[willamette-fcst] Silverton Hills Midday Forecast_Tom

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT THU SEP 24, 2015


BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



An upper-level trough is off the coast of Oregon, and will remain out to sea today as it weakens. Southwesterly flow is present through all levels of the atmosphere, and morning clouds are exiting to the northeast. Mixing heights will rise slowly with limited improvement today. Gradients are still neutral, with +0.7mb onshore Newport to Salem being counteracted by -0.9mb offshore Redmond to Salem. Slower development towards complete onshore flow over the Silverton Hills indicates a late afternoon for potential open burning.



TODAY'S FORECAST:



Mostly Cloudy skies becoming Partly Cloudy this afternoon.



Salem's high temperature today will be near 79°F (average is 74°F).

Relative humidity:  Dropping near 30% by 5 p.m.

Surface winds: W-SW 5-10 mph this afternoon.

Transport winds: S-SW 10-15 mph this afternoon.

Mixing height: Currently near 2000 feet, rising to 3000 feet by 2 p.m. and increasing to 4000 feet by 5 p.m.

Salem's sunset tonight: 7:06 p.m.



(Salem Airport data for Wednesday, September 23rd: High 75°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 60)



EXTENDED DISCUSSION:



The weather pattern will become unstable Friday, as the upper-level trough splits apart and shifts into Washington before sunrise. At the surface, a weak frontal system will bring mostly clouds to the Silverton Hills. Onshore flow will build as mixing is enhanced throughout the day. No rainfall is forecast to impact the Silverton Hills. Open burning opportunities may be limited due to the possibility of smoke emissions down-mixing.



Mild weather sets in for the weekend, as Oregon comes under the influence of an upper-level ridge from Nevada. Skies will clear out, and weak offshore flow will develop, bringing drier air into the Willamette Valley from east of the Cascades. Temperatures will begin to sink down below seasonal averages, with no rainfall projected across the region. An upper-level trough will approach the coast of southern Oregon early next week. A surface frontal system associated with this feature will bring unsettled weather for mid-week, and is highly dependent upon the storm's track.







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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