[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 THURS. JUL 25, 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:



The broad upper level ridge over the Pacific northwest begins to weaken just a bit today. Some mid-level moisture remains over central/eastern OR, but low clouds are confined to the immediate coastline. Pressure gradients are currently weak-onshore with a 2.5 mb onshore gradient from Newport to Salem and a 1.2 mb onshore gradient from Salem to Redmond.



Expect little change from yesterday in the low levels today, as the effects of the movement of the upper ridge will not begin to be realized until late tomorrow. Sunny skies and very warm temperatures will continue to dominate the valley. Transport and surface winds will remain northerly through the day.



(Salem Airport data for Wednesday, July 24th: High 94°F; Rainfall .00")

(Maximum Ventilation Index expected today:  45)

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 40-45% by noon and 30-35% late this afternoon.

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

Transport winds: N at 10-20 mph.

Maximum mixing height:  4000 ft.

Sunset tonight: 8:46 p.m.



EXTENDED DISCUSSION:



The upper level ridge continues to move slowly eastward on Friday, with weak troughing beginning to build along the coastline.  Low level wind flow will remain mostly northerly over the valley through at least mid-afternoon. Sunny skies will likely continue through the day with cooler surface temperatures.



Late on Friday, the weak coastline trough development will increase the chances for a late-day marine push towards the valley. On Saturday and Sunday this trend towards increasing onshore flow continues at all levels with cooler low-level conditions and the likelihood of morning low clouds in the region. On Monday the onshore pattern is likely to continue with cool temperatures and good potential for low clouds and a marine push, although the timing is not clear at this point.



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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