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

Willamette Valley Ag/Burning Forecast willamette-fcst at listsmart.osl.state.or.us
Fri Dec 11 09:13:48 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.

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



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