Area forecast discussion
National Weather Service Medford or
836 PM PDT Tuesday may 21 2013
Discussion...the upper low is now centered just off the northwest Oregon
coast and cold air continues to pour in over our area. Snow levels
are down to around 3500 feet already and we are seeing
accumulating snow on many of the mountain webcams. We will
continue to cool through the night with snow levels bottoming out
around 2000 feet tomorrow morning. The good news is that showers
will lessen overnight which should keep accumulating snow confined
to the mountains and not of sufficient quantity to cause many
problems. Nevertheless, anyone traveling over the mountains and
across the east side should be prepared for slick, icy roadways
and occasional reductions to visibility due to snow.
Other concerns include the potential for freezing temperatures in
agricultural areas. Freezing temperatures in the east side valleys
is virtually certain now and west side valleys are also starting
to worry ME. Deterministic guidance shows lows around 35 in
Medford proper which would bring it close to freezing in the
outlying areas. Current thinking is that cloud cover will hold
temperatures up, but if we get more clearing than expected with
such a cold airmass in place, it could get precariously close to
freezing. Will be evaluating this as new guidance comes in, but
will stick with this thinking for now. Suffice it to say it may
not be a good night for area vegetable gardens, especially outside
the cities and just above the valleys.
Winds are dying down across the east side, so wind advisories have
been lowered. It will be breezy overnight in the normally windier
areas, but nothing too unusual. May do some updates to clean up
the grids, but probably no major changes. -Wright
&&
Aviation...a cold front has passed over the area...but areas of
gusty winds and showers remain. Winds should weaken through the
early evening. Precipitation will continue to reside over the
forecast area throughout the taf period. VFR prevails at this
time...but some MVFR conditions have been reported within areas of
heavier precipitation. This will likely become more common during
the overnight hours. As freezing levels drop to 3000 feet or
below...showers over higher elevations and across the east side
will transition to snow...and visibilities there will suffer
accordingly. Mountain obscuration will occur frequently. -Bpn
&&
Previous discussion... /issued 307 PM PDT Tuesday may 21 2013/
Short term...a cold front has moved inland and east of the
Cascades today. Behind this front a large and unusually cold upper
low will move into place tonight and persist on Wednesday. Then an
upper low remains over the area Thursday into Friday. However
the coldest air will shift east and the air mass will gradually
warm.
Showers have moved inland across areas west of the Cascades and
expect showers to persist tonight and Wednesday as the upper low
remains over the area. Today and this evening...very cold air aloft
is moving into northwest portions of the County Warning Area bringing weak
instability. This will allow some heavier showers with strong gusty
winds and small hail possible. Also a slight chance for thunderstorms
are possible over Coos and Douglas County through this
evening...where models show best instability associated with the
very cold air aloft. Winds across the area will remain gusty into
this evening with strongest winds expected east of the Cascades.
Snow levels are expected to lower to 3000 feet west of the
Cascades in Oregon this evening then down to 2500 to 2800 feet
tonight. Snow levels will rise to around 3500 to 4000 feet during
the day Wednesday. With the cold air mass and continued
showers...snow is expected for mountain locations this evening into
Wednesday. Snow amounts of 3 to 6 inches are possible in the
southern Oregon Cascades. However lighter snow accumulations are
expected at Siskiyou Summit. Also warm ground temperatures may
allow snow to melt quickly especially below 4000 feet elevation.
Locally snow levels may lower to 2000 feet at times tonight. So
lower passes such as Sexton Summit may see brief periods of snow
showers overnight into Wednesday morning...however expect little
or no accumulation.
By early Wednesday morning...unusually cold temperatures are
forecast across the area. Temperatures in many west side
valleys...including portions of the rogue and Illinois
valleys...are expected to lower into the middle 30s and temperatures
in east side and northern California valleys are expected to be
around freezing or below. Mostly cloudy conditions are expected west of
the Cascades in Oregon which will minimize the frost potential.
On Thursday another shortwave disturbance is expected to move into
the area and bring another round of showers Thursday and Thursday
night. Snow levels will be higher around 4000 feet rising to 5000 to
6000 feet late in the day. Additionally models indicate weak
instability east of the Cascades...especially over Lake
County...may bring a slight chance for thunderstorms Thursday
afternoon and evening.
The upper low is forecast to remain in place over the region Thursday
night and Friday with another shortwave rotating around the trough
and into the area. This is expected to bring additional showers
Thursday night into Friday.
Long term...Friday night through Tuesday night...
With the center of the upper low exiting northeast of our area but
still over the Pacific northwest on Friday night, a weak shortwave
in a westerly flow aloft is forecast to continue a slight chance to
chance of light showers. The probability of showers will be highest
over the higher terrain from the northern portion of the southern
Oregon Cascades and northward.
The 12z GFS and European model (ecmwf) agree on a continued active upper level
pattern and slightly warmer temperatures for the Holiday weekend.
But, there are still important differences in the timing and
track/positioning of the features. I broadened the mention of a
slight chance of afternoon and evening thunderstorms across all of
the east side Saturday as the next upper trough is forecast to move
southeast to the southern Oregon coast by Saturday morning then east
across our area through the evening. The GFS is stronger/wetter and
more unstable than the European model (ecmwf). Depending on which way the models
trend...it is forseeable for the probability of rain forecast to
change in either direction.
The model differences increase on Sunday as compared to the ECMWF,
the GFS maintains a stronger version of the trough over our area but
is weaker with an upstream trough offshore from Vancouver Island.
Despite the uncertainty in strength, it is likely that the trough
will still be in place over our area. So, a slight chance to chance
of showers was maintained in the forecast.
Additional slight warming is forecast for Memorial Day with the
potential for a cold front to approach the coast late in the day.
The GFS solution is a bit stronger and about 6 hours faster than the
ECMWF, but the probability of precipitation will increase Monday
night...especially in southwest Oregon and northward.
Model differences are very large by Tuesday with the possibility that
a front will still be lingering over the east side...transitioning
to cool showery Post frontal air mass, or weak ridging could arrive
in between systems. Cool and occasionally wet weather is likely to
continue through next week. /Dw
&&
Mfr watches/warnings/advisories...
or...freeze warning from 2 am to 9 am PDT Wednesday for orz029.
California...freeze warning from 2 am to 9 am PDT Wednesday for caz084.
Freeze warning from 2 am to 9 am PDT Wednesday for caz081.
Pacific coastal waters...hazardous seas warning until 5 PM PDT
Wednesday for pzz350-356.
Small Craft Advisory for winds until 11 PM PDT this evening for
pzz376.
Hazardous seas warning until 11 PM PDT Wednesday for pzz370-376.
Small Craft Advisory for winds until 5 PM PDT Wednesday for
pzz370.
$$
