[willamette-fcst] Willamette-fcst Silverton Hills Forecast

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT WED JUL 6, 2011



BURN ADVISORY:



Recommended burn times for agricultural burning are 10:00 a.m. to 7:00 p.m.



WEATHER DISCUSSION:



A ridge of high pressure will remain over the Pacific Northwest today with light southwesterly flow aloft over Oregon.  Mid-morning visible satellite imagery showed low clouds banked up along much of the coastal strip with minimal inland penetration.  Skies were sunny across the remainder of the state.  Surface pressure gradients will remain strongly northerly today, as a surface thermal trough tries to build northward into southwestern Oregon.



TODAY'S FORECAST:



Sunny and very warm.  Once again, today's high temperature will near 88 degrees.  The mixing height should climb to around 3000 feet between 1 p.m. and 2 p.m. and top out just over 4000 feet around 5 p.m.  Surface winds will be generally northerly at around 10 mph, and transport winds will also be mostly northerly at 15-20 mph.  Surface and transport winds will turn northwesterly in the evening. Relative humidity levels will drop to around 45% by 11 a.m. and to near 30% by 5 p.m.



Salem sunset tonight: 9:01pm



THREE-HOURLY DATA:



                                                       11am     2pm     5pm     8pm

     Temperature:                            76         83         88        80

     Relative Humidity:                 45%      36%     31%     43%

     Surface Wind Direction:      360        355        350       320

     Surface Wind Speed:                8          10          11        10

     Transport Wind Direction:   010      358       355      330

     Transport Wind Speed:           15       17          19        17

     Estimated Mixing Height:   2200     3500    4200    2500

     Ventilation Index:                     33        60          80        42



EXTENDED DISCUSSION:


An upper-level trough is forecast to approach the coastline Thursday and force the surface thermal trough into eastern Oregon.  That will induce a significant "marine push" into western Oregon during the day.  Look for increasing clouds with a slight chance of a light shower in the afternoon.  Temperatures will cool back to near or slightly below normal.

The upper-level trough is forecast to move across Washington and northern Oregon on Friday.  Strong onshore flow will bring considerable marine clouds inland with afternoon clearing.  Temperatures will cool to 5-10 degrees below normal with a chance of morning drizzle or light showers.  The pattern of southwesterly winds aloft, with onshore low-level flow, is forecast to continue into at least early next week.  Generally dry weather with seasonal temperatures can be expected.


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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