[willamette-fcst] Silverton Hills Midday Forecast_Tom

Wed Aug 5 11:51:22 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT WED AUG 05, 2015

***State Fire Marshal Burn-Ban Conditions, due to gusty winds and low humidity, may be observed this afternoon***

BURN ADVISORY:

Agricultural burning is not recommended.

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

WEATHER DISCUSSION:

The atmosphere is cooling aloft while westerly flow continues to increase. Improvement in onshore gradients is occurring, but at a slower pace than originally predicted. The Newport to Salem gradient is onshore +1.2mb while Salem to Redmond is onshore +3.5mb. Northerly winds will become northwesterly in the afternoon hours. Today may provide favorable open burning opportunities once gradients line up, but breezy gusts in the late afternoon hours may produce State Fire Marshal conditions.

TODAY'S FORECAST:

Mostly Sunny.

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

Relative humidity:  Dropping near 25% by 5 p.m.

Surface winds: NW 10-15 mph this afternoon.

Transport winds: NW 10-15 mph this afternoon.

Mixing height: Currently 3500 feet and rising to 5000 feet by 3 p.m.

Salem's sunset tonight: 8:33 p.m.

(Salem Airport data for Tuesday, August 3rd: High 85°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

The remainder of the week may offer further favorable days for open burning, as the Pacific Northwest stays dry under modest onshore flow. An upper-level ridge will approach off the Oregon coast Thursday afternoon. This will build up our surface high near the Oregon coast, increasing onshore flow and gradients. On Friday, the upper-level ridge will weaken and pass over the Cascades, warming the atmosphere and suppressing mixing heights. Our surface high will weaken as well, moving across the Willamette Valley. This will produce our warmest day of the week, but temperatures and wind speeds will remain below State Fire Marshal conditions. Gradients will be the key factor to monitor Friday afternoon to determine if the environment will be favorable for open burning.

The weather pattern over the weekend will return to cooler and cloudier conditions across the Pacific Northwest. An upper-level trough will approach the Canadian coastline, cooling the atmosphere under more southwesterly flow aloft. Another weak surface high will form off the coast of southern Oregon, producing onshore flow with clouds, but deflecting isolated showers further north into Washington. Temperatures will drop just below seasonal averages, with no rainfall over the Silverton Hills this weekend.

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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