[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT FRI SEP 28, 2012



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



Thursday's offshore transported wildfire smoke, from central Oregon, into the region.  Air quality instruments this morning are showing elevated smoke levels across the Willamette Valley.  The upper-level ridge has shifted east, to over the Rockies, with a weak westerly flow aloft over Oregon.  A minor upper-air disturbance is producing considerable middle and high clouds across the state.  A decaying cold front extends from Vancouver Island, British Columbia to about 400 miles off the Oregon coast.



The increase in clouds did not allow for as much overnight cooling.  Salem's sounding this morning showed warming below 3000 feet, but the air above 3000 feet was slightly cooler due to the eastward shift of the upper-level ridge.  That has weakened the inversion over the valley, but mixing heights will still struggle to reach 3000 feet today, even with additional cooling aloft.



A weak surface thermal trough is still over western Oregon this morning.  Winds are light across the Willamette Valley.  The trough is forecast to shift east of the Cascades this afternoon.  That may turn transport winds northwesterly, but ventilation conditions will be only marginal, at best, due to continued warm air aloft and slow wind speeds.



(Salem Airport data for Thursday, Sept. 27th: High 80°F; Rainfall .00")



TODAY'S FORECAST:



Mostly cloudy.  Continued warm.



Salem's high temperature today will be near 78 degrees (normal is 73).

Relative humidity:  Dropping to near 50% by 2 p.m. and to near 45% by 5 p.m.

Surface winds:  Light north; becoming light NW this afternoon.

Transport winds: N 3-6 mph; becoming NW 4-8 mph this afternoon.

Mixing height: Rising to near 2300 feet by 2 p.m. and 3000 feet by 5 p.m.

Salem's sunset tonight: 6:57 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 66       74      77      68

     Relative Humidity:           63%      48%     44%     59%

     Surface Wind Dir/Speed:      N 3     NW 3    NW 5    NW 4

     Transport Wind Dir/Speed:    N 5     NW 4    NW 6    NW 5

     Mixing Height:              1000     2300    3000    1200

     Ventilation Index:            5        9      18       6



EXTENDED DISCUSSION:



A decaying cold front will pass mainly north of the state tonight, with the only precipitation expected to be areas of drizzle, mainly along the coast.  High pressure is forecast to quickly rebuild across the region this weekend.  Northerly winds on Saturday will likely turn offshore on Sunday, which will likely transport more wildfire smoke back over the Willamette Valley.  The upper-level ridge will stay anchored over the Pacific Northwest through at least Tuesday, with the potential for strong offshore flow by Wednesday.  Long-range computer models are now more consistent in maintaining a strong upper-level ridge of high pressure over the Pacific Northwest for the next 10 days, so there is no end in sight to our dry weather pattern.



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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