[willamette-fcst] Silverton Hills Forecast - Fri, Aug 13 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT FRI AUG 13, 2010

…Fire Marshal Conditions are possible this afternoon and likely both Saturday and Sunday
afternoons…

BURN ADVISORY:

     Recommended burn times for agricultural burning are from now to 6:30pm
     Prep burning is not allowed. 
     Propane flaming is not allowed.		
										
WEATHER DISCUSSION:

An upper-level ridge is building just offshore with a dry north-northwesterly flow aloft
over western Oregon.  The late-morning surface chart showed a thermal trough continuing to
expand northward, from southwestern Oregon, into the Willamette Valley.  The low-level
offshore flow now extends westward to the northern Oregon coast, where skies have cleared
and temperatures have jumped to near 80 degrees.  Midday satellite imagery shows the
marine clouds also backing off from the central coast, where the onshore flow is also
reversing.  Onshore flow was keeping skies clouds along the south coast.  Skies are clear
over the interior of western Oregon.

Offshore gradients are increasing over the interior of western Oregon.  As of 11 a.m.
there was 2.2 mb of offshore pressure gradient between Salem and Redmond.  Increasing
northerly gradients, across the Willamette Valley, were already kicking winds up to near
15 mph.  Midday temperatures were already near 80 degrees.  Down-sloping northeasterly
winds and a full day of August sunshine will warm valley temperatures well into the 90s,
with very low humidity levels.  Northeasterly transport winds make the prospect for open
burning poor this afternoon.

TODAY’S FORECAST:

Sunny and warmer.  Salem's high temperature today will be near 93 degrees.  The mixing
height is forecast to rise to 5000 feet by 5 p.m.  Surface winds will turn
north-northeasterly and increase to 10-15 mph this afternoon.  NE transport winds will
increase to about 15 mph this afternoon. Relative humidity will drop to near 21% around 5
p.m.  The ventilation index will climb to 80 this afternoon. Persistent dry offshore winds
will hold evening temperatures up, with the mixing height slowly lowering to about 1500
feet by sunset.

Silverton area sunset tonight: 8:16 pm	
										
EXTENDED DISCUSSION (no changes from the morning forecast):

A strong upper-level ridge of high pressure will build over the Pacific Northwest Saturday
through Monday.  Sunny skies and dry offshore flow will bring near-record warmth to the
Willamette Valley. A weak upper-level trough is forecast to undercut the ridge and move
across northern California on Sunday, possibly bringing some thunderstorms to the southern
Oregon Cascades.

The ridge is forecast to peak over the region on Monday, with a stronger upper-level
trough approaching the southern Oregon Coast by Tuesday.  That system may circulate
thundershower activity further north, along the Cascades, and should initiate a cooling
trend.  That may create an open burning opportunity, as early as Tuesday afternoon, with
transport winds forecast to turn back onshore.

The long-range models are beginning to come into agreement that the upper-level trough
will come onshore Wednesday and/or Thursday.  Increasing onshore flow and continued
cooling may create an enhanced opportunity open burning opportunity one or both of those
days. Temperatures will likely cool back to normal by Thursday.  Strong onshore flow is
forecast on Friday with considerable marine clouds penetrating into the Willamette Valley.
 Gradient-stacking may be too great for open burning by that time.  

EXTENDED FORECAST:	
     
Saturday: Sunny with near-record warmth. NE wind 5-10 mph. 58/97
Sunday: Sunny with near-record warmth. Light wind. 59/98
Monday: Sunny with near-record warmth. Light wind. 61/99
Tuesday: Sunny. A little cooler. Wind possibly becoming W 5-15 mph. 61/94
Wednesday: Partly cloudy and cooler. Wind W 5-15 mph. 57/87
Thursday: Partly cloudy. Slight chance of showers. 54/80
Friday: Morning clouds with afternoon clearing. 51/77

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