[willamette-fcst] Silverton Hills Forecast - FRI, JULY 30 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT FRI JUL 30 2010

BURN ADVISORY:

     Agricultural burning is not recommended.
     Prep burning is not allowed.
     Propane flaming is allowed from 2:00 p.m. until 5:00 p.m.

WEATHER DISCUSSION:

Midday satellite imagery shows low clouds continuing to blanket the northern and central coast but beginning to break up in the extreme northern Willamette Valley.  The central and southern valley avoided marine clouds this morning.  Temperatures are running within a degree or two of 24 hours ago, so the high temperature forecast of 84 degrees looks on track.

The late-morning surface analysis showed the thermal trough had shifted just east of Redmond with weak onshore flow across western Oregon.  There was some mild gradient-stacking, with 2.7 mb from Newport to Salem and 4.2 mb from Salem to Redmond, but daytime heating should reverse that by 3 p.m.

The significant weather feature today is an upper-level trough that is sluggishly advancing closer to the coastline.  The air aloft is slowly cooling, which should provide higher mixing heights today compared with the past couple of afternoons.  It is still uncertain if this system will progress far enough eastward to create an open burning opportunity this afternoon, but at least marginal burning conditions appear likely to develop.

Transport winds will need to be closely monitored, for a forecast shift to northwesterly, this afternoon.  Possible thunderstorm development, over the Cascades, will also need to be closely watched, because that could adversely affect the pressure gradient-stacking and inhibit smoke evacuation.

TODAY'S FORECAST:

Mostly sunny and warm.  Salem's high temperature today will be near 84 degrees.  The mixing height will not climb to 3000 feet until around 2 p.m. but may rapidly lift to near 4500 feet.  Look for a cooling sea breeze to quickly drop to mixing height to around 1000 feet just before sunset.  Light surface and transport winds should increase and turn northwesterly this afternoon. Relative humidity will drop to near 32% by late this afternoon.  The ventilation index should climb to about 36 late this afternoon.

Silverton area sunset tonight: 8:35 pm

EXTENDED DISCUSSION:

The upper-level trough is forecast to move across Oregon over the weekend with the flow aloft turning northwesterly by Sunday.  That will bring a significant surge of marine air into the Willamette Valley with high temperatures cooling to below normal.  Another weak upper-level trough may move into the region early next week, with continued onshore flow only allowing temperatures to warm to near normal.  This weather pattern may provide multiple burning opportunities during the upcoming week.

EXTENDED FORECAST:

Saturday: Morning clouds, then mostly sunny but cooler. 52/75
Sunday: Morning clouds, then mostly sunny.  51/79
Monday: Morning clouds, then mostly sunny.  51/80
Tuesday: Mostly sunny.  52/84
Wednesday: Mostly sunny.  52/83
Thursday: Morning clouds, then mostly sunny.  52/82
Friday: Morning clouds, then mostly sunny.  52/81

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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