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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0639Z Oct 30, 2014)
 
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EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
239 AM EDT THU OCT 30 2014

VALID 12Z SUN NOV 02 2014 - 12Z THU NOV 06 2014

...STRONG COASTAL LOW HEADING FOR ATLANTIC CANADA EARLY IN THE
PERIOD...
...RAIN LIKELY FOR THE WEST AND SOUTHERN PLAINS...

...OVERVIEW...
THE MIGRATION OF TWO MODERATE AMPLITUDE UPPER-LEVEL TROUGHS WILL
PRODUCE THE SENSIBLE WEATHER CHANGES THIS MEDIUM RANGE PERIOD.

..UNCERTAINTY ASSESSMENT...
TO ME---THE WESTERN TROUGH REMAINS CHALLENGING...WITH THE VARIOUS
PIECES OF DETERMINISTIC GUIDANCE GRAPPLING WITH THE NATURE OF THE
UPPER-JET STRUCTURE ALONG THE I-10/I-8 CORRIDOR FROM THE COLORADO
RIVER TO THE RIO GRANDE EARLY ON IN THE EVOLUTION OF THIS FORECAST
PERIOD. AND JUST AS CHALLENGING WILL BE THE TRANSITION OF THE FLOW
BEHIND THIS SYSTEM---LATE IN THE PERIOD ALONG THE WEST COAST.

A SPLIT-FLOW JET LET---DOWNSTREAM AND EAST OF THE DIVIDE PROVIDES
ENOUGH MOMENTUM TO SPIN UP AND MIGRATE THE SURFACE CYCLONE ACROSS
THE UPPER MISSOURI VALLEY ON DAY 4. THIS HAS BEEN A STAPLE OF THE
MEDIUM FORECAST AND MAINTAINS VERY GOOD CONTINUITY WITH RESPECT TO
THIS SURFACE LOW'S DEPTH AND TRACK INTO WESTERN ONTARIO---AND WITH
FRONTAL PROGRESSION ALONG AND NORTH OF I-80--- FROM DENVER TO
CHICAGO.

THE REGION OF DIFFLUENT FLOW IN THE BASE OF THE UPPER-LEVEL TROUGH
(OVER SOUTHWEST NEW MEXICO) IS IMPRESSIVE AND EXPANDS
NORTHEASTWARD INTO MONDAY MORNING. LOOKS TO BE A WIND-MAKER FOR
THE UPPER RIO GRANDE AND SOUTHERN ROCKIES. THIS IS THE PORTION OF
THE WESTERN UPPER-LEVEL TROUGH PROGRESSION THAT CREATES A FORECAST
CHALLENGE WITH RESPECT TO THE QPF OUTPUT AND ALIGNMENT OF THE
UPPER-JET STREAK---PARALLEL TO THE COLD FRONT OVER WEST TEXAS INTO
THE OZARKS. LIKE YESTERDAY...THE MEDIUM RANGE FORECAST FOR THE
SOUTHERN PLAINS...OZARKS AND EASTWARD ALONG THE TRAILING COLD
FRONT (A REGION BOUNDED BY 4 POINTS---FROM ABQ-ELP ON THE WEST END
AND STL-BNA---ON THE EAST END) IS THE BIG QPF QUESTION MARK. IN
THE PLAINS---AS THE SURFACE FRONT SWEEPS SOUTH OF I-80 AND HEADS
SOUTHBOUND TOWARDS I-40...THE UNDER-GLIDING AND SHALLOW NATURE OF
THE SURFACE FRONT IS BECOMING MORE AND MORE STRONGLY-INFLUENCED
AND DOMINATED BY DEVELOPMENTS ABOVE IT. NAMELY...THE STRONG AND
DYNAMIC SOUTHWESTERLY FLOW ALOFT EXITING THE SOUTHERN HIGH PLAINS.

ON DAY 4 AND DAY 5...THE UPPER-LEVEL JET AXIS NO LONGER MAINTAINS
A SPLIT-FLOW CONFIGURATION OVER THE CENTRAL PLAINS---WHICH FORCES
A CHOICE TO BE MADE. TAKE THE BULK OF THE SFC-TO-500MB WAVE
CYCLONE OVER THE DAKOTAS INTO SOUTHEAST CANADA AS A POSITIVE-TILT
TROUGH? OR SHEAR OUT THE SOUTHERN PORTION OF THE FRONT? ACROSS THE
I-40/I-20 CORRIDOR AND ALLOW THE SOUTHERN-STREAM TO RE-DEVELOP A
SECONDARY SURFACE WAVE INVOF THE SOUTHERN OZARKS ON/AFTER 4/12Z.
AND MORE IMPORTANTLY...LET THE SOUTHERN BRANCH OF THIS UPPER-LEVEL
JET (THE BASE OF THE UPPER-LEVEL TROUGH)  BECOME THE DOMINANT
FEATURE OF THE MIGRATORY FLOW PATTERN. TO LET THE JET DYNAMICS
LITERALLY OVER-RIDE AND OVER-RUN THE FRONT MEANS BRIEFLY OPENING
THE WESTERN THIRD OF THE GULF OF MEXICO...AND ALLOWING A LOW-LEVEL
MOISTURE STREAM TO RUN UNINHIBITED FROM EAST CENTRAL TEXAS INTO
THE OHIO VALLEY. 

