Prognostic Meteorological Discussion
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FXUS02 KWBC 211556
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1156 AM EDT SAT OCT 21 2017

VALID 12Z TUE OCT 24 2017 - 12Z SAT OCT 28 2017

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

THE LONGWAVE PATTERN ACROSS THE NATION WILL INITIALLY FEATURE AN
AREA OF WELL BELOW NORMAL HEIGHTS ACROSS THE EASTERN U.S. AS THE
CORE OF LOWER HEIGHTS TRAVERSE THROUGH THE SOUTHEASTERN U.S...THE
00Z GFS SUGGESTS 500-MB HEIGHT ANOMALIES MAY APPROACH THE 3 TO 3.5
STANDARD DEVIATION RANGE EARLY WEDNESDAY. EVENTUALLY THIS SYSTEM
WILL ATTAIN A NEGATIVE TILT AS IT EXITS THE EASTERN SEABOARD ON
THURSDAY BEFORE ACCELERATING TOWARD FAR EASTERN CANADA THEREAFTER.
LOOKING BACK TOWARD THE WESTERN/CENTRAL U.S...A STRONG 594-DM
RIDGE WILL BE IN PLACE OVER CA INTO THE CENTRAL GREAT BASIN ON
TUESDAY. THIS VAST POSITIVE HEIGHT ANOMALY SHOULD RETROGRADE
TOWARD THE EASTERN PACIFIC AS AMPLIFIED FLOW FROM BRITISH
COLUMBIA/ALBERTA BRINGS HEIGHT FALLS INTO THE UPPER INTERMOUNTAIN
WEST LATE WEDNESDAY. MODELS THEN TRACK THIS SYSTEM EITHER TOWARD
THE GREAT LAKES BY EARLY NEXT WEEKEND OR POTENTIALLY ALLOW FOR
MORE DIGGING TOWARD THE FOUR CORNERS REGION.

THERE ARE A NUMBER OF MODEL DIFFERENCES TO ACKNOWLEDGE DURING THE
PERIOD WITH SUCH UNCERTAINTIES NOTED BY AS EARLY AS DAY 3/TUESDAY.
THE 06Z/00Z GFS NOTABLY LAG THE OTHER SOLUTIONS WITH HOW QUICKLY A
SOUTHERN STREAM WAVE BECOMES ABSORBED INTO THE BROADER TROUGH
ACROSS THE GREAT LAKES. THIS LEADS TO THESE GFS RUNS TO BE
SOUTHWARD DISPLACED WITH THE SURFACE LOW OVER THE LOWER GREAT
LAKES ON 24/1200Z. AS THE PARENT UPPER LOW LIFTS TOWARD SOUTHERN
ONTARIO/QUEBEC...THE 00Z CMC/UKMET ARE SLOW TO ALLOW THIS TO OCCUR
WHICH LEADS TO A NEARLY QUASI-STATIONARY MOTION ON
TUESDAY/WEDNESDAY. EVENTUALLY A NEGATIVELY-TILTED UPPER TROUGH
LIFTS UP ALONG THE WESTERN ATLANTIC WHICH IS WELL AGREED UPON
OUTSIDE THE 00Z CMC WHICH IS FASTER WITH THE ENTIRE PATTERN ACROSS
THE U.S.

PROGRESSIVE FLOW ATOP A WESTERN U.S. RIDGE REMAINS A STRUGGLE FOR
THE AVAILABLE GUIDANCE. MODEST PERTURBATIONS WITHIN ZONAL FLOW
ACROSS WESTERN CANADA WILL LEAD TO A PAIR OF POTENTIALLY STRONG
SURFACE WAVES. ONE SUCH SYSTEM MOVING TOWARD THE UPPER MIDWEST ON
WEDNESDAY MORNING IS DEPICTED DIFFERENTLY IN THE 06Z/00Z GFS WHICH
ARE 10-MB STRONGER THAN THE 00Z ECMWF. FARTHER UPSTREAM...THE 00Z
ECMWF FAVOR A MUCH MORE INTENSE SURFACE SOLUTION ACROSS ALBERTA AT
THE SAME TIME. DIFFERENCES EXCEED 15-MB ONLY FOUR DAYS INTO THE
FORECAST PERIOD. THIS TRAILING SHORTWAVE IS THE HANDLED
DIFFERENTLY BY MOST OF THE GUIDANCE WHICH LEADS TO A MESSY ARRAY
OF ENSEMBLE SPAGHETTI PLOTS. ON THURSDAY EVENING...THE SPECTRUM OF
SOLUTIONS ARE NUMEROUS WITH THE 00Z ECMWF AND THEIR ENSEMBLES
BEING NEARLY ON THEIR OWN DIGGING THE TROUGH TOWARD THE FOUR
CORNERS REGION. THE 00Z ECMWF STANDS OUT AS AN OUTLIER AT THIS
POINT BEING WELL TO THE SOUTHWEST WITH POSSIBLY ONLY 3 GEFS
MEMBERS SUGGESTING SUCH A FORECAST. MEANWHILE...THE 06Z/00Z GFS
ARE MORE PROGRESSIVE USHERING LOWER HEIGHTS TOWARD THE GREAT LAKES
LATE IN THE PERIOD. CONSIDERING OTHER MODELS...THE 00Z CMC AND ITS
ENSEMBLES ARE WELL EAST OF THE CONSENSUS WHILE THE 00Z UKMET HAS
MORE SIMILARITIES TO THE GFS CYCLES. THE DIFFERENCES PROPEL THEIR
WAY THROUGH THE REMAINDER OF THE PERIOD WHICH SUGGESTS HEAVY USE
OF ENSEMBLE MEANS DEEPER INTO THE FORECAST.

