[willamette-fcst] Silverton Hills AM Forecast_Tom

Tue Jul 14 08:51:23 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT TUE JUL 14, 2015

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from 11 a.m. to 1 p.m. with a 50 acre limit.

WEATHER DISCUSSION:

A weakened upper-level trough is passing through Oregon, bringing in cooler air from the Pacific Ocean aloft. A surface inversion over the Willamette Valley is quickly heating up and dissipating due to morning heating, which will allow the onshore flow to mix down by mid-day. Mixing conditions should improve rapidly, with convection forming pop-up clouds with little to no rainfall along a dry trough. Surface winds will become more northwesterly in the afternoon and evening hours ahead of a mild sea breeze.

TODAY'S FORECAST:

Partly Cloudy skies today.

Salem's high temperature today will be near 79°F (average is 82°F).

Relative humidity:  Drops to near 60% by 10:30 a.m. and to near 40% by 5 p.m. this afternoon.

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

Transport winds: N 10-15 mph this morning; NW 15-20 mph this afternoon.

Mixing height: Rising to 3000 feet by 11 a.m. and to 5000 feet by 2 p.m.

Salem's sunset tonight: 8:56 p.m.

(Salem Airport data for Monday, July 10th: High 85°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 100)

EXTENDED DISCUSSION:

The weather pattern stabilizes for the middle of the week across the Pacific Northwest, as high pressure strengthens offshore Oregon. Northerly flow will prevail at the surface, with northwesterly onshore flow aloft mixing down in the afternoon hours. Mixing heights will improve steadily with daytime heating, but cooler marine air will moderate temperatures closer to seasonal averages. No rainfall is expected through Thursday evening as the atmosphere dries out.

Friday will be a day of transition, as an upper-level trough from Canada dives south through Washington in the morning hours. Onshore flow at the surface will become more northwesterly, as mixing conditions increase rapidly. Little moisture is associated with the surface front, so no showers or thunderstorms are forecast at this time.

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

Tom Jenkins

ODF Meteorologist
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