[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Wed Aug 6 08:57:13 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT WED AUG 6, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from 11 a.m. until 1 p.m. with a 50 acre limit.

WEATHER DISCUSSION:

An upper-level trough, extending from Idaho to California, is finally shifting eastward in response to an increasing westerly flow aloft pushing over western Oregon.  Increasing onshore flow has cleared wildfire smoke from the skies over the Willamette Valley this morning.  It has also pushed marine clouds inland up the Columbia River into the Portland area and through the central coast range gaps into Eugene.  Some low clouds may penetrate into Marion and Linn Counties later this morning.

The limiting factors for burning today will be wind direction and pressure gradient-stacking.  Cooling aloft and surface warming should help mixing heights climb to near 5000 feet by late this afternoon.  The latest computer guidance is predicting northwesterly winds this afternoon, which would be favorable for burning. If the central Willamette Valley can avoid the marine clouds this morning, that will increase the chances for burning opportunities today.  Daytime heating will be needed to reverse the "stacking" of the onshore pressure gradients.  Clear skies will speed up that process.

TODAY'S FORECAST:

Areas of morning clouds possible.  Sunny and a little cooler.

Salem's high temperature today will be near 83°F (average is 84°F).

Relative humidity:  Dropping to near 60% by 11 a.m. and to near 40% by 5 p.m.

Surface winds: N 5 mph this morning; NNW 5-10 mph this afternoon.

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

Mixing height: Rising to 3000 feet around 11 a.m. and to 5000 feet by 5 p.m.

Salem's sunset tonight: 8:31 p.m.

(Salem Airport data for Tuesday, August 5th: High 86°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

A cooler and stable westerly flow aloft on Thursday is predicted to become more southwesterly on Friday, with continued onshore flow.  Expect some morning marine clouds each day with sunny afternoons.  Transport winds should be mostly northerly but may turn enough northwesterly to allow for limited burning, if pressure gradient-stacking can be balanced by daytime heating.

A weak upper-level trough may trigger some thundershowers over the Cascades this weekend.  It is possible that the flow aloft could turn enough southerly to send thunderstorms over the Willamette Valley Sunday night into Monday morning.  Otherwise, expect a continuation of mild and dry conditions with onshore flow. A little stronger trough is forecast to increase the onshore flow on Tuesday with temperatures cooling to below average.

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

Pete Parsons

ODF Meteorologist
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