[willamette-fcst] Silverton Hills AM Forecast_doug.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT WED JUL 24, 2013



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is allowed from 2:00 p.m. to 4:00 p.m. with a 50 acre limit.



WEATHER DISCUSSION:



A broad upper level ridge, centered over the Pacific northwest, continues over the state today. Some mid-level moisture has worked its way up into central OR overnight, but low clouds are confined to the immediate coastline. Pressure gradients are currently weak-onshore with a 1.7 mb onshore gradient from Newport to Salem and a 1.0 mb onshore gradient from Salem to Redmond.



Expect little change from yesterday with the ridge maintaining strength this afternoon and evening. Mostly sunny skies and warm temperatures will continue to dominate the valley. Isolated light showers will affect the Cascade Range. Transport and surface winds will remain northerly through the day.



(Salem Airport data for Tuesday, July 23rd: High 94°F; Rainfall .00")

(Maximum Ventilation Index expected today:  35)

TODAY'S FORECAST:



Expect mostly sunny skies with above average temperatures continuing.



Salem's high temperature today will be near 90.

Relative humidity will drop to 45-50% by noon and 30-35% late this afternoon.

Surface winds: N to NE at 5-10 mph.

Transport winds: N at 10-15 mph.

Maximum mixing height:  3500 ft.

Sunset tonight: 8:47 p.m.



EXTENDED DISCUSSION:



The upper level ridge persists through Thursday over the region, with only some limited mid-level moisture over southern OR and very warm conditions continuing.  Low level wind flow will remain mostly northerly over the valley.



On Friday the upper ridge appears likely to begin to weaken, as troughing develops on the coast.  This scenario should increase the chances for an onshore gradient to develop late in the day. More precise timing of that change is uncertain at this time.



The National Weather Service's digital forecast is available at:

http://forecast.weather.gov/MapClick.php?lat=44.90549606158295&lon=-122.8106689453125&site=pqr&unit=0&lg=en&FcstType=text



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.



Doug Wesley

ODF Meteorologist


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