Prognostic Meteorological Discussion
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FXUS02 KWBC 221600
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1159 AM EDT MON MAY 22 2017

VALID 12Z THU MAY 25 2017 - 12Z MON MAY 29 2017

MORNING UPDATE
~~~~~~~~~~~~~~

MODEL GUIDANCE IS STARTING TO COME INTO BETTER AGREEMENT WITH THE
DETAILS OF AN ANOMALOUS UPPER TROUGH LIFTING NORTHEASTWARD OUT OF
THE OHIO/TENNESSEE VALLEYS ON THURSDAY...AND ENSEMBLE MEMBERS ARE
ALSO STARTING TO SHOW REASONABLE CLUSTERING WITH AN ASSOCIATED
SURFACE LOW EXPECTED TO TRACK NORTHEASTWARD ALONG THE NEW ENGLAND
COAST FRIDAY INTO SATURDAY (WITH THE MAJORITY OF ECMWF MEMBERS
SUGGESTING THE LOW STAYS SLIGHTLY FARTHER OFFSHORE THAN THE BULK
OF THE GEFS MEMBERS).  HOWEVER...FORECAST UNCERTAINTY REMAINS
RELATIVELY HIGH WITH THE EVOLUTION OF AN UPPER TROUGH PROGRESSING
THROUGH THE NORTH CENTRAL U.S. LATE THIS WEEKEND/EARLY NEXT WEEK.
THE PAST FEW RUNS OF THE GFS (00Z/06Z RUNS) ARE SHOWING THE
POTENTIAL FOR A SLOWER/MORE AMPLIFIED SOLUTION THAN THE ECWMF/GEFS
MEAN/ECMWF ENSEMBLE MEAN...WHICH WOULD RESULT IN A MUCH STRONGER
SURFACE CYCLONE TRACKING THROUGH THE MIDDLE MISSISSIPPI VALLEY
EARLY NEXT WEEK THAN THE CURRENT MODEL CONSENSUS AND CONTINUITY.

THE UPDATED WPC DAY 3-7 FORECAST WAS BASED ON A BLEND BETWEEN THE
GFS/GEFS MEAN/ECMWF/ECMWF ENSEMBLE MEAN/CONTINUITY...WITH LESS
WEIGHTING TOWARDS THE GFS SINCE IT CURRENTLY APPEARS TO BE A
STRONG/SLOW OUTLIER WITH THE UPPER TROUGH CROSSING THE NORTH
CENTRAL U.S. LATE THIS WEEKEND INTO EARLY NEXT WEEK.

GERHARDT


PATTERN OVERVIEW AND MODEL PREFERENCES
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

A VIGOROUS UPPER LEVEL TROUGH ALONG WITH A CLOSED LOW REACHING
SOUTHWEST CANADA AND THE NORTHWEST U.S. AT THE BEGINNING OF THE
PERIOD SHOULD GRADUALLY BECOME BROADER AND MORE DIFFUSE AS IT
DRIFTS EASTWARD WHILE THE AXIS OF AN UPSTREAM RIDGE APPROACHES THE
WEST COAST.  AT THE SAME TIME AN AMPLIFIED UPPER TROUGH INITIALLY
OVER THE OHIO VALLEY WILL LIKELY LIFT AWAY FROM THE EASTERN STATES
BY THE WEEKEND WITH HEIGHTS ALOFT RISING IN ITS WAKE.  OVERALL,
THE MODELS AND ENSEMBLES ARE MODERATELY AGREEABLE WITH THE LARGE
SCALE PATTERN WITH PERSISTENT SPREAD REGARDING THE DETAILS OF THE
EASTERN U.S. TROUGH EVOLUTION, THE SOUTHERN CANADA/WESTERN U.S.
UPPER TROUGH, AND LATE IN THE PERIOD HOW SHORTWAVE ENERGY FLOWING
AROUND THE SOUTHERN PERIPHERY OF THE WESTERN TROUGH MAY INFLUENCE
SURFACE FEATURES FROM THE PLAINS INTO PARTS OF THE EAST.

THROUGH AT LEAST FRIDAY AN OPERATIONAL MODEL BLEND INCLUDING THE
18Z GFS AND 12Z ECMWF/UKMET REFLECTED THE MOST AGREEABLE ASPECTS
OF GUIDANCE WHILE DOWNPLAYING LESS CONFIDENT MEDIUM TO SMALLER
SCALE DETAILS PRESENT IN EACH THOSE MODEL RUNS.  BY THE LATTER
HALF OF THE FORECAST PERIOD, AN INCREASING PERCENTAGE OF THE 18Z
GEFS MEAN AND 12Z EC MEAN WERE INCORPORATED TO ACCOUNT FOR THE
INCREASING MODEL SPREAD.  SINCE THE GEFS MEAN APPEARED TO BE
CLOSER TO THE OPERATIONAL GFS AND ECMWF AND SHOWED GREATER DETAIL
ON THE OVERALL FEATURES, IT WAS WEIGHTED MORE IN THE FORECAST THAN
THE EC MEAN.


WEATHER HIGHLIGHTS
~~~~~~~~~~~~~~~~~~