FORECAST LOGIC SUGGESTS TO ME THAT A SECONDARY SURFACE WAVE
ATTEMPTS TO FORM VERY NEAR THE OK/KS/MO/AR BORDER REGION ON
TUESDAY MORNING. PERHAPS A CONVECTIVELY-INDUCED SURFACE FEATURE.
THE MEANS MAY NOT HELP MUCH HERE. AND MAY NOT QUITE CAPTURE THE
INTERACTION BETWEEN THE RAPIDLY-BUILDING AND EASTWARD-MOVING
PACIFIC SURFACE RIDGE---THAT UNDERGLIDE I SPOKE OF
EARLIER---SPILLING OUT OF THE FRONT RANGE OF THE ROCKIES...AND THE
UPPER-LEVEL JET DYNAMICS LIKELY TO CREATE ELEVATED---.IF NOT
POCKETS OF DEEP CONVECTION---SEE THE DAY 5 SURFACE GRAPHIC.
EVENTUALLY...THE SURFACE HIGH CATCHES UP TO THE COLD FRONT AND
UPPER-JET ASSISTED RAINFALL/CONVECTION. THE FRONT DOES SPLIT...AS
THE DRIER AIR SPREADS DOWNSTREAM---SEE THE DAY 6 GRAPHICS.
NOTE---IF YOU OVERLAY THE DETERMINISTIC 29/18Z GFS & 29/12Z ECMWF
250MB WIND PROG FOR DAY 6 (5/12Z) ON THE DAY 6 SFC
GRAPHIC...YOU'LL SEE THE DILEMMA AND WHERE THE FRONT TRIES TO
SHEAR OUT---IN THE UPPER HALF OF THE OHIO VALLEY AS THE JET EXITS
THE NORTH AMERICAN CONTINENT. JUST IN TIME FOR THE LAST
'UNCERTAINTY' IN THE PATTERN---THE JET STRUCTURE IN THE NORTHERN
ROCKIES...AND THE DAY 5-7 ALBERTA CLIPPER DESCENDING INTO THE
MIDWEST. 

...MODEL PREFERENCES...
THE 29/12Z ECENS/GEFS MEANS WERE GIVEN EQUAL WEIGHT AND CONTINUE
TO MIGRATE THE TWO TROUGHS AT MORE THAN A REASONABLE PACE ACROSS
THE LOWER 48. AND WITH THE RATHER STRONG AND WELL-DEFINED JET
STREAMS TO DEAL WITH PERIOD...THE 29/18Z GFS AND 29/12Z APPEARED
TO BE THE BEST DETERMINISTIC PIECES OF GUIDANCE TO WORK WITH TO
CAPTURE SOME OF THE UNCERTAINTY AND DETAIL OF THE MAJOR WEATHER
FEATURES. THIS IS BASED ON SEVERAL THINGS---THE COASTAL LOW DEPTH
AND LOCATION DAY 3-4...THEIR RATHER DECENT AGREEMENT IN PRINCIPLE
WITH THE SOUTHERN JET AXIS ALONG THE FRONT BETWEEN DAYS 3-6...AND
WITH THEIR POSITIONING OF THE PACIFIC JET AXIS OVER THE SOUTHERN
CANADIAN ROCKIES AND PRESENCE OF AN UPPER-LEVEL RIDGE
AXIS---GENERALLY ALONG 120W FOR DAYS 6-7. IT DOESN'T SOLVE
EVERYTHING---THEIR UPPER-LEVEL 250MB SOLUTIONS DEPICT TWO
DISTINCTLY DIFFERENT SCENARIOS FROM THE CENTRAL MIDWEST TO THE
APPALACHIANS FOR DAY 7. BUT ITS A REASONABLE START.  

...SENSIBLE WEATHER HIGHLIGHTS...
A BRISK NORTH AND NORTHWEST WIND IN THE EAST---WITH MUCH COLDER
AIR AND MOUNTAIN SNOWS POISED TO WRAP AROUND THE DEVELOPING
STORM---JUST EAST OF THE 40N/70W BENCHMARK. THE SENSIBLE WEATHER
EFFECTS OF THIS SYSTEM WILL BE FELT AS FAR SOUTH AS THE GULF COAST
AND FLORIDA...WITH A DISTINCTLY AUTUMN-LIKE DRY AND COOL FEEL TO
THE AIRMASS BEHIND THE COLD FRONT UNTIL DAY 5.

IN THE WEST...AN ACTIVE PACIFIC SYSTEM DIGGING ACROSS THE GREAT
BASIN ON DAY 3 WILL EJECT A SURFACE CYCLONE ACROSS THE NORTHERN
PLAINS FOR DAY 4 AND THE UPPER GREAT LAKES DAY 5. AHEAD OF THE
CYCLONE'S COLD FRONT...A SERIES OF INTENSE JET STREAKS WILL EXIT
THE SOUTHERN ROCKIES AND UPPER RIO GRANDE VALLEY. WHAT STARTS OUT
AS MILD...WINDY AND DRY IN THE CENTRAL AND SOUTHERN PLAINS/ROCKIES
AND LOWER HALF OF THE MISSOURI VALLEY---BECOMES STORMY AND
DISTINCTLY COOLER FOR NEXT TUESDAY AND WEDNESDAY. 

VOJTESAK