
the Pacific northwest.

Post-frontal showers continue this afternoon with a short wave
moving through the area. There may be some isolated thunderstorms
through this evening east of the Cascades. Showers will diminish this
evening as a broad ridge builds over the area. The ridge will push
a warm front the showers will be replaced by more
widespread light precipitation with snow levels rising to
6000-7000 feet. The ridge axis will break to the east Wednesday
night and the next upstream trough will move in Thursday. This
will push another cold front onshore...bringing in another round
of heavier precipitation. Post-frontal showers will persist into
Friday as the long wave trough moves onshore.

The next upstream ridge will weaken as it approaches the coast and
won't amount to much in the zonal flow aloft will develop
Friday night and a short wave embedded in the flow will move
onshore Saturday evening. The associated front will move onshore
Saturday afternoon. This will bring yet another round of heavier
precipitation to the area. -Jrs

Long term...Sunday through Tuesday night.

For the past few days, the main questions regarding the long term
portion of the forecast have been the effects of another couple of
fronts Sunday into Monday and how quickly a high pressure ridge
centered offshore from California will build northward to bring a
period of warmer and drier weather to our area.

Compared to this time yesterday, the GFS and European model (ecmwf) models have each
trended toward a slower arrival of improving conditions. More
recently, compared to the 00z runs, the 12z European model (ecmwf) has trended
slightly toward the wetter GFS regarding the second/stronger
front...a warm front on Sunday night.

First, an upper trough will be in the Gulf of Alaska on Sunday with
a weak cold front forecast to be extending along the Pacific
northwest coast to the northern California coast. I have made only
very minor adjustment to the Sunday forecast with light rain still
expected along the coast and likely into Josephine and Douglas
counties with a chance of rain across the remainder of the area.

For Sunday night and Monday, given the strength of the offshore
ridge and the typical strength of a warm frontal/overrunning rain
producer, I have continued to lean toward the weaker European model (ecmwf) solution.
Needless to say, the probability of rain and rainfall amounts will
be highest near the coast and this system will be watched closely
for any need to make adjustments to the end of the weekend forecast
as it draws nearer.

Confidence is higher in the day 7 forecast than for days 5 and 6. By
Monday night, a large ridge is likely to be building over the
West Coast with a warming and drying trend into the middle of the
week. /Dw


Mfr watches/warnings/advisories...


Pacific coastal waters...Small Craft Advisory for winds from 5 am
Wednesday to 11 am PDT Thursday for pzz350-356-370-376.
Hazardous seas warning from 5 am Wednesday to 11 am PDT Thursday
for pzz350-356-370-376.
Gale Warning from 11 am to 8 PM PDT Wednesday for
Small Craft Advisory for hazardous seas until 11 am PDT Thursday
for pzz356.
Small Craft Advisory for hazardous seas until 5 am PDT Wednesday
for pzz350-370-376.