[willamette-fcst] Willamette Valley Forecast

[Willamette Valley Ag/Burning Forecast]

Oregon Department of Agriculture
Smoke Management Program
Weather Outlook and Field Burning Advisory for Willamette 
Valley Growers and Fire Districts.

Issued:
      Friday, December 11th, 2009 at 9:00am.

Burn Advisory:
      Agricultural burning is not recommended.
      Stack burning is not allowed.

Weather Discussion:
      A very cold and dry air mass remains entrenched 
across
      Washington and Oregon this morning. Clear skies 
allowed
      temperatures to drop into the teens, and locally 
single
      digits, in the Willamette Valley this morning, with 
most
      areas only a couple of degrees warmer than on 
Thursday
      morning.  Skies also remained clear along the 
northern and
      central coast, with most morning temperatures 
dropping into
      the low to mid 20s and locally into the upper teens. 
 Many
      areas east of the Cascades fell below zero again 
this morning.

      As expected, some significant changes in the weather 
pattern
      are taking place this morning.  The flow aloft over 
Oregon
      has become westerly, with a weather system 
undercutting the
      offshore ridge of high pressure and moving into 
California.
      That has continued to warm the air aloft, between 
2000 and
      10,000 feet, with a shallow layer of above-freeezing 
air
      showing up on the morning soundings, over Salem and 
Medford,
      between 2000 and about 5000 feet.  The column of 
above
      freezing air was thicker over Medford than over 
Salem.
      Temperatures were still well below freezing from the 
surface
      up to about 2000 feet over both locations.  That 
will set
      the stage for a wintry mix of precipitation later 
today
      through Saturday, as the feed of moisture moves 
northward
      across western Oregon.

      Clouds spread northward, into southern Oregon, 
overnight,
      and Doppler radar was showing precipitation also 
spreading
      north of the California border at mid-morning.  Rain 
was
      falling in Brookings, on the southern Oregon Coast, 
where
      temperatures had warmed into the upper 30s.  Surface
      temperatures in the southwestern interior were still 
in the
      20s, where the precipitation will fall in the form 
of a
      wintry mix as it spreads northward this morning.

      Computer models differ considerably on both their 
timing and
      intensity of the precipitation, as it moves 
northward,
      across western Oregon today.  Some begin the wintry 
mix, in
      the southern Willamette Valley, as early as 
mid-afternoon,
      with precipitation likely moving into the south 
valley by
      this evening.  The plume of moisture will sweep 
northward to
      the Portland area by about midnight.  High 
temperatures
      today will, once again, struggle to climb into the 
low to
      mid 30s.  Temperatures will rapidly chill back into 
the 20s,
      as soon as the sun sets or if precipitation begins.

      The precipitation will begin mostly as snow or sleet 
in the
      south valley, with a likely transition to sleet and 
freezing
      rain, as the air aloft continues to warm.  Light 
northerly
      surface winds will continue to supply low-level cold 
air
      throughout the valley.  However, there may be enough 
surface
      warming, in the south valley, to change 
precipitation to
      just plain rain as early as Saturday morning. 
 Further
      north, precipitation will be more likely to begin as 
snow,
      due to the deeper layer of cold air, especially in 
the
      Portland area and in the Columbia Gorge.  A 
transition to
      sleet and/or freezing rain is possible in the north 
valley
      during the day Saturday, with portions of the 
central valley
      possibly changing to plain rain.

      Forecast precipitation amounts vary from around 
one-tenth of
      an inch (water equivalent) to around four-tenths of 
an
      inch.  However, even small amounts of snow, sleet, 
and
      freezing rain can make for very hazardous travel 
conditions,
      so be prepared for wintry travel conditions in the
      Willamette Valley tonight and Saturday.  It is 
possible that
      the bands of precipitation will stall over the north 
valley
      Saturday, which could end up producing more 
significant
      amounts of snow and ice there.


Surface Winds:
      NNE 0-6 this morning, NNE 3-6 this afternoon.
Transport Winds:
      NNE 5 this morning, NNE 5 this afternoon.
Atmospheric Ventilation Conditions:
      Maximum mixing height today will be near 1500 feet. 
 Ventilation index 8.
High Temperature:
      Salem's high temperature today will be near 34.
Humidities:
      Minimum relative humidity will be near 51%.
Sunrise/Sunset:
      Salem sunset tonight: 4:31pm; sunrise tomorrow: 
7:41am.

Extended Outlook:
      Precipitation is expected to taper off Sunday, with 
the
      low-level flow turning onshore across most of the 
valley.
      That should help most of the Willamette Valley 
transition to
      just plain rain.  However, there could still be 
enough cold
      air coming out of the Columbia Gorge to keep wintry
      precipitation in the immediate Portland area. 
Another
      weather system is forecast to come onshore late 
Monday.
      There could still be enough cold air remaining in 
the
      Columbia Gorge, and the immediate Portland area, for 
a
      wintry mix of precipitation, but the rest of the 
Willamette
      Valley should get just rain.  Even the Gorge should 
get
      scoured out by Tuesday, with warmer Pacific air 
moving back
      into the region.

Tomorrow (12 Dec):  Snow, Sleet, or Freezing Rain...Likely 
Changing to Rain South.  28/35

Sun (13 Dec):  Showers...Possible Wintry Mix Near the 
Gorge.  Snow above 1500 feet.  32/42

Mon (14 Dec):  Mostly Cloudy.  Chance of Rain...Possible 
Wintry Mix North.  32/40

Tue (15 Dec):  Rain.  Possibly Starting as Freezing Rain 
North.  32/47

Wed (16 Dec):  Mostly Cloudy and Warmer.  Rain Likely 
Late.  37/49

Thu (17 Dec):  Rain Turning to Showers.  42/49

Fri (18 Dec):  Mostly Cloudy.  Chance of Rain.  38/48

ODA Meteorologist
weather at oda.state.or.us