[willamette-fcst] Forecast - Tuesday, September 1st, 2009

Tue Sep 1 08:51:12 PDT 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: 
     Tuesday, September 1st, 2009 at 9:00am.

Burn Advisory:
     Agricultural burning is allowed.  Suggested burn times are from 12:00pm until 7:00pm.
     Preparatory burning is allowed from 12:00pm until 2:00pm with a 50 acre limit.
     Propane flaming is allowed from 12:00pm until 5:00pm.
     Straw stack burning is allowed from 12:00pm until 7:00pm.

Weather Discussion:
     An upper-level trough of low pressure is centered about 650
     miles off the Oregon Coast this morning and is producing
     southerly flow aloft over Oregon and Washington. A weak
     disturbance in the southerly flow brought a few showers and
     a couple of thundershowers to western Oregon Monday night. 
     Sections of the Willamette Valley received from a trace to a
     couple hundredths of an inch of rain overnight.  The Aurora
     Airport was the wettest spot I could find...picking up .08 inches. 

     The main area of clouds and showers was rotating northward,
     into western Washington, at mid-morning, with satellite
     imagery showing areas of clearing across western Oregon.
     Some marine low clouds managed to penetrate into the
     southern Willamette Valley this morning, but the unstable
     southerly flow aloft was helping to break up the low clouds
     along the coast.  Daytime heating will break up the low
     clouds in the south valley but may also trigger a shower or
     two. Under partly sunny skies, coastal temperatures should
     climb into the mid to upper 60s, with valley highs near 80.

     After seeing maximum mixing heights around 3500 feet Monday,
     some cooling aloft should help mixing heights climb to
     around 4500 feet this afternoon. Transport winds are
     forecast to have more of a westerly component this
     afternoon, which may allow for limited open burning.  Note
     that open burning opportunities freqently do not develop
     with southerly flow aloft, for a variety of
     reasons...including weak gradients, lower than expected
     mixing heights, and too much southerly component to the
     transport winds, not to mention the threat of showers or
     even a thunderstorm.  The weather will need to be monitored
     closely this afternoon.

Surface Winds:
     Var 5 this morning, NW 5-10 this afternoon.
Transport Winds:
     SW 4 this morning, W 7 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 4500 feet.  Ventilation index 32.
High Temperature:
     Salem\'s high temperature today will be near 80.
Humidities:
     Relative humidity drops to 50% around 12pm.
     Minimum relative humidity will be near 35%.
Sunrise/Sunset:
     Salem sunset tonight: 7:49pm; sunrise tomorrow: 6:36am.

Extended Outlook:
     The offshore trough will slowly approach the coastline
     Wednesday...increasing the south-southwesterly flow aloft
     over Washington and Oregon.  Skies should remain partly
     sunny, but increasing onshore flow and cooling aloft will
     make for lower temperatures.  Transport winds are forecast
     to become favorable for open burning Wednesday afternoon, so
     that will need to be closely monitored.

     The trough is forecast to come onshore late Wednesday and
     early Thursday...bringing a chance of showers to mainly
     western Washington and extreme northwestern Oregon.  It
     appears that the best chance of showers in the Willamette
     Valley will be from about Salem north.  Rainfall amounts
     should be less than one-tenth of an inch, with much of the
     valley likely receiving little to no rain.  Some clearing is
     expected Thursday afternoon, as a transitory ridge moves
     over the region.  Further cooling aloft, and more of a
     westerly component to the upper-level flow, may combine to
     create a burning opportunity Thursday afternoon.

     The next system appears stronger and is forecast to come
     onshore late Friday.  It appears likely that this system
     will be strong enough to bring rain to the entire Willamette
     Valley Friday night and Satruday.  The latest computer
     guidance has delayed the timing of this system until Friday
     night, which could create yet another burning opportunity
     Friday afternoon, but that is still too too far out to see
     clearly.  A cool upper-level trough is still forecast to
     swing onshore, with cool and showery conditions, late
     Saturday and Sunday.  Dry weather is forecast for Labor Day,
     by most of the computer models, with a ridge of high
     pressure building back over the region.

Tomorrow (02 Sep):  Areas of AM Clouds.  Mostly Sunny in the Afternoon.  54/82

Thu (03 Sep):  Chance of AM Showers North...Partly Cloudy in the Afternoon.  54/77

Fri (04 Sep):  Increasing Clouds.  Rain Likely Late.  53/78

Sat (05 Sep):  Rain Turning to Showers.  Much Cooler.  54/70

Sun (06 Sep):  Mostly Cloudy and Cool.  Chance of Showers.  51/70

Labor Day (07 Sep):  Becoming Mostly Sunny and Warmer in the Afternoon.  49/77

Tue (08 Sep):  Mostly Sunny and Warm.  51/82

ODA Meteorologist
weather at oda.state.or.us