[willamette-fcst] Silverton Hills AM Forecast_Tom

Wed Sep 30 08:25:23 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT WED SEP 30, 2015

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A weak upper-level trough is approaching Oregon this morning. Southwesterly flow aloft will improve mixing slightly, as air is cooled and begins to sink. Down at the surface, a thermal trough near the Oregon coast will be pushed east into the Cascades late this afternoon. Light onshore gradients will develop late, providing a brief window of favorable conditions for open burning.

TODAY'S FORECAST:

Partly Cloudy skies today.

Salem's high temperature today will be near 73°F (average is 72°F).

Relative humidity:  Dropping near 40% by 5 p.m.

Surface winds: S-SW 5-10 mph this morning; W-NW 5-10 mph this afternoon.

Transport winds: SW 5-10 mph this morning; W-SW 5-10 mph this afternoon.

Mixing height: Above 3000 feet by 3 p.m. and increasing to 3500 feet by 5 p.m.

Salem's sunset tonight: 6:55 p.m.

(Salem Airport data for Tuesday, September 29th: High 82°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 35)

EXTENDED DISCUSSION:

The weather pattern Thursday will undergo transition, as the upper-level trough slowly crosses into the desert southeast of Oregon. Cooling air will continue to surge in aloft, increasing mixing of the atmosphere throughout the day. At the surface, onshore gradients will steadily build throughout the day, though wind direction will become more northerly over time.

An upper-level trough will dive south from the Canadian coast on Friday morning, continuing to destabilize the atmosphere. A weak low with surface frontal boundaries will develop ahead of this feature. Clouds will increase, with isolated mountain showers in the Cascades Friday afternoon. Cool, damp northwesterly flow will persist into the weekend, putting western Oregon into a typical fall weather pattern. Morning fog with drizzle will lift and break up mid-day, before reforming after sunset on Saturday. Sunday will see more extensive morning fog. Wetting rains are not expected over the Silverton Hills at this time.

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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