[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT THU JUL 19, 2012



BURN ADVISORY:



Recommended burn times for agricultural burning are from noon to 7:00 p.m.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



An upper-level trough remains parked off the southern Oregon coast with a moist southeasterly flow aloft over Oregon.  The low-level air mass is still stable over the Willamette Valley, but that will change later this afternoon.  The upper-level trough is forecast to slowly move northeastward, which will begin to cool the air mass aloft.  That will combine with daytime heating to destabilize the air mass, especially if we get many sunbreaks.  Look for showers to develop late this afternoon and evening.  There is also a slight chance that southeasterly flow aloft will direct thunderstorms, which form over the Cascades this afternoon, into the Willamette Valley this evening.



TODAY'S FORECAST:



Partial clearing this afternoon with an increasing chance of showers.  Slight chance of late-afternoon and evening thunderstorms. Showers are likely overnight with a continued slight chance of thundershowers. Rainfall totals mostly near one-tenth of an inch, but locally heavier amounts are possible.



Salem's high temperature today will be near 80 degrees (normal is 83).

Relative humidity drops to near 50% around 5 p.m.

Surface winds:  Light; Becoming WNW 5-10 mph late this afternoon and evening.

Transport winds: Light; Becoming W 6-12 mph late this afternoon but shearing to southeasterly at the top of the mixing layer.

Mixing height: Rising to near 3500 feet around 2 p.m. and 4500 feet at 5 p.m.

Salem's sunset tonight: 8:52 p.m.



THREE-HOURLY DATA:

                                2 p.m.  5 p.m.  8 p.m.

     Temperature:                 76      79      72

     Relative Humidity:           52%     47%     66%

     Surface Wind Direction:     340     280     270

     Surface Wind Speed:           3       6       7

     Transport Wind Direction:   320     270     270

     Transport Wind Speed:         3       8       8

     Mixing Height:             3500    4500    1800

     Ventilation Index:           11      36      14



EXTENDED DISCUSSION:



The slow-moving upper-level trough will finally lift northeastward, across Oregon, on Friday.  Showers are likely in the morning, with a continued risk of a thundershower.  The trough will progress into eastern Washington Friday afternoon, with a drier southwesterly flow aloft bringing some clearing.  Temperatures will remain a few degrees below normal.



Another upper-level trough is forecast to drop southeastward, from the Gulf of Alaska to just off the Washington coast, over the weekend.  That will maintain strong onshore flow with morning clouds and afternoon clearing.  Temperatures should remain slightly below normal. The trough is forecast to move across Washington and extreme northern Oregon on Monday. That will further strengthen the onshore flow and introduce a slight chance of light showers.  Temperatures will cool to as much as 10 degrees below normal.



A return to a more typical summer-time pattern is forecast for Tuesday through Friday of next week.  Look for morning clouds and afternoon sunshine, with temperatures recovering to near average.

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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