[willamette-fcst] Silverton Hills Midday Forecast_Tom

Wed Sep 16 11:34:21 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT WED SEP 16, 2015

BURN ADVISORY:

Agricultural burning is recommended between 8 a.m. and 5:30 p.m. today north of Lane County.

Prep burning is not allowed.

WEATHER DISCUSSION:

An upper-level trough is off the coast of Oregon, and it will slow its approach as it deepens today. This will continue to develop a surface frontal system that is approaching the Willamette Valley from southwestern Oregon. Offshore flow is still prevalent, with -2.5mb from Redmond to Newport. Humidity is currently very high and not expected to drop below 60% today before afternoon showers arrive. The temperature has heated enough from partial sunbreaks to reach the convective temperature, so there is a high likelihood that any smoke that is elevated will down-mix and fumigate downstream communities. Surface winds are becoming steady out of the southeast, and may become breezy at times as winds shift to more southwesterly flow with today's showers.

TODAY'S FORECAST:

Overcast with rain becoming showery this afternoon.

Salem's high temperature today will be near 63°F (average is 77°F).

Relative humidity:  Dropping near 60% by 5 p.m.

Surface winds: S-SW 15-20 mph this afternoon.

Transport winds: SW 35-40 mph this afternoon.

Mixing height: Above 5000 feet after 1 p.m.

Salem's sunset tonight: 7:21 p.m.

(Salem Airport data for Tuesday, September 15th: High 65°F; Rainfall: .04")

(Maximum Ventilation Index expected today: 200)

EXTENDED DISCUSSION:

The upper-level trough will cross western Oregon on Thursday, as another disturbance energizes it and enhances mixing. An area of lower pressure off the coast of Oregon will generate another frontal system, bringing another day of breezy winds and showers to the Silverton Hills. Wetting rains up to a quarter of an inch in higher elevations are expected.

Friday begins a transition into more mild weather, as an upper-level ridge begins to develop out over the Pacific Ocean. This will begin to warm and dry the atmosphere ahead of the weekend. Mixing will become suppressed as an area of higher pressure forms near the Oregon coast. Onshore gradients will begin to develop under sunny skies, and light westerly winds will develop over the Silverton Hills on Sunday as flow splits near Newport. This may produce favorable open burning conditions Sunday afternoon after several drying days.

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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