Prognostic Meteorological Discussion
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000
FXUS02 KWBC 201556
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1155 AM EDT WED SEP 20 2017

VALID 12Z SAT SEP 23 2017 - 12Z WED SEP 27 2017

...OVERVIEW/GUIDANCE EVALUATION/PREFERENCES...

A VERY ACTIVE PERIOD AHEAD WITH A LONGWAVE TROUGH SLOWLY ADVANCING
EASTWARD ACROSS THE CENTER OF THE COUNTRY. THE ATTENDANT SURFACE
BOUNDARY WILL BE SLOW MOVING ENHANCING THE RISK FOR HEAVY RAINFALL
ACROSS THE CENTRAL PLAINS UP TOWARD THE UPPER MIDWEST.
MEANWHILE...THE WESTERN ATLANTIC WILL REMAIN A FOCUS FOR TROPICAL
ACTIVITY AS A PAIR OF SYSTEMS CONTINUE TO BE MONITORED. THE LATEST
NATIONAL HURRICANE CENTER FORECASTS ADVERTISE JOSE AS A
POST-TROPICAL CYCLONE WITH LITTLE TO NO MOTION EXPECTED AS IT
WEAKENS IN THE VICINITY OF THE 40N/70W BENCHMARK.
MEANWHILE...HURRICANE MARIA WILL LIFT AWAY FROM PUERTO RICO TODAY
AND EVENTUALLY CROSS EAST OF THE BAHAMAS ON DAY 3/SATURDAY WITH AN
EVENTUAL NORTHWARD TRACK...GENERALLY WEST OF THE 70W LONGITUDE
LINE. THE 06Z/00Z GFS SOLUTIONS REMAIN QUICKER THAN THE CONSENSUS
WHILE THE 00Z CMC RESIDES WELL TO THE EAST. A CHALLENGING FORECAST
IS INDICATED IN THE GUIDANCE WITH SURMOUNTABLE ENSEMBLE SPREAD.
HURRICANE MARIA IS EXPECTED TO MOVE NORTHWARD IN RESPONSE TO A
WEAKNESS CREATED IN THE RIDGE BY JOSE. SOME MODELS SHOW A BRIEF
NORTH-NORTHWESTWARD JOG BEFORE AN ABRUPT TURN NORTHEASTWARD IN
RESPONSE TO STRONG HEIGHT FALLS MOVING THROUGH THE GREAT LAKES. A
COMBINATION OF THE 00Z GEFS MEAN AND 00Z ECMWF/ECMWF ENSEMBLE MEAN
APPEARS TO BE THE BEST FIT TO THE OFFICIAL NHC FORECAST THROUGH
DAY 5/MONDAY WHILE A 00Z ECMWF/GEFS ENSEMBLE MEAN BLEND WILL
SUFFICE THEREAFTER WITH MORE WEIGHT TOWARD THE FORMER.

ACROSS THE CONTINENTAL U.S...LARGE-SCALE AMPLIFICATION ACROSS THE
WESTERN U.S. WILL BE COMPRISED OF SIGNIFICANT ANOMALIES RELATIVE
TO LATE SEPTEMBER CLIMATOLOGY. AT THE START OF THE
FORECAST...23/1200Z...THE 00Z GFS 500-MB HEIGHT ANOMALIES ARE ON
THE ORDER TO 3 TO 3.5 STANDARD DEVIATIONS BELOW AVERAGE ACROSS THE
DESERT SOUTHWEST. THE HIGHLY MERIDIONAL FLOW ACROSS THE CENTER OF
THE COUNTRY WILL PUMP DEEP TROPICAL MOISTURE POLEWARD ACROSS THE
GREAT PLAINS AND INTO THE UPPER MIDWEST. BASED ON THE 00Z PWAT
CLIMATOLOGY...95TH T0 99TH PERCENTILE MOISTURE CONTENT IS EXPECTED
TO CONCENTRATE FROM KS/NEB NORTHEASTWARD INTO EASTERN SD/MN. THE
GUIDANCE CONTINUES TO SHOW GRADUAL EASTWARD PROGRESSION OF THE
MEAN UPPER TROUGH WITH A MYRIAD OF SURFACE LOWS TRAVERSING ALONG
THE FRONTAL ZONE. THE PRIMARY FOCUS OF HEIGHT FALLS WILL BE ACROSS
THE NORTH-CENTRAL U.S. GIVEN POTENTIAL INTERACTION WITH SHORTWAVE
ENERGY PUSHING THROUGH SASKATCHEWAN EARLY NEXT WEEK. THIS WILL
ULTIMATELY DRAG THE STALLED FRONTAL BOUNDARY EASTWARD CARRYING THE
SYSTEM INTO THE LOWER GREAT LAKES/OH VALLEY BY DAY 7/WEDNESDAY.

