[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT WED JUL 20, 2011



BURN ADVISORY:



Recommended burn times for agricultural burning are 10:00 a.m. to 7:00 p.m.

Prep burning is allowed from 11:00 a.m. until 2:00 p.m. with a 50 acre limit.

Propane flaming is allowed from 11:00 a.m. until 5:00 p.m.



WEATHER DISCUSSION:



The Pacific Northwest is still under the influence of a broad upper-level trough.  A deep marine layer remains over western Oregon this morning, but the onshore flow weakened enough overnight to allow for some sunshine this morning across much of the Willamette Valley.  The surface map shows weak northerly pressure-gradients across most of western Oregon.  The exception is along the north coast, where an approaching weather system has already turned gradients weakly southerly and brought in a solid deck of marine clouds.



The Salem sounding this morning showed slight warming above 5000 feet and slight cooling below 5000 feet, compared to yesterday morning.  That makes the atmosphere is a bit more stable today.  Daytime convection will be more limited, so expect just partly cloudy skies this afternoon.



The air aloft is still cooler-than-normal for mid-July, so mixing heights will be quite high this afternoon.  Look for northerly transport winds to turn onshore this afternoon, in response to a weak weather system moving onto the north coast.  That system will thicken the marine layer overnight, with a chance of drizzle or light showers by Thursday morning.



TODAY'S FORECAST:



Sunny early; becoming partly cloudy by this afternoon. Warmer.



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

Relative humidity drops to 60% by 11 a.m. and the near 36% by 5 p.m.

Surface winds:  N 3-7 this morning; becoming NW 5-10 this afternoon.

Transport winds: N 3-7 this morning; becoming NW at 7-13 mph this afternoon.

Mixing height: Rises above 3000 feet by 11 a.m. and above 5000 feet by 2 p.m.

Sunset tonight: 8:52 p.m.





THREE-HOURLY DATA:

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

     Temperature:                 68       75      78      70

     Relative Humidity:           57%      43%     36%     51%

     Surface Wind Direction:     360      320     310     300

     Surface Wind Speed:           4        6       7      10

     Transport Wind Direction:   360      320     300     300

     Transport Wind Speed:         4        7      11      15

     Estimated Mixing Height:   3200     5200    6200    2500

     Ventilation Index:           13       36      68      38



EXTENDED DISCUSSION:



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



The flow aloft will turn southwesterly over the weekend, as the next upper-level trough sets up camp a little farther offshore.  That will bring dry and warmer weather to the region with high temperatures climbing into the mid 80s.  The trough is forecast to move onshore Monday and remain over the region through next Wednesday.  That will renew the threat of showers and cool temperatures back below normal.  Dry and warmer weather is forecast for next Thursday and Friday.



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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