[willamette-fcst] Silverton Hills Midday Forecast.doc

Thu Aug 18 11:56:17 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT THU AUG 18, 2011

BURN ADVISORY:

Agricultural burning is not recommended.

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

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

WEATHER DISCUSSION:

Westerly flow aloft is forecast for today, as a weak upper-level disturbance moves over western Washington.  Late-morning visible satellite imagery showed low clouds from the north coast, up the Columbia River, into the extreme northern Willamette Valley.  Skies were rapidly clearing over Marion County.  The surface map showed high pressure centered offshore with a little stronger northerly pressure-gradients across western Oregon than 24 hours ago.

With the morning influx of cooler marine air, there is some gradient-stacking across western Oregon late this morning.  That is an indication that air is generally sinking over the region.  The latest gradients are: Newport-to-Salem at 0.7mb onshore and Salem-to-Redmond at 2.8mb onshore.  The gradient-stacking will likely reverse by late this afternoon, but transport winds may remain too northerly for open burning.  Pibal readings will begin 2 p.m.

A weak sea breeze will lower mixing heights this evening with some marine air expected to penetrate into the northern Willamette Valley again overnight.

TODAY'S FORECAST:

Mostly sunny but cooler.

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

Relative humidity drops to near 33% by 5 p.m.

Surface winds:  N 10 mph; becoming NNW 8-12 mph late this afternoon.

Transport winds: N 12 mph; becoming NNW 12-16 mph late this afternoon.

Mixing height: Rises to 3000 feet by noon and to 4700 feet by 5 p.m.

Salem's sunset tonight: 8:14 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 75      79      71

     Relative Humidity:           42%     33%     48%

     Surface Wind Direction:     360     350     320

     Surface Wind Speed:           8      10       9

     Transport Wind Direction:   360     345     340

     Transport Wind Speed:        12      14      16

     Mixing Height:             3500    4700    2000

     Ventilation Index:           42      66      32

EXTENDED DISCUSSION:

Little change in the weather pattern is forecast for Friday, with a very weak upper-level trough remaining over Oregon.  Onshore flow will likely bring at least some morning marine clouds into the northern Willamette Valley.  Otherwise, skies will be sunny with northerly transport winds.

The upper-level flow will turn south-southwesterly on Saturday with valley temperatures approaching 90 degrees. Some thunderstorm development is possible over the Cascades.  The flow aloft is forecast to turn westerly on Sunday, with increasing onshore flow capping temperatures in the upper 80s.

A cold front is forecast to move into southern British Columbia and northwestern Washington on Monday.  It is a long way out, but the current forecast of west-southwesterly transport winds could provide very favorable burning conditions Monday afternoon.

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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