[willamette-fcst] Silverton Hills Forecast - WED, SEPTEMBER 15, 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT WED SEP 15, 2010

BURN ADVISORY:

     Agricultural burning burning is not recommended.
     Prep burning is allowed from now to 4:00pm.
     Propane flaming is allowed from now to 2:00pm.

WEATHER DISCUSSION:

A strengthening southwesterly flow aloft is slowly pushing a weak autumn-like cold front onshore today.  Satellite imagery shows middle and high clouds spreading across most of Washington and Oregon at midday, with clouds lowering and thickening along the coastal strip.  Radar is showing light rain slowly advancing to near the coastline, but there were still no reports of rain, inland over Oregon, as of late this morning.  There were some sprinkles making it into northwestern Washington.

The surface map continued to show weak pressure gradients across western Oregon late this morning.  Southerly winds were beginning to increase from about Silverton southward across the Willamette Valley.  Some sunbreaks have helped temperatures climb to near 70 degrees.

Clouds should increase this afternoon along with south-southwesterly winds.  Mixing heights will be slow to rise today, due to warm air aloft and the filtering of sunshine by the clouds.  There could be a brief window of opportunity for open burning this afternoon, if the approaching light rain holds off until this evening and transport winds take on enough of a westerly component.  It is also possible that surface winds may become too strong for good plume development later in the afternoon.  Monitoring of the transport winds, via pibals, will begin at noon.

TODAY'S FORECAST:

Mostly cloudy.  A chance of rain by late afternoon with rain likely this evening.  Increasing clouds should hold high temperature in the mid 70s this afternoon.  The mixing height will likely only climb to between 3000 and 4000 feet in the mid-afternoon, then drop below 3000 feet by 5 p.m.  Relative humidity will drop to near 50% this afternoon with the ventilation index climbing to around 42.

Silverton area sunset tonight: 7:17pm

THREE-HOURLY DATA:

                                 2pm     5pm     8pm
     Temperature:                 74      74      65
     Relative Humidity:           51%     51%     73%
     Surface Wind Direction:     190     210     190
     Surface Wind Speed:          10      12       7
     Transport Wind Direction:   200     230     200
     Transport Wind Speed:        12      15      10
     Estimated Mixing Height:   3500    2500    1000
     Ventilation Index:           42      38      10

EXTENDED DISCUSSION:

A transitory ridge should be just strong enough to provide mostly dry weather on Thursday under weak southwesterly flow aloft.  If fields are dry enough, there is a possibility that transport winds will turn enough onshore to allow for open burning Thursday afternoon.  Another weather system is slated to come onshore by Friday afternoon.  That could also present an opening burning opportunity, Friday afternoon, if the rain holds off long enough.  Wet weather is forecast this weekend...perhaps lasting into Monday.   Long-range computer models are showing a potentially favorable pattern for burning, next Tuesday and Wednesday, if conditions dry out enough.

EXTENDED FORECAST:

     Thursday: Mostly cloudy.  Chance of rain.  High near 75.
     Friday: Rain likely by the afternoon.  High near 72.
     Saturday: Rain likely.  High near 70.
     Sunday: Mostly cloudy.  Chance of rain.  High near 70.
     Monday: A chance of showers.  High near 73.
     Tuesday: Partly sunny.  Slight chance of showers.  High near 76.
     Wednesday: Partly sunny.  Slight chance of showers.  High near 72.

Notes:

     1.  Mixing height, as used here, is the lowest height at which the
         potential temperature exceeds the potential temperature at the
         surface.
         As a practical matter it is the approximate height to which a
         smoke plume will rise assuming good ignition, dry fuels, and
         winds less than about 15mph.
     2.  Transport winds are a layer average through the mixing height,
         weighted slightly toward the winds at the top of the layer.
     3.  Ventilation Index is the height of the mixing layer times
         the transport wind speed divided by 1000.
     4.  Surface wind direction is the general expected wind direction.
         At a specific point surface winds are highly dependent on local
         terrain conditions.

This forecast is provided under an agreement between the Oregon Department of
Agriculture and the Oregon Department of Forestry.  For information contact ODA at 503-986-4701.

Pete Parsons
ODF Meteorologist
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