[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Tue Jul 8 11:51:09 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT TUE JUL 8, 2014

BURN ADVISORY:

Prep burning is not allowed.

Agricultural Burning:

North of Salem -

Recommended times for agricultural burning are from now until 7 p.m.

>From Salem South -

Agricultural burning is not recommended.

WEATHER DISCUSSION:

An upper-level ridge remains locked in place over Oregon.  At the surface, a thermal trough extends from eastern Washington southward across central Oregon, with weak onshore flow into western Oregon.  Marine low clouds are confined to the coastline.  The few patches of low clouds that formed over the western Cascade foothills this morning quickly evaporated this morning, with sunny skies at midday.  Temperatures and north winds are similar what we saw on Monday. Strong surface heating will lead to good mixing again this afternoon, but transport winds should remain mostly northerly across the Willamette Valley.

One significant difference today, however, is that southerly flow aloft is bringing monsoonal moisture into southern Oregon, from Nevada and northern California.  This moisture will progress northward this afternoon; bringing increasing middle and high clouds to NW Oregon.  The increase in moisture will combine with afternoon heating trigger scattered thundershower development over southern Oregon...possibly advancing as far north as the central Cascades.  The possibility of a late evening or overnight thundershower can't be ruled out for the Willamette Valley.

TODAY'S FORECAST:

Increasing middle and high clouds.  Risk of a thundershower late.

Salem's high temperature today will be near 90 degrees (average is 81).

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

Surface winds: N 10-15 mph; becoming NNW 10-15 mph late this afternoon.

Transport winds: N 15-20 mph; becoming NNW 15-20 mph late this afternoon.

Mixing height: Rising to near 5000 feet by 5 p.m.

Salem's sunset tonight: 9:00 p.m.

(Salem Airport data for Monday, July 7th: High 93°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 100)

EXTENDED DISCUSSION:

A weak weather system cutting across SW Canada will flatten the upper-level ridge over Oregon Wednesday and Thursday.  Increasing onshore flow will likely spread some morning marine clouds into western Oregon...cooling temperatures back into the middle 80s and pushing the thundershower threat east of the region.  Transport winds should turn more northwesterly, which could provide limited burning opportunities, if any fields are ready.

The upper-level ridge is expected to rebuild over the region Friday through Sunday, for a return to warmer temperatures and more northerly transport winds.  Southerly flow aloft will increase the chance of afternoon and evening thundershowers again, especially over the weekend.

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
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