[willamette-fcst] Silverton Hills AM Forecast.doc

Fri Sep 9 08:59:17 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT FRI SEP 9, 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 of high pressure is in the process of rebuilding over Oregon, in the wake of a weak weather disturbance that brought some dry thunderstorms to areas mainly east of the Cascades the past two days.  Northerly flow aloft has stabilized the air mass over the state.

At the surface, a thermal trough extends from southeastern Washington to the northern Oregon Cascades, with a new thermal trough building northward along the southern Oregon Coast.  Pressure gradients are still weakly onshore along the northern and central Oregon Coast but becoming mostly northerly.  The flow has turned offshore along the south coast.  Pressure gradients are weak across the Willamette Valley this morning with light winds.

Mid-morning satellite imagery showed low clouds along the northern and central coast but not much penetration inland into the coastal mountain gaps.  Skies were clear over the south coast and the inland valleys of western Oregon.  Some debris clouds from yesterday evening's convective activity were still evident over central Oregon.  Nephelometer readings have dropped slightly, from yesterday morning, but are still elevated across most of the state, due to wildfire smoke.

The Salem sounding this morning was almost identical to yesterday morning with unseasonably warm from about 2500 feet to 6500 feet.  That will keep mixing heights suppressed below 3000 feet today, even though surface temperatures will warm into the low 90s.  North-northeasterly transport winds will increase this afternoon, across the Willamette Valley, into the building surface thermal trough along the south coast.

TODAY'S FORECAST:

Sunny and very warm.

Salem's high temperature today will be near 92 degrees (normal is 79).

Relative humidity drops below 50% by 11 a.m. and to near 25% by 5 p.m.

Surface winds:  N 3-8 mph this morning; N 7-12 mph this afternoon.

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

Mixing height: Rises to only about 2900 feet by 5 p.m.

Salem's sunset tonight: 7:34 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 74       85      91      80

     Relative Humidity:           45%      31%     26%     37%

     Surface Wind Direction:     360      010     010     010

     Surface Wind Speed:           5        8      10       7

     Transport Wind Direction:   010      010     010     020

     Transport Wind Speed:        10       15      17      12

     Mixing Height:             1500     2700    2900    1500

     Ventilation Index:           15       41      49      18

EXTENDED DISCUSSION:

The strong upper-level ridge is forecast to remain over the Pacific Northwest through this weekend.  Sunny skies and very warm air aloft will push high temperatures into the mid 90s. North-northeasterly flow on Saturday will likely circulate more wildfire smoke into the region.  Transport winds will slacken on Sunday, as the surface thermal trough moves over the valley.

A major change to the weather pattern is in store next week, which should finally clear the wildfire smoke from the region and bring favorable burning conditions.  The strong upper-level ridge is forecast to slowly shift eastward and weaken.  That will push the surface thermal trough east of the Cascades by Monday afternoon and initiate a cooling onshore flow pattern into western Oregon.

By Tuesday, the thermal trough is forecast to move into eastern Oregon with strengthening onshore flow likely creating favorable burning conditions.  Increasing westerly flow aloft will maintain onshore flow through the week.  A weak cold front is forecast to approach the coastline by Thursday, which may also create favorable burning conditions.  This system may also be strong enough to bring some light showers to the region, but that is questionable at this time.

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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