[willamette-fcst] Silverton Hills Forecast - Noon Update

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

12:00 PM PDT MON AUG 08 2011



BURN ADVISORY:



Agricultural burning burning is not recommended.

Prep burning is allowed from 12:30pm to 3:00pm with a 50 acre limit.

Propane flaming is allowed from 12:30pm to 5:00pm.





WEATHER DISCUSSION:



A weak upper level trough is swinging over the Pacific Northwest today.  The marine layer

is hanging tough and late morning satellite pictures show very few breaks.  Consequently

valley temperatures have been slow to warm, with low to mid 60's common throughout the

region as of late morning.  I have lowered the forecast high temperature for today.



Strong gradient stacking continues and is unlikely to even out this afternoon.  Pressure

gradients as of 11am included:  Newport to Salem, just 0.9 mb onshore; Salem to Redmond,

4.2 mb onshore; total Newport to Redmond, 5.1 mb onshore.  Meanwhile the Portland to

Medford gradient was 4.3 mb northerly.



Latest models are a bit more optimistic about transport winds with more northwest than

shown earlier.  However the University of Washington models show winds actually going

light northeasterly above a couple thousand feet by about sunset.  This may reflect the

possibility of thunderstorm buildups over the Cascades later this afternoon.  In any case

northeasterly winds developing in the transport region would argue against allowing any

burning this afternoon.





THIS AFTERNOON'S FORECAST:



Cloudy through early afternoon then partly cloudy.

Salem's high temperature today will be near 77. (Average: 85)

Minimum relative humidity will be about 40%.

Surface winds: N to NW 6-10.

Transport winds: N-NNW 5 becoming NW 8-10 late afternoon or early evening.

Maximum mixing height:  3900

Sunset tonight: 8:29 pm





THREE-HOURLY DATA:

                                     2pm     5pm     8pm

     Temperature:                     71      77      70

     Relative Humidity:               53%     40%     49%

     Surface Wind Direction:         340     330     300

     Surface Wind Speed:               6       8      10

     Transport Wind Direction:       350     340     300

     Transport Wind Speed:             5       5       9

     Estimated Mixing Height:       3400    3900    1000

     Ventilation Index:               19      20       9



EXTENDED DISCUSSION:



Latest forecast charts show another weak upper level low approaching the Pacific

Northwest tomorrow for a very similar weather pattern to today's.  At this point computer models do

not show any significant switch to a more westerly transport flow with this feature.



Weak upper ridging should limit the morning marine clouds later in the week, but a

northerly transport wind pattern will continue through the week for very limited, if any,

burn opportunities.





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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