[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 TUE SEP 24, 2013



BURN ADVISORY:



Recommended times for agricultural burning are from now until 5:00 p.m.



WEATHER DISCUSSION:



In the wake of Monday's cold front, a strong early-autumn upper-level trough is forecast to move over the Pacific Northwest Today.  That will maintain showery and quite cool conditions with a chance of thundershowers.  Satellite imagery shows clouds blanketing western Oregon this morning with radar and surface reports indicating numerous showers across the Willamette Valley.  Surface winds are light this morning, but onshore flow is expected to increase later today.



Daytime heating will combine with cold air aloft to destabilize the air mass this afternoon...leading to some thundershower development over western Oregon.

Conditions will be too damp for burning again today, with high relative humidity and continued shower activity.  Additional rainfall totals could exceed one-quarter of an inch in the Silverton Hills.



TODAY'S FORECAST:



Showers and continued unseasonably cool.  Chance of thundershowers.



Salem's high temperature today will be near 62 degrees (average is 74).

Relative humidity:  Remaining above 70% throughout the day.

Surface winds: W 2-5 mph this morning; SW 5-10 mph this afternoon.

Transport winds: SW 10-15 mph.

Mixing height: Rising to above 5000 feet this afternoon.

Salem's sunset tonight: 7:05 p.m.



(Salem Airport data for Monday, Sept. 23rd: High 64°F; Rainfall: .36")

(Maximum Ventilation Index expected today: 75+)



EXTENDED DISCUSSION:



A strong upper-level trough will maintain cool and showery conditions across western Oregon on Wednesday, but the strongest showers will move south of the Silverton Hills.  Rainfall totals should be around one-tenth of an inch.



A dry and cool northerly flow aloft will clear skies on Thursday.  After a chilly morning, daytime temperatures should recover to within about 5 degrees of average.  A weak upper-level ridge is forecast to move over the region on Friday.  Computer models are unclear as to whether it will be a strong enough to keep of weakening cold front from dragging across northern Oregon.  Some clouds are likely with temperatures recovering to near average.



A strengthening trough, in the Gulf of Alaska, is forecast to turn the flow aloft southwesterly over the weekend.  At this point, Saturday looks dry and warmer, but a slow-moving cold front is still slated to bring some rain back to the region by Sunday afternoon.  A flat upper-level ridge is forecast to keep Oregon on the southern edge of the storm track early next week, with sunny and much warmer weather possible beginning a week from Thursday.



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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