[willamette-fcst] Silverton Hills AM Forecast.doc

Wed Jul 27 08:48:09 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:46 AM PDT WED JUL 27, 2011

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from 10:00 a.m. until 2:00 p.m. with a 50 acre limit.

Propane flaming is allowed from 10:00 a.m. until 5:00 p.m.

WEATHER DISCUSSION:

The Pacific Northwest is still under the influence of a broad upper-level trough, but it is beginning to weaken and push to the east.  A minor disturbance rotated across western Oregon early this morning.  It brought just some high clouds but helped reinforce the deep marine layer over the Willamette Valley.  Salem's sounding this morning showed a couple of degrees of cooling in the marine layer, from the surface to about 5000 feet, with minor warming aloft.  Visible satellite imagery showed low clouds covering most of western Oregon, from Coos and Douglas Counties northward

The surface map shows high pressure pushing into Washington and northwest Oregon and a thermal trough starting to build northward into southwestern Oregon.  That is increasing the northerly pressure-gradients across the Willamette Valley.  North winds were already blowing near 10 mph in the northern and central Willamette Valley, at mid-morning, with Eugene getting gusts over 20 mph.  Northerly surface and transport winds will slowly increase through this afternoon.

As the parent upper-level trough shifts east of the region today, the flow aloft will turn northwesterly and continue to slowly warm.  That will help clear the marine clouds from the western valleys, with mostly sunny skies expected this afternoon. The air mass is still cooler than normal, so daytime heating will yield high afternoon mixing heights.

TODAY'S FORECAST:

Morning clouds with afternoon sunshine.

Salem's high temperature today will be near 78 degrees (normal is 83).

Relative humidity drops to 60% about 10 a.m. and to near 35% by 5 p.m.

Surface winds:  N 10 mph; becoming NW 10 mph this evening.

Transport winds: N 12 mph this morning; N 12-15 mph this afternoon.

Mixing height: Rises to 3000 feet about 10 a.m. and to 5500 feet by 5 p.m.

Salem's sunset tonight: 8:44 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 66       74      77      73

     Relative Humidity:           56%      42%     36%     43%

     Surface Wind Direction:     360      360     350     320

     Surface Wind Speed:          10       10      10      10

     Transport Wind Direction:   360      360     360     340

     Transport Wind Speed:        12       12      14      15

     Mixing Height:             3500     5000    5500    3000

     Ventilation Index:           37       60      77      45

EXTENDED DISCUSSION:

Little change in the weather pattern is forecast for Thursday and Friday, with a dry west-northwesterly flow aloft and mostly northerly surface and transport winds.  Very slow warming of the air mass will help temperatures recover to near normal (low to mid 80s).

The flow aloft will turn southwesterly over the weekend, as a weak upper-level trough moves across British Columbia. Temperatures will likely warm a few more degrees with continued mostly sunny skies.  Another system is forecast to take a more southern track and approach the coastline early next week.  That may create burning opportunities, as northerly transport winds turn back onshore.

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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