[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT FRI AUG 26, 2011



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



There has been little change to the overall weather pattern.  A large upper-level ridge of high pressure remains anchored over the south-central United States.  The clockwise flow around it is continuing to direct a warm south to southwesterly flow aloft over Oregon.



The upper-level disturbance that brought the thunderstorms to the Willamette Valley early this morning has moved northeast of the region, and more stable conditions are expected today.  The late-morning surface map showed a thermal trough extending from north-central Oregon through southwestern Oregon with very weak northerly pressure-gradients across the Willamette Valley.



Late-morning satellite imagery showed marine low clouds still banked up along most of the northern and central Oregon coast, but they were dissipating from the coastal mountain gaps.  Patchy low clouds did make it into the extreme northern and southern Willamette Valley, but elsewhere sunny skies prevailed.

With very warm air aloft, mixing heights will struggle to reach 3000 feet today, even with strong surface heating.



TODAY'S FORECAST:



Sunny and very warm.



Salem's high temperature today will be near 90 degrees (normal is 81).

Relative humidity drops to near 37% by 5 p.m.

Surface winds:  N 3-10 mph.

Transport winds: N 10-15 mph.

Mixing height: Rises to 3000 feet by 4 p.m. and to 3200 feet by 5 p.m.

Salem's sunset tonight: 8:00 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 85      89      79

     Relative Humidity:           42%     37%     47%

     Surface Wind Direction:     360     360     330

     Surface Wind Speed:           7       8       7

     Transport Wind Direction:   360     360     350

     Transport Wind Speed:        10      12      12

     Mixing Height:             2700    3200    1500

     Ventilation Index:           27      38      18



EXTENDED DISCUSSION:



The flow aloft is forecast to back to more southerly on Saturday; reintroducing a chance of thunderstorms, especially near and over the Cascades.  High temperatures will only cool a couple of degrees.  The upper-level flow is forecast to turn more southwesterly and stabilize on Sunday.  Weak onshore flow may bring some marine clouds into the valley with temperatures cooling into the mid 80s (still above normal).



Long-range models are showing a possible burning opportunity on Monday, as increasing southwesterly flow aloft pushes the surface thermal trough into Idaho and increases the onshore flow into western Oregon.



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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