[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Wed Aug 28 08:56:58 PDT 2013

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT WED AUG 28, 2013

BURN ADVISORY:

Agricultural Burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

Little change in the overall weather pattern is expected today, so another significant burning opportunity is likely this afternoon.  The most probable timeframe is from 2:00 p.m. until 6:00 p.m.

An autumn-like upper-level trough remains anchored in the eastern Gulf of Alaska, with a warm and moist south-southwesterly flow aloft over Oregon.  Surges of moisture continue to circulate around the trough, bringing some rain to mainly the coast and the NW corner of the state.  A few hundredths of an inch of rain fell in the northern Willamette Valley overnight, including the Silverton Hills.

Satellite imagery shows an impressive cloud-shield, associated with a moist frontal system, stretching from just off the southern British Columbia coast southwestward to off the northern California coast.  Cloudy skies extend inland across western Washington and much of western Oregon, but the light rain that fell overnight has ended.  Skies remain mostly sunny from the Oregon Cascades eastward.  The Silverton Hills are on the eastern edge of the clouds.  With no significant surge of moisture expected to move onshore today, skies should remain partly sunny.

At the surface, a thermal trough extends from central Washington to over the northern Oregon Cascades.  Pressure gradients are weakly onshore from Newport to Salem and weakly offshore from Redmond to Salem.  Like yesterday, daytime heating should shift the thermal trough into central Oregon this afternoon, which will also turn the flow onshore from Salem to Redmond.

Transport wind directions are generally southeasterly this morning but should shift to south-southwesterly this afternoon.  Low-level winds should also turn southwesterly, as the surface thermal trough shifts east of the region.  Those wind-shifts are likely to occur by 2:00 p.m. The air aloft is still cool enough to support mixing heights above 5000 feet, once surface temperatures climb into the 80s.

TODAY'S FORECAST:

Partly sunny and warm.

Salem's high temperature today will be near 84 degrees (normal is 81).

Relative humidity:  Drops to 50% by 2 p.m. and to near 40% by 5 p.m.

Surface winds: SE 3-7 mph this morning; becoming SW 3-7 mph early this afternoon and W 5-10 mph late this afternoon.

Transport winds: SE 5-10 mph this morning; becoming SW 15 mph this afternoon.

Mixing height: Rising to 5000 feet by 2 p.m. and to 6000 feet by 5 p.m.

Salem's sunset tonight: 7:56 p.m.

(Salem Airport data for Tuesday, Aug. 27th: High 85°F; Rainfall Trace)

(Maximum Ventilation Index expected today: 90)

EXTENDED DISCUSSION:

Computer models are consistent in showing the offshore upper-level trough moving closer to the coast on Thursday, with a cold front likely spreading wetting rains inland across the Silverton Hills.  The latest guidance suggests about one-tenth of an inch of rain, beginning Thursday morning.

Things should dry out Friday, with more sunshine helping temperatures recover back into the 80s Saturday and perhaps Sunday.  Another upper-level trough is forecast to spread showers and cooler temperatures back across the region on Monday (Labor Day).

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 (ft) times

         the transport wind speed (mph) 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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