[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
9:00 AM PDT WED SEP 15, 2010

BURN ADVISORY:

     Agricultural burning burning is not recommended.
     Prep burning is allowed from 12:00pm to 2:00pm with a 50 acre limit.
     Propane flaming is allowed from 12:00pm to 2:00pm.

WEATHER DISCUSSION:

A weak cold front about 200 miles offshore has already spread clouds across most of Washington and the western half of Oregon.  Radar is picking up some light rain offshore, but there were no reports of rain inland as of mid-morning.  The surface map shows weak pressure gradients across western Oregon this morning with light winds in the Willamette Valley.

Clouds should lower and thicken today with increasing south-southwesterly winds.  The morning sounding showed fairly warm air aloft, and mixing heights will be slow to rise today due to the cloud-cover.  There could be a brief window of opportunity for open burning this afternoon, if the approaching light rain holds off until this evening.  Transport winds may be too southerly early in the afternoon and become too strong for good plume development later in the afternoon.

TODAY'S FORECAST:

Mostly cloudy.  A chance of rain in the afternoon with rain likely this evening.  After reaching 80 degrees on Tuesday, cloudy skies will hold the high temperature to near 75 degrees this afternoon.  The mixing height will climb to near 3000 feet about noon and likely top out 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:

                                 11am     2pm     5pm     8pm
     Temperature:                 68       74      74      65
     Relative Humidity:           65%      51%     51%     73%
     Surface Wind Direction:     170      190     210     190
     Surface Wind Speed:           6        6       8       5
     Transport Wind Direction:   190      200     230     200
     Transport Wind Speed:        10       12      15      10
     Estimated Mixing Height:   2000     3500    2500    1000
     Ventilation Index:           20       42      38      10

EXTENDED DISCUSSION:

A transitory ridge will bring a break from the rain on Thursday with another weather system slated to come onshore by Friday afternoon.  Wet weather is forecast this weekend and perhaps into early next week.   However, there does appear to be an opportunity for some drying of fields around the middle of next week.

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.

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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