[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:40 AM PDT THU JUL 21, 2011



BURN ADVISORY:



Recommended burn times for agricultural burning are 12:00 p.m. to 7:00 p.m.

Prep burning is allowed from 12:00 p.m. until 2:00 p.m. with a 50 acre limit.

Propane flaming is allowed from now until 5:00 p.m.



WEATHER DISCUSSION:



A weak weather system moved onshore early this morning and deepened the marine layer across western Oregon.  Late-morning visible satellite imagery showed marine low clouds continuing to blanket all of western Washington and the northern two-thirds of western Oregon. Surface pressure gradients are still mostly southerly in the Willamette valley but are turning more westerly along the northern and central coast.



Light rain fell this morning over the northwestern corner of the state elray this morning.  Astoria picked up about one-tenth of an inch of rain with locally a couple of hundredths of an inch falling in the extreme northern Willamette Valley.  Light drizzle fell as far south as the Silverton Hills area.  Not much in the way of precipitation was falling late this morning.  Doppler radar was showing possible sprinkles in the northern coastal range and near the extreme northern Cascades.



Low-level southerly winds will turn more westerly, by late this afternoon, as the parent upper-level trough swings another weak weather system onshore.  Skies should stay mostly cloudy with a slight chance of light showers.  The weak weather system may help to partially break up the marine clouds, which would allow maximum temperatures to climb into the low 70s.



TODAY'S FORECAST:



Mostly cloudy & cooler.  Chance of light showers.



Salem's high temperature today will be near 72 degrees (normal is 83).

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

Surface winds:  S 10 mph; veering to W 10 mph later this afternoon.

Transport winds: SW 10-15 mph; veering to W 10-15 mph later this afternoon.

Mixing height: Rising to 4500 feet by 5 p.m.

Salem's sunset tonight: 8:51 p.m.





THREE-HOURLY DATA:

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

     Temperature:                 69      72      67

     Relative Humidity:           57%     51%     61%

     Surface Wind Direction:     220     270     290

     Surface Wind Speed:          10      10      10

     Transport Wind Direction:   240     270     300

     Transport Wind Speed:        12      13      15

     Mixing Height:             4000    4500    2500

     Ventilation Index:           48      59      38



EXTENDED DISCUSSION:



A drier northwesterly flow aloft is forecast for Friday, as the upper-level trough shifts east of the region.  Morning marine clouds should give way to mostly sunny skies in the afternoon.  Transport winds will turn northerly, and temperatures should recover into the upper 70s.  A flat ridge will move over Oregon Saturday for continued warming and drying of the air mass.



The flow aloft will turn southwesterly on Sunday, as the next upper-level trough approaches the coastline. Sunny skies and warming aloft will combine to lift high temperatures to around 90 degrees.  However, the warm-up will be short-lived.  Transport winds are forecast to turn southwesterly Sunday afternoon, in response to the approaching trough.



A significant marine push will cool temperatures back to normal on Monday and introduce at least a slight chance of showers.  A chance of showers continues through Tuesday morning.  A generally dry westerly flow aloft may present burning opportunities, later in the week, with varying degrees of onshore flow and moderate temperatures.



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



-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20110721/1597af5e/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Silverton Hills Midday Forecast.doc
Type: application/msword
Size: 31744 bytes
Desc: Silverton Hills Midday Forecast.doc
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20110721/1597af5e/attachment.doc>