[willamette-fcst] Silverton Hills AM Forecast_Tom

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT FRI SEP 18, 2015


BURN ADVISORY:



Agricultural burning is recommended between 11 a.m. and 5:30 p.m. today north of Lane County.



Prep burning is not allowed.



WEATHER DISCUSSION:



A flat upper-level ridge is slowly developing over the Pacific Ocean, and will turn flow a bit more northwesterly today as the air begins to warm. At the surface, an area of higher pressure over central Oregon has produced offshore flow. Gradients are -1.4mb from Redmond to Salem, and -0.4mb from Salem to Newport. Mixing will be slow to improve, as morning clouds have moved in over a weak inversion. No rainfall is expected today, beginning a period of dry weather for the Silverton Hills.



TODAY'S FORECAST:



Mostly Cloudy becoming Sunny this afternoon.



Salem's high temperature today will be near 73°F (average is 77°F).

Relative humidity:  Dropping near 40% by 5 p.m.

Surface winds: W-NW 5-10 mph this morning; N-NW 5-10 mph this afternoon.

Transport winds: SW 5-10 mph this morning; N-NW 5-10 mph this afternoon.

Mixing height: Above 3000 feet after 11 a.m. and above 5000 feet after 2 p.m.

Salem's sunset tonight: 7:17 p.m.



(Salem Airport data for Wednesday, September 16th: High 70°F; Rainfall: .59")

(Maximum Ventilation Index expected today: 50)



EXTENDED DISCUSSION:



The upper-level ridge will move east over the Oregon coast on Saturday, warming the air aloft and restricting mixing conditions. The area of higher pressure at the surface will shift east of the Cascades, and clear skies will raise temperatures above seasonal averages over the Willamette Valley. Weak onshore flow will develop after sunset Saturday evening.



Sunday sees the upper-level ridge shift east of the Cascades, and an upper-level trough will be approaching the Washington coastline. Light gradients will become steady onshore, as a weak mid-level trough crosses the Coast Range Sunday afternoon. Transport winds will be southwesterly as a result, with surface winds becoming more southwesterly as the day progresses. Ample sunshine in the morning hours should break the inversion quickly, allowing mixing conditions to become favorable for open burning operations. A mild sea breeze during the evening hours will allow marine air to enter the Willamette Valley through mountain gaps and the Portland area overnight.



Monday morning will start the week off cool, with fog throughout much of the Willamette Valley restricting the amount of warming that occurs before mid-day. The jet stream will be positioned roughly over the Columbia River, keeping progressive westerly flow over Oregon through the week. Skies should be clear to partly cloudy, with most of the moisture further north in Washington. Temperatures are likely to be slightly above seasonal averages. The potential for multiple days of favorable weather for open burning continues to look promising.



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 15 mph.



     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 (ODA) and the Oregon Department of Forestry (ODF).  For

information contact ODA at 503-986-4701.



To add/remove your email address from this list, please go to:



http://listsmart.osl.state.or.us/mailman/listinfo/willamette-fcst



Tom Jenkins, AEM

ODF Meteorologist
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