Preliminary Forecasts
Issued by NWS

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000
FXUS02 KWNH 280629
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
229 AM EDT SUN MAY 28 2017

VALID 12Z WED MAY 31 2017 - 12Z SUN JUN 04 2017

PATTERN OVERVIEW
~~~~~~~~~~~~~~~~
TROUGHING IS EXPECTED IN THE NORTHWEST AND NORTHEAST THIS PERIOD
WITH RIDGING IN BETWEEN.  SYSTEMS MOVING THROUGH THE
SOUTHWEST/SOUTHERN ROCKIES SHOULD MOVE EASTWARD DUE TO A KICKER
SHORTWAVE MOVING TOWARDS SOUTHERN CA, MAINTAINING A BREAK IN THE
SUBTROPICAL RIDGE IN THE WESTERN GULF.  THE DEGREE OF THE
TROUGHING IS SHOWING INCREASING VARIABILITY IN BOTH THE NORTHEAST
AND NORTHWEST.  IN THE CASE OF THE NORTHEAST, THIS CASTS DOUBT ON
MOISTURE RETURN INTO THE REGION NORTH OF THE MID-ATLANTIC STATES.


MODEL PREFERENCES
~~~~~~~~~~~~~~~~~
THE MODEL GUIDANCE DURING THE PERIOD IS BEGINNING TO SQUABBLE ON
DETAILS LARGE AND SMALL, SHOWING LARGER DIFFERENCES WITH EACH
SUCCESSIVE MODEL RUN WITH THE POSITION OF THE DEEP CYCLONE IN
SOUTHEAST CANADA AND WITH THE DEPTH OF TROUGHING MOVING INTO THE
NORTHWEST NEXT WEEKEND.  TO HELP ACCOUNT FOR UNCERTAINTY, THE
PRESSURES/500 HPA HEIGHTS/WIND GRIDS STARTED OUT WITH AN EVEN
SPLIT OF THE 12Z CANADIAN/12Z UKMET/12Z ECMWF/18Z GFS BEFORE USING
INCREASING AMOUNTS OF THE 12Z ECMWF/12Z NAEFS ENSEMBLE MEANS
(WHICH REACHES OVER 50% ON DAY 8).  THE TEMPERATURES, RAIN
CHANCES, CLOUDS, DEW POINTS, AND WEATHER GRIDS WERE HALF 12Z
ECMWF/12Z NAEFS MEAN AND HALF 12Z CANADIAN/12Z ECMWF/18Z GFS.

THE DAYS 4-7 PRECIPITATION FORECAST WAS MORE CHALLENGING THAN EVER
DUE TO AN ECMWF WARM CORE CYCLONE DEVELOPMENT IN THE NORTHWEST
GULF EMANATING FROM NORTHEAST MEXICO LIKELY CAUSING ISSUES ACROSS
THE SOUTHERN PLAINS IN ITS QPF DISTRIBUTION AND THE EVER CHANGING
GFS GUIDANCE OVER THE LOWER 48, WHICH MIGHT BE TOO WET ACROSS
SECTIONS OF THE UPPER OHIO VALLEY AND NEW ENGLAND ON ITS 00Z RUN
DUE TO QUESTIONABLE POSITIONING OF ITS SOUTHEAST CANADA DEEP
CYCLONE.  CHOSE TO COMPROMISE BETWEEN THE LAST TWO RUNS OF THE
IN-HOUSE PSEUDO ENSEMBLE BIAS-CORRECTED QPF, WHICH IS A ROUGH
COMPROMISE OF THE 18Z GFS, 00Z GFS, AND 12Z ECMWF AND SHOULD
ACCOUNT REASONABLY FOR THE UNCERTAINTY.


WEATHER HIGHLIGHTS
~~~~~~~~~~~~~~~~~~
COOLER THAN NORMAL TEMPERATURES ARE EXPECTED TO CONTINUE INTO NEXT
WEEK ACROSS THE GREAT LAKES AND NORTHEAST DUE TO LINGERING UPPER
LEVEL TROUGHING.  MEANWHILE, WARM CONDITIONS ARE EXPECTED ACROSS
MUCH OF INTERMOUNTAIN WEST...WITH AN APPROACHING COLD FRONT
BRINGING SOME ASSOCIATED LATE-SEASON PRECIPITATION.

UNSTABLE SHOWERS ARE EXPECTED ACROSS THE GREAT LAKES DUE TO COLD
AIR ALOFT AND ACROSS FL AND THE LOWER ATLANTIC SEABOARD DUE TO
LOW-LEVEL HEAT AND MOISTURE AVAILABILITY DURING DAYTIME HEATING
NEAR AN AGING STATIONARY FRONT -- THE MODELS SHOW THEIR BEST
AGREEMENT IN THESE AREAS FROM A QPF PERSPECTIVE.  LOCALLY HEAVY
RAINS ARE POSSIBLE ACROSS PORTIONS OF THE SOUTHERN PLAINS AND
MID-MISSISSIPPI VALLEY AHEAD OF A PAIR OF UPPER LEVEL SYSTEMS
EJECTING OUT OF THE SOUTHWEST AND SOUTHERN ROCKIES, BUT ACROSS THE
UPPER OH VALLEY AND NORTHEAST SIGNIFICANT MODEL TO MODEL AND RUN
TO RUN DIFFERENCES ARE CLOUDING THE PICTURE AND LOWERING
CONFIDENCE.

ROTH/PETERSEN
$$




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