[willamette-fcst] Silverton Hills Forecast - Tue, July 27, 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT TUE JUL 27 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:

A slight increase in the onshore flow overnight allowed patchy marine low clouds to penetrate inland, into the extreme northern and southern ends of the Willamette Valley, by early this morning.  Late-morning visible satellite imagery showed the marine clouds had cleared from the interior valleys and were backing out of the coastal mountain gaps.  However, most of the coast was still cloudy.  Temperatures were running several degrees cooler than 24 hours ago and ranged from the mid 50s along the coast to near 70 in the Willamette Valley.  Winds in the valley were light and variable.

The surface map shows weak but even onshore flow across western Oregon, into a thermal trough stretching from east-central Washington, through central Oregon, to southwestern Oregon.  Gradient-stacking is not an issue today for open burning.  However, the onshore flow may be too weak to evacuate smoke over the Cascades and will need to be monitored this afternoon.

A weak upper-level trough is approaching the coastline and backing the winds aloft from southwesterly to more southerly.  That will act to limit the amount of onshore flow today and could also suppress mixing heights.  Afternoon thunderstorm development is likely again over central Oregon, which would raise pressures east of the Cascades and also act to reduce the onshore flow across western Oregon.

TODAY'S FORECAST:

Sunny but not quite as warm.  Salem's high temperature today will be near 87 degrees.  The mixing height will not climb to 3000 feet until around 2 p.m. and will likely top out near 3800 feet late this afternoon.  Surface and transport winds will be NW at just 5-10 mph this afternoon. Relative humidity will drop to near 30% late this afternoon.  The ventilation index will only climb to about 21.

Salem's sunset tonight: 8:44 pm

EXTENDED DISCUSSION:

A weak upper-level trough will slightly increase the southwesterly flow aloft over Oregon Wednesday and Thursday.  That will further increase the onshore flow at the surface and cool high temperatures a couple of more degrees each day.  The onshore flow may still be too weak to allow for open burning but will need to be monitored closely.  The upper-level trough is forecast to finally move onshore Friday, which may generate enough onshore flow t bring morning marine clouds into the Willamette Valley.  That will cool temperatures back to near normal and could also create an open burning opportunity.

The upper-level trough is forecast to move across Oregon over the weekend with the flow aloft turning northwesterly by Sunday.  That will continue to bring onshore flow and near-normal temperatures to the Willamette Valley.  Another weak upper-level trough may move into the region early next week, with little change in the overall weather conditions.

EXTENDED FORECAST:

Wednesday: Mostly sunny. Wind: NW 5-10 mph.  52/85
Thursday: Mostly sunny. Wind: NW 5-10 mph.  52/84
Friday: Morning clouds, then sunny.  Wind:  NW 5-15 mph.  50/82
Saturday: Morning clouds, then sunny.  50/80
Sunday: Morning clouds, then sunny. 50/83
Monday: Morning clouds, then sunny. 52/83
Tuesday: Morning clouds, then sunny. 52/82

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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