[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Mon Sep 22 11:55:43 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT MON SEP 22, 2014

BURN ADVISORY:

Recommended times for agricultural burning are from now until 5 p.m.

Prep burning is not allowed.

WEATHER DISCUSSION:

Autumn officially begins at 7:29 p.m., and our weather will be changing accordingly...right on schedule.  Increasing southwesterly flow aloft induced cooler onshore flow into western Oregon Sunday afternoon.  Marine low clouds still cover most of the Willamette Valley at midday, with increasing middle and high clouds statewide.  Radar is showing possible sprinkles over SW Oregon and a few showers over the mountains of NE Oregon but no rainfall across the Willamette Valley.

The surge of marine air into the western valleys overnight has resulted in some negative stacking of the onshore flow today.  That may not need to get completely balanced to keep smoke elevated, because an approaching weak frontal system will also provide some lift to the air mass this afternoon.  If conditions remain dry, favorable burning conditions are likely to develop this afternoon. Surface and transport winds should remain southwesterly with cooler air aloft providing good mixing.

This may be the last burning opportunity, before fields get wet for an extended period of time.  This first weather system will weaken as it comes onshore tonight, possibly bringing some very light rain to the valley overnight and Tuesday.  Rainfall totals are expected to be less than .10".

TODAY'S FORECAST:

Mostly cloudy and cooler.

Salem's high temperature today will be near 77°F (average is 75°F).

Relative humidity:  Dropping to near 50% by 5 p.m.

Surface winds: SW 5-10 5 mph.

Transport winds: SW 10-15 mph.

Mixing height: Near 3000 feet; rising to near 4000 feet by 5 p.m.

Salem's sunset tonight: 7:09 p.m.

(Salem Airport data for Sunday, September 21st: High 86°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 60)

EXTENDED DISCUSSION:

After a chance of light rain on Tuesday, a strengthening southwesterly flow aloft is forecast to drive a much more potent weather system onshore Wednesday.  That system should dump more than .50" of rain across the Willamette Valley.  An upper-level trough will maintain cool and showery weather Thursday and Friday.  A transitory upper-level ridge is expected to bring drier and warmer weather this weekend, with temperatures recovering to near average.

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 15 mph.

     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 (ft) times

         the transport wind speed (mph) 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 (ODA) and the Oregon Department of Forestry (ODF).  For

information contact ODA at 503-986-4701.

To add/remove your email address from this list, please go to:

http://listsmart.osl.state.or.us/mailman/listinfo/willamette-fcst

Pete Parsons

ODF Meteorologist

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20140922/9970dab7/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Silverton Hills Midday Forecast_Pete.doc
Type: application/msword
Size: 34816 bytes
Desc: Silverton Hills Midday Forecast_Pete.doc
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20140922/9970dab7/attachment.doc>