[willamette-fcst] Silverton Hills Midday Forecast.doc

Thu Jul 14 11:50:38 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT THU JUL 14, 2011

BURN ADVISORY:

Recommended burn times for agricultural burning are from now until 7:00 p.m.

Prep burning is allowed from 12:00 p.m. to 2:00 p.m. with a 50 acre limit.

Propane flaming is allowed from 12:00 p.m. to 5:00 p.m.

WEATHER DISCUSSION:

An upper-level low-pressure area was centered over north-central Washington, late this morning, with an associated trough extending across western Washington and northwestern Oregon.  So far today, showers have mostly been confined to the northern coast and coast range.  However, afternoon heating will further destabilize the air mass and likely lead to more showers.

Midday satellite imagery showed solid cloud-cover over western Washington and northwestern Oregon, from the coast range westward.  Skies were mostly cloudy over the northern Willamette Valley and partly cloudy over the central and south valley.  The Silverton Hills region is on the southern edge of where showers are likely to develop this afternoon.  High temperatures will, once again, be 5-10 degrees below normal.

The upper-level trough is forecast to weaken, as it continues to migrate slowly eastward this afternoon.  Skies should begin to clear this evening, with surface and transport winds veering to more westerly, as the trough moves east of the Willamette Valley.

TODAY'S FORECAST:

Partly sunny.  Slight chance of a shower.  Clearing skies this evening.

Surface winds:  S 6-10 mph; becoming WSW 6-10 mph by this evening

Transport winds: SSW 10 mph; becoming WSW 10 mph by this evening

Salem's forecast high temperature:  74 degrees

Maximum mixing height:  6000 feet

Relative humidity:  Drops to near 43% by 5 p.m.

Salem sunset tonight: 8:56pm

THREE-HOURLY DATA:

                                 2pm     5pm     8pm

     Temperature:                 69      73      69

     Relative Humidity:           53%     43%     48%

     Surface Wind Direction:     210     260     290

     Surface Wind Speed:           8       8      10

     Transport Wind Direction:   210     260     270

     Transport Wind Speed:        10      10      12

     Estimated Mixing Height:   5000    6000    3500

     Ventilation Index:           50      60      42

EXTENDED DISCUSSION:

A drier and slightly warmer westerly flow aloft is forecast on Friday. Low-level winds will remain onshore, so expect morning clouds to give way to at least partly sunny skies in the afternoon.  High temperatures should reach the mid to upper 70s.  High afternoon mixing heights and southwesterly transport winds may provide favorable burning conditions.

Another upper-level trough is forecast set up camp just offshore this weekend.  A weak front will renew the threat of light showers by early Saturday, with another weather disturbance rotating onshore Sunday afternoon.  The trough is forecast to weaken Monday and Tuesday, only to be replaced by another upper-level trough next Wednesday and Thursday.  Look for a continued threat of showers and generally below normal temperatures through next week.

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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