[willamette-fcst] Silverton Hills Midday Forecast.doc

Wed Jul 20 11:54:05 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT WED JUL 20, 2011

BURN ADVISORY:

Recommended burn times for agricultural burning are from now until 7:00 p.m.

Prep burning is allowed from now until 2:00 p.m. with a 50 acre limit.

Propane flaming is allowed from now until 5:00 p.m.

WEATHER DISCUSSION:

The Pacific Northwest is still under the influence of a broad upper-level trough with a deep marine layer over western Oregon.  Daytime heating is already lifting the moist air mass and producing some cumulus cloud development.  However, the aim mass aloft has warmed slightly, since Tuesday, making it more stable.  That should inhibit any shower development today and allow high temperatures to warm about 5-8 degrees compared to yesterday.

The air aloft is still cooler-than-normal for mid-July, so surface heating will lead to high afternoon mixing heights.  Look for light northerly transport winds to increase and back to northwesterly later this afternoon, in response to a weak weather system moving onto the north coast.  As that system comes onshore, it will thicken the marine layer overnight and bring back a chance of drizzle or light showers by Thursday morning.

TODAY'S FORECAST:

Partly cloudy and warmer.

Salem's high temperature today will be near 78 degrees (normal is 82).

Relative humidity drops to near 36% by 5 p.m.

Surface winds:  N 4-8 mph; becoming NW 5-10 mph this afternoon.

Transport winds: N 4-8 mph; becoming NW at 7-13 mph this afternoon.

Mixing height: Rises above 5000 feet by 2 p.m.

Sunset tonight: 8:52 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 75      78      69

     Relative Humidity:           43%     36%     57%

     Surface Wind Direction:     340     330     300

     Surface Wind Speed:           6       7      10

     Transport Wind Direction:   340     320     300

     Transport Wind Speed:         7      11      15

     Estimated Mixing Height:   5200    6200    2000

     Ventilation Index:           36      68      30

EXTENDED DISCUSSION:

A weak weather system will bring a chance of drizzle or light showers Thursday morning, with a second system maintaining a chance of showers Thursday afternoon and evening.  A dry northwesterly flow aloft is forecast for Friday, as the upper-level trough finally lifts into southwestern Canada.  Temperatures will remain below normal.

A warmer southwesterly flow aloft is forecast for this weekend.  That will bring dry weather with high temperatures climbing into the mid to upper 80s.  The trough is forecast to move onshore Monday and remain over the region through next Wednesday.  Although the threat of showers will this system will be minimal, it will increase the onshore flow and drop temperatures back into the 70s.  Dry weather is forecast for next Thursday and Friday with temperatures returning to near normal (low 80s).

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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