893
FXUS02 KWBC 151858
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
258 PM EDT TUE OCT 15 2024
VALID 12Z FRI OCT 18 2024 - 12Z TUE OCT 22 2024
..OVERVIEW
DEEP UPPER TROUGHING WITH A DEVELOPING CLOSED LOW IN THE INTERIOR
WEST WILL LEAD TO A COOL AND WET PATTERN THERE FROM LATE THIS WEEK
INTO THE WEEKEND, INCLUDING HIGHER ELEVATION SNOW AND FROST/FREEZE
CONCERNS IN SOME OF THE VALLEYS. HEAVY RAIN CHANCES COULD INCREASE
IN THE SOUTHERN/CENTRAL PLAINS BY THE WEEKEND. UPPER RIDGING AHEAD
OF THIS TROUGH/LOW WILL PROMOTE WARMER THAN AVERAGE TEMPERATURES
IN THE NORTHERN/CENTRAL PLAINS TO MIDWEST. THE RIDGE SHOULD
GRADUALLY WARM THE EAST AS WELL AND PROMOTE DRY CONDITIONS FOR THE
MOST PART, EXCEPT FOR SOUTH FLORIDA. ADDITIONAL ROUNDS OF
PRECIPITATION MAY COME INTO THE PACIFIC NORTHWEST LATE WEEK INTO
THE WEEKEND.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MOST GUIDANCE STILL AGREES WITH THE LARGE SCALE PATTERN. THIS
EVOLUTION CONSISTS OF EVENTUAL EJECTION OF THE SOUTHWEST/FOUR
CORNERS CLOSED LOW INTO THE PLAINS/MIDWEST AS THE LEADING EDGE OF
AN ELONGATED MEAN TROUGH OVER THE NORTHEASTERN PACIFIC/WESTERN
CANADA PUSHES SOUTHEASTWARD, WHILE WEAK SHORTWAVE ENERGY OVER THE
SOUTHERN APPALACHIANS/SOUTHEAST MAY SEPARATE A COUPLE RIDGES THAT
WILL COMPRISE A LARGER SCALE OVERALL MEAN RIDGE COVERING THE EAST.
THE PRIMARY MODEL DISCREPANCIES INVOLVE THE EVENTUAL PROGRESSION OF
THE UPPER LOW AND DETAILS OF INFLUENCING UPSTREAM SHORTWAVE ENERGY.
MOST MACHINE LEARNING (ML) MODELS OFFER THE BEST SUPPORT FOR THE
INTERMEDIATE TIMING OF RECENT ECMWF RUNS. LATEST GFS RUNS SHIFT THE
UPPER LOW A LITTLE NORTH OF CONSENSUS BY AROUND SUNDAY OR SO AND
THEN STRAY FASTER, THOUGH INCOMING NORTHERN TIER ENERGY ULTIMATELY
YIELDS A MEAN TROUGH NOT TOO FAR OFF THE GUIDANCE AVERAGE FOR
POSITION. BY DAY 7/TUESDAY THE GFS DETAILS ALOFT RESULT IN A
DEEPER SOUTHERN CANADIAN SURFACE LOW COMPARED TO OTHER DYNAMICAL OR
ML MODELS. IN CONTRAST, PER THE ML MODELS THE 00Z ECMWF COULD BE
UNDERDONE WITH SURFACE LOW PRESSURE DUE TO SLOW TIMING OF UPSTREAM
SHORTWAVE ENERGY. CMC RUNS REPRESENT THE SLOWER EXTREME FOR THE
EJECTING UPPER LOW, THOUGH THE NEW 12Z VERSION HAS ADJUSTED CLOSER
TO THE FAVORED INTERMEDIATE TIMING. ALSO OF NOTE BY THE END OF THE
PERIOD, DYNAMICAL/ML GUIDANCE DEVELOPS SIGNIFICANT DIVERGENCE FOR
HOW ENERGY WILL BE DISTRIBUTED WITHIN THE WESTERN CANADA INTO
EASTERN PACIFIC MEAN TROUGH ALOFT. THIS SPREAD BEGINS TO INFLUENCE
SENSIBLE WEATHER OVER THE NORTHWEST BY NEXT TUESDAY BUT HAS THE
POTENTIAL TO PRODUCE MORE SIGNIFICANT SPREAD AND RUN-TO-RUN
VARIABILITY OVER MORE OF THE LOWER 48 THEREAFTER.
