[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT WED SEP 11, 2013



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.



WEATHER DISCUSSION:



A strong ridge of high pressure is over Oregon today.  At the surface, a thermal trough extends from the southwestern interior of Oregon to over the northern coast range.  In response to the shifting trough position, onshore flow has forced marine low clouds and much cooler temperatures onto the coast and into the central and southern coastal range.  However, low-level offshore winds are keeping skies clear, and temperatures unseasonably warm, over the interior of western Oregon and across the northern coast range.



Mixing heights will be quite low today, due to exceptionally warm air aloft.  Winds at the top of the mixing layer will be southeasterly, which is not a favorable pattern for burning.  The upper-level ridge will slowly progress northeastward, to over eastern Washington and northern Idaho, this afternoon.  The surface thermal trough will shift over the Willamette Valley and possibly as far east as the Cascades.  Today should be the warmest day of this hot spell with valley temperatures climbing into the mid 90s.



State Fire Marshal Burn-Ban conditions will be approached this afternoon, but relative humidity levels will likely remain just above 30%.



TODAY'S FORECAST:



Sunny; near-record warm temperatures; light winds.



Salem's high temperature today will be near 94 degrees (normal is 78).

Relative humidity:  Drops below 60% by 11 a.m. and to near 40% by 5 p.m.

Surface winds: Var 2-6 mph this morning; becoming SW 2-6 mph this afternoon.

Transport winds: Var 2-6 mph; possibly turning SW 2-6 mph this afternoon.

Mixing height: Rising to only about 2000 feet by 5 p.m.

Salem's sunset tonight: 7:30 p.m.



(Salem Airport data for Tuesday, Sept. 10th: High 92°F; Rainfall .00")

(Maximum Ventilation Index expected today: 12)



EXTENDED DISCUSSION:



The surface thermal trough is forecast to shift into central Oregon on Thursday and eastern Oregon on Friday.  That will initiate a slow cooling trend, with high temperatures backing off into the mid to upper 80s by Friday.  Burning conditions will not be optimal either day, but northwesterly transport winds should allow for the burning of the few remaining fields, especially by Friday, which is timely considering the outlook for next week.



An upper-level trough is forecast to increase the southerly flow aloft over the weekend, with showers and thundershowers likely by Sunday. A damp and much cooler onshore flow pattern should bring autumn-like weather next week.



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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