[willamette-fcst] Silverton Hills AM Forecast.doc

Wed Aug 17 08:58:17 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT WED AUG 17, 2011

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is allowed from 1:00 p.m. until 5:00 p.m.

WEATHER DISCUSSION:

There has been very little change to the overall weather pattern for the past few weeks.  A massive upper-level ridge remains anchored over the south-central United States with a general weak trough off the west coast.  That is resulting in a dry southwesterly flow aloft over Oregon, with the main jet stream in its usual summer-time location over Canada.

Satellite imagery showed only a few patches of low clouds along the coast this morning with clear skies over the remainder of Oregon.  At the surface, a broad thermal trough extends from central Washington through southwestern Oregon with weak northwesterly pressure-gradients across western Oregon.  This morning's sounding over Salem continued to show a very dry and stable air mass with some minor warming aloft compared to yesterday.

A weak upper-level disturbance is forecast to cut across southern British Columbia this afternoon and slightly increase the onshore flow into western Oregon. Temperatures will likely start off warmer than on Tuesday but cap out at about the same place, due to minor cooling aloft in the afternoon.  Mixing heights will be slow to rise but eventually climb above 4000 feet with the combination of cooling aloft and strong afternoon heating.

Pressure-gradient stacking should be favorable for burning this afternoon, with transport wind direction being the limiting factor.  Computer models show northerly winds this morning shifting slightly to the northwest this afternoon.  Winds will need to be monitored closely for the possibility of limited open burning later this afternoon.  A sea breeze will rapidly lower mixing heights this evening with some marine air expected to penetrate into the Willamette Valley overnight.

TODAY'S FORECAST:

Sunny and warm.

Salem's high temperature today will be near 86 degrees (normal is 82).

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

Surface winds:  N 5 mph this morning; NNW 5-10 mph this afternoon.

Transport winds: N 5 mph this morning; NNW 6-10 mph this afternoon.

Mixing height: Rises to 3000 feet by 1 p.m. and to 4700 feet by 5 p.m.

Salem's sunset tonight: 8:15 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 72       82      85      74

     Relative Humidity:           44%      28%     24%     38%

     Surface Wind Direction:     360      350     340     320

     Surface Wind Speed:           5        6       7       9

     Transport Wind Direction:   360      345     340     330

     Transport Wind Speed:         5        7       9      14

     Mixing Height:             2200     3700    4700    2000

     Ventilation Index:           11       26      42      28

EXTENDED DISCUSSION:

A very weak upper-level trough will be over Oregon on Thursday with onshore flow likely strong enough to bring at least some morning marine clouds into the Willamette Valley.  Skies should clear in the afternoon with daytime highs around 80 degrees.  Gradient-stacking will be less favorable for burning and transport winds are forecast to return to northerly in the afternoon.  Little change is forecast for Friday.

An upper-level ridge is forecast to build over Oregon this weekend and likely produce the warmest temperatures so far this summer.  Valley temperatures could climb into the low 90s, by Sunday, with north to northeasterly winds.  When the upper-level ridge shifts east of the state, early next week, increasing onshore flow could create burning opportunities.

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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