[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Wed Aug 12 08:55:05 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT WED AUG 12, 2015

***  The Red Flag Warning for the western slopes of the northern and central Oregon Cascades has expired, due to a decreased threat of lightning.  ***

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

An upper-level low-pressure system is centered about 400 miles west of the northern California coast.  The counterclockwise circulation around it is still feeding mid-level moisture and instability northward across Oregon, but not as much as the past two days. The air aloft has continued to warm and stabilized since yesterday.  That has decreased, but not completely eliminated, the thundershower threat across the region.

Daytime heating will, once again, destabilize the air mass, so isolated showers and thundershowers may development this afternoon, mainly over the Cascades.  SE flow aloft is slowly becoming more southerly.  At the surface, the thermal trough has shifted east of the Cascades.  Increasing onshore flow may provide a burning opportunity this afternoon, once mixing heights reach about 3000 feet.  However, continued southerly flow aloft and the threat of afternoon shower development are potential limiting factors.

TODAY'S FORECAST:

Partly cloudy.  Slight chance of a shower or thundershower.

Salem's high temperature today will be near 89°F (average is 83°F).

Relative humidity:  Dropping below 50% by 2 p.m. and to near 40% by 5 p.m.

Surface winds: S 5-10 mph this morning; SW 5-10 mph this afternoon.

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

Mixing height: Rising to 3000 feet about 2 p.m. and to 4500 feet by 5 p.m.

Salem's sunset tonight: 8:23 p.m.

(Salem Airport data for Tuesday, August 11th: High 89°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 68)

EXTENDED DISCUSSION:

The upper-level trough is predicted to swing NE across the region Thursday and Friday.  That will bring much cooler onshore flow along with scattered showers and a chance of thundershowers...mainly late Thursday through Friday.  Another burning opportunity is possible, Thursday afternoon, ahead of this transition.  Temperatures will drop back into the mid-to-upper 70s.  Rainfall totals are generally expected to be near .10". Dry weather is predicted to return over the weekend with temperatures warming back into the 80s.

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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