GIVEN UNCERTAINTY IN THE PATTERN...DECIDED TO MAINTAIN 10 PERCENT
OF THE OVERNIGHT SUITE IN THE MIX TO PRESERVE SOME LEVEL OF
CONTINUITY. OTHERWISE...TOOK A DIRECT SPLIT BETWEEN THE 06Z
GFS/00Z ECMWF EARLY ON WHILE GRADUALLY INCORPORATING THEIR
RESPECTIVE ENSEMBLE MEANS BY DAY 4/WEDNESDAY ONWARD. AT THE DAY
6/FRIDAY TIMEFRAME AND THEREAFTER...WENT WITH AN EVEN SPLIT OF THE
06Z GEFS/00Z ECMWF ENSEMBLE MEANS WITH CONFIDENCE BECOMING BELOW
AVERAGE AT THIS POINT.


...SENSIBLE WEATHER HIGHLIGHTS/THREATS...

ON TUESDAY...WELL ABOVE NORMAL TEMPERATURES ARE EXPECTED BOTH
ACROSS THE WEST COAST AS WELL AS THE NORTHEASTERN CORRIDOR OF THE
U.S. A NUMBER OF DAILY TEMPERATURE RECORDS ARE LIKELY TO BE BROKEN
ON TUESDAY/WEDNESDAY ACROSS SOUTHERN CA AS WELL AS ALONG THE
EASTERN SEABOARD. AT THE FORMER...SUCH RECORDS MAY BE POSSIBLE
BOTH FOR WARM MAXIMA AND MILD OVERNIGHT READINGS. TEMPERATURES
JUST OVER THE 100 DEGREE MARK ARE EXPECTED ON TUESDAY ACROSS THE
LOS ANGELES BASIN. UNFORTUNATELY...IDEAL CONDITIONS ARE IN PLACE
TO SUPPORT AN ENHANCED WILDFIRE DANGER GIVEN DRY/GUSTY WINDS VIA
OFFSHORE DOWNSLOPE EFFECTS. BACK TO THE EAST COAST...A HIGHLY
MERIDIONAL FLOW REGIME AHEAD OF THE BARRELING UPPER LOW WILL
TRANSPORT MILD TROPICAL AIR UP ALONG THE I-95 CORRIDOR LEADING TO
SOME VERY WARM OVERNIGHT TEMPERATURES ON TUESDAY/WEDNESDAY.
LOOKING FARTHER AHEAD...THE NEXT UPPER TROUGH MOVING THROUGH THE
CENTRAL U.S. ON THURSDAY/FRIDAY WILL USHER IN TEMPERATURES 10 TO
15 DEGREES BELOW CLIMATOLOGY AS HIGHS STAYS IN THE 40S ACROSS THE
CENTRAL/NORTHERN ROCKIES AND GREAT PLAINS EASTWARD TO THE UPPER
GREAT LAKES.

HEAVY PRECIPITATION WILL BE LIKELY ALONG THE FRONTAL BOUNDARY
EXTENDING BACK TOWARD NEW ENGLAND ON TUESDAY/WEDNESDAY. AS THE
REGION OF LOWER HEIGHTS MOVES THROUGH THE GREAT LAKES/OH
VALLEY...CONVECTIVELY-FORCED SHOWERS WILL BE LIKELY GIVEN THE
STEEPENING LAPSE RATES. EVENTUALLY PRECIPITATION SHOULD BEGIN TO
FILL THE CENTRAL U.S. AS THE NEXT UPPER TROUGH SWEEPS THROUGH THE
REGION. UNCERTAINTIES IN THE FORECAST TRACK WILL LEAD TO DRAMATIC
DIFFERENCES IN PRECIPITATION INTENSITY/TYPE. RETURN FLOW AHEAD OF
THIS TROUGH SHOULD BRING AMPLE GULF MOISTURE BACK INTO THE PICTURE
BY LATE NEXT WEEK.


RUBIN-OSTER

$$





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