[willamette-fcst] Silverton Hills Midday Forecast_Tom

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT THU SEP 10, 2015


***State Fire Marshal Burn-Ban Conditions, from low humidity and breezy winds, may be observed this afternoon.***



BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



An upper-level ridge is currently off the coast of Oregon, suppressing mixing as it warms the air aloft. At the surface, higher pressure over central Oregon is keeping skies clear. An offshore gradient of -1.1mb from Redmond to Salem is producing northeasterly winds, while a "Medford Draw" is strong enough to make winds breezy. Humidity will rapidly drop, and it is possible that State Fire Marshal Burn-Ban criteria will be observed briefly this afternoon.



TODAY'S FORECAST:



Sunny becoming hazy and very warm this afternoon.



Salem's high temperature today will be near 89°F (average is 79°F).

Relative humidity:  Dropping near 25% by 5 p.m.

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

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

Mixing height: Below 3000 feet all day.

Salem's sunset tonight: 7:32 p.m.



(Salem Airport data for Wednesday, September 9th: High 86°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 60)



EXTENDED DISCUSSION:



The upper-level ridge will slowly drift over western Oregon Friday, restricting mixing for one more day. Higher pressure will maintain northerly winds over the Silverton Hills, with Friday being the warmest day of the week in the Silverton Hills. A "Medford Draw" is likely to make winds breezy again and prevent onshore flow from developing.



Early Saturday morning, an upper-level trough will approach the southern coast of Oregon and switch the flow aloft to southwesterly. This will begin to cool the air aloft and weaken the upper-level ridge over our region, pushing it further east into Idaho by Sunday.



Monday will see onshore flow with increasing clouds throughout the day, as a second upper-level trough approaches the Pacific Northwest. An area of lower pressure will form off the coast of Washington. A cold front sweeps across the Silverton Hills early Monday evening with isolated showers. A second storm system mid-week is expected to produce wetting rains across the Silverton Hills.



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, AEM

ODF Meteorologist
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