[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT MON JUL 18, 2016



BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is allowed from 11 a.m. until 1 p.m. with a 50 acre limit.



WEATHER DISCUSSION:



An upper-level trough, centered just off the northern Oregon coast, is producing a cool, moist, and somewhat unstable SW flow aloft over Oregon.  Onshore flow has forced a deep marine layer and a blanket of low clouds across western Oregon this morning.  However, there have been no reports of precipitation across the Willamette Valley.



Onshore pressure gradients are negatively stacked this morning (0.7mb from Newport to Salem and 2.1mb from Salem to Redmond) and will have a difficult time balancing this afternoon.  In addition to gradient-stacking and a threat of light showers, thunderstorm development is possible over the Cascades.  If pressure gradients can become favorable, and showers hold off, cool air aloft will provide excellent mixing this afternoon with SW transport winds.



TODAY'S FORECAST:



Cloudy this morning.  Partial afternoon clearing.  Slight chance of showers.



Salem's high temperature today will be near 74°F (average is 83°F).

Relative humidity:  Dropping to 60% around 11 a.m. and to near 50% by 2 p.m.

Surface winds: SSW 3-6 mph this morning; SW 5-12 mph this afternoon.

Transport winds: SSW 5-10 mph this morning; SW 10-15 mph this afternoon.

Mixing height: Rising to 3000 feet at 11 a.m. to 5000 feet by 2 p.m.

Salem's sunset tonight: 8:53 p.m.



(Salem Airport data for Sunday, July 17th: High 73°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 85)



EXTENDED DISCUSSION:



Very little change is expected on Tuesday with continued SW flow aloft and a deep marine layer remaining across the Willamette Valley.  Gradient-stacking and the chance of light showers will continue to limit the potential for burning, along with the threat of thundershowers over the Cascades. However, mixing will be excellent with SW transport winds.



The offshore upper-level trough will weaken on Wednesday with the air mass warming and stabilizing enough to end the shower activity.  However, the air aloft will remain cool enough for good daytime mixing.  Weaker onshore flow may allow for more favorable gradient-stacking.  Transport winds are also likely to weaken and veer to northwesterly.



An upper-level disturbance is predicted to swing onshore Thursday, which may create a burning opportunity.  However, it could also bring light showers.



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 15 mph.



     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 (ft) times

         the transport wind speed (mph) 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 (ODA) and the Oregon Department of Forestry (ODF).  For

information contact ODA at 503-986-4701.



To add/remove your email address from this list, please go to:



http://listsmart.osl.state.or.us/mailman/listinfo/willamette-fcst



Pete Parsons

ODF Meteorologist
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