[willamette-fcst] Forecast - Friday, October 31st, 2008

Fri Oct 31 08:04:25 PDT 2008

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, October 31st, 2008 at 9:00am.

Burn Advisory:
     Agricultural burning is allowed.  Suggested burn times are from 8:00am until 4:00pm.
     Stack burning is not allowed.

Weather Discussion:
     Increasing southerly flow aloft brought cloudy skies to
     western Oregon Thursday and strengthened the low-level
     temperature inversions over the Willamette Valley.  The
     Salem sounding from late Thursday afternoon showed
     temperaturs at 2000 feet in the upper 60s, while surface
     temperatures were only in the mid 50s.  That made for poor
     ventilation conditions Thursday, with light transport winds
     and mixing heights generally below 1000 feet.

     The first in a series of weather systems ended the extended
     dry spell and brought rain onshore Thursday night.  Light
     rain continued to fall across most of western Oregon this
     morning with amounts in the Willamette Valley generally
     ranging from one-tenth to one-quarter of an inch.  Doppler
     radar showed the main area of rain moving north across the
     Willamette Valley at mid-morning with the back edge of the
     steady precipitation near Eugene.  The rain should taper off
     this afternoon, from south-to-north, across the Willamette
     Valley.  Skies will likely stay mostly cloudy through
     tonight, with satellite imagery not showing many
     cloud-breaks in wake of the steady rain.  At least it look
     mostly dry for the trick-or-treaters tonight, witch is good.

     The Salem sounding this morning showed considerable cooling
     aloft, since Thursday afternoon, with temperatures at 2000
     feet dropping from the upper 60s to the low 50s (about the
     same as currect surface temperatures).  Normally, that would
     promote better mixing of the air mass.  However, a
     developing storm system, about 600 miles off the northern
     California coastline, will maintain southerly flow aloft and
     offshore surface winds this afternoon. That will likely keep
     surface temperatures at or below 60 degrees and reform the
     low-level temperture inversion over the valley.  The
     combination of low mixing heights and south-southeasterly
     low-level winds makes conditions unfavorable for stack
     burning today.

Surface Winds:
     SE 4-8 this morning, SE 5-10 this afternoon.
Transport Winds:
     SE 4 this morning, S 12 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 1800 feet.  Ventilation index 22.
High Temperature:
     Salem\'s high temperature today will be near 60.
Humidities:
     Minimum relative humidity will be near 69%.
Sunrise/Sunset:
     Salem sunset tonight: 6:01pm; sunrise tomorrow: 7:51am.

Extended Outlook:
     The next weather system will spilt as it move onshore late
     Saturday, with the main jet stream energy aimed at northern
     California.  It will still bring increasing rain to western
     Oregon, with total rainfall amounts around one-quarter of an
     inch (similar to today).  Offshore surface winds will likely
     keep high temperatures in the upper 50s Saturday afternoon.

     An upper-level trough will bring cooling aloft Sunday and
     Monday with the flow aloft, and at the surface, turning
     onshore.  That should promote better mixing of the air mass
     and improve ventilation conditions.  It will also maintain
     shower activity with enhanced precipitation over the
     mountains.  More rain will also make it east of the
     Cascades.  Colder air aloft will drop the snow level to
     about 5000 feet, in the Cascades, by late Sunday and to 4000
     feet by late Monday.

     The upper-level trough is forecast to push east of the
     region Tuesday afternoon with the flow aloft turning
     northwesterly and beginning to dry out.  A warm front will
     likely be strong enough to bring light rain to western
     Oregon Wednesday, as it pushes the snow level back above the
     Cascade passes.  The long-range computer model forecasts
     diverge late next week, with some calling for ridging and
     dry conditions and others maintaining a more damp westerly
     flow aloft.  The ODA Seasonal Climate Outlook favors drier
     conditions returning, so I have leaned the forecast in that
     direction for next Thursday and Friday.

Tomorrow (01 Nov):  Increasing Light Rain in the Afternoon. 50/59

Sun (02 Nov):  Showers Early...Increasing Rain Late.  Snow Level Dropping to 5000 Feet. 48/57

Mon (03 Nov):  Rain Likely with Snow in the Mountains.  Snow Level Near 4000 Feet.  43/54

Tue (04 Nov):  Showers.  Snow Level 3500 feet.  42/52

Wed (05 Nov):  Light ain Likely...Mainly North.  Snow Level Rising to 5500 Feet Late.  38/52

Thu (06 Nov):  Mostly Cloudy.  Areas of Fog.  Chance of Light Rain...Mainly North.  40/55

Fri (07 Nov):  Mostly Cloudy.  Areas of Fog.  Chance of Light Rain...Mainly North.  40/55

ODA Meteorologist
weather at oda.state.or.us