[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Fri Aug 22 08:45:35 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT FRI AUG 22, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A "Marine Push" is in progress across western Oregon...initiated by a cool but relatively dry upper-level trough that came onshore overnight and will remain over Washington and Oregon today.  Low clouds cover most of the Willamette Valley at mid-morning and will likely continue to expand in coverage and depth, before giving way to partly cloudy skies this afternoon.

With a cool upper-level trough over the region, daytime heating will allow mixing to the top of the marine layer this afternoon, which is about 6000 feet deep.  However, the air mass may become unstable enough to trigger thunderstorm development over the Cascades.  The deep marine layer should also create some gradient-stacking issues, and transport winds will maintain a significant northerly component.

TODAY'S FORECAST:

Cloudy this morning with possible areas of drizzle.  Partly cloudy this afternoon.  Risk of afternoon thunderstorms over the Cascades.

Salem's high temperature today will be near 79°F (average is 82°F).

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

Surface winds: Light this morning; NNW 5-12 mph this afternoon.

Transport winds: N 3-6 mph this morning; NNW 10-15 mph this afternoon.

Mixing height: Rising to near 3000 feet at noon; to 6000 feet by 5 p.m.

Salem's sunset tonight: 8:06 p.m.

(Salem Airport data for Thursday, August 21st: High 82°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 100)

EXTENDED DISCUSSION:

The upper-level trough currently over the region is forecast to move over the Rockies this weekend.  A dry northwesterly flow aloft will clear skies and warm temperatures back into the mid-80s.

A strengthening upper-level ridge may warm valley temperatures into the low-90s by the middle of next week, with generally north to NE transport winds.  The ridge is expected to give way to a fairly strong upper-level trough late next week, which could provide burning opportunities.

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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