[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. JUL 12, 2013



BURN ADVISORY:



Recommended burn times for agricultural burning from now until 7:00 p.m.

Prep burning is allowed from now until 3:00 p.m. with a 50-acre limit.



WEATHER DISCUSSION:



A moderately strong upper trough continues to be entrenched along the Pacific coastline today, with generally dry southwesterly flow aloft over western Oregon. Some energy within the trough is pushing eastward and will increase our cloud cover as well as assist an onshore eastward surge late this afternoon and evening. The leading edge of the cloud cover is currently moving into northwestern Oregon.



The upper trough remains in place for the next several days, with more weak waves of energy skirting western Oregon as they periodically progress eastward.



Cloud cover over western Oregon and the surge will limit valley maximum temperatures to the middle to upper 70s this afternoon.  Transport winds already have a weak westerly component, and some increase in NW flow is expected this afternoon and evening.



TODAY'S FORECAST:



Becoming partly cloudy with seasonably cool temperatures.



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

Relative humidity:  Will drop to 40-45% by 5 p.m.

Surface winds:  NW 5-10 through the afternoon; NW 7-15 mph this evening.

Transport winds: NW 5-10 increasing to 10-15 this afternoon.

Mixing height: Rising to near 5000 feet this afternoon.

Salem's sunset tonight: 8:58 p.m.



(Salem Airport data for Thursday, July 11th: High 79°F; Rainfall .00")

(Maximum Ventilation index expected today: 60)



EXTENDED DISCUSSION:



The upper-level trough just to our west should persist through the weekend, with a portion of the trough progressing eastward into eastern OR and WA on Saturday. Expect some morning clouds and continued partly cloudy conditions through the day with maximums in the upper 70s.



Upper-level troughing will persist just west of Oregon Sunday.  No showers are expected, but the threat of some morning low clouds over the Willamette Valley persists. Mostly sunny conditions are likely in the afternoon.  Maximum temperatures will likely warm to the low 80s. On Monday and Tuesday the slow warming is likely to continue with dry conditions persisting.



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.



Doug Wesley

ODF Meteorologist

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