806
FXUS02 KWBC 260658
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
158 AM EST TUE DEC 26 2023
VALID 12Z FRI DEC 29 2023 - 12Z TUE JAN 02 2024
...LOCALLY HEAVY PRECIPITATION POSSIBLE FOR NORTHERN CALIFORNIA
LATE THIS WEEK...
..OVERVIEW
WESTERN U.S. INTO CANADA MEAN RIDGING ALOFT THAT SHOULD BE MOST
PRONOUNCED LATE THIS WEEK INTO THE WEEKEND WILL LIKELY SUPPORT
MEAN TROUGHING OVER EASTERN NORTH AMERICA DURING THE PERIOD. THE
PRIMARY FEATURES WITHIN THE MEAN TROUGH WILL BE A DEEP UPPER LOW
TRACKING OUT OF KENTUCKY/TENNESSEE AS OF FRIDAY, EASTERN CANADA
ENERGY THAT MAY DIG ACROSS NEW ENGLAND, AND THEN AN UPSTREAM
SHORTWAVE EXPECTED TO DROP INTO THE NORTHERN TIER BY SATURDAY AND
CROSS THE EAST AROUND EARLY NEXT WEEK. MEANWHILE GUIDANCE EXPECTS
A COUPLE PACIFIC SHORTWAVES TO PUSH INTO THE LONG-TERM MEAN RIDGE,
WITH SOME CONTINUED UNCERTAINTY IN THE DETAILS. THE LEADING ONE
ARRIVING BY SATURDAY MAY BRING A BRIEF EPISODE OF ENHANCED
PRECIPITATION TO THE CENTRAL WEST COAST. THE SOUTH AND ESPECIALLY
FLORIDA WILL SEE BELOW NORMAL TEMPERATURES LATE THIS WEEK INTO THE
WEEKEND WHILE MOST OTHER AREAS OF THE LOWER 48 SHOULD SEE NEAR TO
ABOVE NORMAL READINGS.
..GUIDANCE/PREDICTABILITY ASSESSMENT
GUIDANCE IS CONTINUING TO HAVE A DIFFICULT TIME RESOLVING VARIOUS
DETAILS WITHIN THE EASTERN U.S. MEAN TROUGH. ALREADY AS OF DAYS
3-4/FRIDAY-SATURDAY THERE IS SIGNIFICANT DIVERGENCE OVER THE
EVOLUTION OF EASTERN CANADA ENERGY WHICH PER LATEST SPREAD COULD
TRACK ANYWHERE BETWEEN THE GREAT LAKES/UPPER OHIO VALLEY (00Z
UKMET) OR HAVE THE RESULTING CLOSED LOW STAY NORTH OF MAINE
(18Z/00Z GFS) THE PREFERRED MODEL AVERAGE WOULD HAVE AN UPPER LOW
TRACKING OVER NEW ENGLAND, WHICH WOULD BE SLIGHTLY WEAKER AND
NORTHWARD OF THE 12Z ECMWF. LIKELY DUE IN PART TO THE LACK OF
NORTHERN STREAM INTERACTION, THE 00Z GFS HAS TRENDED NOTICEABLY
SLOWER AND SOUTHWARD FOR ITS UPPER LOW EJECTING FROM TENNESSEE.
SURFACE DETAILS ALONG THE EAST COAST WILL BE SENSITIVE TO THESE
DIFFERENCES ALOFT. FOR THE UPPER TROUGH DROPPING INTO THE UPPER
MIDWEST BY SATURDAY AND CONTINUING INTO THE EAST THEREAFTER, THERE
HAVE AT LEAST BEEN SOME FAVORABLE TRENDS AWAY FROM MORE EXTREME
COMPONENTS OF THE PRIOR SPREAD. THE 12Z ECMWF REDUCED THE
SOUTHWESTWARD ELONGATION THAT EARLIER RUNS HAD EXHIBITED, WHILE BY
DAY 7 TUESDAY THE NEW 00Z GFS TRENDS VERY CLOSE TO THE 12Z ECMWF
VERSUS 18Z/12Z RUNS THAT WERE SLOWER AND HAD A FARTHER SOUTH
CLOSED LOW TRACK. BY LATE IN THE PERIOD THE PAST COUPLE DAYS OF
ECMWF-INITIALIZED MACHINE LEARNING MODELS HAVE BEEN ADVERTISING A
FAIRLY PROGRESSIVE TROUGH, PERHAPS EVEN FASTER THAN THE CURRENT
MAJORITY CLUSTER OF THE ENSEMBLE MEANS AND 00Z GFS/12Z ECMWF. THE
NEW 00Z CMC IS ONE SUCH SOLUTION FROM THE OPERATIONAL MODEL REALM.
