[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Fri Jul 11 11:53:13 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT FRI JUL 11, 2014

BURN ADVISORY:

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

Prep burning is not allowed.

WEATHER DISCUSSION:

An upper-level ridge is strengthening over the Pacific Northwest with weak southwesterly flow aloft over NW Oregon.  At the surface, weak onshore flow forced low clouds onto the coast this morning but with little penetration east of the coast range.  By midday, the low clouds had backed off to just along the immediate coastline.  Under mostly sunny skies, Willamette Valley temperatures are 4-8 degrees warmer than 24 hours ago, and winds are light.

An upper-level disturbance off the northern California coast is circulating considerable moisture and instability across southern Oregon.  Radar and surface reports indicate widespread mostly light shower activity south of about Roseburg.  Daytime heating will make the air mass more unstable, so thundershower development is likely this afternoon across SW and south-central Oregon.

The upper-level winds are forecast to turn from SW to SE this afternoon, which will help to draw moisture from southern Oregon northward into the region.  Increasing clouds should cap temperatures in the Willamette Valley in the low 90s but may also develop into late-afternoon and evening thundershowers.  These storms have the potential to turn both the pressure gradients and transport winds unfavorable for burning later this afternoon.

TODAY'S FORECAST:

Increasing clouds and quite warm.  Chance of evening thundershowers.

Salem's high temperature today will be near 92 degrees (average is 81).

Relative humidity:  Dropping to near 30% by 5 p.m.

Surface winds: NW 2-6 mph but locally gusty and variable near thundershowers.

Transport winds: NW 3-7 mph but stronger and variable near thundershowers.

Mixing height: Rising to 3000 feet by 1 p.m. and to near 4000 feet by 5 p.m.

Salem's sunset tonight: 8:58 p.m.

(Salem Airport data for Thursday, July 10th: High 90°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 28)

EXTENDED DISCUSSION:

An upper-level ridge is expected to amplify over the Pacific Northwest this weekend.  South to SE flow aloft will increase the available moisture and instability for afternoon and evening thundershower development, especially on Sunday, when locally strong to severe storms are possible.  Even with very warm air aloft, considerable clouds should cap valley highs in the mid 90s and keep overnight temperatures mostly in the 60s.

A drier and more stable west to NW flow aloft should push the thunderstorm threat well east of the region Monday afternoon and Tuesday.  Very warm air aloft and more sunshine may help valley temperatures climb into the mid to upper 90s, but overnight periods should be a little cooler.

The strong upper-level ridge is forecast to begin breaking down about next Wednesday or Thursday, with a shift to a cooler onshore flow weather pattern possibly creating 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 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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