US Final Extended Weather thru Dec 1

Preliminary Update...

Most Of The Deterministic Model Solutions Have Been Volatile Over
The Past Few Cycles, Particularly Over The Eastern United States.

The Issue Can Be Boiled Down To Phasing- When, Where, How Much, Etc. The Most Unstable Models Have Been The Gem Global And Gfs, With Their Solutions Ranging From Sending A Deep Low Northward West Of The Appalachians, To Flatter, Suppressed Solutions, To Phasing Off The Northeast Coast Resulting In A Major Winter Storm For That Region. The Deterministic Ecmwf Has Been More Stable During The Past Four Cycles, Though Even It Has Displayed Meaningful Synoptic Fluctuation. The Most Consistent Guidance Has Been The 50-Member European Centre Ensemble Mean, Which For The Past 6 Cycles Has Not Shown A Strong Cyclogenetic Signal Over The East. The Ecens Mean Was Also Flat In The Face Of Other Developed Solutions Over The Western Atlantic Last Week, A Scenario That Verified Well (I.E. Storm Closer To Bermuda Than The United States East Coast).

Additionally, The Ecens Mean Has Shown Skill At Days 6 And 7 Comparable To Other Guidance At Day 5 Since The Onset Of The Fall, Further Bolstering Its Reliability. Indeed, The 06z/24 Gfs Backed Off Considerably On The Phasing Over The Northeastern United States Early In The Medium Range, Toward A Solution Much Closer To The 00z/24 Deterministic Ecmwf- Itself Comfortably Within The Envelope Of The European Centre Mean. In Light Of The Flakiness Of The Deterministic Solutions, And The Stability Of The Ecens Mean, Elected To Update The Preliminary Fronts And Pressures Using A Modified Version Of The 00z/24 Ecens Mean.

The Modest Trend In That Mean From Early Yesterday Toward A Slightly Slower, Wavier Boundary Crossing The Mid Atlantic States Day 3 Should Allow For A Little More Precipitation Astride The Front From The Appalachians Eastward, With Eastern New England Netting More Precipitation With The Resulting Synoptic Low Deepening Closer To That Region. The Ecens Mean's Stability Extends To The Eastern Pacific, Its Mass Fields Thus Affording The Western And Central United States A Better Bet As Well.


As Per Coordination With The Short Range Desk, Squashed The Day
3/3.5 Primary Surface Low Southwestward From The Update Package To Emphasize The Collective Trend Toward Giving More Credit To The Trailing Shortwave Crossing The East.

This Adjustment, As Per The Trend Across The Board In The 12z/24 Models, Significantly Reduces The Threat Of A Significant Winter Storm For New York State And Central And Northern New England. There Is Still Some Uncertainty Whether The Region From Philadelphia To Southeast New England Could See A Quick Shot Of Accumulating Snow. The Limiting Factor There Would Be The Confluent Flow Over The Region- Related To The Timing Of The Impulse.

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