[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT MON AUG 26, 2013



BURN ADVISORY:



Agricultural Burning is not recommended.

Prep burning is not allowed.



WEATHER DISCUSSION:



A moist south-southwesterly flow aloft directed a weather system across Oregon Sunday afternoon and evening.  The bulk of the shower and thundershower activity was focused across a path from SW Oregon, along the Cascades, and over central Oregon.  However, some showers did make their way into the Willamette Valley, where rainfall totals were mostly around one-tenth of an inch.



An autumn-like upper-level trough remains anchored in the eastern Gulf of Alaska this morning, with brisk southwesterly flow aloft over Oregon. However, the region is between weather systems today, and some breaks in the clouds are already appearing across much of western Oregon.  The combination of slight warming aloft and some sun-breaks today will help afternoon temperatures climb back into the upper 70s.  In addition, south to SW winds should also aid in the drying of damp fields.  Brisk southwesterly transport winds and high afternoon mixing heights should be favorable for burning this afternoon, if fields can dry out enough.  Fluffing may be necessary.



Another weather disturbance will likely spread rain back onto the north coast by late this afternoon.  It may bring some light rain to the northern Willamette Valley this evening and tonight, as it stalls over NW Oregon.



TODAY'S FORECAST:



Mostly cloudy but warmer.  Chance of light rain late by evening.



Salem's high temperature today will be near 77 degrees (normal is 81).

Relative humidity:  Drops to 60% by noon and to near 45% by 5 p.m.

Surface winds:  S 5-10 mph this morning; SSW 10-15 mph this afternoon.

Transport winds: SWW 10-15 mph this morning; SW 15-20 mph this afternoon.

Mixing height: Rising to 3000 feet by noon and to near 6000 feet by 5 p.m.

Salem's sunset tonight: 7:59 p.m.



(Salem Airport data for Sunday, Aug. 25th: High 71°F; Rainfall .07")

(Maximum Ventilation Index expected today: 120)



EXTENDED DISCUSSION:



The flow aloft will turn more southerly by Tuesday, with a decaying frontal system keeping skies mostly cloudy over NW Oregon.  Southwesterly transport winds will likely remain favorable for burning, but continued warming aloft will begin to suppress mixing.  Dampness of fields may also limit burning.



A mostly dry day is forecast for Wednesday, ahead of another weather system slated to come onshore Thursday afternoon.  Some burning opportunities are possible each day, but that will depend of dampness of fields and the timing of incoming weather systems.



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 (ft) times

         the transport wind speed (mph) 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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