[willamette-fcst] Silverton Hills Forecast - Mon, August 2 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT MON AUG 2 2010

BURN ADVISORY:

     Agricultural burning burning is not recommended.
     Prep burning is allowed from now to 2:00pm with a 50 acre limit.
     Propane flaming is allowed from now to 5:00pm.

WEATHER DISCUSSION:

Visible satellite imagery shows marine low clouds still covering much of western Washington and extending along the Washington coast and most of the northern and central Oregon coast.  Patches of low clouds made it inland into the extreme northern Willamette Valley this morning but were evaporating at midday.

Satellite imagery also shows elevated wildfire smoke continuing to cover much of central and eastern Washington.  Some of that smoke got rotated south and west, to over northwestern and north-central Oregon, by the upper-level trough over the region.  Skies were mostly sunny over the remainder of the state, at midday.  Temperatures in the Willamette Valley are generally running 5-8 degrees warmer than 24 hours ago.

The morning sounding over Salem showed several degrees of warming aloft, since Sunday morning.  The marine air is very shallow with quite dry conditions above 2500 feet.  The low-level temperature profile is stable, with a temperature inversion at about 4700 feet that should cap maximum mixing heights this afternoon.

A weak upper-level disturbance that was over northwestern Oregon this morning is forecast to continue eastward into central Oregon this afternoon.  Daytime heating is already triggering some minor cumulus cloud development over the southern Cascades and over central Oregon, but the very dry air mass will limit afternoon thunderstorm development.  Northwesterly flow aloft should promote an afternoon sea breeze into the Willamette Valley.  Transport winds and pressure gradients will need to be monitored for the possibility of limited open burning later this afternoon.

TODAY'S FORECAST:

Smoke and haze aloft, otherwise mostly sunny.  Salem's high temperature today will be near 84 degrees.  The mixing height will climb to around 4700 feet later this afternoon.  A cooling sea breeze will drop the mixing height to 1500 feet by shortly before sunset.  Surface and transport winds become NW 6-10 this afternoon. Relative humidity will drop to 50% by 1 p.m. and to near 36% by late this afternoon.  The ventilation index will climb to about 36 late this afternoon.

Silverton area sunset tonight: 8:32 pm

EXTENDED DISCUSSION:

Little change to the overall weather pattern is expected through Friday, with a weak upper-level trough remaining over the Pacific Northwest.  Varying degrees of onshore flow will maintain at least a chance of open burning each afternoon.  The onshore flow may weaken a bit on Wednesday, as the upper-level flow turns southwesterly, and then strengthen late Thursday and Friday.  The best chances for open burning opportunities later this week appear to be Tuesday and Thursday afternoons, but that may change.

EXTENDED FORECAST:

Tuesday: Patchy morning clouds, then mostly sunny. Wind: NW 5-12 mph.  55/84
Wednesday: Mostly Sunny. Wind: NNW 5-12 mph.  55/86
Thursday: Mostly sunny. Wind: NW 5-15 mph.  56/84
Friday: Morning clouds, then mostly sunny.  55/79

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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