Preliminary Forecasts
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FXUS02 KWNH 260503
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
103 AM EDT MON SEP 26 2016

VALID 12Z THU SEP 29 2016 - 12Z MON OCT 03 2016

USED THE 25/12Z ECENS/NAEFS/GEFS MEANS FOR 80-90% OF THE FORECAST
--- AND SOME OF THE DETAILS OF THE 25/18Z GFS THROUGH DAY 4. STILL
SOME QUESTION MARKS OVER THE EAST CENTRAL US THAT FORCE ME TO
DEFER TO THE ENSEMBLE MEANS IN A WEST-TO-EAST FASHION ACROSS THE
LOWER 48.

PLEASE SEE THE REGIONAL BREAK DOWNS BELOW.

...UNCERTAINTY ASSESSMENTS...
OUT WEST---THE 25/12Z ECENS, NAEFS AND GEFS MEANS CONTINUE TO
HANDLE THE AMPLIFIED FLOW PATTERN ACROSS THE WEST WITH RELATIVE
EASE --- AND IT`S REASONABLE TO ASSUME THAT THE 500MB TROUGH AXIS
WILL MOVE INLAND (INSIDE OF 125W) VERY NEAR THE 2/12Z TIME FRAME.
THOUGH THE SHORTWAVE DETAILS MOVING SOUTHWEST TO NORTHEAST AHEAD
OF THE TROUGH AXIS DO PROVIDE SOME CONCERNS AT DAY 6 JUST HOW FAR
SOUTH PRECIPITATION WILL BREAK OUT ALONG THE CENTRAL CALIFORNIA
COAST AND DOWNSTREAM---THERE ARE SUFFICIENT HEIGHT FALLS (THE
564DM LINE ALONG THE OREGON/CALIFORNIA BORDER) TO BELIEVE THE
DETERMINISTIC GUIDANCE HAS THE GENERAL CONCEPT. THE TREND HAS BEEN
FOR THE FLOW BEHIND THE TROUGH AXIS ---OVER THE GULF OF ALASKA ---
TO BE LOOKING MORE PROGRESSIVE SO --- `EARLY WET SEASON` ONSHORE
FLOW CONDITIONS MAY FOLLOW FOR COASTAL WASHINGTON AND OREGON (TO
THE WINDWARD CASCADES) AT/BEYOND DAY 7.

FOR THE CONTINENTAL DIVIDE/GREAT BASIN---THE SENSIBLE WEATHER
CHANGES ARE TIED TO THE SOUTHWESTERLY FLOW ALOFT STREAKING OUT
ACROSS THE CENTRAL GREAT BASIN AND NORTHERN DIVIDE EARLY IN THE
PERIOD. PART OF THIS PARTICULAR PATTERN AND AMPLIFIED FORECAST ---
IS RECOGNITION OF THE BROAD DOWNSTREAM RIDGE AND THE VERY SLOW
WEST-TO-EAST PROGRESSION OF THE PACIFIC FLOW ACROSS IDAHO, MONTANA
AND WYOMING THROUGH DAY 6. HERE AGAIN, THE DETERMINISTIC GUIDANCE
DOES A REASONABLY DECENT JOB OF TIMING THE SOUTHWESTERLY FLOW
INTRUSION INTO THE CENTRAL DIVIDE OF COLORADO AND EASTERN UTAH.
ONE THING TO CONSIDER IS --- THE VERY SLOW MOVING NATURE OF THE
MID-LEVEL AND UPPER-LEVEL FLOW COULD VERY WELL MAINTAIN A
PROLONGED PERIOD OF BREEZY TO WINDY CONDITIONS FOR PORTIONS OF THE
INTERMOUNTAIN WEST. THE DETERMINISTIC GFS/ECMWF WERE NOT PERFECT
PROG AT/AFTER DAY 6 BUT THEY DO INDICATE SOME CYCLONIC FLOW WILL
REACH THE CENTRAL GREAT BASIN OF NORTHERN NEVADA, SOUTHERN IDAHO
AND NORTHWEST UTAH. THE BREEZY CONDITIONS COULD VERY WELL BE
LOCALIZED INVOF OF A DRY SLOT --- AS THE BASE OF THE PACIFIC
TROUGH MIGRATES EAST OF THE SIERRA AND OREGON CASCADES.

