[willamette-fcst] Silverton Hills Midday Forecast.doc

Fri Sep 28 11:58:16 PDT 2012

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT FRI SEP 28, 2012

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.

WEATHER DISCUSSION:

The strong upper-level ridge that has been over the Pacific Northwest this week has shifted east of the region.  A minor upper-air disturbance is producing considerable middle and high clouds across the state but little to no precipitation.  Even though a weak westerly flow aloft has developed over Oregon, the air aloft remains quite warm and fairly stagnant.  Air quality instruments are still showing elevated smoke levels across the Willamette Valley, from wildfires in central Oregon.

A weak surface thermal trough, over western Oregon late this morning, is showing signs of shifting east, to over the Cascades.  Winds continue to be light across the Willamette Valley.  The thermal trough should slide east of the Cascades this afternoon.  That may induce some northwesterly transport flow, but ventilation conditions will be only marginal, at best, due to continued warm air aloft and slow wind speeds.  Transport winds will be monitored this afternoon, with pibals beginning at 1 p.m.

TODAY'S FORECAST:

Mostly cloudy.  Continued warm.

Salem's high temperature today will be near 78 degrees (normal is 73).

Relative humidity:  Dropping to near 50% by 2 p.m. and to near 45% by 5 p.m.

Surface winds:  Light north; becoming light NW later this afternoon.

Transport winds: N 3-6 mph; becoming NW 4-8 mph later this afternoon.

Mixing height: Rising to near 2300 feet by 2 p.m. and 3000 feet by 5 p.m.

Salem's sunset tonight: 6:57 p.m.

THREE-HOURLY DATA:

                                2 p.m.  5 p.m.  8 p.m.

     Temperature:                 74      77      68

     Relative Humidity:           48%     44%     59%

     Surface Wind Dir/Speed:     NW 3    NW 5    NW 4

     Transport Wind Dir/Speed:   NW 4    NW 6    NW 5

     Mixing Height:              2300    3000    1200

     Ventilation Index:            9      18       6

EXTENDED DISCUSSION:

A very weak cold front will pass mainly north of the state tonight, with only some areas of drizzle expected, mainly along the coast.  High pressure is forecast to quickly rebuild across the region this weekend.  Northerly winds on Saturday will turn offshore on Sunday, which will likely transport more wildfire smoke into the Willamette Valley.

An upper-level ridge will stay anchored over the Pacific Northwest through at least Tuesday.  Computer models are showing a strong offshore flow developing on Wednesday, which would maintain low mixing heights and likely transport even more wildfire smoke into western Oregon.  The strong upper-level ridge is expected to maintain dry conditions through next week.

The National Weather Service's digital forecast is available at:

http://forecast.weather.gov/MapClick.php?lat=44.90549606158295&lon=-122.8106689453125&site=pqr&unit=0&lg=en&FcstType=text

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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