[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:43 AM PDT FRI JUL 29, 2011



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



A weak system cutting across British Columbia has turned the flow aloft more westerly this morning.  Salem's morning sounding showed westerly winds above 9000 feet and northwesterly winds from 9000 feet down to about 5000 feet.  Below 5000 feet, north to northeasterly winds still prevailed.


The surface map shows weak onshore flow across the northwestern corner of Oregon, with the flow turning light northerly from about Marion County southward across the central and southern Willamette Valley.  Northerly pressure-gradients are stronger across southwestern Oregon, into a weak thermal trough trying to build northward from California.



Visible satellite imagery showed marine clouds had advanced back onto the northern and central coastline.  Weak onshore flow had also pushed low clouds up the Columbia River into sections of the extreme northern Willamette Valley.  Elsewhere across western Oregon, north to northeasterly low-level winds were keeping skies clear.



The weather system moving across British Columbia is too far away to change the wind-flow to onshore across the central and southern Willamette Valley today.  North-northeasterly low-level winds are forecast to turn due northerly this afternoon.  That should keep skies clear with similar temperatures and mixing heights to yesterday.



TODAY'S FORECAST:



Sunny and warm.



Salem's high temperature today will be near 86 degrees (normal is 83).

Relative humidity drops to 50% about 11 a.m. and to near 30% by 5 p.m.

Surface winds:  NNE 5-10 mph this morning; N 8-12 mph this afternoon.

Transport winds: NNE 8-12 mph this morning; N 15 mph this afternoon.

Mixing height: Rises to 3000 feet about noon and to 4500 feet by 5 p.m.

Salem's sunset tonight: 8:42 p.m.





THREE-HOURLY DATA:

                               11 a.m.   2 p.m.  5 p.m.  8 p.m.

     Temperature:                 71       80      85      78

     Relative Humidity:           51%      38%     31%     42%

     Surface Wind Direction:     010      360     360     350

     Surface Wind Speed:           6        8      10      10

     Transport Wind Direction:   015      360     360     360

     Transport Wind Speed:        11       13      15      18

     Mixing Height:             2500     4000    4500    2800

     Ventilation Index:           28       52      68      50



EXTENDED DISCUSSION:



The flow aloft will turn southwesterly over the weekend, as an upper-level trough moves inland and across southern British Columbia. Low-level winds will likely stay northerly across the Willamette Valley with mostly sunny skies and little change in temperatures.  Weak onshore flow, at times, next week will keep temperatures near normal and could create some favorable 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 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 times

         the transport wind speed 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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