[willamette-fcst] Silverton Hills AM Forecast.doc

Fri Oct 12 08:52:44 PDT 2012

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT FRI OCT 12, 2012

BURN ADVISORY:

Recommended burn times for agricultural burning are 8:00 a.m. until 4:30 p.m.

Prep burning is not allowed.

Propane flaming is not allowed.

WEATHER DISCUSSION:

The upper-level ridge that has been over the Pacific Northwest since July has flattened in response to a strengthening westerly jet stream.  The first in a series of progressively stronger weather systems is moving onshore this morning.  Radar and surface reports indicate that areas of light rain have already advanced from the northern and central coast into the northern Willamette Valley.

What was left of a thermal trough has progressed into eastern Oregon, which has turned transport winds strongly onshore across northwestern Oregon.  That has helped to clear the haze from the valley this morning, with air quality instruments mostly returning to baseline readings.  Surface winds have turned southerly at 5-10 mph in the valley and should increase slightly later today.

The air mass will likely remain too moist for burning today, with areas of light rain increasing in coverage this afternoon.

(Salem Airport data for Thursday, Oct. 11th: High 69°F; Rainfall .00")

TODAY'S FORECAST:

Cloudy and much cooler with light rain at times.

Salem's high temperature today will be near 56 degrees (normal is 66).

Relative humidity:  Remaining above 75% all day.

Surface winds:  S 5-15 mph.

Transport winds: SW 15 mph; becoming SW 20-25 mph this afternoon.

Mixing height: Rising to near 3500 feet this afternoon.

Salem's sunset tonight: 6:31 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 52       55      56      53

     Relative Humidity:           83%      77%     80%     89%

     Surface Wind Dir/Speed:      S 7     S 10    S 10     S 7

     Transport Wind Dir/Speed:  SW 15    SW 20   SW 22   SW 25

     Mixing Height:              2800     3000    3500    3000

     Ventilation Index:           42       60      77      75

EXTENDED DISCUSSION:

Continued westerly flow aloft will maintain mostly cloudy skies Saturday and  Sunday with a chance of light rain at times.  Valley temperatures should warm back into the 60s, as the main storm track migrates northward, to over Washington.  The jet stream will drop south again late Sunday, with a couple of potent storms forecast to bring soaking rains to western Oregon Sunday night and Monday.  That should effectively end field-burning for the season.

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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