939
FXUS02 KWBC 230700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 AM EST SAT DEC 23 2023
VALID 12Z TUE DEC 26 2023 - 12Z SAT DEC 30 2023
...HEAVY RAIN THREAT REACHING THE MID-ATLANTIC BY TUESDAY AND
POSSIBLY EXTENDING NORTHEASTWARD INTO WEDNESDAY...
..OVERVIEW
LATEST GUIDANCE SHOWS THE LARGE SCALE PATTERN SETTLING INTO A
FAIRLY AMPLIFIED AND BLOCKY REGIME DOMINATED BY MEAN RIDGING OVER
THE WESTERN U.S. INTO WESTERN/WEST-CENTRAL CANADA, WITH TROUGHING
OVER THE EASTERN HALF OF NORTH AMERICA AND PACIFIC SHORTWAVES
WEAKENING AS THEY ENCOUNTER THE MEAN RIDGE. SERVING TO ESTABLISH
THE EASTERN UPPER TROUGH WILL BE AN INITIAL PLAINS/MIDWEST UPPER
LOW WHOSE ASSOCIATED SURFACE SYSTEM WILL SPREAD RAINFALL OF
VARYING INTENSITY OVER THE EAST EARLY-MID WEEK ALONG WITH THE
POTENTIAL FOR LINGERING SNOW OVER THE NORTH-CENTRAL PLAINS.
PRECIPITATION OVER THE EAST SHOULD TREND LIGHTER AND MORE
SCATTERED LATER IN THE WEEK BUT WITH UNCERTAINTY IN SPECIFICS.
OVER THE WEST, MOST PRECIPITATION SHOULD FOCUS NEAR THE WEST COAST
WITH A COUPLE FRONTAL SYSTEMS TENDING TO PRODUCE THE HIGHEST
TOTALS OVER/NEAR NORTHERN CALIFORNIA.
..GUIDANCE/PREDICTABILITY ASSESSMENT
RECENT MULTI-DAY D+8 MEANS HAVE BEEN FAIRLY AGREEABLE IN SHOWING A
DOMINANT CORE OF POSITIVE HEIGHT ANOMALIES POSITIONED OVER CENTRAL
CANADA, WITH CORRESPONDING TELECONNECTIONS CONSISTENT FOR
SURROUNDING NEGATIVE HEIGHT ANOMALIES ASSOCIATED WITH MEAN
TROUGHING OVER THE EASTERN U.S. AND EASTERN PACIFIC. THIS ADDS TO
CONFIDENCE IN THE FORECAST LARGE SCALE PATTERN, BUT INDIVIDUAL
SOLUTIONS SUGGEST RAPIDLY INCREASING DETAIL UNCERTAINTIES FROM
MIDWEEK ONWARD--ESPECIALLY FOR THE EASTERN U.S. TROUGH.
EARLY IN THE PERIOD RECENT GUIDANCE TRENDS HAVE BEEN PERSISTENT
TOWARD A DEEPER MIDWEST UPPER LOW INTO MIDWEEK. THEN GUIDANCE HAS
RAPIDLY DIVERGED OVER HOW DIGGING CANADIAN FLOW MAY INTERACT
WITH/REPLACE THE INITIAL UPPER LOW, AND AT WHAT POINT THE OVERALL
TROUGH COULD OPEN UP (WHILE STAYING AMPLIFIED). AT THE MOMENT, BY
THURSDAY THE 00Z GFS AND FARTHER NORTH 12Z ECMWF (A NORTHERN
EXTREME) ARE THE MOST PRONOUNCED WITH THE IDEA OF UPPER LOW
REPLACEMENT WHILE THE UKMET/CMC MAINTAIN THE INITIAL LOW EVEN WITH
SOME SOME POTENTIAL FOR NORTHERN STREAM INPUT. CONSENSUS TRENDS
HAVE BEEN A LITTLE SOUTH FOR THE UPPER LOW BY LATE WEEK, AND
INTERESTINGLY FOUR OF THE FIVE ECMWF-INITIALIZED MACHINE LEARNING
MODELS SHOW AN UPPER LOW TRACK THAT WOULD BE SOUTH OF MOST
DYNAMICAL MODELS ASIDE FROM THE NEW 00Z UKMET. OF COURSE THE
UNCERTAINTY ALOFT TRANSLATES TO DECREASING CONFIDENCE IN SURFACE
DETAILS BY LATE WEEK. THERE IS A GENERAL CONSENSUS TOWARD BEST
DEFINED/WETTER LOW PRESSURE TRACKING NEAR THE EAST COAST
TUESDAY-THURSDAY WHILE THE MIDWEST SYSTEM WEAKENS, WITH POTENTIAL
FOR SOME RE-DEVELOPMENT SOMEWHERE NEAR THE NORTHERN HALF OF THE
EAST COAST AROUND FRIDAY BUT WITH LESS MOISTURE TO WORK WITH.
HOWEVER PLENTY OF NON-CONSENSUS DAY-TO-DAY SOLUTIONS ARE PLAUSIBLE
WITHIN THE OVERALL MEAN PATTERN.
NEAR THE WEST COAST, THE PAST COUPLE CMC RUNS HAVE FINALLY JOINED
OTHER GUIDANCE FOR THE SHORTWAVE ARRIVING WEDNESDAY-THURSDAY BUT
THE NEW 00Z UKMET STRAYS TO THE SLOW SIDE. BY DAY 7 SATURDAY THE
12Z ECMWF SHOWS MORE CLOSING OF ENERGY WITHIN THE NEXT ARRIVING
TROUGH VERSUS MOST OTHER GUIDANCE, FAVORING EMPHASIS OF OTHER
MODELS/MEANS.
