US Extended Forecast thru Sep 1

Models Project A Similar Longwave Pattern Through The Medium Range Period As In Recent Days...With The Anomalously Strong Upper High Over The Plains Slowly Retrograding...And Mean Troughs Continuing Along The Pacific Northwest And Western Atlantic.

However...Considerable Spread...Particularly Beyond Day 5/Friday... Continues To Be Displayed Within The Deterministic And Ensemble Member Solutions.

The Largest Uncertainty Is Across The Pacific...Where Interactions Involving Multiple Streams Produce A Multitude Of Differing Solutions ...With Likely Implications For The Western Us By Next Weekend. The 00z Gefs/Ecmwf Ensemble Means Have Switched Trough Positions Along The West Coast By Day 7/Sunday Compared To 24 Hours Ago...Which Is Indicative Of This Uncertainty.

If The Gefs Mean Were To Verify...Heights Would Fall More Quickly Across The Northwest And Erode The Northern Periphery Of The Upper High Over The Plains...While If The Ecmwf Ensemble Mean Were To Verify...The Trough Would Possibly Amplify Or Slow.

However...The 00z Gefs Mean Is Nearly Out Of Phase With The Model
Consensus Across The Northern Pacific...Which May Explains Its
Faster Solution...Thus Making It Difficult To Use. The Canadian
Ensemble Members On The Other Hand Are Indeed Slower And More
Amplified Like The Ecmwf Ensemble Members Except A Little
Faster...About Halfway Toward The Gefs...Which Helps To Account
For The Speed Uncertainty. While The Canadian Ensembles Are
Unavailable From A Wpc Blending Perspective...The Naefs Bc Mean
Is...Which Uses The Canadian Ensembles.

Another Reason To Account For The Canadian Ensemble Members Is Its Different Dynamical Core And Physics Choices...Which Help To Expand The Solution Envelope When The Uncertainty Is Unusually High. Therefore...The Overall Model Choice Or Blend Is A Mixture Of The 00z Ecmwf Ensemble Mean/Naefs Bc Mean...With Slight Contributions From The 06z Gfs/00z Ukmet Day 3/Wed...When And Where The Best Clustering Occur. Another Possibility For Model Choice Is The 12z Ecmwf Ensemble Mean From Yesterday (Aug 24)...Which Is Close To The 00z Ensemble Consensus.

This Approach Maintains Good Overall Continuity...With Hot
Temperatures Likely To Continue Across Portions Of The Plains/ Midwest...While Rainfall Chances Remain Highest On The Perimeter Of The Upper High...Particularly The Pacific Northwest With Onshore Slow...And Also Across Portions Of The Northeast/New England.

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