[willamette-fcst] Silverton Hills AM Forecast.doc

Wed Aug 31 08:56:07 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT WED AUG 31, 2011

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from 11 a.m. until 2 p.m. with a 50 acre limit.

Propane flaming is allowed from 11 a.m. until 5 p.m.

WEATHER DISCUSSION:

A cool but generally dry upper-level trough is over the Pacific Northwest this morning.  An embedded disturbance rotated across northern Oregon overnight and deepened the marine layer over the Willamette Valley.  The Salem sounding this morning showed an inversion at the top of the marine layer, at about 7000 feet, with some drying of the air mass below 5000 feet, compared to Tuesday morning.  Winds were light southwesterly near the surface and veered to northwesterly up to 7000 feet. The freezing level was at just 10,000 feet, which is well below normal for the end of August.

Mid-morning satellite imagery showed low clouds blanketing the Willamette Valley, but there were breaks in the clouds along the coast and over the coastal range.  No rain was being reported in the valley, with temperatures in the mid to upper 50s.  Since the lower layers of the air mass are not saturated, at most only patchy morning drizzle is possible.

It appears as if conditions will become favorable for open burning today.  With the air aloft being so cool, daytime heating will rapidly lift mixing heights above 3000 feet later this morning.  Another upper-level disturbance is forecast to rotate across Washington and northern Oregon this afternoon, which will maintain the cool air aloft and likely allow mixing heights to rise to the top of the marine layer (around 7000 feet).  Transport winds are forecast to slowly veer from WNW to NW this afternoon.

Possible limiting factors for burning will be minor morning gradient-stacking, which should at least balance out this afternoon, and high surface humidity.  There is also a slight chance of an afternoon shower.

TODAY'S FORECAST:

Mostly cloudy.  Slight chance of morning drizzle and/or afternoon showers.

Salem's high temperature today will be near 70 degrees (normal is 81).

Relative humidity drops to near 50% from 2 p.m. through 5 p.m.

Surface winds:  W 3-6 mph this morning; NW 6-12 mph this afternoon.

Transport winds: WNW 5-10 mph this morning; NW 10-15 mph this afternoon.

Mixing height: Rises to 3000 feet by 11 a.m. and to near 7000 feet by 2 p.m.

Salem's sunset tonight: 7:51 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 63       67      68      63

     Relative Humidity:           60%      50%     50%     60%

     Surface Wind Direction:     270      310     320     330

     Surface Wind Speed:           4        6      10      10

     Transport Wind Direction:   300      315     330     340

     Transport Wind Speed:         6       12      15      20

     Mixing Height:             3000     7000    6500    3000

     Ventilation Index:           18       84      98      60

EXTENDED DISCUSSION:

The upper-level trough is forecast to push east of the Cascades by Thursday morning with the flow aloft warming and turning more northerly Thursday afternoon.  Transport winds will likely become mostly northerly, so the potential for open burning is poor.

An upper-level ridge will build over the Pacific Northwest Friday through Sunday for much warmer conditions.  Transport winds are forecast to become northerly on Friday and possibly offshore this weekend.  Increasing southerly flow aloft may introduce a chance of thunderstorms into the region by Sunday.

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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