A TENDENCY TOWARD GREATER PROGRESSION WOULD REDUCE THE POTENTIAL
FOR WESTERN ATLANTIC LOW PRESSURE BY NEXT TUESDAY TO HAVE A
SIGNIFICANT EFFECT ON THE EAST COAST.
OVER THE WEST, GUIDANCE IS MAINTAINING GOOD RELATIVE AGREEMENT FOR
THE NEGATIVELY TILTED SHORTWAVE FORECAST UP TO THE POINT THAT IT
REACHES THE WEST COAST AROUND SATURDAY. THERE HAS BEEN A LOT OF
DIVERGENCE FOR THE DETAILS AFTER THAT POINT, BUT LATEST RUNS SEEM
TO BE CONSOLIDATING TOWARD THE SENSIBLE IDEA OF HAVING THE
SOUTHERN PART OF THE TROUGH QUICKLY UNDERCUT THE CORE OF THE
WESTERN RIDGE CENTERED OVER/NORTH OF THE NORTHERN ROCKIES WHILE
THE NORTHERN ENERGY SHEARING OUT AS IT PUSHES INTO/AROUND THE
REMAINING RIDGE. THE NEXT APPROACHING AREA OF SHORTWAVE ENERGY IS
MORE DIFFUSE WITH SOME RECENT TIMING DIFFERENCES. IN PARTICULAR
THE GFS/GEFS MEAN HAVE BEEN ON THE SLOWER SIDE WITH WHAT SHOULD BE
THE BEST DEFINED TROUGHING THAT REACHES THE SOUTHWESTERN U.S. AND
NORTHWESTERN MEXICO, WHILE ECMWF-INITIALIZED MACHINE LEARNING
MODELS HAVE GENERALLY BEEN RUNNING ON THE FAST SIDE OF MOST
GUIDANCE. THE NEW 00Z GFS HAS TRENDED FASTER TO MATCH THE 00Z
ECMWF MEAN.
BASED ON OVERALL GUIDANCE COMPARISONS/PREFERENCES BASED ON 12Z/18Z
GUIDANCE, THE UPDATED FORECAST STARTED WITH A 12Z/18Z MODEL
COMPOSITE EARLY IN THE PERIOD AND THEN TRENDED TOWARD 60 PERCENT
TOTAL ENSEMBLE MEAN INPUT (EQUALLY DIVIDED AMONG THE 18Z GEFS, 12Z
ECENS, AND 12Z CMCENS) WITH THE GFS CONTRIBUTION ELIMINATED BY DAY
7 TUESDAY (LEAVING THE 12Z ECMWF/CMC).
..WEATHER/HAZARDS HIGHLIGHTS
THE COMBINATION OF THE UPPER LOW TRACKING OUT OF
KENTUCKY/TENNESSEE FROM EARLY FRIDAY ONWARD AND POTENTIAL DIGGING
EASTERN CANADA ENERGY (WITH ASSOCIATED SURFACE FEATURES) MAY
PRODUCE AREAS OF PRECIPITATION FROM THE OHIO/TENNESSEE VALLEYS
INTO THE NORTHEAST LATE THIS WEEK INTO THE WEEKEND. THE
RELATIVELY BETTER CHANCE FOR SOME SNOW SHOULD BE OVER AND
WEST/SOUTHWEST OF THE CENTRAL APPALACHIANS, NEAR THE INITIAL UPPER
LOW, AND NORTHERN NEW ENGLAND DEPENDING ON DETAILS OF CANADIAN
DYNAMICS. SPECIFIC DETAILS OF COVERAGE AND AMOUNTS HAVE LOWER
THAN AVERAGE CONFIDENCE AT THIS TIME. THE WEAK SYSTEM TRACKING
THROUGH THE UPPER MIDWEST/GREAT LAKES DURING THE WEEKEND MAY
PRODUCE SOME MOSTLY LIGHT SNOW NEAR ITS PATH WHILE SOME RAIN MAY
FALL FARTHER SOUTH ALONG THE TRAILING COLD FRONT. SOUTHERN
RAINFALL TOTALS ARE UNCERTAIN AT THIS POINT, MOST LIKELY LIGHT TO
MODERATE BUT WITH HEAVIER ACTIVITY POSSIBLE IF THE MINORITY
SCENARIO OF STRONGER SOUTHERN STREAM ENERGY AND/OR INTERACTION
WITH SLOWER NORTHERN STREAM DYNAMICS WERE TO OCCUR.
