Preliminary Forecasts
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FXUS02 KWNH 240622
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
222 AM EDT THU AUG 24 2017

VALID 12Z SUN AUG 27 2017 - 12Z THU AUG 31 2017

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


OVERVIEW
~~~~~~~~
UPPER RIDGING IS FORECAST TO LINGER ACROSS THE WESTERN US,
SUPPORTING TROUGHING IN THE MID-MISSISSIPPI AND OHIO VALLEYS. IN
THE GULF OF MEXICO, THE NATIONAL HURRICANE CENTER IS WRITING
ADVISORIES ON TROPICAL STORM HARVEY WITH A TRACK AIMED TOWARD THE
TEXAS COAST. PLEASE CONSULT THE LATEST NHC FORECAST FOR DETAILS.
SERIOUS FLOODING IS A DISTINCT POSSIBILITY OVER SOUTHEASTERN TEXAS
INTO LOUISIANA NEXT WEEK AS HARVEY MAY VERY SLOWLY WOBBLE BETWEEN
THE COAST AND THE GULF OF MEXICO BEFORE POSSIBLY BEING PICKED UP
BY THE TROUGH TO THE NORTH.


GUIDANCE EVALUATION/PREFERENCES
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
DETERMINISTIC GUIDANCE CONTINUED TO WAVER A BIT ON THE TRACK OF
HARVEY, DEPENDING ON THE UPPER FLOW AND SUBTLE DETAIL DIFFERENCES.
NHC ADJUSTED THE TRACK AT 03Z TO A HALF CYCLONIC LOOP NEAR CORPUS
CHRISTI PER THE 12Z GFS/ECMWF AND ENSEMBLE GUIDANCE. USED THOSE
MODELS WITH THE 12Z ECMWF ENSEMBLE MEAN TO FORM THE FORECAST.
FARTHER NORTH, THE IMPORTANT SHAPE/AMPLITUDE OF THE TROUGH WAS
MOST CONSISTENTLY DEPICTED BY THE ECMWF ENSEMBLE MEAN WHICH HAS
TRENDED SHARPER IN THE RIDGE/TROUGH CONFIGURATION, BUT PERHAPS NOT
SW ENOUGH TO PICK UP HARVEY BEFORE LATER NEXT WEEK. 12Z GFS
APPEARED TO BE MOST REASONABLE BY NEXT WED/THU, LIFTING HARVEY
NORTHWARD ACROSS LOUISIANA SLOWER THAN THE 12Z ECMWF (GIVEN THE
TREND).

BACK TO THE NORTH AND WEST, ECMWF ENSEMBLES BACKED OFF ON LOWERING
HEIGHTS AS MUCH INTO THE PAC NW TUE/WED (GEFS HAD THAT IDEA 24 HRS
AGO) DUE IN PART TO THE STRONGER RIDGE IN THE INTERIOR. 50/50
SPLIT BETWEEN THE 12Z GFS AND ECMWF ENSEMBLE MEAN ROUNDED OUT THE
FORECAST FOR NEXT THURSDAY.


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 SFC 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 AND A WEAKENING SFC COLD FRONT. 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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