CONSIDERING THE SOUTHERN EXTENT OF THE WESTERN U.S. LONGWAVE
TROUGH...THERE CONTINUES TO BE SUPPORT FOR THE FEATURE TO BRIEFLY
STALL IN THE VICINITY OF BAJA CA/NORTHWESTERN MX. THE 00Z GEFS
MEMBERS FROM YESTERDAY FIRST SHOWED SUCH A SOLUTION BUT TRENDS
HAVE MOVED EASTWARD DURING THE PAST 24 HOURS. ULTIMATELY THIS MAY
LEAD TO A NEGATIVE HEIGHT ANOMALY THAT WOULD SLOWLY DRIFT EASTWARD
TOWARD THE FOUR CORNERS REGION EARLY/MIDDLE OF NEXT WEEK.

SYNOPTIC-SCALE AGREEMENT IS ABOVE AVERAGE GIVEN THE AMPLIFICATION
WITHIN THE FLOW ACROSS THE MUCH OF THE NATION. ALL SOLUTIONS
CLUSTER WELL WITH THE MEAN RIDGE INITIALLY PARKED OVER THE GREAT
LAKES/INTERIOR NEW ENGLAND WHILE EXHIBITING SOME TEMPORAL SPREAD
WITH THE EVOLVING WESTERN U.S. TROUGH. MUCH OF THE UNCERTAINTY
SITS OVER THE WESTERN ATLANTIC WITH THE FATE OF HURRICANE MARIA.
THE MORNING PACKAGE WAS INITIALLY DRIVEN BY A 50/50 COMBINATION OF
THE 00Z GFS/GEFS MEAN AND 00Z ECMWF/ECMWF ENSEMBLE MEAN. BY DAY
5...MONDAY...ONLY 40 PERCENT OF THE BLEND WAS DRIVEN BY
OPERATIONAL GUIDANCE. EVENTUALLY SHIFTED TOWARD A MORE ENSEMBLE
BASED APPROACH...70 PERCENT TOWARD THE 00Z ECMWF ENSEMBLE MEAN
GIVEN THE SLOWER SOLUTION MATCHED THE LONGER RANGE NHC TRACK.


...SENSIBLE WEATHER/HIGHLIGHTS...

THE INITIAL HEAVY RAINFALL THREAT WILL EXTEND FROM THE CENTRAL
PLAINS INTO THE UPPER MIDWEST GIVEN FAVORABLE/PERSISTENT
SOUTH-SOUTHWESTERLY FLOW ALOFT IN THE PRESENCE OF ANOMALOUS
MOISTURE. SOME HYDROLOGIC ISSUES MAY TAKE SHAPE GIVEN IT WILL BE A
MULTI-DAY EVENT BEFORE THE TROUGH FINALLY MOVES DOWNSTREAM. THE
FOCUS SHOULD EVENTUALLY SHIFT SOUTHWARD ACROSS THE SOUTHERN PLAINS
NEXT WEEK AS A FAVORABLE UPPER DIFFLUENCE PATTERN SETS UP IN
ADVANCE OF THE SLOW MOVING TROUGH ACROSS THE SOUTHWESTERN U.S.
ELSEWHERE...IMPACTS STILL REMAIN UNCERTAIN WITH MARIA AS ANY
WESTWARD JOG IN THE TRACK WOULD BRING HEAVY RAINFALL/GUSTY WINDS
TO THE COASTAL CAROLINAS NORTHWARD. REGARDLESS...ROUGH SEAS AND
DANGEROUS RIP CURRENTS WOULD BE LIKELY GIVEN THE PRESENCE OF MARIA
IN THE OFFSHORE WATERS.

FROM SATURDAY THROUGH MONDAY...WELL BELOW NORMAL TEMPERATURES ARE
EXPECTED ACROSS MUCH OF THE WESTERN U.S...PARTICULARLY OVER THE
UPPER INTERMOUNTAIN WEST/NORTHERN ROCKIES. HIGH TEMPERATURES IN
THE LOW 50S WILL BE THE NORM WHILE THE COLD SPOT SHOULD BE ACROSS
THE TETONS WITH READINGS IN THE UPPER 30S. SUCH FORECASTS INDICATE
DEPARTURES FROM NORMAL IN THE 20 TO 25 DEGREE RANGE. THESE
ANOMALIES SHOULD GRADUALLY COME DOWN AS THE UPPER TROUGH
ACCELERATES AND WEAKENS INTO NEXT WEEK. ELSEWHERE...VERY MILD
TEMPERATURES WILL PREVAIL ACROSS THE GREAT LAKES WITH READINGS UP
TO 20 DEGREES ABOVE CLIMATOLOGY...THUS SPREADING MID/UPPER 80S TO
THE REGION. SOME RECORD WARM MINIMA ARE POSSIBLE ON SATURDAY
ACROSS THE MIDDLE MS VALLEY/UPPER MIDWEST/UPPER GREAT LAKES.


RUBIN-OSTER

$$




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