BASED ON 12Z/18Z GUIDANCE COMPARISONS, THE UPDATED FORECAST
STARTED WITH AN OPERATIONAL MODEL COMPOSITE EARLY BUT THEN BY
MID-LATE PERIOD PLACED MORE EMPHASIS ON THE GFS/GEFS MEAN, ECENS
MEAN, AND CMC RUNS DUE TO LESSER CONFIDENCE IN ECMWF DETAILS BOTH
OVER THE EAST AND LATE IN THE PERIOD ALONG THE WEST COAST.
..WEATHER/HAZARDS HIGHLIGHTS
THE SYSTEM AFFECTING THE CENTRAL AND EASTERN U.S. THROUGH MIDWEEK
WILL SPREAD LOCALLY HEAVY RAINFALL OVER PORTIONS OF THE EAST, WITH
LIGHTER RAIN EXTENDING BACK INTO THE GREAT LAKES/UPPER MIDWEST AND
LINGERING SNOW WHERE THE AIR IS COLD ENOUGH OVER THE NORTH-CENTRAL
PLAINS AND THEN POSSIBLY WRAPPING AROUND INTO THE MIDWEST. THE
DAY 4 EXCESSIVE RAINFALL OUTLOOK VALID ON TUESDAY MAINTAINS A
MARGINAL RISK AREA OVER THE MID-ATLANTIC AS A WAVY FRONTAL SYSTEM
LIFTS NORTHWARD. SOUTHEASTERLY LOW LEVEL FLOW MAY HELP TO ENHANCE
TOTALS OVER FAVORED TERRAIN ALONG THE APPALACHIANS WHILE POTENTIAL
FOR SOME MODEST INSTABILITY MAY CONTRIBUTE TO ANOTHER RELATIVE
MAXIMUM NEAR THE COAST. CONFIDENCE IN SYSTEM DETAILS AT THE
SURFACE AND ALOFT DECREASES AS MOISTURE LIFTS NORTHEASTWARD BY
WEDNESDAY. WITH SOME LINGERING SENSITIVITY OVER THE REGION AND A
LOOSE SIGNAL FOR POTENTIAL ENHANCEMENT FROM THE NORTHERN
MID-ATLANTIC INTO SOUTHEASTERN MAINE, THE NEW DAY 5 ERO INTRODUCES
A MARGINAL RISK AREA FROM THE NEW JERSEY VICINITY NORTHEASTWARD.
PRECIPITATION SHOULD BECOME LIGHTER AND MORE SCATTERED WITH TIME,
WITH A TENDENCY TOWARD LAKE EFFECT/TERRAIN ENHANCED SNOW AS
TEMPERATURES GRADUALLY DECLINE.
ALONG THE WEST COAST, THE PRIMARY PRECIPITATION EVENTS SHOULD BE
OVER THE CENTRAL WEST COAST AROUND WEDNESDAY AND FRIDAY WITH
ARRIVING UPPER SHORTWAVES/SURFACE FRONTS. A MARGINAL RISK AREA IN
THE DAY 5 ERO REFLECTS FAIRLY GOOD GUIDANCE CLUSTERING TOWARD A
BRIEF EPISODE OF POTENTIALLY HEAVY RAINFALL CENTERED OVER NORTHERN
CALIFORNIA AND FAR SOUTHWESTERN OREGON. CURRENT CONSENSUS THEN
FOCUSES HIGHEST TOTALS OVER NORTHERN CALIFORNIA FOR FRIDAY. IN
BOTH EVENTS EXPECT SNOW OVER HIGHER ELEVATIONS. THE PACIFIC
NORTHWEST WILL LIKELY SEE SOME PRECIPITATION FROM THESE EVENTS BUT
WITH LIGHTER AMOUNTS.
CONTINUE TO EXPECT UNSEASONABLY WARM TEMPERATURES AHEAD OF THE
SYSTEM FORECAST TO EXTEND FROM THE MIDWEST INTO SOUTHEAST AS OF
EARLY TUESDAY. MORNING LOWS SHOULD BE PARTICULARLY ANOMALOUS WITH
READINGS 20-30F ABOVE NORMAL FROM THE UPPER MIDWEST INTO A DECENT
PORTION OF THE EAST FOR ONE OR MORE DAYS, WITH SUCH ANOMALIES
PERSISTING OVER NEW ENGLAND AT LEAST INTO THURSDAY. LOCATIONS
FROM THE GREAT LAKES/OHIO VALLEY INTO THE NORTHEAST WILL SEE THE
BEST POTENTIAL OF DAILY RECORDS FOR WARM LOWS. PLUS 10-20F
ANOMALIES SHOULD BE MORE COMMON FOR HIGHS. COOLER AIR BEHIND THE
SYSTEM WILL INITIALLY STILL BE ON THE MILD SIDE, WITH ONLY
SLIGHTLY BELOW NORMAL READINGS OVER THE CENTRAL-SOUTHERN
ROCKIES/PLAINS ON TUESDAY. AS UPPER TROUGHING BECOMES ESTABLISHED
OVER THE EAST LATER IN THE WEEK, THE SOUTHEAST AND VICINITY SHOULD
TREND COOLER WITH TEMPERATURES DROPPING TO 5-10F OR SO BELOW
NORMAL. MUCH OF THE WEST SHOULD SEE MODERATELY ABOVE NORMAL
TEMPERATURES THROUGH THE PERIOD.
RAUSCH
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
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page