[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Wed Aug 13 11:56:19 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT WED AUG 13, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

Clusters of showers and thunderstorms are rotating around an upper-level trough that extends from eastern Washington to NW California.  One large area of showers currently stretches from western Washington across the NW tip of Oregon, with another area of showers extending from SW to north-central Oregon.  The Willamette Valley is in a relatively dry slot between these two shower clusters.  However, showers are advancing northward along the Cascades and may move over the valley later this afternoon.

The air aloft is cool, so daytime heating will yield high mixing heights.  Onshore pressure gradients are currently favorable for burning but will need to be monitored.  Transport winds are predicted to be westerly, shower development over the Cascades will tend to decrease the onshore flow from Salem to Redmond, which is needed for smoke evacuation.  Burning chances today will also depend upon dryness of fields and relative humidity levels.

TODAY'S FORECAST:

Mostly cloudy.  Chance of showers and thundershowers.

Salem's high temperature today will be near 78°F (average is 83°F).

Relative humidity:  Dropping to near 50% by 5 p.m.

Surface winds: SW 4-8 mph.

Transport winds: W 10-15 mph this afternoon.

Mixing height: Rising to near 5000 feet by 5 p.m.

Salem's sunset tonight: 8:21 p.m.

(Salem Airport data for Tuesday, August 12th: High 76°F; Rainfall: .13")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

The upper-level trough is forecast to move over eastern Oregon on Thursday and eastern Washington on Friday.  The moist circulation around it should maintain mostly cloudy skies, but the threat of showers will decrease.  Onshore flow and good mixing may provide burning opportunities both days, if humidity levels are low enough and fields are not too wet.

A drier westerly flow aloft is forecast for Saturday through Monday, with onshore flow at the surface.  Mostly sunny skies will help temperatures recover to slightly above 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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