[willamette-fcst] Silverton Hills AM Forecast_Tom

Tue Jul 21 08:50:06 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT TUE JUL 21, 2015

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

An upper-level trough is approaching from northwest of Vancouver Island, and will be providing southwesterly onshore flow aloft all day. This will cause a surface ridge along the Oregon coast to weaken and move further south, with onshore flow becoming northwesterly by mid-day. Mostly Cloudy skies over the Willamette Valley will break up and begin scattering over the southern valley, becoming Partly Cloudy after noon. This delayed heating will limit mixing conditions and favorable gradients from developing until mid-afternoon. Open burning opportunities may develop today, but all indications point towards this event being a later afternoon window for field burning. A moderate sea breeze will move in to the Silverton Hills before sunset, bringing cool, marine air Tuesday evening.

TODAY'S FORECAST:

Mostly Cloudy skies, becoming Partly Cloudy and breezy this evening.

Salem's high temperature today will be near 77°F (average is 83°F).

Relative humidity:  Drops to near 60% by 11 a.m. and near 35% by 5 p.m.

Surface winds: NW 5-10 mph this morning; NW 10-20 mph this afternoon.

Transport winds: N-NW 5-10 mph this morning; NW 15-20 mph this afternoon.

Mixing height: 3500 feet by 11 a.m. and rising to 5000 feet by 5 p.m.

Salem's sunset tonight: 8:51 p.m.

(Salem Airport data for Monday, July 20th: High 89°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 100)

EXTENDED DISCUSSION:

A deep marine layer will be in place over the Willamette Valley on Wednesday, bringing more clouds in from the Pacific Ocean. This will delay heating again, while trapped moisture keeps the humidity high into the afternoon hours. An upper-level trough will sweep across Washington during the day, producing onshore flow aloft. Light onshore flow will occur at the surface as well, but the overcast conditions will prevent a true sea breeze from forming Wednesday afternoon.

Thursday marks a brief period of stable weather, as an upper-level ridge moves in from the Pacific Ocean. This will help to dry out the atmosphere, but will limit mixing conditions while onshore flow weakens. A surface ridge will begin to form near the Oregon coast that will also stack gradients unfavorably for open burning operations. Friday will see conditions become more favorable again, as a strong upper-level trough cruises down the Canadian coastline, weakening the upper-level ridge in the Pacific Ocean. Onshore flow aloft increases, as straight westerly winds develop at all levels. The surface ridge near the Oregon coast will be pushed south, increasing onshore flow with west-by-northwesterly winds. Mixing conditions will improve, as well as gradients, to provide another possible open burning opportunity ahead of the 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

ODF Meteorologist
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