[willamette-fcst] Silverton Hills Forecast - Fri, Jul 23 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT FRI JUL 23 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from 11:00am to
     7:00pm.
     Prep burning is not allowed. 
     Propane flaming is not allowed.		
										
WEATHER DISCUSSION:

An upper-level ridge of high pressure is forecast to build over the region today with
transport winds turning northerly and temperatures warming to above normal.  Visible
satellite imagery shows marine low clouds covering much of the Willamette Valley at
mid-morning.  However, low clouds were already clearing from all but the extreme north
coast, with northerly winds rapidly drying the air mass.  Skies were sunny over
southwestern Oregon and all areas from the Cascades eastward.

The surface map shows the onshore flow is weakening and becoming northerly across western
Oregon.  There is just 0.7 mb of onshore gradient from Newport to Salem and 2.0 mb onshore
gradient from Salem to Redmond.  A thermal trough was beginning to build into southwestern
Oregon with 6.0 mb of northerly gradient from Astoria to Medford.  North winds have
already increased to around 15 mph along the southern coast and in the southern Willamette
Valley, where gusts to around 25 mph are likely later this afternoon.

TODAY’S FORECAST:

Morning clouds, then sunny and warmer with brisk north winds.  After reaching 81 degrees
on Wednesday, Salem's high temperature today will be near 88 degrees.  The mixing height
will climb to near 3000 feet around noon and top out near 4000 feet around 5 p.m.  Surface
and transport winds will be northerly at 5-12 mph this morning…increasing to 10-15 mph,
with higher gusts, this afternoon. Relative humidity will drop to 50% by noon and bottom
out near 27% late this afternoon.  The combination of low relative humidity and increasing
northerly winds may put the valley into State Fire Marshal conditions later this
afternoon.  The ventilation index will climb to 60.

Salem's sunset tonight: 8:49 pm	
										
THREE-HOURLY DATA:		

                                 11am     2pm     5pm     8pm			
     Temperature:                 68       79      87      83  			
     Relative Humidity:           51%      35%     27%     34%     			
     Surface Wind Direction:     350      360     360     360 		
     Surface Wind Speed:          10       10      13       9    			
     Transport Wind Direction:   360      010     010     360 
     Transport Wind Speed:        10       14      15      13  
     Estimated Mixing Height:   2500     3300    4000    2000
     Ventilation Index:           25       46      60      26			
										
EXTENDED DISCUSSION:

The upper-level ridge of high pressure is forecast to continue building over the region
Saturday.  Transport winds will turn slightly offshore, with valley temperatures climbing
into the 90s.  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 but may also circulate
afternoon showers and thunderstorms northward along the Cascades.  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:	
     
Saturday: Sunny and very warm.  Wind: NNE 5-15 mph.  56/94  
Sunday: Mostly sunny. Very warm. Chance of t-storms near the Cascades. 58/94  
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: Morning clouds, then mostly sunny. 52/78
Friday: Morning clouds, then sunny.  50/83

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