Prognostic Meteorological Discussion
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000
FXUS02 KWBC 221601
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1200 PM EDT SUN OCT 22 2017

VALID 12Z WED OCT 25 2017 - 12Z SUN OCT 29 2017

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

AT THE START OF THE PERIOD...A BROAD LONGWAVE TROUGH WILL EXTEND
OVER THE EASTERN U.S. WHILE A FLATTENING UPPER RIDGE IS NOTED FROM
THE HIGH PLAINS WESTWARD. AMPLIFYING FLOW ATOP THIS RIDGE ACROSS
WESTERN CANADA IS FORECAST TO ACCELERATE SOUTH-SOUTHEASTWARD WHICH
HELPS CARVE OUT A MEAN TROUGH WHICH WILL TRANSLATE FROM THE
ROCKIES TOWARD THE APPALACHIANS FROM DAY 4 THROUGH 7...OCTOBER
26-29. THIS ULTIMATELY WILL HELP RE-ESTABLISH UPPER RIDGING ALONG
THE WEST COAST AND MAINTAIN THE AMPLIFIED MID-LATITUDE PATTERN.
ALONG THE EASTERN SEABOARD...THE MENTIONED UPPER TROUGH SHOULD BE
SLOW TO EXIT THE COAST AS IT ATTAINS A NEGATIVE-TILT AND FORMS A
POTENT AREA OF LOW PRESSURE LIFTING INTO NEW ENGLAND BY LATE
WEDNESDAY/EARLY THURSDAY. MODELS CONTINUE TO CARRY THIS FEATURE
TOWARD THE HIGHER LATITUDES OF CANADA AS HEIGHTS RAPIDLY BUILD
THEREAFTER ACROSS THE EASTERN STATES.

REGARDING THE INITIAL UPPER TROUGH ACROSS THE EASTERN U.S...THE
00Z CMC/UKMET CONTINUE TO BE SLOWER TO EJECT THE PARENT SURFACE
LOW TOWARD THE ONTARIO/QUEBEC BORDER EARLY ON. CONSIDERING OTHER
SOLUTIONS...GENERAL AGREEMENT EXISTS IN ALLOWING FOR A
NEGATIVELY-TILTED TROUGH TO EXIT THE MID-ATLANTIC EARLY THURSDAY
WHICH MAY BE ACCOMPANIED BY A STRONG CLOSED UPPER LOW AS NOTED IN
THE 00Z GFS/ECMWF. BOTH SOLUTIONS FAVOR A SUB 1000-MB LOW MOVING
TOWARD NOVA SCOTIA WITH SOME NORTH/SOUTH SPREAD IN THE ENSEMBLE
LOW PLOTS. LOOKING FARTHER UPSTREAM...THE SPATIAL SPREAD IN THE
GUIDANCE HAS DIMINISHED SOME RELATIVE TO YESTERDAY. HOWEVER...THE
GLOBAL MODEL CAMPS DIVERGE BY DAY 4/THURSDAY AS SEEN IN RECENT
ENSEMBLE SPAGHETTI PLOTS. MULTI-DAY COMPARISONS SHOW A GRADUAL
EASTWARD SHIFT IN THE ECMWF ENSEMBLE MEMBERS WHICH TIES IT A BIT
MORE CLOSELY WITH THE GEFS MEMBERS AS WELL AS THE 00Z UKMET. THE
00Z ECMWF AND ITS ENSEMBLE MEMBERS CONTINUE TO BE OFF TO THE WEST
AS THEY DIG THE TROUGH MORE SIGNIFICANTLY TOWARD THE FOUR CORNERS
REGION WHILE THE GEFS FAVORS MORE EASTWARD PROGRESSION IN TIME.
THEY HAVE REMAINED ON THESE OPPOSING SIDES THE PAST COUPLE OF DAYS
ALTHOUGH THE GAP HAS BEEN BRIDGED SOMEWHAT MOST RECENTLY. THIS
DOES LEAD TO TIMING DIFFERENCES WITH THE NORTH/SOUTH ORIENTED
FRONTAL ZONE WHICH MARCHES TOWARD THE EAST. ONE SET OF SOLUTIONS
NOT CONSIDERED HERE IS THE 00Z CMC/CMC ENSEMBLE MEAN WHICH ALSO
AFFECTS THE 00Z NAEFS ENSEMBLE MEAN AS THEY ARE MUCH MORE
PROGRESSIVE WITH THE ENTIRE SYNOPTIC-SCALE PATTERN. FURTHER...THE
FORMER CONJURES UP A TROPICAL CYCLONE LATER IN THE PERIOD WHICH
WOULD MOVE TOWARD CUBA AND THE BAHAMAS.

