[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT THU SEP 8, 2011



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



A strong and broad upper-level ridge continues to encompass most of the western half of the country this morning.  A very weak weather disturbance slid across northern California and southern Oregon overnight.  That provided enough mid-level instability to trigger a few high-based thunderstorms over the southern Cascades and south-central Oregon, with very little rainfall.



Satellite imagery showed considerable debris clouds are over north-central Oregon this morning, with a few showers possibly developing near The Dalles.  Low clouds are banked up along the northern and central coast with scattered mid-level clouds over the interior of western Oregon.  Skies remain somewhat smoky, from area wildfires, over western Oregon, with elevated nephelometer readings, especially at Lyons.



The Salem sounding this morning showed continued slight warming aloft of the air mass and light north to northeast winds below 5000 feet. At the surface, a thermal trough extends from near The Dalles, through the Willamette Valley, to the southern Oregon coast.  Pressure gradients are onshore from Newport to Salem and offshore from Redmond to Salem.



The air mass is unseasonably warm between 2500 feet and 6500 feet, so high temperatures will be able to climb well into the 90s again today across the Willamette Valley.  The lower atmosphere is quite stable, and mixing heights will not likely even reach 3000 feet this afternoon A few more clouds and higher humidity levels should cap high temperatures at about where they were yesterday.



A weak upper-level disturbance is forecast to slowly move into central Oregon today, so afternoon thunderstorm development will likely be confined from the Cascades eastward.  However, an evening thunderstorm can't be ruled-out, especially near the Cascade foothills.



TODAY'S FORECAST:



Partly cloudy and continued very warm. Slight chance of an evening thunderstorm, mainly near the Cascades.



Salem's high temperature today will be near 93 degrees (normal is 79).

Relative humidity drops to 50% by 11 a.m. and to near 30% by 5 p.m.

Surface winds:  Light this morning; NNW 5 mph this afternoon.

Transport winds: Light this morning; N 10 mph this afternoon.

Mixing height: Only rises to about 2800 feet by 5 p.m.

Salem's sunset tonight: 7:36 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 75       85      92      79

     Relative Humidity:           50%      36%     29%     46%

     Surface Wind Direction:     Var      350     350     350

     Surface Wind Speed:           3        5       5       6

     Transport Wind Direction:   Var      350     360     360

     Transport Wind Speed:         3       10      10      12

     Mixing Height:             1200     2000    2800    1000

     Ventilation Index:            4       20      28      12



EXTENDED DISCUSSION:



The strong upper-level ridge is forecast to remain over the Pacific Northwest through this weekend.  The flow aloft will turn north-northeasterly, which should decrease the thunderstorm threat.  High temperatures will likely remain in the low to mid 90s.  North-northeasterly transport winds will continue to circulate smoke, from area wildfires, over the region.



A major change to the weather pattern is in store next week, which will likely bring more favorable burning conditions.  The strong upper-level ridge is forecast to finally weaken and shift east of the region about next Tuesday.  That will force the surface thermal trough into eastern Oregon and initiate a "Marine Push" into western Oregon.  The timing of this transition to onshore flow could create a significant burning opportunity.  A weak cold front may also create a burning opportunity around mid-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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