996
FXUS02 KWBC 181900
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 PM EDT MON SEP 18 2023
VALID 12Z THU SEP 21 2023 - 12Z MON SEP 25 2023
...HEAVY RAINFALL POTENTIAL ACROSS PARTS OF THE PLAINS AS WELL AS
THE EAST COAST LATE THIS WEEK AND WEEKEND...
..OVERVIEW
AN UPPER LEVEL CLOSED LOW DROPPING THROUGH THE WEST THURSDAY AND
FRIDAY SHOULD EVENTUALLY SHIFT EAST AS THE LEADING EDGE OF
AMPLIFYING/BROADENING NORTHEAST PACIFIC TROUGHING EXTENDS INTO THE
PACIFIC NORTHWEST AND WESTERN CANADA BY THE WEEKEND NEXT WEEKEND.
MOST GUIDANCE SUGGESTS THE UPPER LOW WILL SLOW DOWN FOR A TIME
ACROSS THE CENTRAL U.S. AS UPPER RIDGING BUILDS INTO THE WESTERN
ATLANTIC AND PARTS OF SOUTHERN CANADA, CREATING A BLOCKY PATTERN
ACROSS THE CONUS DURING THE SECOND HALF OF THE PERIOD. ELSEWHERE,
A SURFACE LOW FORMING OFF THE FLORIDA COAST LATE THIS WEEK WILL
BRING POTENTIAL FOR HEAVY COASTAL RAINS TO PARTS OF THE SOUTHEAST
AS MID-ATLANTIC AS IT MOVES SLOWLY UP THE COAST. FROM THE WEEKEND
ONWARD THERE IS INCREASING UNCERTAINTY IN THE TRACK OF THIS
SURFACE SYSTEM AND THE COVERAGE OF ASSOCIATED RAINFALL,
CORRESPONDING TO SIGNIFICANT DIFFERENCES THAT ARISE FOR UPPER
PATTERN DETAILS OVER SOUTHERN CANADA AND THE NORTHEASTERN U.S.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
THERE'S DECENT AGREEMENT ON THE UPPER PATTERN FORECAST ACROSS THE
CONUS FROM A MULTI-DAY MEAN PERSPECTIVE, BUT SOME OF THE
DAY-TO-DAY DETAILS VARY SIGNIFICANTLY AND HAVE BIG IMPLICATIONS
FOR RAINFALL AMOUNTS/QPF. WITH THE UPPER LOW INITIALLY OVER THE
WEST, A GENERAL MODEL BLEND SUFFICED FOR THE EARLY PART OF THE
PERIOD. THE MODELS HAVE BEEN SIGNALING POTENTIAL FOR THE UPPER LOW
TO WEAKEN/OPEN AROUND THE HIGH PLAINS AS OF SATURDAY, WHILE DETAIL
ISSUES FOR AN INITIAL SOUTHERN CANADA TROUGH AND TRAILING RIDGE
THAT BUILDS IN BECOME AN ISSUE BY MID-LATE PERIOD. THE 00Z CMC AND
NEW 12Z GFS APPEAR TO HAVE LOWER PROBABILITY SOLUTIONS FOR A
SOUTHERN CANADA UPPER LOW, WITH THE 00Z GFS/ECMWF AND NEW 12Z CMC
(PLUS AN EXTRAPOLATION OF THE SLIGHTLY SLOWER 12Z UKMET THROUGH
EARLY DAY 6 SUNDAY) OFFERING THE BEST CLUSTERING FOR THE UPPER LOW
THAT SHOULD MAINTAIN DEFINITION INTO NEXT MONDAY. THIS GENERALLY
FITS THE TROUGH POSITION FROM THE ENSEMBLE MEANS WHOSE VARIED
MEMBERS LEAD TO A MORE OPEN MEAN DEPICTION. SOUTHERN CANADA FLOW
UNCERTAINTY CONTRIBUTES TO THE DRAMATIC DIVERGENCE OF SOME
GUIDANCE FOR THE EAST COAST SYSTEM. LATEST CMC RUNS AND THE
06Z/12Z GFS TRACK THE SURFACE LOW WELL INLAND WHILE ECMWF RUNS AND
THE 12Z UKMET MAINTAIN A MORE SUPPRESSED WAVE. THUS FAR THE
ENSEMBLE MEANS HAVE SUPPORTED THE MORE SUPPRESSED/COASTAL
SCENARIO. CURRENT PREFERENCE IS IN LINE WITH THE ECMWF/00Z
GFS/ENSEMBLE MEAN SOLUTION THAT WOULD MAINTAIN CONTINUITY WHILE
AWAITING ANY DRAMATIC SHIFTS IN GUIDANCE CLUSTERING. THE NATIONAL
HURRICANE CENTER IS MONITORING THIS SYSTEM FOR POTENTIAL
SUB-TROPICAL DEVELOPMENT, BUT REGARDLESS OF CHARACTERISTICS, THIS
SURFACE LOW HAS THE POTENTIAL TO BRING SIGNIFICANT RAINFALL TO
PARTS OF THE SOUTHEAST AND MID-ATLANTIC COAST THIS COMING WEEKEND.
OVER THE PACIFIC NORTHWEST THE GUIDANCE SPREAD FOR THE LEADING
EDGE OF NORTHEAST PACIFIC UPPER TROUGHING APPEARS FAIRLY TYPICAL,
WITH SHORTWAVE DIFFERENCES HAVING LOW PREDICTABILITY FOR THE TIME
FRAME IN QUESTION. HOWEVER FAIRLY MINOR DIFFERENCES DO HAVE A MORE
SIGNIFICANT INFLUENCE ON EXACTLY HOW MUCH MOISTURE REACHES THE
REGION.
