[willamette-fcst] Silverton Hills AM Forecast.doc

Wed Sep 12 08:48:15 PDT 2012

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT WED SEP 12, 2012

BURN ADVISORY:

...Fire Marshal conditions may be reached this afternoon...

(relative humidity is expected to drop well below 30% and wind speeds may increase to at or above 15 mph)

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.

WEATHER DISCUSSION:

An upper-level ridge is building over the Pacific Northwest. At the surface, a thermal trough is building northward along the southern Oregon Coast, with increasing offshore flow across western Oregon.  The Salem sounding this morning showed considerable warming aloft, compared to 24 hours ago, with brisk northeasterly winds from just above the surface up to about 6000 feet.

Satellite imagery shows clear skies over all of Washington and Oregon, with the exception of areas of wildfire smoke.  Some elevated smoke from the wildfire near Sisters, Oregon appeared to be streaming westward over the southern Willamette Valley.  Some smoke from wildfires in south-central Washington may get transported across northwestern Oregon this afternoon.

Warmer air aloft will suppress mixing heights today.  In addition, increasing northeasterly winds and very low relative humidity will make for unfavorable burning conditions this afternoon.

(Salem Airport data for Tuesday, Sept. 11th: High 74°F; Rainfall .00")

TODAY'S FORECAST:

Sunny and warmer.  Turning breezy and possibly a little hazy this afternoon.

Salem's high temperature today will be near 84 degrees (normal is 78).

Relative humidity drops well below 30% this afternoon.

Surface winds:  N 5-12 mph; becoming NE 10-15 mph this afternoon.

Transport winds: NE 12-18 mph.

Mixing height: Rising to 2000 feet by noon and to near 3000 feet by 5 p.m.

Salem's sunset tonight: 7:27 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 64       76      82      72

     Relative Humidity:           43%      25%     19%     29%

     Surface Wind Dir/Speed:     N 10    NE 12   NE 15   NE  8

     Transport Wind Dir/Speed:  NE 15    NE 15   NE 18   NE 12

     Mixing Height:              1800     2500    3200    1500

     Ventilation Index:           27       38      58      18

EXTENDED DISCUSSION:

The center of the broad upper-level ridge is forecast to shift over Idaho on Thursday, with a strong surface thermal trough remaining over western Oregon.    Valley temperatures will likely climb into the low 90s with slackening northeasterly transport winds.  The thermal trough will likely shift into eastern Oregon on Friday, as a weak upper-level trough turns the flow aloft southwesterly.  That should create more favorable burning conditions, as the transport winds turn onshore and cooling aloft improves mixing.  Valley temperatures will cool back into the mid 80s.

The dry upper-level trough is forecast to move across Oregon on Saturday, with increasing onshore flow likely bringing some morning marine clouds into the valley and helping to cool high temperatures back to near normal.  The flow aloft is forecast to turn more northerly on Sunday, which appears to be the beginning of another period of quite warm and continued dry weather.  Sunny skies and offshore flow is forecast for much of next week, which would be unfavorable for burning.

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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