[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 08 2011



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is allowed from 12:30pm to 5:00pm with a 50 acre limit.

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





WEATHER DISCUSSION:



In the upper atmosphere flow is very light with a weak trough off the Oregon and northern

California coastline.  The morning ODF surface analysis shows onshore flow with a thermal

trough in the central valley of California and another thermally induced trough centered

in eastern Washington extending southward into central Oregon.



The morning Salem sounding showed a marine layer similar to yesterday - topping out

around 3500 feet.  The cloud layer is much thinner, however, with bases around 1900 feet

and top sat about 2300 feet.  The clouds should begin to breakup later this morning and

by noon expect mostly sunny skies.



The morning sounding showed a more unstable atmosphere (compared to yesterday) above the

marine layer and with the weak trough offshore moving eastward there's a good chance of

isolated afternoon or evening thunderstorms over the southern and central Oregon

Cascades.



The weather pattern is conducive to northerly winds all day and computer models show just

that:  winds north or north-northeast through most of the transport layer then backing to

very light west-southwest at the top of the mixing layer and above.  Conditions are not

favorable for good smoke evacuation to the east today.  If any opportunity arose it

wouldn't be until early evening and then it would be marginal at best.





TODAY'S FORECAST:



Morning clouds, mostly sunny by afternoon.  Slight chance of thunderstorms over the

Cascades.

Salem's high temperature today will be near 80.

Relative humidity drops to 50% by 11am.  Minimum relative humidity 35%.

Surface winds:  N 4-7 mph slowly becoming NW 4-7 late afternoon or early evening.

Transport winds: N 5 mph slowly becoming NW 5-9 late afternoon or early evening.

Maximum mixing height:  4100 feet.

Sunset tonight: 8:29 pm





THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm

     Temperature:                 69       76      80      72

     Relative Humidity:           55%      40%     35%     46%

     Surface Wind Direction:     350      340     330     290

     Surface Wind Speed:           5        6       7       8

     Transport Wind Direction:   350      350     340     300

     Transport Wind Speed:         5        5       5       9

     Estimated Mixing Height:   2800     3800    4100    1000

     Ventilation Index:           14       19      20       9



EXTENDED DISCUSSION:



Forecast charts indicate Tuesday will be similar to today as a second weak upper level

trough moves in from the west.  At this point computer models do not show any significant

switch to a more westerly transport flow with this feature.



Weak upper level 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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