[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Thu Aug 28 11:54:04 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT THU AUG 28, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A flattening upper-level ridge is allowing weather systems that have been sweeping across SW Canada to get progressively closer to Oregon.  One such system will bring some high clouds to NW Oregon today.  West winds aloft are increasing, with the surface thermal trough over central Oregon.

Cooling aloft will yield higher mixing heights today, and increasing onshore flow should make for favorable pressure gradients to keep smoke elevated.  However, transport winds were NNE this morning and may not turn enough onshore to allow for much burning.  The first PIBAL is scheduled for noon.

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 near 35% by 5 p.m.

Surface winds: NNE 4-8 mph; possibly NNW 5-10 mph this afternoon.

Transport winds: NNE 5-10 mph; possibly NNW 8-15 mph this afternoon.

Mixing height: Rising to near 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.  Another dry weather system may bring favorable burning conditions around the middle of 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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