..WEATHER/HAZARDS HIGHLIGHTS
ANOMALOUS MOISTURE ASSOCIATED WITH THE UPPER LOW IN THE WEST LATE
THIS WEEK WILL ALLOW FOR A POTENTIAL OF HEAVY RAINFALL FROM THE
NORTHERN ROCKIES INTO THE HIGH PLAINS. THERE COULD ALSO BE SOME
SNOW IN THE HIGHEST ELEVATIONS DEPENDING ON THE DEPTH OF THE UPPER
LOW. AS THE LOW MOVES EAST INTO THE PLAINS, RAINFALL SHOULD BEGIN
TO EXPAND IN COVERAGE ACROSS THE CENTRAL AND NORTHERN PLAINS
THROUGH SATURDAY. THE SET UP IS FAVORABLE FOR HEAVY TO EXCESSIVE
RAINFALL, GIVEN AMPLE MOISTURE AND INSTABILITY. BASED ON THE
PATTERN AND LATEST GUIDANCE SIGNALS, THERE APPEAR TO BE TWO AREAS
FAVORABLE ENOUGH FOR A SLIGHT RISK AREA WITHIN THE BROADER
MARGINAL RISK AREAS DEPICTED IN THE DAYS 4-5 EXCESSIVE RAINFALL
OUTLOOKS THAT COVER THE THURSDAY-FRIDAY NIGHT PERIOD. ON DAY 4,
THERE IS A PROPOSED SLIGHT RISK AREA CENTERED OVER THE CENTRAL
DAKOTAS IN THE VICINITY OF A STALLED SURFACE FRONT. FOR DAY 5,
THERE IS SOME CLUSTERING AMONG MODEL/ENSEMBLE GUIDANCE FOR AN AXIS
OF HEAVY RAINFALL POTENTIAL FROM PARTS OF SOUTHERN
MONTANA/NORTHERN WYOMING INTO WESTERN SOUTH DAKOTA--JUST NORTH OF
THE FORECAST UPPER LOW TRACK--WHERE A SLIGHT RISK AREA HAS BEEN
PROPOSED.
A STALLED FRONTAL BOUNDARY WILL CONTINUE A FAIRLY WET PATTERN OVER
THE FLORIDA PENINSULA INTO LATE WEEK, THOUGH CONFIDENCE IN
PLACEMENT AND AMOUNTS OF RAINFALL ARE SUFFICIENTLY LOW TO INCLUDE
EVEN A MARGINAL RISK AREA ON THE EROS. THE SURFACE LOW OFF THE
COAST BY FRIDAY COULD TAP TROPICAL MOISTURE, INCREASING THE
POTENTIAL FOR HEAVY RAINFALL ALONG THE ATLANTIC COAST FROM FLORIDA
TO THE MID-ATLANTIC. AMOUNTS AND INLAND EXTENT OF THE HEAVIER
RAINS CONTINUE TO BE EXTREMELY UNCERTAIN AT THIS POINT, BUT A
MARGINAL RISK AREA SEEMS TO BE A REASONABLE STARTING POINT IN THE
ERO ALONG THE SOUTHEAST COAST FOR DAY 5 (FRIDAY-FRIDAY NIGHT).
SLIGHT ADJUSTMENT TO THE AREA IN THIS AFTERNOON'S UPDATE KEEPS THE
OUTLOOK IN THE MIDDLE OF THE GUIDANCE SPREAD THAT STARTS TO
BROADEN AT THAT TIME.
THROUGH THE REST OF THE WEEKEND INTO EARLY NEXT WEEK, RAINFALL
OVER THE CENTRAL U.S. SHOULD CONTINUE GRADUALLY EASTWARD ALONG
WITH THE UPPER LOW AND SURFACE SYSTEM. SOME ACTIVITY MAY CONTINUE
TO BE LOCALLY HEAVY. EXPECT ADDITIONAL RAINFALL OVER PARTS OF THE
MID-ATLANTIC AND NORTHEAST, BUT UNCERTAINTY FOR AMOUNTS/COVERAGE
CONTINUES TO INCREASE WITH TIME. THE PACIFIC NORTHWEST SHOULD SEE
GRADUALLY MORE RAINFALL WITH TIME, BUT TO WHAT EXTENT IS STILL A
QUESTION MARK THAT WILL TAKE ADDITIONAL TIME TO RESOLVE. THE AXIS
OF MOISTURE MAY BE FAIRLY NARROW, WITH MINOR CHANGES IN POSITION
BEING THE DIFFERENCE BETWEEN MINIMAL TOTALS OR SIGNIFICANTLY MORE.
TEMPERATURES AS MUCH AS 10-20 DEGREES BELOW NORMAL ASSOCIATED WITH
THE DEEP WESTERN UPPER LOW FROM CALIFORNIA TO MONTANA SHOULD
MODERATE BY SUNDAY AS THE LOW SHIFTS EAST. ELSEWHERE, TEMPERATURES
COULD BE MODERATELY ABOVE NORMAL ACROSS THE UPPER MIDWEST/GREAT
LAKES AND PARTS OF TEXAS THURSDAY AND FRIDAY. BY MONDAY, MUCH OF
THE COUNTRY SHOULD BE NEAR NORMAL OR WITHIN A FEW DEGREES OF
NORMAL.
RAUSCH/SANTORELLI
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC
MEDIUM RANGE HAZARDS OUTLOOK CHART AT:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,
QUANTITATIVE PRECIPITATION FORECAST, EXCESSIVE RAINFALL OUTLOOK,
WINTER WEATHER OUTLOOK PROBABILITIES, HEAT INDICES AND KEY
MESSAGES ARE AT:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=ERO
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=OVW
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