[willamette-fcst] Silverton Hills AM Forecast.doc

Thu Sep 1 08:54:05 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:52 AM PDT THU SEP 1, 2011

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.

WEATHER DISCUSSION:

The dry upper-level disturbance that moved across northern Oregon Wednesday afternoon was cutting across Idaho this morning. In its wake, the flow aloft has turned northerly over western Oregon, and low-level winds have turned offshore.  The Salem sounding this morning revealed a much shallower and drier marine layer than 24 hours ago.  Temperatures have warmed several degrees, above 2000 feet, with northeasterly winds extending from the surface up to 10,000 feet.

Mid-morning visible satellite imagery showed only patchy low clouds in the extreme northern Willamette Valley and over the northern coastal region.  Otherwise, skies were mostly clear across the state.  The surface map shows high pressure over western Washington and northwestern Oregon, with a thermal trough beginning to build northward into southwestern Oregon.

An upper-level disturbance is forecast to move across British Columbia today.  Although it will stay well north of Oregon, it will retard to northward movement of the surface thermal trough into southwestern Oregon.  Therefore, temperatures will warm today but stay slightly below normal.  Warming aloft will lead to much lower mixing heights compared to Wednesday.  NE transport winds are forecast to strengthen and back to more northerly this afternoon.

TODAY'S FORECAST:

Sunny and warmer.

Salem's high temperature today will be near 78 degrees (normal is 80).

Relative humidity drops to near 50% by noon and to 34% by 5 p.m.

Surface winds:  N 5-10 mph this morning; N 10-15 mph this afternoon.

Transport winds: NE 10 mph this morning; NNE 15 mph this afternoon.

Mixing height: Rises to 3000 feet by 2 p.m. and to near 4000 feet by 5 p.m.

Salem's sunset tonight: 7:49 p.m.

THREE-HOURLY DATA:

                               11 a.m.   2 p.m.  5 p.m.  8 p.m.

     Temperature:                 62       71      77      70

     Relative Humidity:           54%      41%     34%     44%

     Surface Wind Direction:     360      360     360     350

     Surface Wind Speed:           5       10      12      10

     Transport Wind Direction:   025      010     360     360

     Transport Wind Speed:        10       12      17      17

     Mixing Height:             2000     3000    4000    1500

     Ventilation Index:           20       36      68      26

EXTENDED DISCUSSION:

An upper-level ridge is forecast to build onshore Friday and over Oregon Saturday.  Temperatures will continue to warm with highs likely climbing to near 90 by Saturday.  As the ridge slides east of the state, increasing southerly flow aloft may bring isolated thunderstorms to the Cascades on Sunday.  Temperatures should climb into the low 90s.

The forecast weather pattern for next week is not favorable for open burning, with a broad ridge of high pressure likely staying parked over the region.   Expect a continuation of dry and quite warm conditions.  If the ridge shifts far enough to the east, then afternoon sea breezes could provide limited burning opportunities.

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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