[willamette-fcst] Silverton Hills Midday Forecast.doc

Thu Sep 8 11:57:01 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT THU SEP 8, 2011

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.

WEATHER DISCUSSION:

A strong and broad upper-level ridge continues to encompass most of the western half of the country this morning.  A very weak weather disturbance is producing a wide band of mainly mid-level clouds that extends from northwestern to southeastern Oregon. Doppler radar is showing what could be some sprinkles near The Dalles.

Late-morning satellite imagery also shows low clouds banked up along the northern and central coast.  Skies were generally clear south of northern Linn County across western Oregon.  Area nephelometer readings are still elevated due to wildfire smoke, especially at Lyons and Government Camp.

At the surface, a thermal trough extends from near The Dalles to the southern Oregon coast.  It appears anchored over the central Cascades, because pressure gradients continue to be onshore from Newport to Salem and offshore from Redmond to Salem.

With unseasonably warm air aloft, between 2500 feet and 6500 feet, high temperatures will be able to climb well into the low 90s again today across the Willamette Valley.  The lower atmosphere is quite stable, so mixing heights will likely remain below 3000 feet this afternoon.  However, mid-level instability will combine with daytime heating to trigger high-based (dry) thunderstorms over the Cascades.  It is possible that some storms could drift over the Willamette Valley, due to light and variable winds aloft.

Transport winds should be light today with poor mixing.  A pibal is scheduled for 2 p.m.  Transport winds are forecast to increase, from the north-northeast, late today and tonight but become variable near thunderstorms.

TODAY'S FORECAST:

Partly cloudy and continued very warm. Slight chance of an evening thunderstorm, mainly near the Cascades.

Salem's high temperature today will be near 93 degrees (normal is 79).

Relative humidity drops to near 30% by 5 p.m.

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

Transport winds: Light and variable; becoming N 6-10 mph this afternoon.

Mixing height: Only rises to about 2800 feet by 5 p.m.

Salem's sunset tonight: 7:36 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 85      92      79

     Relative Humidity:           36%     29%     46%

     Surface Wind Direction:     Var     350     350

     Surface Wind Speed:           3       5       4

     Transport Wind Direction:   Var     360     360

     Transport Wind Speed:         4       8      10

     Mixing Height:             2000    2800    1000

     Ventilation Index:            8      22      10

EXTENDED DISCUSSION:

The strong upper-level ridge is forecast to remain over the Pacific Northwest through this weekend.  The flow aloft, and at transport level, will turn north-northeasterly on Friday.  That will end the thunderstorm threat.  High temperatures will remain in the low to mid 90s.  North-northeasterly winds will likely circulate more wildfire smoke into the region.

A major change to the weather pattern is in store next week, which should finally bring favorable burning conditions.  The strong upper-level ridge is forecast to weaken and shift east of the region by next Tuesday.  That will force the surface thermal trough into eastern Oregon and initiate a "Marine Push" into western Oregon.  The timing of this transition to onshore flow could create a significant burning opportunity Tuesday afternoon.  A weak cold front is forecast to approach the coastline by late Wednesday; possibly creating another burning opportunity Wednesday afternoon.

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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