[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Wed Jul 23 08:56:11 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT WED JUL 23, 2014

BURN ADVISORY:

Recommended times for agricultural burning are from 8 a.m. until 7 p.m.

Prep burning is not allowed.

WEATHER DISCUSSION:

An unseasonably strong upper-level trough, just off the Oregon and Washington coast, will continue to circulate bands of showers across the region today.  One potent line of showers already traversed the Willamette Valley this morning, dropping about one-tenth of an inch of rain at the Salem Airport.  Daytime heating will increase the instability over the region, leading to a chance of thundershowers this afternoon.  Some storms could produce small hail.  Rainfall totals will vary but should generally be from one-quarter to one-half of an inch or greater.  Temperatures will only reach the lower 70s.

TODAY'S FORECAST:

Showers and unseasonably cool.  Chance of thundershowers.

Salem's high temperature today will be near 71°F (average is 84°F).

Relative humidity:  Dropping to 60% by 2 p.m. and to near 50% by 5 p.m.

Surface winds: S 5-10 mph this morning; SSW 10-15 mph this afternoon.

Transport winds: SW 15-20 mph this morning; SW 17-25 mph this afternoon.

Mixing height: Rising to above 5000 feet this afternoon.

Salem's sunset tonight: 8:48 p.m.

(Salem Airport data for Tuesday, July 22nd: High 76°F; Rainfall: .11")

(Maximum Ventilation Index expected today: 150)

EXTENDED DISCUSSION:

Showers may linger into Thursday morning, followed by a drying and warming trend into the weekend.  With clearing skies, 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.  Increasing southerly flow aloft may circulated enough mid and high level monsoonal moisture and instability into the region for thundershower development by Sunday evening.

The upper-level ridge axis is forecast to shift slightly east, to over Idaho, early next week.  South to SW flow aloft will maintain a threat of afternoon and evening thunderstorms.  The ridge is forecast to shift further east by the middle of next week.  SW flow aloft and onshore low-level winds may be strong enough to push the thundershower threat east of Cascades and possibly create 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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