[willamette-fcst] Forecast - Thursday, December 10th, 2009

Willamette Valley Ag/Burning Forecast willamette-fcst at listsmart.osl.state.or.us
Thu Dec 10 09:15:34 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: 
     Thursday, December 10th, 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 will remain in place over
     Washington and Oregon today.  With the exception of a few
     mid and high-level clouds, satellite imagery and surface
     reports show mostly clear skies over both states.  The ODA
     surface analysis showed high pressure centered near the
     eastern end of the Columbia Gorge, with continued weak
     offshore flow across western Oregon.  Light winds and clear
     skies allowed temperatures to locally drop into the single
     digits across the Willamette Valley early this morning. 
     Even the southwestern valleys dropped well down into the
     teens, with coastal minimums mostly in the low 20s.

     The very cold northerly jet stream, that brought the Arctic
     air into the region, is slowly shifting east...allowing
     temperatures aloft to begin warming.  The Salem sounding
     from early this morning showed several degrees of warming,
     between 2000 feet and 16,000 feet.  A strengthening
     low-level temperature inversion is is making for poor
     ventilation conditions.  Even though the air mass aloft is
     warming, temperatures were still below freezing.

     Another full day of sunshine is expected today, but the
     shortened day and low sun-angle will not allow for enough
     surface warming to break the strong low-level temperature
     inversion.  Valley highs, once again, will struggle to get
     to the freezing mark.  Winds will remain light from the
     north-northeast, except for near the western end of the
     Columbia Gorge, where easterly winds will gust to around 30
     mph...making for very cold windchill temperatures.

Surface Winds:
     NNE 0-6 this morning, NNE 3-8 this afternoon.
Transport Winds:
     NNE 6 this morning, NNE 6 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 1700 feet.  Ventilation index 10.
High Temperature:
     Salem\'s high temperature today will be near 31.
Humidities:
     Minimum relative humidity will be near 40%.
Sunrise/Sunset:
     Salem sunset tonight: 4:31pm; sunrise tomorrow: 7:40am.

Extended Outlook:
     Skies should remain mostly clear overnight across the
     Willamette Valley, making for very cold temperatures, again,
     Friday morning.  However, some changes will begin to show up
     by Friday afternoon.  The flow aloft over Oregon will turn
     westerly, with clouds increasing across southern Oregon, as
     a Pacific weather system moves into California.  Clouds will
     begin to increase, from the south, across the Willamette
     Valley, Friday afternoon and evening.  Continued
     north-northeastery surface winds will maintain the cold
     layer of air, below about 2000 feet, while the upper-levels
     of the atmosphere warm above freezing.  That will set the
     stage for a wintry mix of precipitation Friday night and Saturday...

     By early Saturday morning, enough moisture is forecast to
     spread northward into the Willamette Valley for
     precipitation to begin.  The air mass aloft will be
     continuing to warm, but low-level cold air will reamin
     entrenched across the Willamette Valley.  The cold air mass
     will be deepest in the north valley, where precipitation
     will likely begin as all snow.  However,the air aloft over
     the central and south valley will be warmer, so some sleet
     and/or freezing rain is likely to at least mix with snow. 
     Precipitation amounts are not forecast to be heavy, with
     most of the moisture staying south of the region.  However,
     even small amounts of freezing rain can make for very
     hazardous travel conditions.

     Precipitation is expected to taper off Sunday, with the
     low-level flow turning weakly onshore.  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 (11 Dec):  Mostly Sunny and Cold Early.  Increasing Clouds.  15/34

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

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

Mon (14 Dec):  Mostly Cloudy.  Chance of Rain...Possible Freezing Rain 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

ODA Meteorologist
weather at oda.state.or.us



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