[willamette-fcst] Silverton Hills Forecast - Fri, September 17, 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT FRI SEP 17, 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from now to 5:00p.m.
     Prep burning is not allowed.
     Propane flaming is not allowed.

WEATHER DISCUSSION:

An autumn-like weather pattern has set up across the Pacific Northwest.  A strong upper-level trough is centered about 800 miles off the Oregon Coast with a very moist south-southwesterly flow aloft continuing to feed surges of rain northward across the western portion of western Oregon...mainly from the coast range westward.

Skies remain cloudy across all of western Oregon late this morning, but radar shows that the main area of rain has not advanced eastward since mid-morning.  Only some sprinkles or very light rain has advanced eastward into the Willamette Valley today, while the coast and coastal range has picked up between one-tenth and one-quarter of an inch in just the past 6 hours.

This area of rain will be very slow to move eastward today, with locally moderate rainfall totals likely from the coast range westward.  I have downgraded how much rain I think will fall today in the Willamette Valley.  The Silverton Hills region will likely see some increase in the rain this afternoon, but amounts may not top one-tenth of inch, before this latest surge of moisture begins tapering off tonight.

TODAY'S FORECAST:

Cloudy with light rain likely spreading slowly eastward across the region this afternoon.  Today's high will only climb to 72, which is 3 degrees below normal.  The mixing height may briefly climb to near 3000 feet in the mid-afternoon, before increasing rain spreads across the valley.  With more rain moving in and with the relative humidity likely remaining above 65% all day, fields will remain too damp for burning.  The ventilation index will only climb to about 20 this afternoon.

Silverton area sunset tonight: 7:13pm

THREE-HOURLY DATA:

                                 2pm     5pm     8pm
     Temperature:                 69      70      64
     Relative Humidity:           71%     68%     84%
     Surface Wind Direction:     180     190     200
     Surface Wind Speed:           3       5       3
     Transport Wind Direction:   190     190     200
     Transport Wind Speed:         5       8       7
     Estimated Mixing Height:   2500    2000    1000
     Ventilation Index:           13      16       7

EXTENDED DISCUSSION:

Rain should be widespread across western Oregon over the weekend, as the parent upper-level trough moves onshore.  Some showers could last into Monday.

Long-range computer models show weak weather systems possibly bringing a few light showers to the valley Tuesday and again Wednesday, but both days could easily remain dry.  Any dry weather may not last long, with computer models showing a more active pattern late next week.  It is possible that fields could dry out enough for a burning opportunity Thursday or Friday, if the rain holds off.  That is way too far out to call at this time.

EXTENDED FORECAST:

     Saturday: Rain likely.  High near 72.
     Sunday: Rain turning to showers.  High near 70.
     Monday: Mostly cloudy with some showers.  High near 70.
     Tuesday: Mostly cloudy.  Slight chance of showers north.  High near 72.
     Wednesday: Partly sunny.  Slight chance of showers.  High near 74.
     Thursday: Increasing clouds.  Chance of rain late.  High near 75.
     Friday: Chance of light rain.  High near 70.

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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