238
FXUS02 KWBC 261901
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 PM 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 STRUGGLES RESOLVING THE EVOLUTION OF AN UPPER LEVEL
TROUGH PATTERN OVER THE EASTERN HALF OF THE COUNTRY EARLY NEXT
WEEK. THERE'S BROAD AGREEMENT REGARDING THE EVOLOUTION OF THE
SOUTHEAST/MID-ATLANTIC SYSTEM ON DAY 3. SOME VARIANCE DEVELOPS ON
DAY 4 WITH RESPECT TO A SHORTWAVE TROUGH DESCENDING INTO THE
NORTHERN PLAINS/UPPER MIDWEST AS WELL AS THE CONDITION OF WHAT
WILL THEN BE THE EAST COAST TROUGH. INTERACTION BETWEEN THE
NORTHERN AND SOUTHERN STREAM SYSTEMS WITH POTENTIAL PHASING AS
THEY EXIT INTO THE WESTERN ATLANTIC HAS INTRODUCED SPREAD IN THE
ENSEMBLE AND DETERMINISTIC GUIDANCE ON DAY 5. THE 06Z GFS IS
NOTABLY SLOWER THAN THE REST OF THE GUIDANCE WITH RESPECT TO THE
POSITIVELY TILTED TROUGH PROPAGATING THROUGH THE CENTRAL PART OF
THE COUNTRY. THUS, A GENERAL MODEL BLEND WAS UTILIZED THROUGH DAY
5 WITH INCREASING ENSEMBLE MEANS THROUGH DAY 7.
..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. LOW
TEMPERATURES IN THE LOW TO MID 30S MAY LEAD TO FROST OR FREEZES
OVER PORTIONS OF NORTHERN FLORIDA SATURDAY AND SUNDAY MORNING. 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.
KEBEDE/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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