[willamette-fcst] Silverton Hills AM Forecast.doc

Thu Sep 20 08:57:12 PDT 2012

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT THU SEP 20, 2012

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.

WEATHER DISCUSSION:

A strong upper-level ridge of high pressure remains over the Pacific Northwest but is shifting eastward this morning in response to a weak trough approaching the Oregon coast. The surface thermal trough has also shifted east, to over the Cascades, which has turned pressure gradients weakly onshore into the Willamette Valley.

Visible satellite imagery shows marine low clouds extending from the coast to near the crest of the Cascades and as far south as Roseburg.  Surface and area pilot reports indicate that the marine low clouds are about 2000 feet thick.  They will likely begin to break up around midday.  The Salem sounding this morning showed very warm air aloft, with a strong surface-based inversion extending up about 5000 feet.  Smoke levels remain elevated across northwestern Oregon this morning from area wildfires.

The upper-level ridge will continue to slide eastward today with increasing southerly flow aloft.  The surface thermal trough will likely progress no farther east than central Oregon this afternoon.  With very warm air aloft, mixing heights will struggle to reach 3000 feet this afternoon.  Even though the low-level flow is onshore, winds at the top of the mixing layer are forecast to be southerly or southeasterly this afternoon, which is not a favorable direction for open burning.  In addition, there may not be enough mid-level onshore flow to evacuate existing wildfire smoke from the Cascades.

(Salem Airport data for Wednesday, Sept. 19th: High 77°F; Rainfall .00")

TODAY'S FORECAST:

Morning clouds; mostly sunny in the afternoon.

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

Relative humidity drops to near 50% from 2 p.m. until 5 p.m.

Surface winds:  Light and variable; turning NW 5-10 mph late this afternoon.

Transport winds: Light and variable; Becoming S 7-12 mph this afternoon.

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

Salem's sunset tonight: 7:12 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 63       72      75      67

     Relative Humidity:           65%      51%     49%     63%

     Surface Wind Dir/Speed:    VAR 5    VAR 5    NW 5    NW 8

     Transport Wind Dir/Speed:  VAR 5      S 7     S 9   NW 10

     Mixing Height:              1600     2500    3000    1500

     Ventilation Index:             8      18      27      15

EXTENDED DISCUSSION:

A weak upper-level trough is forecast to approach the Oregon Coast on Friday; further increasing the southerly flow aloft.  That will introduce a chance of afternoon and evening showers or thunderstorms over the Cascades and possibly even over the Willamette Valley.  Winds at the top of the mixing layer are forecast to remain southerly or southeasterly.

The upper-level trough is forecast to come onshore Saturday; deepening the marine layer over western Oregon for further cooling.  There is also a chance of morning drizzle or light showers.  The trough is forecast to stall over Oregon and slowly weaken on Sunday.  That will maintain a slight chance of showers and slightly below normal temperatures.

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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