GUIDANCE COMPARISONS LED TO STARTING THE UPDATED FORECAST WITH A
COMPOSITE OF 00Z/06Z OPERATIONAL MODELS, REPLACING THE GFS WITH THE
GEFS MEAN AROUND MID-PERIOD AND THEN SPLITTING ECMWF INPUT BETWEEN
THE 00Z/15 AND 12Z/14 RUNS. THE 00Z ECENS RUN WAS NOT AVAILABLE
LOCALLY FOR BLENDING PURPOSES, SO THE LAST PART OF THE FORECAST
CONSISTED OF THOSE ECMWF RUNS, THE 06Z GEFS MEAN, AND WPC
CONTINUITY.
..WEATHER/HAZARDS HIGHLIGHTS
THE UPPER LOW CLOSING OFF ACROSS THE SOUTHWEST LATE WEEK INTO THE
WEEKEND WILL PUSH PRECIPITATION INTO THE FOUR CORNERS STATES,
ROCKIES, AND HIGH PLAINS. EXPECT ENHANCED PRECIPITATION TOTALS IN
TERRAIN WITH HIGHER ELEVATION SNOW, WHICH WILL BE THE FIRST SNOW OF
THE SEASON IN SOME LOCATIONS GIVEN THE WARM FALL IN THE WEST THUS
FAR. THE DAY 4/FRIDAY EXCESSIVE RAINFALL OUTLOOK HAS INTRODUCED A
SMALL MARGINAL RISK AREA CENTERED OVER THE CENTRAL PORTION OF
NORTHERN NEW MEXICO BASED ON POTENTIALLY HIGH LOCALIZED RAIN RATES
OVER SENSITIVE TERRAIN CARRYING OVER FROM THE DAY 3 PERIOD. SOME
PRECIPITATION COULD CONTINUE ACROSS THE CENTRAL/SOUTHERN ROCKIES
INTO THE WEEKEND. BY LATER FRIDAY INTO THE WEEKEND, GULF MOISTURE
MAY STREAM IN JUST AHEAD OF THE UPPER LOW FOR INCREASING RAIN
CHANCES IN THE CENTRAL/SOUTHERN HIGH PLAINS. WHILE FORECAST DETAILS
VARY, MODELS HAVE BEEN FAIRLY PERSISTENT IN SHOWING SOME MODERATE
TO HEAVY RAIN POTENTIAL FOCUSED IN NORTHEASTERN NEW MEXICO AND
NEARBY, WITH SOME MODEST INSTABILITY AND SLOW MOVEMENT OF THE UPPER
LOW AROUND SATURDAY IN PARTICULAR. THUS THE DAY 5/SATURDAY ERO
CONTINUES TO DEPICT A MARGINAL RISK IN THAT REGION AND HEAVY RAIN
WOULD BE MOST CONCERNING ATOP SENSITIVE BURN SCAR AREAS. LATEST
GUIDANCE STILL SUGGESTS THAT THE SOUTH- CENTRAL PLAINS WILL
CONTINUE TO BE A FOCUS FOR ENHANCED PRECIPITATION ON SUNDAY BEFORE
RAINFALL FINALLY MOVES EASTWARD MONDAY, THOUGH WITH DECREASING
INTENSITY.
