[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 WED AUG 3, 2011



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is allowed from noon until 5:00 p.m.



WEATHER DISCUSSION:



Late-morning visible satellite imagery showed patchy marine clouds along the northern and central coastline and clear skies over the Willamette Valley.  A weak thermal trough extends from central Oregon into south-central Oregon.  Onshore gradients are slowly increasing across western Oregon, with Newport to Salem at 1.3mb onshore and Salem to Redmond 1.5mb onshore, as of 11 a.m.



Onshore gradients should continue increase this afternoon, as an offshore upper-level trough slowly approaches the coastline.  Cooling aloft will yield favorable mixing heights for burning this afternoon.  However, considerable shear of the transport winds is likely again today and will be the limiting factor for burning.



Light northwesterly low-level winds are forecast to abruptly shift to south-southwesterly near the mixing height this afternoon. Pibals are scheduled to begin at 2 p.m.  Increasing onshore flow should rapidly drop mixing heights this evening with northwesterly winds dominating the transport flow.





TODAY'S FORECAST:



Sunny and warm.



Salem's high temperature today will be near 84 degrees (normal is 84).

Relative humidity drops to near 30% by 5 p.m.

Surface winds:  NW 5-10 mph this afternoon.

Transport winds: NW 4-8 mph; shifting to SSW 10 mph near the mixing height.

Mixing height: Rises 5500 feet by 5 p.m.; then drops to 2300 feet by 8 p.m.

Salem's sunset tonight: 8:36 p.m.





THREE-HOURLY DATA:

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

     Temperature:                 78      83      74

     Relative Humidity:           35%     30%     48%

     Surface Wind Direction:     320     310     300

     Surface Wind Speed:           4       7      10

     Transport Wind Direction:   300     310     300

     Transport Wind Speed:         5       7      15

     Mixing Height:             5000    5500    2300

     Ventilation Index:           25      39      35



EXTENDED DISCUSSION:



Gradient-stacking may inhibit burning on Thursday, if too much marine air pours into the valley tonight.  Otherwise, conditions for burning should be favorable on Thursday, as a weak upper-level trough moves across western Oregon.  A dry westerly flow aloft may create marginal burning conditions on Friday.  Temperatures are only expected to cool a couple of degrees each day.



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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