[willamette-fcst] Corrected Date Silverton Hills AM Forecast_Pete.doc

Thu Sep 7 08:58:57 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT THU SEP 7, 2017

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A strong upper-level ridge has progressed eastward, to over Idaho, with S-SE winds aloft transporting elevated wildfire smoke, moisture, and unstable air northward over Oregon.  Showers and thundershowers were concentrated mainly over SW Oregon on Wednesday, but a few very light showers survived as far north as the central Willamette Valley.  The air mass is becoming increasingly moist and unstable, so scattered showers and thundershowers will likely advance northward across the Willamette Valley today.

At the surface, the thermal trough has stalled over central Oregon.  Very weak onshore will slowly strengthen today, west of the Cascades, which will begin to clear low-level wildfire smoke from the Willamette Valley.  However, elevated smoke will persist at least through Friday.  Clouds and smoke will inhibit daytime heating, which will decrease, but not eliminate, the chance of afternoon and evening thunderstorm development.

TODAY'S FORECAST:

Mostly cloudy and smoky.  Chance of showers and thundershowers.  Rainfall totals likely less than a tenth of an inch.

Salem's high temperature today will be near 82°F (average is 79°F).

Relative humidity:  Dropping to 55% by 5 p.m.

Surface winds: NW-N 5-10 mph.

Transport winds: NE 10 mph this morning; NW-N 10-15 mph this afternoon.

Mixing height: Rising to only 3000 feet by 5 p.m.

Salem's sunset tonight: 7:37 p.m.

(Salem Airport data for Wednesday, September 6th: High 85°F; Rainfall: Trace)

(Maximum Ventilation Index expected today: 45)

EXTENDED DISCUSSION:

The state will continue to be under the influence of a weak upper-level trough on Friday, but winds aloft are expected to turn more southwesterly.  That will reduce the threat of thunderstorms but maintain a chance of light showers.  Increasing onshore flow will hold temperatures near average and could create a burning opportunity for dry fields.  However, elevated wildfire smoke will likely persist over the region.

An upper-level trough is predicted to move across southern British Columbia on Saturday, turning the winds aloft more westerly over Oregon.  Strong onshore flow should significantly improve air quality.  Temperatures will cool into the mid-70s with a slight chance of a light shower.  A building upper-level ridge is expected to turn transport winds northerly on Sunday, and offshore early next week, as temperatures climb back to above normal.

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.

     5. In accordance with OAR 837-110-0090, all field burning shall be
         banned when any two of the following criteria are present:
         A. Temperature of 95 degrees Fahrenheit or above
         B. Relative humidity of 30 percent or below
         C. Wind speed of 15 miles per hour or higher

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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