[willamette-fcst] Silverton Hills Midday Forecast.doc

Fri Sep 14 11:58:06 PDT 2012

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT FRI SEP 14, 2012

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.

WEATHER DISCUSSION:

The axis of a broad upper-level ridge of high pressure has shifting eastward, to over the Rockies. The corresponding surface thermal trough is also expected to shift eastward but was still over the interior of western Oregon late this morning.  Satellite imagery shows sunny skies west of the Cascades, except for a solid deck of low clouds and fog banked up along the coastline.  DEQ air quality monitors are still showing elevated particulate levels, due to wildfire smoke, across all of northwestern Oregon.

An approaching very weak upper-level trough will increase the southwesterly flow aloft over Oregon this afternoon.  A strong low-level inversion made for poor ventilation conditions this morning, but minor cooling aloft and daytime heating should combine to lift mixing heights to near 4000 feet by late this afternoon.  Favorable burning conditions should develop later this afternoon, once the thermal trough moves east of the Willamette Valley.  Increasing onshore flow will also help to clear wildfire smoke from the valley.  Pibals are scheduled to begin shortly.

TODAY'S FORECAST:

Mostly sunny and a little cooler.

Salem's high temperature today will be near 86 degrees (normal is 78).

Relative humidity drops to near 30% by 5 p.m.

Surface winds:  Light and variable; turning light NW this evening.

Transport winds: Becoming W 7-10 mph this afternoon.

Mixing height: Rising to 3000 feet by 3 p.m. and to near 4000 feet by 5 p.m.

Salem's sunset tonight: 7:24 p.m.

THREE-HOURLY DATA:

                                2 p.m.  5 p.m.  8 p.m.

     Temperature:                 81      84      71

     Relative Humidity:           34%     31%     51%

     Surface Wind Dir/Speed:    VAR 3    NW 3    NW 5

     Transport Wind Dir/Speed:    W 7     W 9     W 8

     Mixing Height:              2800    4000    1500

     Ventilation Index:           20      36      12

EXTENDED DISCUSSION:

A very weak and dry upper-level trough is forecast to move across Oregon on Saturday.  Skies should remain mostly sunny, but onshore flow may be just strong enough to bring some morning marine clouds into the valley.  Temperatures should cool back close to 80°F.  The flow aloft, and at transport level, is forecast to turn northerly on Sunday.

A strong upper-level ridge is forecast to return next week; bringing another round of offshore flow and very warm weather.  Air quality may deteriorate, with the potential for wildfire smoke to get directed back over western Oregon.  The chances for burning opportunities next week look slim.

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 15mph.

     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 times

         the transport wind speed 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 and the Oregon Department of Forestry.  For information contact ODA at 503-986-4701.

Pete Parsons

ODF Meteorologist
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