[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Tue Jul 29 08:49:04 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT TUE JUL 29, 2014

BURN ADVISORY:

...State Fire Marshal Burn-Ban Conditions may be reached late this afternoon due to high temperatures and low humidity...

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A strong upper-level ridge is centered over the Rockies with a warm SW flow aloft over Oregon.  A surface thermal trough extends from eastern Washington through central and SW Oregon.  That is allowing weak onshore flow to bring marine clouds onto the coast, but skies are sunny across the interior of NW Oregon this morning.

Monsoonal moisture is flowing northward over Oregon, and isolated thunderstorms were already evident this morning over the Cascades.  These storms will continue to develop and expand in coverage today but should stay east of the valley.  However, they will act to keep pressures higher over the Cascades, which will inhibit onshore flow this afternoon.

Salem's upper-air sounding today was very similar to Monday, so temperatures should track very closely to what we saw yesterday...likely topping out in the low to mid 90s.  Transport winds should be mostly northerly.

TODAY'S FORECAST:

Sunny and warm.  Scattered thunderstorms over the Cascades.

Salem's high temperature today will be near 95°F (average is 84°F).

Relative humidity:  Dropping below 50% by 11 a.m. to below 30% by 5 p.m.

Surface winds: N 5-10 mph this morning; N 8-12 mph this afternoon.

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

Mixing height: Rising to 3000 feet by 2 p.m. and to 4000 feet by 5 p.m.

Salem's sunset tonight: 8:42 p.m.

(Salem Airport data for Monday, July 28th: High 96°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 60)

EXTENDED DISCUSSION:

Very little change in the overall weather pattern is predicted this week.  A strong upper-level ridge of high pressure will stay anchored over the Rockies with a south to SW flow aloft over Oregon.  That will keep skies mostly sunny with well above normal temperatures.  It will also maintain a threat of afternoon and evening thunderstorms...mainly over the Cascades.  Onshore flow will be generally weak but could provide limited 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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