[willamette-fcst] Silverton Hills AM Forecast_Tom

Wed Jul 22 08:44:46 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT WED JUL 22, 2015

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed between 12 p.m. to 2 p.m. with a 50 acre limit.

WEATHER DISCUSSION:

An upper-level trough is over Oregon, with a second upper-level trough near Vancouver Island that will pass across the Pacific Northwest mid-afternoon. The southwesterly onshore flow aloft is weakening a surface ridge offshore Oregon, which will slide south this afternoon, turning surface flow more northwesterly. Partly Cloudy skies will allow the southern Willamette Valley to heat rapidly, improving mixing conditions early in the afternoon hours. Gradients are currently stacked unfavorably, with +0.7mb onshore from Newport to Salem, and +2.7mb from Salem to Redmond, but this is significantly improved from 24 hours ago and expected to line up favorably by mid-afternoon. Open burning opportunities may develop at this time, and will extend into the early evening hours if wind direction and gradients remain in sync with each other. A mild sea breeze is expected to push into the Willamette Valley just before sunset.

TODAY'S FORECAST:

Mostly Cloudy early becoming Partly Cloudy skies this afternoon.

Salem's high temperature today will be near 77°F (average is 84°F).

Relative humidity:  Drops below 60% by 10 a.m. and near 45% by 2 p.m.

Surface winds: N 5-10 mph this morning; NW 8-12 mph this afternoon.

Transport winds: N 5-10 mph this morning; NW 10-15 mph this afternoon.

Mixing height: 4000 feet by 10 a.m. and rising to 5000 feet by 2 p.m.

Salem's sunset tonight: 8:50 p.m.

(Salem Airport data for Tuesday, July 21st: High 79°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

The upper-level trough from Wednesday will linger over eastern Oregon Thursday morning, with a weak upper-level ridge grazing the Oregon coast as it passes south into California. Westerly onshore flow aloft will develop, while surface flow becomes more northerly as a low forms in Washington on the eastern slopes of the Cascades. The gradients will not be favorably stacked either, so open burning opportunities are not likely to materialize Thursday afternoon.

Friday proves to be a horse of a different color, as onshore flow develops early at both the surface and aloft, with transport winds becoming southwesterly in the afternoon hours. Clouds with no rainfall will move across Oregon from the Pacific, limiting daytime heating initially. Mixing will develop slowly, but gradients should stack favorably by mid-day and open burning opportunities may develop for the afternoon and early evening hours. A strong upper-level trough will curve down from the Canadian coast and across the U.S./Canadian border through the weekend. It will keep the onshore flow westerly, and moisture it gathers could produce some pop-up showers over the Oregon Cascades with afternoon heating, but rainfall on Saturday and Sunday is expected to be less than 0.10 inches of rain for western Oregon.

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

ODF Meteorologist
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