[willamette-fcst] Silverton Hills Midday Forecast.doc

Fri Aug 12 11:56:51 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 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:

A dry southwesterly flow aloft will bring sunshine and average mid-August temperatures to the Willamette Valley again this afternoon.  However, a significant trough is dropping into the Gulf of Alaska and will bring cooler temperatures over the weekend and a good chance of light rain by Monday.

The mainly coastal marine low clouds were beginning to dissipate late this morning with generally sunny skies over the Willamette Valley.  The late-morning surface map continues to show a weak thermal trough extending from central Washington to southwestern Oregon with slowly increasing north-northwesterly gradients across western Oregon.  The orientation of the onshore pressure-gradients is favorable for burning today.  The question will be whether to transport winds assume enough of a westerly component for valley smoke evacuation.

Slightly warmer temperatures aloft will suppress mixing somewhat this afternoon, but warmer surface temperatures should help maximum mixing heights reach 4000 feet late in the day. As the southwesterly flow aloft increases this afternoon, so will the low-level onshore flow.  Computer models are forecasting northerly transport winds to slowly back to north-northwesterly this afternoon, which would be more favorable for valley smoke evacuation.

Potentially shifting transport winds will need to be monitored closely today, and pibals are scheduled to begin at 1 p.m.  Marine air is forecast to penetrate into the Willamette Valley this evening, which would rapidly lower mixing heights.

TODAY'S FORECAST:

Sunny and warmer.

Salem's high temperature today will be near 84 degrees (normal is 83).

Relative humidity drops to near 30% by 5 p.m.

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

Transport winds: N 5-8 mph; backing to 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:

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

     Temperature:                 79      83      74

     Relative Humidity:           38%     31%     45%

     Surface Wind Direction:     355     345     320

     Surface Wind Speed:           6       7       9

     Transport Wind Direction:   350     335     310

     Transport Wind Speed:         8       8      13

     Mixing Height:             3200    4000    2000

     Ventilation Index:           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 marine clouds and several degrees of cooling both days.

An active 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 return, on Tuesday, and persist 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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