[willamette-fcst] Silverton Hills Midday Forecast.doc

Thu Sep 22 11:57:12 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT THU SEP 22, 2011

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is allowed from 2:00 p.m. until 5:00 p.m.

WEATHER DISCUSSION:

A persistent upper-level ridge of high pressure, centered over the Rockies, is beginning to weaken with increasing southwesterly flow aloft over Oregon

Midday satellite imagery shows a broad band of clouds extending from southern British Columbia across most of Washington and northwestern Oregon.  Radar and surface observations show that the areas of light rain have decreased across western Washington.  Some light rain was still falling along the Washington coast, and a trace of rain fell as far south as Astoria.

The Salem sounding this morning still showed a very warm air mass aloft, so mixing heights will be slow to rise this afternoon.  At the surface, a weak thermal trough remains over western Oregon with light southeasterly pressure-gradients across the Willamette Valley.

The offshore frontal system is weakening but will continue to spread considerable clouds over western Oregon today.  Temperatures should be about 5 degrees cooler, with low-level winds forecast to turn weakly onshore this afternoon.  That could create a burning opportunity, if the flow turns enough westerly to keep smoke out of the northern Willamette Valley.  Pibals are scheduled to begin at noon.

TODAY'S FORECAST:

Partly cloudy and warm.

Salem's high temperature today will be near 83 degrees (normal is 75).

Relative humidity drops below 50% by 11 a.m. and to near 35% by 5 p.m.

Surface winds:  SE 4-7 mph; becoming SSW 4-8 mph this afternoon.

Transport winds: SSW 10-15 mph this afternoon.

Mixing height: Rising to 3000 feet by 2 p.m. and to near 3500 feet by 5 p.m.

Salem's sunset tonight: 7:10 p.m.

THREE-HOURLY DATA:

                                2 p.m.  5 p.m.  8 p.m.

     Temperature:                 78      82      70

     Relative Humidity:           41%     36%     55%

     Surface Wind Direction:     200     200     230

     Surface Wind Speed:           5       5       4

     Transport Wind Direction:   220     220     230

     Transport Wind Speed:        12      14      10

     Mixing Height:             3000    3500    1000

     Ventilation Index:           36      49      10

EXTENDED DISCUSSION:

A very similar day is forecast for Friday, with the potential for an afternoon burning opportunity if the transport winds can take on enough of a westerly component.  The flow will become more southerly, on Saturday, as an upper-level trough digs offshore.  Computer models are showing rain-making weather systems moving onshore Sunday through Tuesday of next week with a possible break from the rain next Wednesday and Thursday.

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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