[willamette-fcst] Forecast - Tuesday, September 23rd, 2008

Tue Sep 23 07:55:12 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: 
     Tuesday, September 23rd, 2008 at 9:00am.

Burn Advisory:
     Agricultural burning is allowed.  Suggested burn times are from 2:00pm until 6:00pm.
     Preparatory burning is not allowed.
     Propane flaming is not allowed.
     Stack burning is not allowed.

Weather Discussion:
     A transitory ridge of high pressure is forecast to bring
     some sunshine and warmer temperatures to Oregon today. 
     Willamette valley highs should climb int the low 70s, which
     is still slightly below aferage but much warmer than the mid
     60s we saw Monday afternoon.  The sounding over Salem this
     morning showed considerable warming above 3500 feet compared
     with Monday morning and more warming aloft is forecast
     today.  That will cap afternoon mixing heights near 3500. 
     Forecast northerly transport winds this afternoon makes open
     burning unlikely.

     The ODA surface analysis showed a weak thermal trough trying
     to build northward, across Western Oregon, in reponse to
     transitory upper-level ridge moving over the state. 
     Pressure gradients were weak northerly across the Western
     Oregon this morning with a slight offshore component. 
     Clearing skies overnight allowed temperatures to drop to
     their lowest values since last spring for many Western
     Oregon locations.  Hillsboro dipped to at least 36 degrees
     this morning, and Eugene dropped to 37.  McMinnville was not
     much warmer at 38.  Recent rains and cool surface
     temperatures helped  create areas of shallow valley fog this
     morning, but it should be short-lived this morning.

     Infrared satellite imagery showed an impressive-looking
     cloud-shield from an approaching weather system about 300
     miles offshore at mid-morning.  Some middle and high clouds,
     from this system, had already advanced onshore and over the
     Willamette Valley this morning.  The latest computer models
     speed up the timing of this system, which does not bode well
     for creating a burning opportunity.  It will likely stay too
     far offshore today, with transport winds remaining northerly
     and the air aloft staying too warm.  By Midday Wednesday, it
     may be too close, with stong southwesterly winds near the
     surface and rain moving into the Willamette Valley.

     The best chance for a burning opportunity will be in the
     early afternoon Wednesday, if the winds stay weak enough and
     the rain has not moved onshore by that time.  Fluffing of
     damp fields would be required to take advantage of this
     potential burning opportunity.  It is looking more likely,
     however, that the timing of this system will not allow for a
     burning opportunity.  We will monitor transport winds this
     afternoon for the unlikely event that they react early to
     the approaching storm and turn more westerly.  That is not
     forecast to happen until around sundown or shortly
     thereafter. 

Surface Winds:
     NNE 5-10 this morning, NNE 5-10 this afternoon.
Transport Winds:
     NE 10 this morning, N 7 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 3500 feet.  Ventilation index 35.
High Temperature:
     Salem\'s high temperature today will be near 71.
Humidities:
     Relative humidity drops to 50% by 1pm.
     Minimum relative humidity will be near 38%.
Sunrise/Sunset:
     Salem sunset tonight: 7:07pm; sunrise tomorrow: 7:02am.

Extended Outlook:
     Computer models had been holding off rain until late
     Wednesday, which may have allowed for a burning opportunity
     Wednesday afternoon.  However, the latest guidance is
     suggesting that light rain will move into the Northern
     Willamette Valley as early as tomorrow morning with rain
     slowly spreading south during the afternoon.  The associated
     upper-level trough should maintani a little shower activity
     through Thursday...mainly in the north valley.

     Interestingly, the bulk of the rain from this storm is
     forecast to stay north of Oregon with only about one-tenth
     of an inch projected to fall in the Willamette Valley.  This
     system has entrained some tropical moisture, and will
     produce impressive low-level southwesterly winds, so I
     suspect total rainfall amounts will be closer to one-quarter
     of an inch in the upslope regions of the north valley, near
     the Cascade Foothills.

     The weak upper-level trough that was forecast to approach
     the coastline Friday now appears as if it will stay north of
     the region, with weak westerly flow aloft.  However, it may
     come close enough to turn transport winds onshore and create
     a burning opportunity, for dry fields, Friday afternoon.

     The long-range models are still forecasting a stronger ridge
     of high pressure to build over the Pacific Northwest
     beginning Saturday with a return to above normal
     temperatures.  Weak west-northwesterly flow aloft is
     forecast Saturday.  The upper-level ridge to forecast to
     amplify just east of the region Sunday, putting Western
     Oregon under weak southwesterly flow aloft with possible
     offshore flow at the surface.

     The upper-level ridge is forecast to slowly slide eastward,
     early next week, with a series of weather systems coming
     onshore beginning as early as late Monday.  This transition may
     bring another burning opportunity early next week, and then
     there is every indication that the 2008 burn season will
     come to a wet conclusion.

Tomorrow (24 Sep):  Cloudy and Blustery with Rain Spreading South During the Day.  48/69

Thu (25 Sep):  Mostly Cloudy North with a Chance of Showers.  Partly Cloudy South.  50/67

Fri (26 Sep):  Partly Sunny.  46/73

Sat (27 Sep):  Partly Sunny.  47/76

Sun (28 Sep):  Sunny and Warm.  47/80

Mon (29 Sep):  Increasing Clouds.  48/77

Tue (30 Sep):  Increasing Chance of Rain.  49/69

ODA Meteorologist
weather at oda.state.or.us