THE PLAINS AND MS/MO VALLEYS---MILD AND SEASONAL IF NOT SLIGHTLY
WARMER THAN CURRENT GUIDANCE SUGGESTS AS THE RIDGE AXIS MAKES A
VERY SLOW EASTWARD MIGRATION ACROSS THE CENTRAL PORTION OF THE
CONTINENT BY THE END OF THIS MEDIUM RANGE PERIOD. A DOWNSLOPING
WEST TO SOUTHWEST WIND REGIME ACCOMPANYING THE WESTERN FLANKS OF
THIS RIDGE --- GENERATING LOCALLY BREEZY CONDITIONS AND DRY/WARM
CONDITIONS FROM WEST TO EAST ACROSS THE MISSOURI VALLEY AND PLATTE
VALLEY THROUGH TIME.

EASTERN US/GULF COAST---
THINGS WOULD BE MUCH SIMPLER --- IF THE CYCLONIC FLOW CURRENTLY
OVER THE GREAT LAKES WOULD BE `FOLLOWED UP` BY THE UPSTREAM
WESTERN RIDGE --- BUT IT DOESNT LOOK LIKE THIS WILL OCCUR FOR
ANOTHER 120 HOURS. MEANWHILE...THE DETERMINISTIC GUIDANCE AND
INCREASINGLY LARGER NUMBERS OF INDIVIDUAL ENSEMBLE MEMBERS (EC,
GEFS AND CMCE) ARE SUGGESTING THE MID-LEVEL CYCLONE IN MINNESOTA
SETTLES OVER EASTERN KENTUCKY AND WEST VIRGINIA (THURSDAY AND
FRIDAY) AND MEANDERS (MORE OR LESS) OVER THE SAME GENERAL AREA
INTO NEXT WEEKEND. TWO THINGS TO CONSIDER WITH THIS `MEANDERING`
TRACK SCENARIO. ONE IS THE ROLE OF A TROUGH AXIS IN ATLANTIC
CANADA THAT SEEMS TO BE `BLOCKED` FROM PROGRESSING DOWNSTREAM INTO
THE NORTHERN ATLANTIC. AND TWO, WHAT BECOMES OF THE `TWIN` CUTOFF
CIRCULATIONS INVOF OF 30N ANCHORED OVER WRN FL AND NE FLORIDA
COAST.

BEST I CAN TELL --- HAVING WATCHED THIS PATTERN UNFOLD OVER THE
PAST 4 MIDS --- THE MINNESOTA TO EASTERN KY/WV "ALLEY WAY" (A
SOUTHEASTWARD MIGRATION)--- IS REALLY THE ONLY PATHWAY THIS SYSTEM
HAS NOW. AND NOT UNTIL THE PATTERN RELEASES DOWNSTREAM (TO ITS
NORTHEAST) OR THE TWIN LOWS TO ITS SOUTHEAST START MOVING
OUT...WILL THERE BE A MEANINGFUL CHANGE IN THE LOW`S PATH. AGAIN
THOUGH... THE CORE IS ONLY A MODEST 564DM-570DM SYSTEM --- SO IT
DOESN`T HAVE MUCH INFLOW TO WORK WITH "JUST YET" --- AND ITS
SURFACE REFLECTION APPEARS TO BE RATHER WEAK. A COUPLE OF NIGHT
AGO --- I HAD NO REAL GOOD EXPLANATION FOR THIS SOLUTION. TONIGHT
I "THINK" I DO. VOJTESAK



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