[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Mon Sep 15 11:50:54 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT MON SEP 15, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A strong upper-level ridge over the Pacific Northwest will slowly progress eastward today, with an increasing southwesterly flow aloft.  A surface thermal trough remains near the coast this morning.  Onshore flow has returned to the central and southern coast, but continued weak offshore flow is forcing smoke, from the "36 Pit" wildfire near Estacada, into the Willamette Valley.  No relief from the smoke is likely today, as the thermal trough shifts over the Willamette Valley; producing light and variable winds, low mixing heights, and very warm temperatures.

A very weak upper-level disturbance, currently off the northern California coast, will move over western Oregon by this evening.  Along with increasing middle and high clouds, this system could trigger high-based thunderstorms.  Since the low-levels of the air mass remain very dry, any storms that form would product little to no rain, adding to an already elevated wildfire risk.

Due to the very dry conditions and the chance of dry thunderstorms, the National Weather Service has extended the Red Flag Warning for most of NW Oregon, including the Willamette Valley, through 5 a.m. Tuesday.

TODAY'S FORECAST:

Partly cloudy, warm, and smoky.  Slight chance of a dry thunderstorm late.

Salem's high temperature today will be near 90°F (average is 77°F).

Relative humidity:  Dropping to below 20% by late this afternoon.

Surface winds: Var 5 mph.

Transport winds: Var 5 mph.

Mixing height: Only rising to near 2500 feet by late this afternoon.

Salem's sunset tonight: 7:23 p.m.

(Salem Airport data for Sunday, September 14th: High 89°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 13)

EXTENDED DISCUSSION:

As the upper-level ridge progresses eastward, increasing southwesterly flow aloft will force the surface thermal trough east of the Cascades on Tuesday.  That will turn low-level winds onshore; initiating a cooling trend and dramatically improving the air quality across the Willamette Valley.

The switch to onshore flow could create burning opportunities Tuesday and/or Wednesday, with a little stronger weather system possibly bringing light showers to the region later Wednesday and Thursday.  A return to dry and warmer weather begins on Friday; possibly lasting through next week.

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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