Preliminary Forecasts
Issued by NWS

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000
FXUS02 KWNH 230605
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
204 AM EDT WED AUG 23 2017

VALID 12Z SAT AUG 26 2017 - 12Z WED AUG 30 2017

...EXTREMELY HEAVY RAIN POSSIBLE ALONG THE NW GULF COAST THIS
WEEKEND INTO NEXT WEEK...


OVERVIEW
~~~~~~~~

UPPER RIDGING IS FORECAST TO BUILD/EXPAND INTO THE WESTERN US,
SUPPORTING TROUGHING IN THE CENTRAL/EASTERN STATES. IN THE GULF OF
MEXICO, THE NATIONAL HURRICANE CENTER CONTINUES TO ADVERTISE THE
LIKELY REDEVELOPMENT OF TROPICAL STORM HARVEY WITH A TRACK AIMED
TOWARD THE TEXAS COAST LATER THIS WEEK. PLEASE CONSULT THE LATEST
NHC FORECAST IF/WHEN HARVEY REDEVELOPS. A SLOWLY MOVING TROPICAL
SYSTEM (INCLUDING A TROPICAL DEPRESSION) HAS THE CAPABILITY OF
PRODUCING SEVERAL INCHES OF RAIN EVERY 6-12 HOURS WHICH CAN CAUSE
SERIOUS FLOODING.


GUIDANCE EVALUATION/PREFERENCES
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

LATEST 12-18Z/22 MODEL CONSENSUS (EXCEPT THE 12Z UKMET) WAS RATHER
WELL-CLUSTERED TO START THE PERIOD (SATURDAY), INCLUDING THE L ON
THE MAP OVER TEXAS THAT SIGNIFIES RESURRECTED HARVEY. THEREAFTER,
MODELS/ENSEMBLES DIVERGE ON HOW TO HANDLE HARVEY -- WILL IT GET
PICKED UP BY TROUGHING TO ITS NORTH OR BE FAR ENOUGH AWAY TO
WANDER ALONG THE NW GULF COAST? OR, IF YOU BELIEVE THE 00Z
CANADIAN, WILL IT TURN WESTWARD ACROSS SOUTHERN TEXAS AND
DISSIPATE OVER NORTHERN MEXICO? TREND HAD BEEN TO ALLOW HARVEY TO
BE PICKED UP AND LIFTED NORTHEASTWARD, BUT 12Z/18Z GUIDANCE WAS
SLOW ENOUGH TO ALLOW THE NORTHERN STREAM TROUGH AND HARVEY TO STAY
SEPARATE THROUGH THE PERIOD. MAJORITY OF ENSEMBLES KEPT THE
CIRCULATION OVER SE TEXAS OR THE NW GULF THROUGH TUESDAY BEFORE
MOVING IT MORE NORTHEASTWARD, SO OPTED TO RELY ON THE GFS/ECMWF
AND ESPECIALLY THE GEFS MEAN (WHICH WAS DEEPER AND FARTHER
NORTHEAST THAN THE 12Z ECMWF ENSEMBLE MEAN). QPF FROM SUCH A TRACK
WOULD BE EXTREMELY HEAVY NEAR ITS TRACK, BUT CONFIDENCE IN
PLACEMENT/TIMING AND AMOUNTS IS LOW.

OUTSIDE THE GULF, HIGH PRESSURE WILL BE IN CONTROL OVER THE
NORTHEAST QUARTER OF THE COUNTRY AND A LINGERING SURFACE BOUNDARY
WILL SINK THROUGH THE PLAINS SUN/MON. RIDGING WILL PREVENT AND
SYSTEM FROM IMPACTING THE WEST. BY NEXT TUE/WED, ENSEMBLES
DISAGREE ON EASTWARD EXTENT OF TROUGHING OUT OF THE NE PACIFIC
INTO THE PAC NW -- GEFS MEMBERS HELD THE TROUGHING BACK WHILE THE
ECMWF MEMBERS LOWERED HEIGHTS INTO WA/OR/ID. PUT A BIT MORE WEIGHT
IN THE GFS/GEFS SOLUTIONS THAT MAINTAINED A STRONGER UPPER RIDGE
IN THE WEST.


SENSIBLE WEATHER HIGHLIGHTS
~~~~~~~~~~~~~~~~~~~~~~~~~~~

EXPECT SHOWERS AND THUNDERSTORMS TO FOCUS ACROSS THE SOUTHEAST AND
THE GULF COASTS ALONG A LINGERING STATIONARY FRONTAL BOUNDARY.
FLORIDA AND THE SOUTHEAST COAST SHOULD ALSO SEE AN INCREASE IN
RAINFALL AS A TROPICAL WAVE INTERACTS WITH THIS FRONTAL BOUNDARY
TO PRODUCE A WAVE THAT SLOWLY MOVES OFFSHORE THE SOUTHEAST AND
MOISTURE FROM THE FRINGE OF HARVEY ROTATE INTO THE REGION. TO THE
NORTH, SHOWERS AND THUNDERSTORMS WILL DEVELOP ACROSS THE CENTRAL
U.S. AND THE NORTHERN PLAINS/UPPER MIDWEST AHEAD OF UPPER
TROUGHING.  AFTERNOON SHOWERS AND THUNDERSTORMS SHOULD ALSO
DEVELOP ACROSS THE CENTRAL/SOUTHERN ROCKIES (CO/NM PRIMARILY) JUST
TO THE EAST OF THE UPPER HIGH.

THE WEST WILL SEE WARMER THAN AVERAGE TEMPERATURES WITH RECORD
HIGHS POSSIBLE IN THE INTERIOR NW. BELOW AVERAGE TEMPERATURES ARE
EXPECTED OVER MUCH OF THE CENTRAL/EASTERN STATES, ESPECIALLY IN SE
TEXAS INTO LOUISIANA GIVEN AN EXPANSIVE RAIN SHIELD.


FRACASSO


$$




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