A LATE WEEK PACIFIC SHORTWAVE/WEAKENING FRONTAL SYSTEM WILL
PROVIDE THE BEST FOCUS FOR MEANINGFUL PRECIPITATION ALONG THE
CENTRAL WEST COAST. THE DAY 4/FRIDAY EXCESSIVE RAINFALL OUTLOOK
MAINTAINS A MARGINAL RISK AREA COVERING PORTIONS OF NORTHERN
CALIFORNIA WITH NO CHANGE FROM YESTERDAY'S DAY 5 UPDATE. IN
GENERAL THE FORECAST SCENARIO HAS NOT CHANGED MUCH, WITH SOME
VARIANCE IN MODEL QPFS BUT WITH AN INDICATION OF SOME INSTABILITY
REACHING THE CALIFORNIA COAST AND THE SYSTEM'S ORIENTATION
POTENTIALLY ALLOWING FOR SOME SOUTH-TO-NORTH TRAINING BANDS OF
ENHANCED RAINFALL. ANOTHER EVENT A COUPLE DAYS EARLIER SHOULD
LEAD TO DAMP ANTECEDENT CONDITIONS AS WELL. SOME OF THE LATEST
MODEL RUNS ARE HINTING AT THE POSSIBILITY OF A SLIGHTLY SOUTHWARD
FOCUS AND LOWER SPOT MAXIMA BUT THE ENSEMBLES MAINTAIN SUPPORT FOR
KEEPING THE MARGINAL RISK AREA AS-IS FOR THE TIME BEING. SOME
ACTIVITY SHOULD LINGER INTO SATURDAY WITH THE BEST MOISTURE
REACHING THE SIERRA NEVADA. SOME SNOW WILL BE POSSIBLE AT HIGH
ELEVATIONS. THIS EVENT WILL BRING LIGHTER TOTALS TO THE PACIFIC
NORTHWEST. ANY PRECIPITATION EXTENDING FARTHER EAST INTO THE
WESTERN U.S. SHOULD BE LIGHT AND SCATTERED AS THE UPPER DYNAMICS
WEAKEN, WHILE AN UPPER TROUGH ARRIVING EARLY NEXT WEEK SHOULD
PRODUCE RELATIVELY LIGHT PRECIPITATION.
EXPECT CHILLY TEMPERATURES ACROSS THE GULF COAST/SOUTHEAST LATE
THIS WEEK AND INTO THE WEEKEND, WITH FLORIDA SEEING THE BEST
POTENTIAL FOR SOME READINGS AT LEAST 10F BELOW NORMAL. A
MODERATING TREND SHOULD CONFINE NEGATIVE ANOMALIES JUST TO FLORIDA
BY SUNDAY. A FRONTAL PASSAGE EARLY NEXT WEEK MAY LOWER
TEMPERATURES AGAIN OVER THE SOUTHEAST BY NEXT TUESDAY. IN
CONTRAST, PARTS OF THE FAR NORTHERN PLAINS AND UPPER MIDWEST
(EXCLUDING NORTH-CENTRAL PLAINS LOCATIONS THAT MAY STILL HAVE SNOW
COVER) WILL BE CONSISTENTLY ABOVE NORMAL BY 10F OR MORE AND
MORNING LOWS SHOULD STAY ABOVE NORMAL OVER THE NORTHEAST. MUCH OF
THE WEST WILL SEE MODERATELY ABOVE NORMAL READINGS DURING THE
PERIOD BUT WITH A GRADUAL COOLING TREND AS THE WESTERN U.S. MEAN
RIDGE ALOFT WEAKENS AT LEAST TEMPORARILY. THE SOUTHERN PLAINS
WILL SEE SOME VARIABILITY BUT SHOULD BE ABOVE NORMAL OVERALL WITH
THE WEEKEND OFFERING THE WARMEST TEMPERATURES.
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
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