[willamette-fcst] Silverton Hills Midday Forecast.doc

Fri Sep 21 11:56:22 PDT 2012

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT FRI SEP 21, 2012

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.

WEATHER DISCUSSION:

A weak offshore upper-level trough is slowly approaching the coastline and is producing southerly flow aloft over Oregon.  Pilot reports indicate that the solid deck of marine clouds covering all of northwestern Oregon is about 4000 feet thick.   Areas of drizzle continued into the late morning across the northern and central Willamette Valley.  As of 11 a.m., the Salem Airport had received just a trace of rain, but there have been reports of up to a few hundredths of an inch to the east of Salem, in the Cascades foothills.

As is typically the case with a "marine push," pressure gradient stacking is highly unfavorable for burning today.  One benefit of the increased onshore flow is an improvement in air quality west of the Cascades.  However, the southerly flow aloft is maintaining significant levels of wildfire smoke over the Cascades and across central Oregon, especially in and near Sisters, OR.

The approaching upper-level trough should keep skies cloudy today over the northern and central Willamette Valley, along with areas of drizzle.  High temperatures will likely stay in the mid 60s (well below normal).  As the trough moves onshore tonight, it could also produce areas of light rain.  Additional rainfall totals should not exceed a few hundredths of an inch.

TODAY'S FORECAST:

Cloudy and cool with areas of drizzle.  Rainfall totals ranging from a trace to a few hundredths of an inch.

Salem's high temperature today will be near 65 degrees (normal is 76).

Relative humidity: Likely staying above 65% all day.

Surface winds:  Light and variable; becoming NW 3-8 mph late this afternoon.

Transport winds: Light and variable; Becoming N-NW 5-10 mph this afternoon.

Mixing height: Rising to near 3000 feet by 5 p.m.

Salem's sunset tonight: 7:10 p.m.

THREE-HOURLY DATA:

                                2 p.m.  5 p.m.  8 p.m.

     Temperature:                 61      64      59

     Relative Humidity:           81%     73%     87%

     Surface Wind Dir/Speed:    VAR 4    NW 5    NW 5

     Transport Wind Dir/Speed:  VAR 4    NW 5    NW 5

     Mixing Height:              2500    3000    2000

     Ventilation Index:           10      15      10

EXTENDED DISCUSSION:

A weak upper-level trough will park over NW Oregon on Saturday.  Expect mostly cloudy and cool conditions to continue with areas of drizzle or light showers.  Rainfall totals should be less than one-tenth of an inch.  Ventilation conditions will improve.  However, gradient stacking will likely be unfavorable for burning and transport winds are forecast to be mostly northerly.  Some thunderstorms are likely from the Cascades eastward, with a high potential for ignition of new wildfires.

The upper-level trough is forecast to slowly drift to over south-central Oregon on Sunday.  That will maintain a deep marine layer over western Oregon.  Skies should remain mostly cloudy with areas of morning drizzle.  Unfavorable gradient stacking should continue with northerly transport winds.

The upper-level trough is forecast to move east of the state on Monday.  Expect some afternoon clearing with weak westerly flow aloft possibly creating more favorable burning conditions.

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 15mph.

     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 times

         the transport wind speed 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 and the Oregon Department of Forestry.  For information contact ODA at 503-986-4701.

Pete Parsons

ODF Meteorologist
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