[willamette-fcst] Silverton Hills AM Forecast.doc

Fri Aug 12 08:44:11 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:40 AM PDT FRI AUG 12, 2011

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is allowed from 2 p.m. until 5:00 p.m.

WEATHER DISCUSSION:

The upper-level flow has turned southwesterly, with a fairly strong trough dropping into the Gulf of Alaska.  At the surface, a weak thermal trough extends from central Washington to southwestern Oregon with north-northwesterly gradients across western Oregon.  Onshore flow has strengthened along the coast but is still weak in the western interior.  Visible satellite imagery shows low clouds along the entire Oregon coastal strip this morning with some penetration into the coastal mountain gaps and up the Columbia River into the Portland area.  Skies were clear from Marion County southward across the Willamette Valley.

Salem's sounding this morning showed a few degrees of warming, below 6000 feet, compared to 24 hours ago.  That will make for slightly lower mixing heights and warmer surface temperatures today.  As the southwesterly flow aloft increases this afternoon, so will the low-level onshore flow.  Unlike yesterday, computer models are forecasting northerly transport winds to back to northwesterly by mid-afternoon, which would be more favorable for smoke evacuation from the valley.  Some marine air is forecast to penetrate into the Willamette Valley this evening, which would rapidly lower mixing heights.

TODAY'S FORECAST:

Sunny.

Salem's high temperature today will be near 85 degrees (normal is 83).

Relative humidity drops below 50% by 11 a.m. and to near 30% by 5 p.m.

Surface winds:  N 5 mph this morning; NW 5-10 mph this afternoon.

Transport winds: N 5-8 mph this morning; NW 6-10 mph this afternoon.

Mixing height: Rises to 3000 feet by 2 p.m. and to 4000 feet by 5 p.m.

Salem's sunset tonight: 8:23 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 71       80      84      74

     Relative Humidity:           50%      37%     30%     45%

     Surface Wind Direction:     360      350     330     300

     Surface Wind Speed:           5        6       7       9

     Transport Wind Direction:   360      340     320     300

     Transport Wind Speed:         7        8       8      13

     Mixing Height:             2100     3200    4000    2000

     Ventilation Index:           15       26      32      26

EXTENDED DISCUSSION:

Further strengthening of the southwesterly flow aloft is forecast over the weekend, as a broad upper-lever trough, in the Gulf of Alaska, slowly moves closer to our coastline.  Expect an increase in morning marine clouds and several degrees of cooling on Saturday.

A fairly impressive summer cold front is forecast to spread light rain onto the northern Oregon coast by Sunday evening and likely into the Willamette Valley by Monday.  Valley rainfall amounts, on Monday, could be on the order of one-tenth of an inch.  Dry weather is forecast to quickly return, on Tuesday, with a more typical summertime pattern of morning clouds and afternoon sunshine for the remainder 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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