[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Tue Jul 22 11:56:18 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT TUE JUL 22, 2014

BURN ADVISORY:

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

Prep burning is not allowed.

WEATHER DISCUSSION:

The light rain that moved through sections of the northern Willamette Valley this morning has progressed east of the region.  Satellite imagery shows a band of clouds, about 200 miles wide, extending from north-central Oregon to over the southern Oregon coast.  Radar is indicating some light showers continuing under that cloud-shield, which is pushing just east of the northern Willamette Valley at midday.  In its wake, some clearing is evident along the northern and central coast, which should move over the valley this afternoon.

However, an unseasonably strong upper-level trough remains parked offshore, with an increasing moist and unstable southwesterly flow aloft over the region.  Slightly cooler air aloft and more cloud-cover should continue the recent cooling trend, with temperatures only climbing into the mid to upper 70s this afternoon.  The threat of showers will continue this afternoon, but the next significant surge of moisture is not expected to come onshore until Wednesday.  Additional rainfall totals today should be light, if any.

Southwesterly transport winds and high mixing heights are expected this afternoon, if any fields become dry enough for burning.

TODAY'S FORECAST:

Partly sunny but slightly cooler.  Chance of a shower.

Salem's high temperature today will be near 78°F (average is 84°F).

Relative humidity:  Dropping to near 45% by 5 p.m.

Surface winds: SW 5 mph this afternoon.

Transport winds: SW 10-15 mph this afternoon.

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

Salem's sunset tonight: 8:49 p.m.

(Salem Airport data: Rainfall So Far Today: .10")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

A potent upper-level trough will approach the Oregon coast on Wednesday, which will increase the moisture and instability over the region.  Expect widespread showers with a chance of thunderstorms.  Rainfall totals will vary but should be on the order of one-quarter to one-half of an inch.  Temperatures will only reach the low to mid 70s.

Showers may linger through Thursday morning, followed by a drying and warming trend into the weekend.  Temperatures will recover into the upper 70s Thursday afternoon and into the mid 80s on Friday.  Expect sunny and quite warm conditions Saturday and Sunday, with highs in the low 90s. The ridge is forecast to weaken next week, with possible 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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