[willamette-fcst] Silverton Hills AM Forecast_doug.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT FRI AUG 23, 2013



BURN ADVISORY:



Recommended times for agricultural burning are 11:00 a.m. to 6:00 p.m.

Prep burning is not allowed.



WEATHER DISCUSSION:



Our meteorological setup is quite different today. We have mostly cloudy conditions over the Willamette Valley and Silverton Hills this morning, behind the upper wave that moved off to the north last night. Rain late yesterday has left us with wet fields.  Low clouds will be dominant through most of the morning as onshore flow is prevailing.

Generally southwesterly transport winds will likely prevail through the afternoon, with partly cloudy skies expected by early afternoon and cooler surface conditions. If the low cloud does burn off significantly, mixing heights from mid-afternoon through early evening will be favorable. The pressure gradient pattern will slowly improve this afternoon in this scenario and surface humidities should decrease to below 50% by mid afternoon. Ventilation will be significantly improved compared to yesterday after the low cloud coverage decreases. However conditions will be closely monitored as the low cloud may not burn off sufficiently.



(Salem Airport data for Thursday, August 22nd: High 86°F; Rainfall .02")

(Maximum Ventilation Index expected today:  30)

TODAY'S FORECAST:



Cooler today; mostly cloudy this morning becoming partly cloudy this afternoon.



Salem's high temperature today will be in the upper 70s.

Relative humidity drops to below 50% during the early afternoon. Late afternoon 40-45%.

Surface winds: Weak southerly during the morning.  SW to W at 5-10 mph by late afternoon.

Transport winds: WSW 10 mph by afternoon with some strengthening late this afternoon.

Maximum mixing height:  3000-3500 ft.

Sunset tonight: 8:04 p.m.



EXTENDED DISCUSSION:



Cooler than average surface temperatures and onshore conditions will likely prevail through the weekend, with the likelihood of morning low clouds. These low clouds should burn off at least partially by early afternoon over the area on both days, yielding to partly cloudy afternoons but with scattered showers developing late over the Cascades and coastal ranges. However in general widespread afternoon clouds over the Silverton Hills is not expected either Saturday or Sunday.



This onshore, cool pattern with periods of sunshine and a westerly component to afternoon transport winds looks to persist into early next week with potential favorable burning conditions.





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.



Doug Wesley

ODF Meteorologist


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