US Preliminary Extended Forecast thru May 19

...Heavy Rainfall Possible For The Upper Missouri And Mississippi
River Basins...

...Vigorous Upper-Level Low Expected To Stall Along The Pacific
Northwest Coast...

Amplified Pattern Will Prevail Across The Lower 48 This Period With Much Of The Guidance Suggesting 2 Very Slow-Moving And Unstable Cutoffs Generating Generous Amounts Of Deformation And Convective Precipitation Along And North Of 45n Latitude From The
Pacific Coast To The Great Lakes This Forecast Period.

Their Associated Surface Fronts Appear To Be Most Active Across The
West...Northern Rockies And Southern Mid Atlantic Region...With
Warm Sector Advection And Spotty/Localized H85-H7 Moisture
Advection For The Conus East Of The Mississippi River To The
Atlantic Shoreline.

By Day 7...The Central High Plains Could Become An Active Area For Convective Initiation As A Stationary Front Interacts With A Deepening Lee Side Trough And Pacific Energy Exiting The Central Rockies.

Preferences Continued To Be A Blend Of The 16/12z Gefs And 16/00z
Ecmwf Means Aloft To Handle The Spread In The East Beyond Day 5.

Much Of The Spread Beyond Day 5...Seems To Be Tied To The Slow
Amplification (Ridging) Of The Flow In South Central Canada That
Develops Out Ahead Of The Pacific System And Behind The Great
Lakes System.

What Does Stand Out In The Recent 17/00z Guidance Is The Energy
That Rotates Around The Cutoff In The Pacific Northwest Will Have
A Very Difficult Time Ejecting Out Onto The Plains. In The Southern And Central Plains Especially...Deep-Layered Warm Advection And Increasing Stability Will Be A Limiting Factor For Precipitation That Attempts To Develop Along The Canadian Surface Front That Spreads Through The Plains On Day 4-5-6.

On A Related Topic...The Pacific System Will Have No Problem Creating A Favorable Environment Of Deep Convection And Advecting Mid-Level Moisture Across The Cascades And Into The Northern Rockies To The Continental Divide. In Fact...By Day 6-7...Lee Side Cyclogenesis In Central Montana And An Upslope East To Southeasterly Flow Looks To Develop As A Result Of The Persistence Of The Trough/Ridge Couplet West Of 100w.

Of The 17/00z Deterministic Guidance...The Gfs/Canadian/Ukmet/Ecmwf Reasonably Handled The Cutoffs Along The Pacific Northwest Coast...North Of Hawaii And Over The Mid-Missouri Valleys Through Day 5. Compared To The Wpc H5 Graphics (A H5 Low Center Invof 47n123w)...The Ecmwf And Gfs Are The Closest To The Same Latitude (46n) Along The West Coast On Day 5 With The Canadian At (49n) And The Ukmet (43n).

Aside From The Ecmwf...All Are Faster Bringing The System's H5 Center Across The Washington/Northern Oregon Cascades Cascades.

For The Cutoff North Of Of Hawaii...The 17/00z Canadian Holds On To The Cutoff Longer Than The Consensus And Would Be The Outlier
Rolling Into Day 6.

The Canadian/Ukmet/Ecmwf/Gfs Deterministic Runs Do A Decent Job With The Mid-Missouri Valley System For Day 4 And 5.

Once Again On Day 6...The 17/00z Deterministic Ecmwf Is A Deeper System Over The Great Lakes Vs The Canadian And Gfs.

By Day 7 (Valid 24/12z)...Its Very Deep With Its Cutoff Solution 'Attempt' South Of James Bay In Ne Ontario. Two Days Ago (Its 15/12z Run)...It Attempted A Cutoff Solution Over Wrn Kentucky On That Specific Day 7 Forecast...(Also Valid For 24/12z). In A Nutshell...The Deterministic Ecmwf Should Not Be Totally Accepted With This Amplified Pattern.

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