[willamette-fcst] Silverton Hills Midday Forecast.doc

Wed Aug 22 08:56:14 PDT 2012

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT WED AUG 22, 2012

BURN ADVISORY:

Recommend agricultural burning be limited to the period from 11:00 a.m. until 6:00 p.m., from Salem north.  Agricultural burning is not recommended south of Salem.

Prep burning is allowed from 11:00 a.m. until 2:00 p.m. with a 50 acre limit.

Propane flaming is allowed from 11:00 a.m. until 5:00 p.m.

WEATHER DISCUSSION:

A dry and stable westerly flow aloft continues over the Pacific Northwest.  A weak upper-level trough moved across Washington and northern Oregon overnight, bringing a little cooler air mass into the region.  In its wake, onshore flow forced marine low clouds into western Washington and the extreme northwestern corner of Oregon this morning, with dry north-northeasterly transport winds keeping skies clear across most of the Willamette Valley.

Cooler air aloft and plenty of sunshine will help mixing heights quickly climb to 3000 feet by late this morning.  Moderate gradient-stacking this morning should get balanced out by daytime heating, but transport winds are forecast to remain generally northerly, which is unfavorable for open burning.

(Salem Airport on Tuesday: High 82°F; Rainfall .00")

TODAY'S FORECAST:

Sunny skies with breezy north winds.

Salem's high temperature today will be near 80 degrees (normal is 82).

Relative humidity drops to 55% by 11 a.m. and to near 35% by 5 p.m.

Surface winds:  N 5-10 mph this morning; N 8-13 mph this afternoon.

Transport winds: NE 8-12 mph this morning; N 12-18 mph this afternoon.

Mixing height: Rising to 3000 feet around 11 a.m. and to 5000 feet by 5 p.m.

Salem's sunset tonight: 8:06 p.m.

THREE-HOURLY DATA:

                               11 a.m.   2 p.m.  5 p.m.  8 p.m.

     Temperature:                 67       74      79      69

     Relative Humidity:           55%      40%     34%     49%

     Surface Wind Dir/Speed:     N  5     N 10    N 10   NW 10

     Transport Wind Dir/Speed:  NE 10     N 12    N 14   N  16

     Mixing Height:              3000     4000    5000    2500

     Ventilation Index:           30       48      70      40

EXTENDED DISCUSSION:

A stronger upper-level trough is forecast to move onshore late Thursday, with some increasing clouds but likely no precipitation as far south as the Silverton Hills.  Transport winds are forecast to become northwesterly, which will likely make for favorable open burning conditions Thursday afternoon.  The trough will push east of the region on Friday with transport winds likely turning northerly again.

The National Weather Service's digital forecast is available at:

http://forecast.weather.gov/MapClick.php?lat=44.90549606158295&lon=-122.8106689453125&site=pqr&unit=0&lg=en&FcstType=text

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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