[willamette-fcst] Silverton Hills AM Forecast_doug.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT WED SEP 4, 2013



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.



WEATHER DISCUSSION:



The upper level low continues to spin off the Pacific Northwest coast this morning bringing cloudy conditions and deep southerly winds aloft to the region.  Surface pressure gradients are weakly favorable this morning but winds are very weak.

The upper low will edge just a bit closer today, and as a result mostly cloudy conditions should dominate over the region with surface temperatures a bit cooler than yesterday and some isolated showers over the Silverton Hills.  Pressure gradients will slowly increase onshore this afternoon due to inland heating, and weak northwesterly surface winds are expected late today.  Gradient stacking may be a factor though late as the pressure gradient from Salem to Redmond will increase this afternoon.  Mixing levels should again rise to near 5000 ft by mid- to late-afternoon.



(Salem Airport data for Tues., Sept. 3rd: High 83°F; Rainfall .00")

(Maximum Ventilation Index expected today:  35)

TODAY'S FORECAST:



Mostly cloudy with weak surface winds and some late-day isolated showers.



Salem's high temperature today will be near 80.

Relative humidity drops to near 40% during mid-afternoon.

Surface winds: Light and variable, becoming NW 5-10 during the afternoon.

Transport winds: Light and variable, becoming NW 5-10 during the afternoon.

Maximum mixing height: 5000 ft.

Sunset tonight: 7:43 p.m.



EXTENDED DISCUSSION:



The upper level low energizes and moves inland on Thursday, centered just south of the Silverton Hills creating widespread showers and thunderstorms and cooler conditions. Significant wetting rains are expected by mid-afternoon.  Expect mixing heights of 2500-3500 feet with northwesterly transport winds increasing during the day.



The low will continue to move to the east on Friday, leaving cool temperatures and wet fields.  Expect cloudy skies Friday morning with some clearing during the afternoon.  Wind flow will be mostly N to NW.



Cloudy skies are also expected early on Saturday, but with clearing during the afternoon and warmer conditions.



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.



Doug Wesley

ODF Meteorologist


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