[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 THU SEP 17, 2015


BURN ADVISORY:



Agricultural burning is recommended between 8 a.m. and 5:30 p.m. today north of Lane County.



Prep burning is not allowed.



WEATHER DISCUSSION:



An upper-level trough is moving into the Oregon coast, and will transit across the Silverton Hills this afternoon. Cool, moist air aloft should mix down to the surface throughout the day, bringing scattered showers. Wetting rains fell before sunrise, and cloud cover with additional showers are likely to keep soil damp today. Onshore flow will be breezy today, becoming steady west-by-northwest winds as showers taper off after sunset.



TODAY'S FORECAST:



Mostly Cloudy with showers.



Salem's high temperature today will be near 67°F (average is 77°F).

Relative humidity:  Dropping near 55% by 5 p.m.

Surface winds: SW 12-18 mph this morning; SW 10-15 mph this afternoon.

Transport winds: W 15-20 mph this morning; SW 15-20 mph this afternoon.

Mixing height: Above 5000 feet all day.

Salem's sunset tonight: 7:19 p.m.



(Salem Airport data for Wednesday, September 16th: High 71°F; Rainfall: .31")

(Maximum Ventilation Index expected today: 100)



EXTENDED DISCUSSION:



Friday begins a transition into more mild weather, as an upper-level ridge begins to develop out over the Pacific Ocean. This will begin to warm and dry the atmosphere ahead of the weekend. Mixing will start out poor under morning inversions, but the September sun will improve mixing quickly after sunrise. Onshore gradients will begin to develop under sunny skies, and light westerly winds will develop over the Silverton Hills on Sunday as flow splits near Newport. These factors continue to indicate ample potential for favorable open burning opportunities Sunday afternoon.



An upper-level trough will pass across northern Washington, returning clouds to Oregon as autumn "officially" begins on Monday. Cooler, onshore flow will continue across the Silverton Hills, keeping temperatures near seasonal averages. Mild weather with onshore gradients is likely to persist through the end of the week, as an upper-level trough remains in the Gulf of Alaska. The air aloft will cool and mix easily after daytime heating, as transport winds turn more southwesterly. This suggests the possibility of multiple days of favorable open burning conditions next week.



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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