[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Thu Aug 21 08:58:04 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT THU AUG 21, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from 11 a.m. until 1 p.m. with a 50 acre limit.

WEATHER DISCUSSION:

Northwesterly flow aloft is spreading some high clouds across NW Oregon this morning.  Low-level onshore flow is weaker, compared to yesterday morning, with only patchy marine clouds in the Willamette Valley. That allowed valley temperatures to drop into the upper 40s and low 50s.

An upper-level disturbance will slide southward along the British Columbia coastline today, turning the flow aloft more westerly and increasing the low-level onshore flow.  Cooling aloft will combine with daytime heating to lift mixing heights to at or above 6000 feet by late this afternoon with WNW transport winds likely providing very favorable burning conditions.  A significant "Marine Push" is expected tonight with areas of drizzle possible Friday morning throughout the valley.

TODAY'S FORECAST:

Mostly sunny.

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: NW 3-6 mph this morning; WNW 5-10 mph this afternoon.

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

Mixing height: Rising to near 3000 feet at 11 a.m.; to 6000 feet by 5 p.m.

Salem's sunset tonight: 8:08 p.m.

(Salem Airport data for Wednesday, August 20th: High 79°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 100)

EXTENDED DISCUSSION:

An upper-level trough will move over the region on Friday.  After areas of morning drizzle, expect partial afternoon clearing with a threat of showers.  High temperatures will likely remain in the 70s.  The upper-level trough is forecast to move east of the state over the weekend with a dry northwesterly flow aloft clearing skies and warming temperatures back into the mid to upper 80s.  Expect continued dry weather with a slight cool-down early next week.

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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