US Preliminary Extended Weather thru Apr 27

...Overview And Preferences...

Medium Range Guidance Continues To Show A Broad Mean Trough
Covering Most Of Noam... Supported By A Fairly Strong Ridge Over
The Ern Pac And Another Ridge That Builds Over The Nrn Atlantic
During The Period.

Within The Mean Trough The Most Agreeable Solns Suggest There Should Be Three Discrete Shrtwvs... One Progressing From The Cntrl Conus Into The East Tue Into Thu... A Second Dropping From Canada Into The Nrn Tier/Grtlks Wed-Thu... And The Third Reaching The Nrn Conus/Srn Canada By Fri-Sat.

At Least In The Large Scale There Is Good Agreement During The First Half Of The Fcst But Models/Ensembles Still Diverge With Some
Aspects Of Ern Pac/Wrn Noam Flow By Days 6-7. A 12z
Gfs/Ecmwf/Ukmet Blend Provides A Good Intermediate Soln For
Individual Systems Days 3-4 Tue-Wed. After That Time Prefer A
70/30 Weighting Of The 12z Ecmwf Mean/12z Gefs Mean As An Initial
Base Map To Account For The Increased Detail Uncertainty During
The Latter Half Of The Period.

...Guidance Evaluation...

During Days 3-4 Tue-Wed The Primary Features Of Interest Will Be A
Sfc Wave Tracking Newd From The Grtlks... The Trailing Cold Front
Crossing The Ern States... And Wave That May Track Nwd Just Off
The East Coast. While The Various Operational Models Offer Some
Question Marks With Their Respective Solns For Each Feature... An
Avg Of The 12z Gfs/Ecmwf/Ukmet Yields A Good Intermediate Starting
Point And Keeps Continuity Changes Fairly Modest. For The Grtlks
Wave The 18z Gfs And Slow 12z Cmc Appear To Be Over-Developed.
The Trailing Front Has Trended A Little Faster In The Ecmwf Mean
Compared To 24 Hrs Ago While The 18z Gefs Mean And Especially 18z Gfs Become Faster Than Desired Relative To The Full Guidance Mean. At The Same Time A Consensus Soln Has Held Up Fairly Well For The Wave Off The East Coast. The 12z Cmc Was The Deep Extreme For This Feature.

Continuity Is Good With A Weak Frontal System Expected To Affect
Areas From The Nrn Plains Into Grtlks Wed-Thu... Though Solns
Differ With The Track Of Anchoring Low Pressure. Best Indications
At This Time Point Toward A Sfc Low Track Over Or Slightly Nwd Of
The Upr Grtlks.

By The Latter Half Of The Period Guidance Continues To Diverge
With Ern Pac/Wrn Noam Flow Details... Related To How Quickly
Energy Within An E-Cntrl Pac Closed Low Ejects Ewd. The 18z
Gfs/Gefs Mean And To A Lesser Extent 12z Gfs Are Earliest To
Flatten The Overall Pattern. On The Other Hand The 12z Ecmwf
Maintains A Very Stg Ern Pac Ridge Through Day 7 Sat... Leading To
A Sharper Shrtwv From The Nrn Plains Swwd And Stg High Pres
Building Into Nrn-Cntrl High Plains. Trends Over The Past Couple
Days And Occasional Tendency For Models To Weaken Stg Ridges Too Quickly Favor Emphasizing The 12z Ecmwf Mean Which Maintains The Ern Pac Ridge Better Than Most Gfs-Based Guidance And Has A More Moderate Cntrl Conus Sfc Evolution Than The Past Two Operational Ecmwf Runs. The 12z Gefs Mean Was Sufficiently Close With The Ern Pac/Noam Pattern Aloft To Give It Minority Weighting In The Blend.

Meanwhile The Idea Of Some Hgt Falls Reaching The East Is
Consistent With Teleconnections Relative To The Ridge Building
Over The Nrn Atlc. Continuing From Earlier In The Period The 18z
Gfs/Gefs Mean May Push The Front Too Far Ewd.

Favored Blends Through The Period Hold Onto The Middle/Slower
Portion Of The Spread With The Upr Low Reaching Srn Ca/Nrn Baja...
Given A Continued Lack Of A Pronounced Upstream Kicker. The 12z
Ecmwf Is Closest To The Preferred Timing At Least Into Day 6 Fri
While The 12z Gfs And Especially 18z Gfs Stray To The Faster Side.

...Sensible Weather Highlights...

Most Locations Over The West Should Remain Warm And Dry.

The Plains Will See Below Normal Temps During The Period... With The Most Extreme Anomalies For Daytime Highs Of 20-30 F Below Normal Occurring At Some Locations On Tue.

Some Of This Cool Air Will Spread Ewd But In Modified Form.

Early In The Fcst There Is Still Uncertainty With The Wwd Extent Of Mstr Assoc With The Wave Tracking Nwd Off The East Coast... Favoring An Intermediate Soln.

Grtlks To Plains Front Heading Toward The East Coast Tue-Thu Will
Be Accompanied By Some Areas Of Locally Mdt-Hvy Rnfl. Trailing
System Brushing The Nrn Tier Should Have Only Modest Amts Of Mstr
With It... But Then Enough Low Lvl Flow From The Gulf May Become
Established To Support Development Of Another Area Of Rnfl Over
The Plains/E-Cntrl Conus By Next Fri-Sat.

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