[willamette-fcst] Silverton Hills Midday Forecast

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

12:00 PM PDT MON AUG 01 2011



BURN ADVISORY:



Agricultural burning burning is not recommended.

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

Propane flaming is allowed from now until 6:00pm.





WEATHER DISCUSSION:



Latest satellite pictures show clear skies over virtually all of Oregon with no evidence

yet of thunderstorm development east of the Cascades.



Marion County and vicinity temperatures were mostly in the upper 60's and low 70's as of

late morning, although an amateur weather station in Sublimity was reporting 76 degrees.

Surface wind reports were all light - under 5 mph - and were generally northerly.



Pressure gradients remain onshore.  Pressure has risin slightly at Newport but has fallen

at Redmond during the late morning hours.  Pressure gradients as of 11am included:

Newport to Salem, 1.3 mb onshore; Salem to Redmond, 2.2 mb onshore; and Newport to

Redmond, 3.5 mb onshore.  Thus gradient stacking to the east continues.



Short range computer models are split on the gradient stacking issue one showing a

continuation, the other showing slightly stronger gradients to the west of the Willamette

Valley late this afternoon.  We will continue to monitor.



THIS AFTERNOON'S FORECAST:



Sunny and warm.



Salem's high temperature today will be near 86.

Minimum relative humidity 29%.

Surface winds: N 5-7mph slowly backing to NW 7-9 during the afternoon

Transport winds: N 5-8 slowly backing to NW during the afternoon

Maximum mixing height: 5000 feet

Sunset tonight: 8:39 pm





THREE-HOURLY DATA:

                                     2pm     5pm     8pm

     Temperature:                     80      85      77

     Relative Humidity:               37%     30%     40%

     Surface Wind Direction:         340     330     300

     Surface Wind Speed:               6       7       8

     Transport Wind Direction:       340     330     300

     Transport Wind Speed:             5       5       5

     Estimated Mixing Height:       4200    5000    1000

     Ventilation Index:               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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