[willamette-fcst] Silverton Hills AM Forecast_Tom

Fri Jul 31 08:49:45 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT FRI JUL 31, 2015

***State Fire Marshal Burn-Ban Conditions, due to high temperatures and low humidity, will be observed this afternoon***
***A Red Flag Warning is in effect now through 11 p.m. tonight for the Willamette Valley and western Cascades***

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

An upper-level ridge over the Pacific Northwest will begin to weaken and shift east across the Cascades later today. At the surface, the thermal trough over western Oregon will strengthen with intense heating and move into the foothills of the Cascades this afternoon. These are signs that our heatwave is coming to an end, but northerly winds and restricted mixing heights will persist into Friday evening. State Fire Marshal conditions are expected again due to above-average temperatures and low humidity. Clear skies early today will become partly cloudy in the afternoon, as elevated smoke from wildfires in southern Oregon and some wispy high clouds drift north across our area.

TODAY'S FORECAST:

Clear skies becoming Mostly Sunny with very hot temperatures.

Salem's high temperature today will be near 103°F (average is 84°F).

Relative humidity:  Dropping near 15% by 5 p.m.

Surface winds: N-NE 5-10 mph this morning, becoming N-NE 5-10 mph this afternoon.

Transport winds: N-NE 10-15 mph this morning, becoming N-NE 15-20 mph this afternoon.

Mixing height: 3000 feet by 3 p.m. and rising to 4000 feet by 5 p.m.

Salem's sunset tonight: 8:40 p.m.

(Salem Airport data for Thursday, July 30th: High 105°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 80)

EXTENDED DISCUSSION:

The weekend will be a transition period that starts off slowly on Saturday, with more changes apparent by Sunday. The upper-level ridge will continue to move east and exit Oregon Saturday evening, ushering in southwesterly flow aloft with more cloud coverage. At the surface, a mild sea breeze Sunday afternoon will return temperatures closer to seasonal averages throughout the Willamette Valley. An upper-level trough off the coast of California will continue to pump moisture into southern Oregon, creating isolated thunderstorms over the Cascades in the afternoon hours. No rainfall is expected for the Silverton Hills.

On Monday, an upper-level trough will remain off the coast of California while a second trough approaches from the Canadian coastline, increasing the southwesterly flow and cooling air aloft. A surface high near the Oregon coast will increase onshore gradients ahead of an afternoon sea breeze, with mixing heights rising rapidly throughout the day. Isolated thunderstorms with scattered showers are expected along the Cascades and across eastern Oregon. This suggests possibilities for open field burning, but it may start early and end early ahead of the sea breeze. The remainder of the week sees the Silverton Hills under continued onshore flow, as the upper-level trough slows and strengthens as it nears the Washington coast. These conditions could potentially provide several days of favorable open burning conditions, tapering off towards the weekend as daily sea breezes progressively deepen the marine layer over western Oregon.

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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