[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT FRI AUG 10, 2012



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is allowed from noon until 2 p.m. with a 50 acre limit.

Propane flaming is allowed from noon until 5 p.m.



WEATHER DISCUSSION:



The weak upper-level trough that moved onshore this morning was pushing into central Oregon at midday.  A reinforced marine layer, across western Oregon, is making for a slow break-up of the morning clouds, especially along the western foothills of the Cascades.  Expect sunny skies by mid-afternoon.



Moderate gradient stacking, which is not conducive to good smoke ventilation from the valley, may get balanced out by afternoon heating. However, transport winds are forecast to remain northerly and a bit brisk.  Pibals are scheduled to begin at 2 p.m., to verify the forecast of northerly winds.  An unlikely shift to northwesterly winds would be more favorable for burning.



TODAY'S FORECAST:



Turning sunny this afternoon.  A little breezy.



Salem's high temperature today will be near 82 degrees (normal is 83).

Relative humidity drops to 50% by 2 p.m. and to near 40% by 5 p.m.

Surface winds:  N 8-13 mph this afternoon.

Transport winds: N 12-18 mph this afternoon.

Mixing height: Rising to 4500 feet by 5 p.m.

Salem's sunset tonight: 8:25 p.m.



THREE-HOURLY DATA:

                                2 p.m.  5 p.m.  8 p.m.

     Temperature:                 76      82      74

     Relative Humidity:           47%     38%     50%

     Surface Wind Dir/Speed:     N 10    N 12   NW 12

     Transport Wind Dir/Speed:   N 14    N 16   N  18

     Mixing Height:              3800    4500    2500

     Ventilation Index:           53      72      45



EXTENDED DISCUSSION:



A warm-up is expected this weekend, as an upper-level ridge builds over the region.  Sunshine and offshore flow will likely push valley highs into the 90s.  A very weak upper-level trough is forecast flatten the ridge Monday and Tuesday; turning the low-level flow onshore and possibly creating favorable burning conditions.  Temperatures will likely cool back to normal by Tuesday.



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

http://forecast.weather.gov/MapClick.php?lat=44.90549606158295&lon=-122.8106689453125&site=pqr&unit=0&lg=en&FcstType=text



Notes:



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

         potential temperature exceeds the 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.



Pete Parsons

ODF Meteorologist
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