[willamette-fcst] Forecast - Friday, December 11th, 2009

Willamette Valley Ag/Burning Forecast willamette-fcst at listsmart.osl.state.or.us
Fri Dec 11 12:17:10 PST 2009




Daily Smoke Management Forecast




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

     ...Next Update Not Until Wednesday, December 23rd, 2009...

NOON UPDATE

Issued: 
     Friday, December 11th, 2009 at 12:00pm.

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

Weather Discussion:
     The very cold and dry air mass that has been entrenched
     across Washington and Oregon will get replaced by a much
     more mild and moist Pafcific air mass over the next 2-4 days,
     but the process will likely bring at least some wintry
     precipitation to much of the region. Clear skies allowed
     temperatures to drop into the teens, or colder, in the
     Willamette Valley this morning.  Hillsboro dipped to 8
     degrees, with McMinnville and Corvallis dipping to 9. 
     Eugene\'s minimum was 11, Salem bottomed out at 12, and
     Portland dropped down to 14 degrees.  The cold temperatures
     extended to the northern and central coast.  Astoria dropped
     down to 20 degrees this morning. Many areas east of the
     Cascades fell below zero again this morning.  Burns was the
     cold spot in the state with a low of -8 and Redmond dropped to -3.

     As expected, some significant changes in the weather pattern
     are currently underway.  The flow aloft over Oregon has
     become westerly, with a weather system undercutting the
     offshore ridge of high pressure and moving into California.
     Continued warming aloft has created a layer of
     above-freezing air above about 2000 feet over most of
     western Oregon.  The column of above freezing is more thick
     over southern Oregon.  Temperatures are still below
     freezing, from the surface up to about 2000 feet, across the
     interior of western Oregon.  That has the stage set for a
     wintry mix of precipitation later today through Saturday, as
     moisture feeds northward across western Oregon.

     Clouds have already spread northward, into southern Oregon,
     and Doppler radar was showing precipitation as far north as
     Grants Pass and Medford.  Surface reports indicate a wintry
     mix of precipitation over southwestern Oregon, where
     temperatures were near 30 degrees.  Plain rain was falling in
     Brookings, on the southern Oregon Coast, where temperatures
     had warmed into the upper 30s.

     Computer models show the clouds and precipitation slowly
     migrating northward later today and tonight.  The wintry mix
     will likely make it into the southern Willamette Valley by
     late this afternoon and into the extreme north valley by
     around midnight.  Ahead of it, some sunshine should lift
     northern and central Willamette Valley temperatures into the
     low 30s.  Clouds had already spread over the south valley,
     where temperatures were in the mid 20s.

     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 north of
     about Salem.  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-quarter
     to more than one-half inch, so snow and ice accumulations
     could be significant, especially in the north valley. 
     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,
     where several inches of snow are possible by Sunday morning.

Surface Winds:
     NNE 3-6 this afternoon.
Transport Winds:
     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 44%.
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 Snow 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



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