[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Thu Sep 14 08:59:01 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT THU SEP 14, 2017

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from 11 a.m. until 2 p.m. with a 50-acre limit.

WEATHER DISCUSSION:

A dry upper-level trough will bring seasonal temperatures today under partly cloudy skies. Cool air aloft will provide excellent mixing.  Weak onshore gradients have some negative-stacking this morning but should become balanced this afternoon.  However, transport winds are predicted to remain mostly northerly and will be the limiting factor for burning.

TODAY'S FORECAST:

Partly cloudy and mild.

Salem's high temperature today will be near 76°F (average is 78°F).

Relative humidity:  Dropping to 50% by 11 a.m. and to 30% by 5 p.m.

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

Transport winds: N 5-10 mph this morning; NNW 8-13 mph this afternoon.

Mixing height: Rising to 3000 feet by 11 a.m. and to 6000 feet by 5 p.m.

Salem's sunset tonight: 7:24 p.m.

(Salem Airport data for Wednesday, September 13th: High 77°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 40)

EXTENDED DISCUSSION:

A weak and transitory upper-level ridge will clear skies on Friday with afternoon temperatures warming to near 80°F.  The air aloft will remain cool enough for good daytime mixing with northerly transport winds turning northeasterly Friday night.

Low-level winds are expected to veer from NE to SE on Saturday, as the weak upper-level ridge breaks down.  Increasing westerly flow aloft should push some clouds into the area by Saturday afternoon.

A major change in the weather pattern is expected on Sunday, as the first fall-like storm of the season gets pushed onshore.  Ahead of the cold front, low-level winds will increase and veer to the S-SW Sunday morning.  Light rain is predicted to spread across the Willamette Valley Sunday afternoon.  The timing of this system does not appear optimal for field-burning, as it may come in too early in the day.

An even stronger weather system is expected to bring more significant rain on Monday with temperatures cooling into the low-60s.  Rainfall totals may exceed one-half inch.  Cool and showery weather is expected to extend at least through the middle of 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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