[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT THU JUL 24, 2014



BURN ADVISORY:



Recommended times for agricultural burning are from 8 a.m. until 7 p.m.



Prep burning is not allowed.



WEATHER DISCUSSION:



The strong upper-level trough that generally dumped between one-quarter and one-half inch of rain across the northern Willamette Valley on Wednesday is rapidly moving east of the region.  A few lingering showers this morning should give way to clearing skies this afternoon.  Brisk southerly surface winds will slowly relax and veer to the NW in the afternoon.  Very cool air aloft will provide excellent mixing but also maintain at least partly cloudy skies.  Even with some sunshine, temperatures will remain well below average.



TODAY'S FORECAST:



Showers tapering off with afternoon clearing.  Continued unseasonably cool.



Salem's high temperature today will be near 73°F (average is 84°F).

Relative humidity:  Dropping to 60% by noon and to near 40% by 5 p.m.

Surface winds: S 10 mph this morning; shifting to NW 5-10 by late afternoon.

Transport winds: SW 15-20 mph; shifting to NW 10-15 mph by late afternoon.

Mixing height: Above 5000 feet throughout the afternoon.

Salem's sunset tonight: 8:47 p.m.



(Salem Airport data for Wednesday, July 23rd: High 68°F; Rainfall: .26")

(Maximum Ventilation Index expected today: 90)



EXTENDED DISCUSSION:



A drying and warming trend will continue into the weekend, as an upper-level ridge of high pressure builds over the western US.  With mostly clear skies, temperatures will recover into the low 80s on Friday, then warm into the upper 80s on Saturday and low 90s on Sunday.



The axis of the upper-level ridge is forecast to set-up over Idaho and Montana.  Increasing southerly flow aloft may circulate enough mid and high level monsoonal moisture and instability over Oregon for thundershower development as early as Sunday evening...including west of the Cascades.



Very little change in the overall weather pattern is predicted though next week.  South to SW flow aloft will maintain a threat of afternoon and evening thunderstorms with above normal temperatures.  The main upper-level ridge axis is now predicted to stay close enough to the region to limit onshore flow into western Oregon; reducing the chance of burning opportunities.



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 15 mph.



     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 (ODA) and the Oregon Department of Forestry (ODF).  For

information contact ODA at 503-986-4701.



To add/remove your email address from this list, please go to:



http://listsmart.osl.state.or.us/mailman/listinfo/willamette-fcst



Pete Parsons

ODF Meteorologist
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