[willamette-fcst] Silverton Hills AM Forecast.doc

Fri Aug 19 08:57:58 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT FRI AUG 19, 2011

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is allowed from 11:00 a.m. until 5:00 p.m.

WEATHER DISCUSSION:

A dry and seasonably warm weather pattern has established itself over the Pacific Northwest.  A very weak upper-level trough remains over this morning with light winds aloft.  At the surface, high pressure is centered offshore with a thermal trough beginning to build northward, from California, into southwestern Oregon.  That has weakened the onshore flow with increasingly northerly pressure-gradients across western Oregon.

Visible satellite imagery showed only patchy low clouds on the north coast and up the Columbia River into the Portland area. Elsewhere, skies were clear.  A few low clouds could form as far south as the Silverton Hills but should quickly dissipate with daytime heating.

This morning's Salem sounding was almost identical to 24 hours ago, with north to north-easterly winds from the surface up to 5000 feet.  Warming aloft is forecast today, as the weak upper-level trough over the state sags southwestward to over northwestern California.  That will allow temperatures to climb into the middle 80s.  Pressure gradients are forecast to remain northerly, with increasing wind speeds this afternoon.

TODAY'S FORECAST:

Sunny and warmer.

Salem's high temperature today will be near 85 degrees (normal is 82).

Relative humidity drops to near 60% by 11 a.m. and to near 35% by 5 p.m.

Surface winds:  N 5-10 mph this morning; N 8-12 mph this afternoon.

Transport winds: NNE 10-15 mph this morning; NNE 12-20 mph this afternoon.

Mixing height: Rises to 3000 feet by noon and to 4500 feet by 5 p.m.

Salem's sunset tonight: 8:12 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 69       79      84      77

     Relative Humidity:           61%      44%     35%     45%

     Surface Wind Direction:     360      010     360     360

     Surface Wind Speed:           8       10      10       9

     Transport Wind Direction:   010      010     010     360

     Transport Wind Speed:        12       15      20      16

     Mixing Height:             2800     3800    4500    2500

     Ventilation Index:           34       57      90      40

EXTENDED DISCUSSION:

With a weak cut-off upper-level trough centered over northwestern California, the upper-level flow will turn weakly south-southwesterly on Saturday.  That will further warm the air mass with valley temperatures approaching 90 degrees. Some mid-level moisture could circulate far enough northward for isolated thunderstorm development over the Cascades, especially south.

The system over northern California will continue to weaken on Sunday, and Oregon will come under the influence of a continued dry and more stable westerly flow aloft.  The air mass will remain quite warm, with similar temperatures to Saturday.

A weakening cold front is forecast to move into southern British Columbia, and extreme northwestern Washington, on Monday.  That will increase the onshore flow and could create favorable burning conditions Monday afternoon.  The front is forecast to stall and weaken, well north of the region, on Tuesday, with the flow aloft warming and turning more southerly.

South-southwesterly flow aloft is forecast for the middle of next week, which will warm temperatures into the middle and upper 80s and circulate enough moisture into the region for a chance of thunderstorms, mainly over the Cascades.  It is possible that a weather system will turn the flow more southwesterly by Friday, which would push the thunderstorm threat eastward and possibly create a burning opportunity.

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

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20110819/e8cf5490/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Silverton Hills AM Forecast.doc
Type: application/msword
Size: 33280 bytes
Desc: Silverton Hills AM Forecast.doc
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20110819/e8cf5490/attachment.doc>