Weather US Extended Outlook Through Sep 24

Extended Forecast Discussion
Nws Hydrometeorological Prediction Center Camp Springs Md
220 Pm Edt Fri Sep 17 2010

Valid 12z Mon Sep 20 2010 - 12z Fri Sep 24 2010

Preliminary And Now Final Hpc Days 3-7 Fronts/Sea Level Pressure
And 500 Mb Progs Maintain Continuity And Remain Primarily Derived
From A Blend With Majority Weighting Toward The 00z Ecmwf And
Lesser Input From 00 Utc Ecmwf Ensemble Mean/Gfs Days 3-5
Mon-Wed...Transitioning To A Near Even Blend Of The 00 Utc Gfs/
Ecmwf And Their Respective Ensemble Means Days 6-7 Thu-Fri. We See Minimal Compelling Reason To Make Significant Adjustments To
Continuity Based On Latest 12 Utc Guidance.

The Models Offer Highest Solution Spread In The Flow Over The Nrn
Pacific Days 4-7...Where The Pattern Transitions Towards
Development Of A Deep Cyclone. Shortwaves Rotating Around The
Parent Cyclone Spawn Mesoscale Areas Of Low Pressure. A More Even Blend Of The Gfs/Ecmwf With Their Ensemble Means Was Used To Address Smaller Scale Differences...But These Remain Compounded By Inherent Error Resulting From Potent Undercutting Of Ample Upper Jet Energy.

Well Downstream...Models/Ensemble Means Show A Deep Layer Trough Emerging Out Of The Wrn Us Mon That Moves Across The Plains Tue And Great Lakes/Se Can By Midweek. An Associated Low Should Produce Widespread Moderate To Locally Heavy Rain Across Portions of The N-Central Plains/Upper Ms Valley/Upper Great Lakes And Convection Down With The Trailing Cold Front. The 00 Utc Ecmwf
Seems To Offer The Best Run-Run Continuity And Was Accordingly
Given More Weight In Our Blended Guidance Solution. The 00 Utc
Gfs/Canadian Lag Behind Slightly And Seem To Maintain Slightly Too
Much Amplitude Aloft Given Progressive Flow.

Later Across The Central Us...The 00 Utc Ecmwf Showed A Strong
Slowing Trend From Their 12 Utc Thu Run And Making The Current 00
Utc Gfs/Ecmwf Now In Unusually Close Agreement With The Track Of
The Next Low Starting Thu Over Nebraska And Continuing Near Nrn
Lake Huron By 12 Utc Fri. This Position Has Good Multi-Model
Ensemble Mean Support...Favoring Continued Use Of A Consensus
Approach Consistent With A Pattern With Lingering Uncertainty.

The 00 Utc Canadian Was An Outlier In Forecasting A Cyclone To
Develop In The Gulf Of Mexico South Of The Southeast Us Ridge Thu.
The Gfs/Ecmwf/Respective Ensemble Means Maintain No Developments Are Expected In The Gulf And That Matches Tpc Coordination. The 12 Utc Canadian Has Also Significantly Backed-Off On This Development.


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