THE BEGINNING OF THE MEDIUM RANGE PERIOD ON THURSDAY WILL FEATURE
HOT AND DRY CONDITIONS FROM THE DESERT SOUTHWEST TO WESTERN TEXAS,
WITH HIGHS 5 TO 10 DEGREES ABOVE NORMAL.  AFTER A FEW DAYS OF
COOLER TEMPERATURES, THE HEAT RETURNS TO THE INTERIOR PACIFIC
NORTHWEST BY NEXT WEEKEND WITH HIGHS IN THE 80S TO NEAR 90.
UNSEASONABLY COOL CONDITIONS ARE EXPECTED ACROSS THE NORTH-CENTRAL
U.S. AND THE OHIO VALLEY FOR THURSDAY WITH THE UPPER LEVEL TROUGH
IN PLACE, WITH DEPARTURES OF 5 TO 15 DEGREES BELOW CLIMATOLOGICAL
AVERAGES.  A GRADUAL MODERATING TREND IS EXPECTED THROUGH NEXT
WEEKEND AS TEMPERATURES REBOUND CLOSER TO NORMAL AS THE UPPER
LEVEL TROUGH LIFTS OUT OF THE REGION.  ANOTHER ROUND OF COOLER
WEATHER IS LIKELY FOR THE WESTERN HIGH PLAINS BY THE BEGINNING OF
NEXT WEEK.

CONCERNING PRECIPITATION PROSPECTS DURING THIS TIME PERIOD, THE
HEAVIEST RAINFALL IS LIKELY ACROSS THE FLORIDA PENINSULA ON
THURSDAY AS THE COLD FRONT ADVANCES EASTWARD BEFORE STALLING AND
EVENTUALLY DISSIPATING OVER THE FLORIDA STRAITS.  AN ADDITIONAL
AREA OF MODERATE TO LOCALLY HEAVY RAIN AND THUNDERSTORMS IS LIKELY
FROM THE CENTRAL APPALACHIANS TO NEW ENGLAND IN ASSOCIATION WITH
THE MAIN SURFACE LOW.  SCATTERED SHOWERS AND STORMS WILL BE
POSSIBLE ACROSS THE ROCKIES AND EXTENDING EASTWARD TO THE UPPER
MIDWEST AND CENTRAL PLAINS WITH THE OTHER FRONTAL BOUNDARY THAT
CROSSES THE WESTERN U.S. AND THEN REACHES THE CENTRAL U.S. BY NEXT
WEEKEND.  AN ENHANCED AREA OF HEAVIER RAINFALL WILL BE POSSIBLE
ACROSS PARTS OF ARKANSAS AND MISSOURI AS DEEP MOISTURE INTERACTS
WITH A FRONTAL BOUNDARY IN THE REGION.

HAMRICK

$$





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