[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 TUE AUG 02 2011



BURN ADVISORY:



Agricultural burning burning is not recommended.

Prep burning is not allowed.

Propane flaming is allowed from 1:00pm to 5:00pm.





WEATHER DISCUSSION:



A weak upper level disturbance moving through a broad but weak trough has brought mid and

high clouds to Washington but only some widely scattered clouds to Oregon.  The extensive

low cloudiness seen earlier on satellite pictures has mostly dissipated and fair weather

is the rule over the region.



Temperatures are in the upper 60's and low 70's as of late morning in the central

Willamette Valley.



Gradient stacking east is still quite strong.  As of 11am there was a total of 4.7mb

between Newport and Redmond but 3.3mb of that was to the east of Salem and the

Newport/Salem gradient was only 1.4 mb.



As temperatures warm in the Valley pressures should drop and the gradient stacking should

even out somewhat.  We will monitor pressures and gradients carefully through the

afternoon.





THIS AFTERNOON'S FORECAST:





Salem's high temperature today will be near 80.

Minimum relative humidity today should be about 31%.

Surface winds:  Mostly westerly this afternoon 5-7mph.

Transport winds:  Becoming WNW 3-6 and increasing during the early evening.

Estimated mixing height as of noon: 2100 feet.  Forecast maximum 4000 feet.

Sunset tonight: 8:38 pm





THREE-HOURLY DATA:

                                     2pm     5pm     8pm

     Temperature:                     77      80      73

     Relative Humidity:               40%     34%     44%

     Surface Wind Direction:         300     300     290

     Surface Wind Speed:               5       6       7

     Transport Wind Direction:       290     300     300

     Transport Wind Speed:             3       6      13

     Estimated Mixing Height:       3700    4000    1500

     Ventilation Index:               11      24      20



EXTENDED DISCUSSION:



The general pattern of weak troughiness over the western United states will continue

through the week.  Weather should stay dry.  A marine push today  or tonight would leave

a

strong inversion over the valley and strong gradient stacking east would likely preclude

burning tomorrow, but the pattern is generally favorable for potential burn opportunities

Thursday or Friday.



The National Weather Service's digital forecast is available at:

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







The National Weather Service's digital forecast is available 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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