[willamette-fcst] Corrected Silverton Hills Forecast Fri August 6 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT FRI AUG 6 2010

...Corrected for prep burning (not allowed)...

BURN ADVISORY:

     Recommended burn times for agricultural burning are from
     1:00pm to 6:30pm.
     Prep burning is not allowed.
     Propane flaming is allowed from 1:00pm to 5:00pm.

WEATHER DISCUSSION:

An upper-level trough stretches from western Washington, across western Oregon, to northern California this morning.  Wildfire smoke from both the Rooster Rock Fire, in central Oregon, and from fires in southern British Columbia is still making skies hazy over western Oregon.  Nephelometer readings remain slightly elevated this morning in the northern Cascades and in the Willamette Valley.  Some improvement in air quality is likely today, as the upper-level trough slowly migrates eastward and the winds aloft turn onshore.

The Salem sounding this morning still showed easterly winds between 2000 and 3000 feet, but all other levels had turned onshore.  However, the onshore flow is still fairly weak this morning with just 1.6 mb from Newport to salem and 1.6 mb from Salem to Redmond.  Visible satellite imagery shows low clouds along the coast with some inland penetration up the Columbia River into the extreme northern Willamette Valley.  The remainder of the valley had hazy sunshine.  Temperatures were generally running only a degree or two cooler than 24 hours ago.

The flow aloft is forecast to turn onshore, at all levels, by this afternoon with very slight cooling aloft likely taking a few degrees off of valley high temperatures.  Since the smoke plumes from area wildfire extend close to 100 miles offshore, expect the increasing westerly flow aloft to only slowly clear the hazy conditions across western Oregon.  Transport winds will need monitoring this afternoon for the possibility of ventilation conditions improving enough for limited open burning.

TODAY'S FORECAST:

Mostly sunny and hazy.  A touch cooler.  After reaching 88 degrees on Thursday, Salem's high temperature today will be near 84 degrees.  The mixing height will climb to 3000 feet about 2 p.m. and likely top out near 4000 feet around 5 p.m.  An evening sea breeze should drop the mixing height to near 2000 feet by 8 p.m.  Surface and transport winds will be NW 3-6 mph this morning, then increase to NW 5-10 this afternoon and NW 7-12 this evening. Relative humidity is forecast to drop to 50% by 1 p.m. and to near 34% by 5 p.m.  The ventilation index will climb to about 32 late this afternoon.

Silverton area sunset tonight: 8:26 pm

THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 71       80      84      75
     Relative Humidity:           55%      41%     34%     48%
     Surface Wind Direction:     320      310     320     290
     Surface Wind Speed:           5        6       8       9
     Transport Wind Direction:   310      320     320     310
     Transport Wind Speed:         4        6       8      10
     Estimated Mixing Height:   2200     3000    4000    2000
     Ventilation Index:            9       18      32      20

EXTENDED DISCUSSION:

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 Monday for continued cooler than normal conditions and moderate to strong onshore flow.   That may create an open burning opportunity and will need to be monitored closely.  The trough is forecast to weaken on Tuesday with decreasing onshore flow leading warmer temperatures and decreasing the chances for open burning opportunities.  Long-range computer models are inconsistent.  Some are showing a cool northwesterly flow aloft for later next week, while others have temperatures warming to above normal.

EXTENDED FORECAST:

Saturday: Morning clouds, then partly cloudy. 55/79
Sunday: Chance of morning drizzle.  Partly sunny in the afternoon. 54/75
Monday: Morning clouds, then mostly sunny. 54/79
Tuesday: Morning clouds, then mostly sunny.  53/84

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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