[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Tue Sep 22 11:51:48 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT TUE SEP 22, 2015

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from noon until 2 p.m. with a 50 acre limit.

WEATHER DISCUSSION:

A SW flow aloft spread considerable clouds across most of NW Oregon this morning.  Water vapor imagery is showing this zone of mid-level moisture pushing east of the region, at midday, with much drier air advancing across NW Oregon.  While this may clear skies this afternoon, allowing for more heating, it also greatly increases the chances for smoke down-mixing.

At the surface, a deep marine layer covers the valley with light, mostly northerly, winds.  Onshore pressure-gradient-stacking worsened during the morning.  At 11 a.m., the Newport-to-Salem gradient was 0.6mb onshore, and the Salem-to-Redmond gradient was 3.5mb onshore.  Even if winds turn to the NNW later today, it is unlikely that these pressure gradients will balance-out enough to allow for open burning.  A PIBAL is scheduled for 2 p.m.

TODAY'S FORECAST:

Becoming mostly sunny but slightly below average temperatures.

Salem's high temperature today will be near 71°F (average is 75°F).

Relative humidity:  Dropping to near 35% by 5 p.m.

Surface winds: N 5-10 mph.

Transport winds: N 10 mph at midday; becoming NNW 10 mph this afternoon.

Mixing height: Near 3000 feet at midday; rising to 4500 feet by 5 p.m.

Salem's sunset tonight: 7:10 p.m.

(Salem Airport data for Monday, September 21st: High 73°F; Rainfall: .05")

(Maximum Ventilation Index expected today: 45)

EXTENDED DISCUSSION:

Wednesday and Thursday, increasing SW flow aloft should provide better afternoon burning conditions.  Onshore gradients are expected to become more favorable with mostly SW transport winds.  A weak weather system may bring a few showers onshore Thursday night.

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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