[willamette-fcst] Silverton Hills Forecast - Wed, August 4 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT WED AUG 4 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from now to 6:30pm.
     Prep burning is not allowed.
     Propane flaming is allowed from 2:00pm to 5:00pm.

WEATHER DISCUSSION:

An upper-level disturbance triggered thundershowers over the northern Washington Cascades early this morning, while a more stable west-southwesterly flow aloft kept conditions dry over Oregon, except for a few patches of drizzle.  Little change in the upper-level weather pattern is forecast today with a few afternoon showers and thundershowers likely developing over the Washington Cascades and possibly as far south as extreme northern Oregon.

Marine low clouds continued to blanket much of the Willamette Valley at midday but were breaking up over the coast range.  Skies were generally sunny over the remainder of the state.  Over in central Oregon, smoke from a wildfire, just south of Sisters Oregon, was covering much of northern Deschutes County and streaming northward over southern Jefferson County.  It is unlikely that any smoke from that fire will push west of the Cascades today.  Skies should soon begin clearing over the Willamette Valley with considerable sunshine likely after about 2 p.m.

The late-morning ODA surface analysis showed only minor gradient-stacking of the onshore flow across western Oregon, which could be overcome by afternoon heating.  However, a thermal trough over extreme southwestern Oregon was helping to create a strong northerly gradient, known as the "Medford Draw," across western Oregon.  That will likely keep surface and transport winds too northerly for open burning this afternoon.   However, direct monitoring of the transport winds, via pibals, will begin at 1 p.m.

TODAY'S FORECAST:

Mostly cloudy early this afternoon with clearing skies in the mid-to-late afternoon.  Salem's high temperature today will be near 80 degrees.  The mixing height will not climb to 3000 feet until about 2 p.m. and will likely peak near 3800 feet around 5 p.m.  A cooling sea breeze will drop the mixing height to 2000 feet by shortly before sunset.  Surface and transport winds will be light NW to NE through early this afternoon, then become mostly northerly and increase to 5-12 mph in the mid-to-late afternoon.  Surface and transport winds will become NW 5-12 mph this evening. Relative humidity will drop to 50% by 3 p.m. and to near 42% by late this afternoon.  The ventilation index will climb to about 30 late this afternoon.

Silverton area sunset tonight: 8:29 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.  The onshore flow is forecast to increase, which may give afternoon transport winds more of a westerly component both Thursday and Friday.  That increases the chances for open burning opportunities.

Slightly cooler temperatures are forecast for this weekend, as the upper-level trough strengthens and swings a weak cold front onshore.  Moisture with this system is very limited, but it may bring some drizzle to the coast and possibly the Willamette Valley Sunday morning.  A dry and mild northwesterly flow aloft is forecast for early next week with continued onshore flow possibly providing enough ventilation for open burning.

EXTENDED FORECAST:

Thursday: Morning clouds, then partly cloudy. Wind: NW 5-15 mph.  55/83
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

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

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20100804/965986f6/attachment.html>