[willamette-fcst] Silverton Hills Midday Forecast.doc

Fri Sep 23 11:47:22 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT FRI SEP 23, 2011

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is allowed from 1:00 p.m. until 5:00 p.m.

WEATHER DISCUSSION:

The first day of autumn will bring a continuation of summer-like weather across the state.  A persistent upper-level ridge of high pressure remains centered over the Rockies with a warm and dry southwesterly flow aloft over Oregon.

The weak frontal system that spread middle and high clouds across northwestern Oregon on Thursday dissipated overnight.  Satellite imagery this morning shows a broad band of clouds, associated with a strong southwesterly jet stream, extending from northern Vancouver Island, British Columbia to about 450 miles west of the northern Oregon coast. With the exception of coastal low clouds, skies are mostly sunny across Oregon.

The Salem sounding this morning was nearly identical to Thursday morning; showing a very warm air mass and continued southwesterly winds aloft.  At the surface, a weak thermal trough remains over southwestern Oregon with light pressure-gradients across the Willamette Valley.

The offshore frontal system is forecast to remain nearly stationary today.  Sunny skies and warm air aloft will help valley temperatures climb to a few degrees warmer than on Thursday and to more than 10 degrees above normal.  Like yesterday, south-southwesterly transport winds could create favorable afternoon burning conditions.

TODAY'S FORECAST:

Sunny and unseasonably warm.

Salem's high temperature today will be near 86 degrees (normal is 75).

Relative humidity drops below 50% by 1 p.m. and to near 40% by 5 p.m.

Surface winds:  S 3-6 mph this morning; becoming SSW 4-8 mph this afternoon.

Transport winds: SSW 10 mph this morning; SW 10-15 mph this afternoon.

Mixing height: Rising to 3000 feet by 1 p.m. and to near 4000 feet by 5 p.m.

Salem's sunset tonight: 7:08 p.m.

THREE-HOURLY DATA:

                               11 a.m.   2 p.m.  5 p.m.  8 p.m.

     Temperature:                 72       82      85      76

     Relative Humidity:           57%      45%     39%     55%

     Surface Wind Direction:     180      200     200     230

     Surface Wind Speed:           4        5       5       4

     Transport Wind Direction:   200      220     220     230

     Transport Wind Speed:        10       12      12      10

     Mixing Height:             1700     3200    4000    1000

     Ventilation Index:           17       38      48      10

EXTENDED DISCUSSION:

The flow aloft, and at transport level, will become more southerly, on Saturday, as an upper-level trough digs offshore.  The southerly flow will act to depress mixing heights.  Computer models are showing a slow-moving frontal system finally making it onshore Saturday night with rain likely spreading across the Willamette Valley by midday Sunday.  At least one-tenth of an inch of rain is likely Sunday with a stronger front bringing more than one-quarter of an inch of rain on Monday.  Long-range models are inconsistent after that time, but drier weather is likely for the middle of 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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