[willamette-fcst] Silverton Hills Forecast - Thu, Jul 08 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT THU JUL 08 2010

BURN ADVISORY:

Agricultural burning is not recommended.
Prep burning is not allowed. 
Propane flaming burning is not allowed.									
										
WEATHER DISCUSSION:

A strong ridge of high pressure remains over the Pacific Northwest, but there are some
changes in the weather pattern today.  The strong surface thermal trough that was just
off
the northern Oregon coast on Wednesday has shifted eastward into the Willamette Valley. 
That has changed the wind-flow pattern from offshore to onshore along the coast with
light
winds in the Willamette Valley.

Midday visible satellite imagery showed the marine low clouds, which briefly penetrated
into some of the western coastal range gaps this morning, had backed off to the immediate
coastline.  Skies were sunny over the remainder of Oregon, except for some shower and
thunderstorm activity beginning to build up again across extreme southwestern and
south-central Oregon.  Midday temperatures were running about 5-8 degrees cooler than 24
hours ago, across the Willamette Valley, and today’s forecast high temperature has been
lowered a few degrees.

A weak upper-level trough, just off the northern California coastline, will continue to
circulate enough moisture northward for afternoon showers and thunderstorms over mainly
southern Oregon.  A few storms could push as far north as the central Cascades, but the
convective activity is forecast to stay south and east of the Willamette Valley.  The
surface thermal trough is forecast to remain over the valley through this afternoon and
then shift eastward to over the Cascades this evening.  Light winds may turn
northwesterly
this evening, with some cooler marine air beginning to filter into the Willamette Valley.

TODAY:

Sunny and hot.  Light north winds turning northwesterly this evening.  High near 95
degrees.  Minimum relative humidity will be near 24%.

Salem Sunset: 8:55 pm	
							
										
THREE-HOURLY DATA:		

                                 2pm     5pm     8pm			
     Temperature:                 89      94      86  			
     Relative Humidity:           28%     24%     32%     			
     Surface Wind Direction:     350     340     330 		
     Surface Wind Speed:           7       7       7    			
     Transport Wind Direction:   360     350     340 
     Transport Wind Speed:         5      10       7  
     Estimated Mixing Height:   3000    3800    1500
     Ventilation Index:           15      38      10				
										
EXTENDED DISCUSSION:

A weak upper-level trough is forecast to push across Oregon Friday with a return of
onshore flow cooling temperatures a few more degrees.  There may still be a few
thunderstorms over the Cascades, but most of the convective activity should remain well
east of the Willamette Valley.  A dry northwesterly flow aloft will prevail Saturday
through Tuesday with temperatures progressively cooling back to near normal.  Increasing
onshore flow may bring patchy marine clouds into the Willamette Valley by Sunday morning.

The latest long-range models are now showing a more significant trough moving into the
region Wednesday and Thursday of next week.  That would further cool temperatures and
possibly bring some light showers.

EXTENDED FORECAST:	
     
Friday:    Mostly Sunny and a touch cooler. Wind: NW 5-12 mph.  58/92
Saturday:  Mostly Sunny but cooler.  Wind: NW 5-15 mph.  59/87
Sunday:    Patchy morning clouds.  Sunny in the afternoon.  56/85
Monday:    Patchy morning clouds.  Sunny in the afternoon.  55/85
Tuesday:   Patchy morning clouds.  Sunny in the afternoon.  53/82
Wednesday: Mostly cloudy.  Slight chance of showers north.  52/77
Thursday:  Mostly cloudy.  Chance of showers…mainly north.  52/74
										
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.