[willamette-fcst] Silverton Hills Midday Forecast_doug.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

12:00 PM 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 today, bringing cloudy conditions and deep southerly winds aloft to the region.  Locally, surface pressure gradients are 2.0 mb Newport-Salem and 1.9 Salem-Redmond. The latter may increase significantly this afternoon and cause some local downward motion which would not be favorable for burning. However, large-scale upward motion is expected to offset this to some degree.



Mostly cloudy conditions have dominated over the region so far today, and this is expected to continue through the afternoon. Thus far scattered showers have been confined to west of the region; however, some isolated showers over the Silverton Hills are expected.  Currently, surface winds are weak S to SW, but this is expected to switch to weak W-NW late today.  Mixing levels, currently above 3000 feet, should continue to rise and get to 4500 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-afternoon isolated showers.



Salem's high temperature today will be near 80.

Relative humidity drops to 40-45% by mid-afternoon.

Surface winds: S to SW 3-7 mph becoming W-NW 5-10 during the afternoon.

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

Maximum mixing height: 4500 ft.

Sunset tonight: 7:43 p.m.



EXTENDED DISCUSSION:



The upper level low energizes and moves inland on Thursday, centered over southwestern OR during the afternoon, likely creating widespread showers and thunderstorms and cooler conditions for the Silverton Hills. Significant wetting rains are expected by mid-afternoon or earlier.  Expect mixing heights of 2000-3000 feet with W to NW transport winds increasing during the day.



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



Cloudy skies are also expected early on Saturday, but clearing should take over during the afternoon with 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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