[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Wed Jul 9 11:52:39 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT WED JUL 9, 2014

BURN ADVISORY:

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

Prep burning is not allowed.

WEATHER DISCUSSION:

A weak weather system cutting across SW Canada is flattening the upper-level ridge over the Pacific Northwest and shifting the feed of monsoonal moisture into eastern Oregon.  Scattered showers and thundershowers will develop this afternoon near the Idaho border, but skies will remain sunny across western and central Oregon, with the exception of low marine clouds along sections of the coast.

The air aloft is still quite warm, so high temperatures will only back off into the upper 80s today.  Daytime heating should lift mixing heights to near 5000 feet later this afternoon, but increasing north to NW transport winds may be too strong and too northerly to allow for the burning of fields.  PIBALS are scheduled to begin at 1 p.m. to monitor the low-level winds.

Surface pressure gradients are onshore from Newport to Redmond but not yet favorably stacked to keep smoke elevated.  Continued surface heating may turn gradients more favorable for burning this afternoon.  Another potential limiting factor for burning today will be the increasing wildfire risk.  Relative humidity values are already below 50% and may drop below 30% by late this afternoon.  The combination of low humidity values and increasing winds may put sections of the Willamette Valley into State Fire Marshal Burn-Ban Conditions later this afternoon, for the third straight day.

TODAY'S FORECAST:

Sunny and only slightly cooler this afternoon.

Salem's high temperature today will be near 88 degrees (average is 81).

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

Surface winds: NNW 7-15 mph this afternoon.

Transport winds: NNW 10-17 mph this afternoon.

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

Salem's sunset tonight: 8:59 p.m.

(Salem Airport data for Tuesday, July 8th: High 94°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 85)

EXTENDED DISCUSSION:

Only minor changes in the overall weather pattern are expected on Thursday.  The weather system cutting across SW Canada will maintain a dry and stable westerly flow aloft over Oregon.  Weak onshore low-level flow may briefly bring marine clouds inland Thursday morning, but expect another sunny and warm afternoon, with temperatures climbing back into the upper 80s.  Daytime mixing will be good, but afternoon transport winds are forecast to revert back to northerly, which is not favorable for burning.

An upper-level ridge is expected to rebuild over the region Friday through Sunday.  Valley temperatures will likely climb back into the 90s.  Low-level winds may turn northwesterly again Friday afternoon, but the threat of thundershowers will need to be closely monitored, as the flow aloft backs from weak westerly to southerly.  The chance of thundershowers continues through the weekend.

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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