[willamette-fcst] Silverton Hills Forecast - Thu, Jul 22 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT THU JUL 22 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from now to 7:00pm.
     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 dry cold front is moving across northeastern Oregon, at midday, and initiating moderate
to strong onshore flow into northwestern Oregon.  Visible satellite imagery shows marine
low clouds continuing the cover the extreme northwestern corner of Oregon.  Late-morning
temperatures were only in the upper 50s along the north coast, with some areas of light
drizzle.  Meanwhile, patchy morning low clouds had cleared from the northern Willamette
Valley, and temperatures had climbed into the upper 60s and low 70s. Skies were sunny over
the remainder of the state, except for a few clouds pushing across northeastern Oregon, in
association with the dry cold front.

The surface map still showed some gradient-stacking across western Oregon late this
morning, with 2.6 mb onshore from Newport to Salem and 4.3 mb onshore from Salem to
Redmond.  Like yesterday, daytime heating should balance out the onshore flow, but
transport winds will likely remain too northerly for open burning.  The air aloft is
cooler than on Wednesday, so even though midday temperatures are slightly warmer than 24
hours ago, a cooling sea breeze should cap afternoon highs short of Wednesday’s readings.

TODAY’S FORECAST:

Sunny but a touch cooler.  After reaching 82 degrees on Wednesday, Salem's high
temperature today will be near 80 degrees.  The mixing height will climb to near 3000 feet
in the early afternoon with a maximum near 3800 feet around 5 p.m.  Surface and transport
winds will be NNW at 8-15 mph this afternoon and early evening. Relative humidity will
drop to near 38% by late this afternoon.  The maximum ventilation index will be near 50
today.

Salem's sunset tonight: 8:50 pm	
										
EXTENDED DISCUSSION:

An upper-level ridge of high pressure is forecast to build over the region Friday and
Saturday.  Transport winds will turn northerly Friday, and perhaps slightly offshore
Saturday, with temperatures climbing to well above normal.
The ridge is forecast to slide east of the region by Sunday with increasing
south-southwesterly flow aloft introducing a chance of showers or afternoon thunderstorms
to southern Oregon.  These storms should stay south and east of the Willamette Valley.

Increasing southwesterly flow aloft is forecast to eventually turn transport winds onshore
early next week.  That could present open burning opportunities, if the flow aloft does
not become too southerly and bring thunderstorms into the valley.  The flow is forecast to
turn more westerly by the middle of next week, with temperatures cooling back to near
normal.  That transition could also create open burning opportunities.

EXTENDED FORECAST:	
     
Friday: Sunny and turning much warmer.  Wind: N 5-15 mph.  50/88
Saturday: Sunny and very warm.  Wind: NNE 5-15 mph.  56/94  
Sunday: Mostly sunny. Very warm. Chance of t-storms near the Cascades. 58/92  
Monday: Partly cloudy.  Slight chance of showers or t-storms.  58/86  
Tuesday: Partly cloudy.  Slight chance of showers or t-storms.  55/82  
Wednesday: Morning clouds, then mostly sunny.  53/80
Thursday: Patchy morning clouds, then sunny. 52/84

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