[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:35 AM PDT WED AUG 22, 2012



BURN ADVISORY:



Recommend agricultural burning, from Salem north, be limited to the period from now until 6:00 p.m.

Agricultural burning is not recommended south of Salem.

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

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



WEATHER DISCUSSION:



A dry and stable westerly flow aloft continues over the Pacific Northwest.  In its wake of Tuesday's trough, onshore flow forced marine low clouds into western Washington and the northern Willamette Valley this morning.  Patchy low clouds have penetrated inland as far south as the Silverton Hills, with dry north-northeasterly transport winds keeping skies generally clear across the southern Willamette Valley.



Cooler air aloft and plenty of sunshine will help mixing heights climb to 5000 feet by late this afternoon.  Moderate midday gradient-stacking may get balanced out by afternoon heating, but brisk northerly winds will likely maintain unfavorable open burning conditions.  Pibal monitoring will begin at 2:00 p.m., to ensure that no burning opportunities are missed.



TODAY'S FORECAST:



Partly cloudy and cooler with breezy north winds.



Salem's high temperature today will be near 80 degrees (normal is 82).

Relative humidity drops to near 35% by 5 p.m.

Surface winds:  N 8-14 mph this afternoon.

Transport winds: N 12-18 mph this afternoon.

Mixing height: Rising to 5000 feet by 5 p.m.

Salem's sunset tonight: 8:06 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 74      79      69

     Relative Humidity:           40%     34%     49%

     Surface Wind Dir/Speed:     N 12    N 12   NW 12

     Transport Wind Dir/Speed:   N 15    N 15   N  16

     Mixing Height:              4000    5000    2500

     Ventilation Index:           60      75      40



EXTENDED DISCUSSION:



A stronger upper-level trough is forecast to increase the onshore flow on Thursday.  Morning marine clouds should be more expansive across the Willamette Valley, but no precipitation is expected as far south as the Silverton Hills.  Valley temperatures should only climb into the mid 70s with skies becoming partly cloudy in the afternoon.  Northwesterly transport winds and cool air aloft will likely make for favorable open burning conditions.



On Friday, the trough will push east of the region.  Drying northerly winds will clear skies in the afternoon with temperatures recovering into the upper 70s.  An even stronger trough is forecast to dig offshore Saturday with southwesterly flow aloft maintaining dry weather, onshore flow, and near-normal temperatures.  Long-range computer models have been inconsistent on their timing of bringing that trough onshore.  It may spread showers across the region as soon as Sunday afternoon.



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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