[willamette-fcst] Silverton Hills Forecast_Joint.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT WED JUL 29, 2015



***State Fire Marshal Burn-Ban Conditions, due to winds, high temperatures, and low humidity are expected to be met this afternoon***



BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



An upper-level ridge is building over the Pacific Northwest. Warming aloft will suppress mixing this afternoon.  At the surface, a thermal trough is building northward along the Oregon coast.  Increasing offshore gradients across western Oregon will make for very dry and breezy conditions this afternoon. Clear skies and very warm air aloft will allow surface temperatures to rapidly rise into the 90s this afternoon with highs approaching 100°F.



TODAY'S FORECAST:



Sunny, breezy, and hot.



Salem's high temperature today will be near 99°F (average is 84°F).

Relative humidity:  Dropping to near 20% by 2 p.m.

Surface winds: NE 5-10 mph this morning; N-NE 10-20 mph this afternoon.

Transport winds: NE 15 mph this morning; N-NE 15-20 mph this afternoon.

Mixing height: 3000 feet by 2 p.m. and rising to 4000 feet by 5 p.m.

Salem's sunset tonight: 8:42 p.m.



(Salem Airport data for Tuesday, July 28th: High 87°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 80)



EXTENDED DISCUSSION:



A strong upper-level ridge is expected to produce near-record heat across western Oregon through Friday.  Willamette Valley highs will be near 100°F.  Hot temperatures and low humidity levels will likely put the valley into State Fire Marshal Burn-Ban conditions in the afternoons.



The upper-level ridge will begin to weaken and shift east of the region over the weekend with increasing SW flow aloft over Oregon.  A switch to onshore surface winds will initiate slow a cooling trend, with high temperatures dropping back into the mid-90s.  The air mass will remain quite dry, but there is a risk of late-day thunderstorms...mainly over the Cascades.



SW flow aloft is predicted to strengthen next week.  Stronger onshore flow will provide additional cooling and possibly some burning opportunities, although the chance for showers will also increase.



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



Tom Jenkins/Pete Parsons

ODF Meteorologists
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