[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Fri Sep 1 11:57:33 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT FRI SEP 1, 2017

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A building upper-level ridge will keep mixing suppressed today.  Meanwhile, a strengthening surface thermal trough, along the coast, will maintain offshore flow.  Hot surface temperatures, low humidity, and increasing wind speeds will likely combine to put the valley into State Fire Marshal Burn-Ban Conditions this afternoon.

TODAY'S FORECAST:

Sunny, hot, and breezy.

Salem's high temperature today will be near 96°F (average is 80°F).

Relative humidity:  Dropping to near 30% by 2 p.m.

Surface winds: N 10-15 mph.

Transport winds: NE 15-20 mph this afternoon.

Mixing height: Near 2000 feet at midday; only rising to 2500 feet by 5 p.m.

Salem's sunset tonight: 7:48 p.m.

(Salem Airport data for Thursday, August 31st: High 86°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 50)

EXTENDED DISCUSSION:

Temperatures will climb to around 100°F Saturday through Tuesday.  The National Weather Service has issued a "Heat Advisory" for the entire Willamette Valley from 2 p.m. Saturday until 10 p.m. Tuesday.  In addition, a developing southerly flow aloft will likely transport wildfire smoke, from fires across SW Oregon and northern California, back over NW Oregon.

The axis of the strong upper-level ridge is expected to shift eastward, to over eastern Oregon, on Wednesday and progress to over the Rockies by Friday.  That will turn transport winds onshore and initiate a cooling trend.  However, SE-S flow aloft will transport increasing amounts of moisture and unstable air northward over Oregon.  Thundershower development may extend as far north as NW Oregon as soon as Wednesday evening, with a high probability of showers and thundershowers Thursday and Friday.  That may signal a major change to what has been a very dry weather pattern.  It is too soon to tell if this weather pattern change will create burning opportunities next week.

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.

     5. In accordance with OAR 837-110-0090, all field burning shall be
         banned when any two of the following criteria are present:
         A. Temperature of 95 degrees Fahrenheit or above
         B. Relative humidity of 30 percent or below
         C. Wind speed of 15 miles per hour or higher

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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