MEANWHILE, THE PACIFIC NORTHWEST WILL LIKELY SEE MORE
PRECIPITATION DURING THE PERIOD. INITIAL FOCUS FOR ENHANCED TOTALS
WILL BE OVER THE OLYMPICS AND VICINITY FRIDAY INTO THE WEEKEND, ON
THE SOUTHERN PERIPHERY OF A STRONG JET PUSHING INTO BRITISH
COLUMBIA. WITH TIME, EXPECT UPSTREAM SHORTWAVE ENERGY TO EXPAND THE
PRECIPITATION SHIELD SOUTHWARD AND EASTWARD BUT WITH GRADUALLY
LESSER MAX AMOUNTS. UNCERTAINTY ABOUT SPECIFICS OF EASTERN
PACIFIC/WEST COAST FLOW ALOFT DECREASES CONFIDENCE IN PRECIPITATION
DETAILS OVER THE NORTHWEST BY NEXT TUESDAY.
MEANWHILE, THE EASTERN U.S. IS FORECAST TO BE GENERALLY DRY LATE
WEEK INTO EARLY NEXT WEEK, WITH THE EXCEPTION OF PARTS OF THE
FLORIDA PENINSULA. THERE, RETURN FLOW OF MOISTURE NEAR A MEANDERING
FRONT COULD INCREASE RAIN AND THUNDERSTORM CHANCES ESPECIALLY FOR
SOUTH FLORIDA BY FRIDAY AND INTO THE WEEKEND. THERE COULD BE A
NONZERO CHANCE FOR FLASH FLOODING WITH THIS ACTIVITY ESPECIALLY IF
HEAVY RAIN RATES OCCUR IN THE SENSITIVE URBAN AREAS. HOWEVER, AS
MODELS CONTINUE TO VARY WITH HOW MUCH RAIN MAY FALL ONSHORE VERSUS
OFFSHORE, THE DAYS 4-5/FRIDAY-SATURDAY EROS CONTINUE TO DEPICT NO
RISK AREAS. WILL CONTINUE TO MONITOR THIS REGION FOR ANY TRENDS IN
MODEL CLUSTERING.
AN UPPER RIDGE ALIGNED OVER THE EAST-CENTRAL U.S. FRIDAY WILL
PROMOTE ABOVE NORMAL TEMPERATURES IN ITS VICINITY. THE BIGGEST
ANOMALIES OF PLUS 10-20 DEGREES ARE FORECAST ACROSS THE MIDWEST.
MEANWHILE ANOTHER ROUND OF CHILLY MORNING LOWS MAY CAUSE
FROST/FREEZE CONCERNS FOR THE INTERIOR NORTHEAST AND APPALACHIANS.
NORTHEASTERN TEMPERATURES WILL REBOUND DURING THE WEEKEND WHEN
UPPER RIDGING MOVES OVERHEAD, BRINGING ANOMALIES UP TO 5-10
DEGREES ABOVE NORMAL. THEN THE RIDGE REBUILDING OVER THE CENTRAL
INTO EASTERN U.S. EARLY NEXT WEEK WILL LEAD TO WARMER THAN AVERAGE
TEMPERATURES IN THE NORTHERN PLAINS TO UPPER MIDWEST ONCE AGAIN,
WITH EXPANDING COVERAGE OF PLUS 5-15 DEGREE ANOMALIES OVER THE EAST.
ON THE OTHER HAND, THE UPPER TROUGH AND FRONTAL SURGES IN THE WEST
WILL CAUSE BELOW NORMAL TEMPERATURES (BY UP TO 10-20 DEGREES FOR
HIGHS) TO SPREAD ACROSS THE GREAT BASIN/INTERMOUNTAIN WEST AND
SOUTHWEST ON FRIDAY WITH SLOW MODERATION INTO THE WEEKEND. THESE
COLDER TEMPERATURES MAY ALSO LEAD TO ADDITIONAL FROST/FREEZE
CONCERNS FOR ANY REMAINING SUSCEPTIBLE GROWING AREAS/VALLEY
LOCATIONS.
RAUSCH/TATE
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 (QPF), EXCESSIVE RAINFALL
OUTLOOK (ERO), WINTER WEATHER OUTLOOK (WWO) PROBABILITIES, HEAT
INDICES, AND KEY MESSAGES CAN BE ACCESSED FROM:
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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