[willamette-fcst] Silverton Hills Forecast Thu August 5, 2010

Thu Aug 5 11:58:47 PDT 2010

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT THU AUG 5 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from
     1:00pm to 6:30pm.
     Prep burning is allowed from 1:00pm to 3:00pm with a 50 acre limit.
     Propane flaming is allowed from 1:00pm to 5:00pm.

WEATHER DISCUSSION:

An upper-level trough remains over the Pacific Northwest.  The main circulation center is over western Washington.  A weaker mid-level circulation center is over southwestern Oregon.  The Salem sounding this morning showed easterly winds from 5000 feet up to about 8000 feet, which circulated some wildfire smoke, from the Rooster Rock Fire in central Oregon, over the Cascades and across the Willamette Valley during the night. Nephelometer readings in the northern Cascades and in the Willamette Valley remain slightly elevated late this morning, and skies are still a bit hazy.

The surface flow is onshore with 2.7 mb from Newport to Salem and 1.4 mb from Salem to Redmond.  Visible satellite imagery shows low clouds have retreated to just along the coast with sunny skies over the interior of the state.  Temperatures in the Willamette Valley are running as much as 10 degrees warmer than 24 hours ago.  Increased sunshine will result in valley temperatures climbing into the mid 80s this afternoon.

Air quality should begin improving this afternoon, as both upper-level disturbances slowly drift eastward.  Winds will shift to northwesterly, below about 5000 feet, as onshore flow increases across westerly Oregon.  However, the winds above 5000 feet may continue to circulate some elevated wildfire smoke over the region through this evening, before turning onshore tonight and Friday.   Transport winds will need to be monitored for the possibility of open burning this afternoon.

TODAY'S FORECAST:

Mostly sunny, hazy, and warmer.  Salem's high temperature today will be near 86 degrees.  The mixing height will climb to 3000 feet about 1 p.m. and will likely top out near 4500 feet around 5 p.m.  A stronger evening sea breeze is expected today with the mixing height dropping to near 2000 feet by sunset.  Surface and transport winds will increase to NW 5-10 this afternoon and NW 7-12 this evening. Relative humidity is forecast to drop to 50% by 2 p.m. and to near 36% by 5 p.m.  The ventilation index will climb to about 45 late this afternoon.

Silverton area sunset tonight: 8:28 pm

EXTENDED DISCUSSION:

The upper-level troughs are both forecast to finally shift east of the Cascades Friday with the winds at transport level and aloft turning onshore.  That should clear the skies of wildfire smoke and may provide an open burning opportunity Friday afternoon.  Morning clouds will likely penetrate inland to the Cascade crest with high temperatures cooling back into the low 80s across the Willamette valley.

Stronger onshore flow will bring further cooling over the weekend, with a weak cold front forecast to come onshore Saturday evening.  Marine clouds will be extensive by Sunday morning with local drizzle possible.  High temperatures should fall to about 10 degrees below normal by Sunday.

Another upper-level trough is forecast to drop into the region early next week for continued cooler than normal conditions and strong onshore flow.   Open burning opportunities may be possible, if the deep marine layer does not create too much gradient-stacking.

EXTENDED FORECAST:

Friday: Morning clouds, then partly cloudy.  Wind: NW 5-15 mph.  55/82
Saturday: Morning clouds, then partly cloudy. 55/77
Sunday: Chance of morning drizzle.  Partly sunny in the afternoon. 54/75
Monday: Morning clouds.  Afternoon clearing. 54/78

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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