[willamette-fcst] Silverton Hills Forecast

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT MON AUG 01 2011



BURN ADVISORY:



Agricultural burning burning is not recommended.

Prep burning is allowed from 12:00pm to 6:00pm with a 50 acre limit.

Propane flaming is allowed from 12:00pm to 6:00pm.





WEATHER DISCUSSION:



A broad but weak upper level trough covers western North America and the northeastern

Pacific this morning.  Satellite pictures show nearly clear skies over northwestern

Oregon with just a few areas of patchy morning fog.



Pressure gradients are weakly onshore and should remain onshore throughout the day,

although gradient stacking to the east will continue.



Low level winds are generally northerly this morning, but should turn more northwesterly

this afternoon.



The morning Salem sounding was similar to yesterday's morning sounding above 5000 feet

but showed some cooling below 3000 feet.  This has made the atmosphere a little more

stable today.  Inversions were noted from about 1700 feet to 2200 feet, from 2900 feet to

3000 feet and from 4100 feet to 4800 feet.  With ample heating today these inversions

should mix out, however,  and the maximum mixing height this afternoon should reach 5000

feet. A 3000 foot mixing height should be reached by about noon.



Good mixing heights this afternoon and a potential shift to a more favorable wind

direction late this afternoon may provide a burning opportunity.  The limiting factor

would be gradient stacking east, which implies atmospheric subsidence, which implies

smoke hanging up in the Cascades and drifting back into the Willamette Valley during the

overnight hours.



Winds and pressure gradients will be monitored carefully throughout the day.



TODAY'S FORECAST:



Sunny and warm.

Salem's high temperature today will be near 86.

Relative humidity drops to 50% by 10am.  Minimum relative humidity 29%.

Surface winds:  Northerly 4 to 6 mph this morning shifting to northwesterly 6-9 after

about 3pm.

Transport winds: Northerly about 5 mph this morning gradually backing to northwesterly

late this afternoon then westerly at about 5 this evening.

Maximum mixing height:  5000 feet.

Sunset tonight: 8:39 pm.





THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm

     Temperature:                 73       81      85      77

     Relative Humidity:           48%      35%     29%     39%

     Surface Wind Direction:     010      340     310     280

     Surface Wind Speed:           4        5       7       7

     Transport Wind Direction:   360      340     320     300

     Transport Wind Speed:         5        5       5       5

     Estimated Mixing Height:   2800     4200    5000    1000

     Ventilation Index:           14       21      25       5



EXTENDED DISCUSSION:



An onshore flow pattern should continue through the week with minor upper level

disturbances moving west to east mostly to the north of the area.  This should present

occasional burning opportunities that need to be assessed on a day to day basis.

Temperatures should be near to slightly above average and no rain is likely through

through the week and into the weekend.





The National Weather Service's digital forecast is available at

http://forecast.weather.gov/MapClick.php?lat=44.905&lon=-122.810.



Notes:

1.  Mixing height, as used here, is the lowest height at which the

    potential temperature exceeds the equivalent 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.



Jim Little

ODF Meteorologist













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