[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Thu Aug 28 08:56:41 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT THU AUG 28, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

The strong upper-level ridge over Oregon is continuing to flatten in response to weather systems cutting across SW Canada.  Westerly winds aloft have increased since yesterday; finally forcing the surface thermal trough into central Oregon.

Onshore flow is slowly increasing across NW Oregon this morning.  Marine clouds cover much of the coast and have made their way up the Columbia River into the Portland area.  The sounding over Salem this morning showed several degrees of cooling below 6000 feet, but skies have remained mostly clear across the central and southern Willamette Valley.

Expect mostly sunny skies today, with the southern edge of a dry cold front likely dragging some middle and high clouds across NW Oregon in the afternoon.  With cooler air aloft, mixing heights should climb to near 3000 feet around noon, when surface temperatures reach 80°F, and to near 5000 feet by late this afternoon.  Increasing onshore flow may turn transport winds enough northwesterly to create a burning opportunity.

TODAY'S FORECAST:

Mostly sunny but cooler.

Salem's high temperature today will be near 85°F (average is 81°F).

Relative humidity:  Dropping to 50% around 11 a.m. and to near 35% by 5 p.m.

Surface winds: NNW 3-6 mph this morning; NNW 5-10 mph this afternoon.

Transport winds: N 5-10 mph this morning; NNW 8-15 mph this afternoon.

Mixing height: Rising to near 3000 feet at noon and to 5000 feet by 5 p.m.

Salem's sunset tonight: 7:56 p.m.

(Salem Airport data for Wednesday, August 27th: High 93°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

Marine clouds may penetrate into much of the Willamette Valley Friday morning, with some negative pressure-gradient-stacking.  A stronger dry cold front is forecast to approach the coastline Friday afternoon, with a strengthening west-southwesterly flow aloft.  Additional cooling aloft and increasing WNW transport winds should make for very favorable burning conditions, once onshore pressure gradients are "balanced out."

An upper-level trough is forecast to move across Oregon Friday night with a cool NW flow aloft Saturday through Monday.  This pattern looks mostly dry, but some morning drizzle or a light shower can't be ruled out.  The main impact will be temperatures cooling to the mid-70s and good ventilation.

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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