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Extended Forecast Discussion
 
(Latest Discussion - Issued 1854Z Feb 23, 2018)
 
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Abbreviations and acronyms used in this product
 
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
153 PM EST FRI FEB 23 2018

VALID 12Z MON FEB 26 2018 - 12Z FRI MAR 02 2018

...HEAVY RAIN/FLOOD THREAT FOR SATURATED SOILS/SWELLING RIVERS
ACROSS PORTIONS OF THE MID-MISSISSIPPI VALLEY AND MID-SOUTH...


PATTERN OVERVIEW
~~~~~~~~~~~~~~~~
THE FLOW PATTERN DRAMATICALLY TRANSFORMS THIS PERIOD AS A LONGWAVE
TROUGH ACROSS NORTH AMERICA BECOMES MORE ALIGNED WITH LITTLE TILT
BETWEEN A STRONG MID-LEVEL VORTEX NORTH OF ALASKA AND TROUGHING
ACROSS THE WEST, WHICH DEVELOPS DOWNSTREAM OF AN INCREASINGLY
AMPLIFIED RIDGE MOVING INTO WESTERN AK.  DOWNSTREAM, THIS
DISLODGES THE VORTEX NEAR HUDSON BAY AND LEADS TO RETROGRADING AND
STRONG RIDGING MOVING WESTWARD OUT OF EUROPE INTO SOUTHERN
GREENLAND.  AS ENERGY IN THE FORM OF DEEP LAYER CYCLONES UNDERCUTS
THE GREENLAND BLOCK, THE PATTERN ASSUMES A REX BLOCK CONFIGURATION
ON TUESDAY WHICH EVOLVES INTO AN OMEGA BLOCK OVER THE NORTH
ATLANTIC/NORTHEAST UNITED STATES BY NEXT FRIDAY.


MODEL ASSESSMENT & PREFERENCES
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
IN GENERAL, THE MODELS/ENSEMBLES SHOWED DECENT AGREEMENT WITH
RESPECT TO THE LARGE SCALE PATTERN THROUGHOUT THE PERIOD. 
SIGNIFICANT DIFFERENCES AMONG THE GUIDANCE WERE CONSTRAINED TO THE
NORTHEAST PACIFIC EARLY AND NEAR THE EAST COAST LATE.  ENSEMBLE
SPREAD IS RATHER HIGH AND COVERS THE ENTIRE RANGE OF
POSSIBILITIES.  THE BIGGEST ISSUE LIES ACROSS THE NORTHEAST, WHERE
THE 00Z ECMWF SHOWS A SOLUTION WITH A CLOSED LOW NEAR THE GREAT
LAKES IN MULTIPLE PIECES AND WEAKER THAN ITS OWN ENSEMBLE MEAN.

THE WPC PRESSURES, WINDS, AND FRONTS WERE BASED HEAVILY ON
DETERMINISTIC GUIDANCE DURING DAYS 3-5, WITH A 00Z ECMWF/06Z
GFS/00Z UKMET/00Z CANADIAN BLEND. AFTER THAT TIME, DUE TO
INCREASING UNCERTAINTY, ENSEMBLE MEAN (00Z ECENS/NAEFS) WEIGHTING
WAS GRADUALLY BOOSTED THROUGH TIME, EVENTUALLY MAKING UP 50%+ OF
THE DISTRIBUTION BY NEXT FRIDAY.  THE TEMPERATURES/RAIN
CHANCES/CLOUDS/DEW POINTS/WEATHER GRIDS WERE BASED ON AN EVEN
BLEND OF THE ENSEMBLE AND DETERMINISTIC GUIDANCE, WITH A
PREFERENCE SLIGHTLY MORE IN THE DIRECTION OF THE 00Z ECMWF
ENSEMBLE MEAN THAN THE 00Z NAEFS MEAN DUE TO DIFFERENCES IN THE
SOUTHEAST.  THE QPF CHOICE WAS A COMPROMISE OF THE 00Z ECMWF/06Z
GFS/12Z GFS SOLUTIONS.


WEATHER HIGHLIGHTS/THREATS
~~~~~~~~~~~~~~~~~~~~~~~~~~
FOR THE WEST...
PERSISTENT TROUGHING ALONG THE WEST COAST WILL KEEP CONDITIONS
UNSETTLED THERE, WITH THE THREAT FOR HEAVY PRECIPITATION
INCREASING BY MID TO LATE NEXT WEEK AS THE NEXT TROUGH/UPPER LOW
DIGS ALONG THE WEST COAST. HEAVY PRECIPITATION IS POSSIBLE FROM
THE CASCADES SOUTHWARD INTO NORTHERN & CENTRAL CA IN THE WAKE OF A
COLD FRONT THAT PUSHES INLAND.  PERSISTENT TROUGHING AND
PRECIPITATION WILL KEEP TEMPERATURES BELOW AVERAGE ACROSS MUCH OF
THE WEST THROUGH NEXT WEEK.

FROM THE PLAINS EASTWARD...
WIDESPREAD CONVECTION AND POTENTIALLY HEAVY RAINS WILL ONCE AGAIN
POSE A THREAT ACROSS PORTIONS OF THE LOWER MS VALLEY, TN VALLEY,
AND PERHAPS THE SOUTHEAST/OHIO VALLEY FROM LATE TUE THROUGH THU
NIGHT AS MOIST LOW-LEVEL SOUTHERLY FLOW WITHIN A CYCLONE'S WARM
CONVEYOR BELT SPREADS AMPLE MOISTURE NORTHWARD OVER A RETURNING
WARM FRONT.  FARTHER NORTH, A LOW PRESSURE SYSTEM ON
WEDNESDAY/THURSDAY TRACKING ACROSS THE CENTRAL
PLAINS/MIDWEST/GREAT LAKES WOULD BRING CHANCES OF SNOWFALL TO
PORTIONS OF THOSE REGIONS, ALTHOUGH CONFIDENCE IS LOW IN THE
SPECIFICS AT THIS POINT. BROAD POSITIVE HEIGHT ANOMALIES WILL KEEP
TEMPERATURES ABOVE AVERAGE FOR MUCH OF THE CENTRAL/EASTERN U.S.,
WITH HIGHS REACHING DOUBLE DIGITS ABOVE AVERAGE ACROSS PORTIONS OF
THE MID-MS VALLEY/MIDWEST BY TUESDAY.

ROTH/RYAN