[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Fri Aug 1 08:54:56 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT FRI AUG 1, 2014

BURN ADVISORY:

State Fire Marshal Burn-Ban Conditions may be reached this afternoon.

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

There have been subtle changes to the overall weather pattern in the past 24 hours.  A warm southwesterly flow aloft is continuing to periodically send upper-air disturbances across the state...triggering clusters of thunderstorms.  The daily focus of these storms has been along a nearly stationary thermal trough axis, extending from SW Oregon to north-central Oregon.  However, that focus has shifted westward, with one such system moving northward across the Willamette Valley overnight, producing significant lightning and gusty winds from Eugene to Portland.  Dry air in the lower levels of the atmosphere lead to the gusty winds and also held rainfall totals to generally around one-tenth of an inch or less.

Radar and lightning detection were both indicating active thundershowers continuing over the Cascade foothills, from east of Salem northward to east of Portland, at mid-morning.  These storms should slowly weaken as they migrate northward.

The upper-air sounding over Salem this morning looked very similar to Thursday's sounding, except for being slightly more unstable in the mid-levels of the atmosphere.  The surface thermal trough, which has been over central Oregon, has shifted westward to over the western Cascade foothills.  That has turned pressure gradients offshore from Redmond to Salem, with onshore gradients from Newport to Salem.  As skies clear later this morning, temperatures should rapidly warm, with afternoon highs once again in the low to mid 90s.  Transport winds should be northerly this afternoon, and State Fire Marshal Burn-Ban Conditions may be reached.

Daytime heating will, once again, destabilize the lower levels of the air mass, leading to additional thundershower development.  The focus of the storms will be mainly over the Cascades, but it is possible that storms could move over the Willamette Valley again later today through tonight.

TODAY'S FORECAST:

Lingering thunderstorms this morning...  Partly cloudy and hot again this afternoon with a chance of thunderstorms returning late.

Salem's high temperature today will be near 94°F (average is 84°F).

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

Surface winds: N 5-10 mph this morning; N 8-15 mph this afternoon.

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

Mixing height: Rising to near 4000 feet by 5 p.m.

Salem's sunset tonight: 8:38 p.m.

(Salem Airport data for Thursday, July 31st: High 94°F; Rainfall: Trace)

(Salem Airport rainfall so far today: .15")

(Maximum Ventilation Index expected today: 60)

EXTENDED DISCUSSION:

A strong upper-level ridge of high pressure will remain anchored just east of the state through early next week.  That will maintain a warm south to SW flow aloft over Oregon, with continued above normal temperatures and clusters of mainly afternoon and evening thunderstorms.  There will continue to be an elevated chance of thunderstorms over the Willamette Valley through Saturday, with the storm focus shifting to mainly from the Cascades eastward by Sunday.

The upper-level ridge is forecast to weaken slightly by Tuesday.  Increasing onshore flow will likely shift the thundershower threat well east of the region and cool temperatures back into the upper 80s.  Transport winds may turn enough onshore to allow for some burning.  Additional weakening of the ridge may further increase the onshore flow by Friday.

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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