[willamette-fcst] Silverton Hills AM Forecast.doc

Fri Sep 14 08:58:15 PDT 2012

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 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:

A broad upper-level ridge of high pressure remains over the Pacific Northwest but is weakening and shifting eastward this morning. A surface thermal trough extends across the interior of western Oregon.  Low-level winds have turned onshore along the immediate coastline and light in the Willamette Valley.  The Salem sounding this morning showed very warm air aloft with a strong surface-based inversion extending up about 1800 feet.  Winds above the inversion were southwesterly.

Satellite imagery shows partly cloudy skies across most of the state, with a solid deck of low clouds and fog banked up along the entire coastline.  Yesterday's offshore flow transported smoke, from wildfires in the Cascades, into northwestern Oregon. DEQ air quality monitors are still showing somewhat elevated smoke levels across all of northwestern Oregon this morning.

The upper-level ridge will continue to slide eastward and weaken today, in response to an approaching very weak upper-level trough.  Increasing southwesterly flow aloft should force the surface thermal trough east of the Cascades later this afternoon.  Only minor cooling aloft is expected, so mixing heights will be slow to climb above 3000 feet.  In addition, the westerly wind push forecast by the computer models is weak.  That said, conditions will likely be favorable for some open burning this afternoon. The westerly transport flow should also help to clear wildfire smoke from the valley.  High temperatures will likely get capped in the mid 80s.

(Salem Airport data for Thursday, Sept. 13th: High 91°F; Rainfall .00")

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 below 50% by noon and 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 3500 feet by 5 p.m.

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

THREE-HOURLY DATA:

                               11 a.m.   2 p.m.  5 p.m.  8 p.m.

     Temperature:                 72       81      84      71

     Relative Humidity:           44%      34%     31%     51%

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

     Transport Wind Dir/Speed:  VAR 3      W 7     W 9     W 8

     Mixing Height:              1200     2500    3500    1500

     Ventilation Index:            7       18      32      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 is forecast to turn more northerly on Sunday, with temperatures beginning to warm again.

A strong upper-level ridge is forecast to return next week; bringing another round of offshore flow and very warm weather.  Air quality is likely to deteriorate, as wildfire smoke gets transported back across northwestern 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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