[willamette-fcst] Silverton Hills Forecast - Fri, Jul 16 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT FRI JUL 16 2010

BURN ADVISORY:

Recommended burn times for agricultural burning are from now to 6:00pm.
Prep burning is allowed from now to 2:00pm with a 50 acre limit. 
Propane flaming is allowed from now to 5:00pm.	
										
WEATHER DISCUSSION:

A weak upper-level trough, cutting across southern British Columbia, has induced an
onshore flow into western Oregon.  Areas of morning low clouds, across northwestern
Oregon, were giving way to sunshine except for over the extreme northwest corner of the
state.  The remainder of Oregon had sunny skies.  Morning low clouds were also beginning
to give way to sunshine across western Washington.  

Late-morning valley temperatures are running 5-10 degrees cooler than 24 hours ago with
most locations still in the 60s.  The morning sounding also showed several degrees of
cooling aloft, up to about 8000 feet.  However, the air aloft is still relatively warm, so
mixing heights will be slow to rise today.  The surface analysis shows the thermal trough
extending from eastern Washington, across central Oregon, to southwestern Oregon.  Surface
winds are generally northerly across the Willamette Valley, with considerable
gradient-stacking between Salem and Redmond.

As the upper-level trough pushes eastward, westerly winds aloft will maintain the
low-level onshore flow across northwestern Oregon.  Daytime heating will promote an
afternoon sea breeze, which should turn transport winds from northerly to northwesterly. 
The significant gradient-stacking will not likely be overcome this afternoon, but
conditions look favorable for limited early afternoon prep burning.

TODAY’S FORECAST:

Sunny but cooler.  After reaching 89 degrees on Thursday, Salem's high temperature today
will be near 82 degrees.  The mixing height will climb to 3000 feet in the early afternoon
with a maximum near 4000 feet around 5 p.m.  Surface winds will be north at 8-15 mph early
this afternoon, becoming NW 10-15 mph late this afternoon and evening.  Northerly
transport winds will also becoming NW at 10-15 mph in the late afternoon and evening.
Relative humidity will drop to around 35% this afternoon.

Salem's sunset tonight: 8:55 pm	
										
EXTENDED DISCUSSION:

A weak upper-level trough will maintain a typical summer-time pattern over the weekend
with morning clouds giving way to afternoon sunshine and near-normal temperatures.  The
trough is forecast to strengthen early next week with the onshore flow possibly becoming
strong enough to produce areas of morning drizzle or light showers.  Otherwise dry
conditions should prevail, but temperatures will cool to below normal with considerable
marine low clouds.  The trough is forecast to back away from the coastline by late next
week with southwesterly flow aloft and temperatures returning to normal.  The deep marine
layer may create too much gradient-stacking for open burning, during much of the extended
period, but there should be opportunities for limited prep burning.

EXTENDED FORECAST:	
     
Saturday: Morning clouds, then mostly sunny.  50/80  
Sunday: Morning clouds, then mostly sunny.  50/77  
Monday: Morning clouds, then partly sunny.  49/79  
Tuesday: Morning clouds, then partly sunny.  51/77  
Wednesday: Morning clouds, then partly sunny.  51/76
Thursday: Brief morning clouds, then mostly sunny.  52/82
Friday: Mostly sunny.  54/85 
										
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.