[willamette-fcst] Silverton Hills Midday Forecast_doug.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

12:00 PM PDT FRI AUG 23, 2013



BURN ADVISORY:



Recommended time for agricultural burning is from now to 6:00 p.m.

Prep burning is not allowed.



WEATHER DISCUSSION:



Mostly cloudy conditions continue late this morning over the Willamette Valley and Silverton Hills, with initiation of cloud dissipation confined to the higher Cascades. Wet fields continue and drying will be dependent on the cloud evolution.  Low clouds will be dominant through at least early afternoon and possibly later. Due to the lack of solar heating today, the gradient pattern in place is not favorable and mixing heights are currently rather low. We may be partly cloudy at best late this afternoon.

Generally southwesterly transport winds will likely prevail through the afternoon, with some clearing expected by mid afternoon and cool surface conditions. If the low cloud does burn off significantly, mixing heights from mid-afternoon through early evening will be somewhat favorable. The pressure gradient pattern will slowly improve this afternoon in this scenario and surface humidities should decrease to below 50% by mid afternoon if we get some clearing. However conditions will need to be closely monitored as the low cloud is not burning off yet and if this continues conditions for burning will not occur.



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

(Maximum Ventilation Index expected today:  30)

TODAY'S FORECAST:



Mostly cloudy early this afternoon, becoming partly cloudy late afternoon.



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

Relative humidity drops to below 55% during the early afternoon. Late afternoon 45-50%.

Surface winds: Weak southerly, becoming SW-W 5-10 mph by late afternoon.

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

Maximum mixing height:  2500-3000 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. Once again, for Saturday these low clouds may not burn off completely; the highest chance of burning off would be the higher hills. At this point, widespread cloudy conditions are doubtful for early afternoon, but a complicating factor is that cloudiness and showers will increase in those areas late in the day also as the heating increases. For Sunday, low clouds are also likely in the morning, and latest model guidance suggests an increased coverage of late-day showers and thunderstorms.



This onshore, cool pattern with periods of clouds and a westerly component in the transport winds looks to persist into early next week. On Monday and Tuesday, potential favorable burning conditions will likely exist if clearing and surface warming occurs during the early afternoon hours. At this point it appears that the better chance of this occurring is on 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.



Doug Wesley

ODF Meteorologist


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