[willamette-fcst] Silverton Hills AM Forecast.doc

Fri Aug 26 08:56:57 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT FRI AUG 26, 2011

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.

WEATHER DISCUSSION:

There has been little change to the overall weather pattern.  A large upper-level ridge of high pressure remains anchored over the south-central United States.  The clockwise flow around it is continuing to direct a warm south to southwesterly flow aloft over Oregon.

A small upper-level disturbance produced quite a lightning show early this morning.  Thunderstorms rolled across the Willamette Valley, from SW to NE, before exiting over the northern Cascades. CoCoRaHS stations showed scattered rainfall totals, mostly less than one-tenth of an inch, across the southern valley and the east side of the north valley.  The strongest storms were over the east side of the valley, with numerous rainfall reports of around one-tenth of an inch along the Cascade foothills.  The greatest reported rainfall total was .26" at a site 6 miles ENE of Stayton, in Marion County.

The upper-level disturbance that brought the thunderstorms early this morning has moved northeast of the region, and more stable conditions are expected today.  Doppler radar was still showing some shower activity over the northern Oregon Cascades at mid-morning, but skies had mostly cleared over the Willamette Valley.  Satellite imagery showed marine low clouds banked up along the coastal strip with some inland penetration into the coastal mountain gaps.  Patchy low clouds are possible in the Willamette Valley this morning but should quickly give way to sunshine.

The Salem sounding this morning showed northeasterly winds, and several degrees of warming compared to 24 hours ago, from about 1500 feet to 6000 feet.  With quite warm air aloft, mixing heights will struggle to reach 3000 feet today, even with strong surface heating.  The mid-morning surface map showed a thermal trough extending from southeastern Washington through southwestern Oregon with a weak north to northwest flow across the Willamette Valley.  Surface and transport winds are forecast to be northerly today.

TODAY'S FORECAST:

Sunny and very warm.

Salem's high temperature today will be near 91 degrees (normal is 81).

Relative humidity drops to near 50% around 11 a.m. and to near 33% by 5 p.m.

Surface winds:  N 5-10 mph.

Transport winds: NNE 10 mph this morning; N 12-15 mph this afternoon.

Mixing height: Rises to 3000 feet by 4 p.m. and to 3200 feet by 5 p.m.

Salem's sunset tonight: 8:00 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 76       86      90      79

     Relative Humidity:           52%      38%     33%     47%

     Surface Wind Direction:     360      360     360     330

     Surface Wind Speed:           6        7       8       7

     Transport Wind Direction:   010      360     360     350

     Transport Wind Speed:        10       12      15      15

     Mixing Height:             1500     2700    3200    1500

     Ventilation Index:           15       32      48      23

EXTENDED DISCUSSION:

The flow aloft is forecast to back to more southerly on Saturday and reintroducing a chance of thunderstorms, especially near and over the Cascades late in the day.  High temperatures will only cool a couple of degrees.  The upper-level flow is forecast to turn more southwesterly and stabilize on Sunday.  Weak onshore flow may bring some marine clouds into the valley with temperatures cooling into the mid 80s (still above normal).

Long-range models are showing a possible burning opportunity on Monday, as a system moving into southern British Columbia increasing both the southwesterly flow aloft and the onshore flow in the transport layer.

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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