AS THE OVERALL FORECAST SPREAD HAS DECREASED...WAS ABLE TO
MAINTAIN MORE OF AN OPERATIONAL INFLUENCE DURING MUCH OF THE
PERIOD. TOOK A DIRECT SPLIT BETWEEN THE 00Z GFS/ECMWF FOR THE
FIRST FEW DAYS. DID SELECT THE 00Z GFS OVER THE 06Z RUN AS THE
LATTER WAS A BIT QUICKER WHICH LED TO LARGER SPREAD BETWEEN IT AND
THE 00Z ECMWF. BY DAY 5/FRIDAY ONWARD...ADDED MORE ENSEMBLE
CONTRIBUTIONS TO THE PICTURE WHILE STILL KEEPING AROUND 30 PERCENT
OF THE OPERATIONAL MODELS INTO DAY 7/OCTOBER 29.


...SENSIBLE WEATHER HIGHLIGHTS/THREATS...

WITH AMPLIFIED FLOW IN PLAY...SOME DAILY TEMPERATURE RECORDS MAY
BE BROKEN GIVEN THE VERY MILD OVERNIGHT LOWS EXPECTED ON
WEDNESDAY/THURSDAY. IN PARTICULAR...THE UPPER RIDGE CENTERED OVER
THE DESERT SOUTHWEST WILL BRING VERY WARM CONDITIONS TO THE REGION
AS LOWS STAY WELL INTO THE MID/UPPER 60S ACROSS SOUTHERN CA. AND
FROM THE MID-ATLANTIC NORTHWARD...STRONG WARM ADVECTION AHEAD OF
THE CLOSED LOW WILL ALLOW MILD TROPICAL AIR TO INFILTRATE THE
EASTERN SEABOARD. EVENTUALLY THE PATTERN SHIFTS TOWARD COOLER
READINGS AS THE UPPER TROUGH DIGS THROUGH THE CENTRAL U.S. LATER
NEXT WEEK. DEPARTURES FROM NORMAL ON THE ORDER OF 10 TO 15 DEGREES
WILL BE LIKELY AS HIGHS REMAIN ONLY IN THE 40S AS FAR SOUTH AS THE
TX PANHANDLE. THE AIR MASS SHOULD MODIFY SOME ON ITS EASTWARD
APPROACH ALTHOUGH IT WILL FEEL MUCH MORE LIKE AUTUMN AS THIS
SYSTEM SPREADS ACROSS THE CENTRAL TO EASTERN U.S.

THERE WILL BE AN INITIAL HEAVY RAINFALL THREAT ACROSS UPPER NEW
ENGLAND AS THE SURFACE LOW PIVOTS THROUGH THE REGION. SOME FAIRLY
HEFTY TOTALS MAY BE POSSIBLE GIVEN THE 850-700 MB MOISTURE FLUXES
BEING AROUND 4 TO 5 STANDARD DEVIATIONS ABOVE CLIMATOLOGY.
GRADUALLY THE FOCUS OF THIS BAND WILL MOVE INTO QUEBEC WITH
SCATTERED SHOWERS IN ITS WAKE. FARTHER WEST...WITH THE NEXT SYSTEM
MOVING THE CENTRAL U.S...SOME MOUNTAIN SNOWS WILL BE POSSIBLE
ACROSS THE NORTHERN/CENTRAL ROCKIES WITH A GRADUAL HEAVY RAINFALL
THREAT BY PERHAPS FRIDAY ACROSS THE RED RIVER NORTHEASTWARD ALONG
THE FRONT. THE DEGREE OF RETURN FLOW AND HOW MUCH MOISTURE CAN
MOVE BACK TOWARD THE GREAT PLAINS IS STILL AN UNKNOWN.
ADDITIONALLY...DEPENDING ON THE STRENGTH OF THE PARENT SURFACE LOW
AND AMOUNT OF COLD AIR PUSHING ACROSS THE CANADIAN BORDER...SOME
SNOW MAY BE POSSIBLE OVER NORTHERN MN INTO THE UPPER GREAT LAKES.


RUBIN-OSTER

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