851
FXUS02 KWBC 161859
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
159 PM EST FRI FEB 16 2024
VALID 12Z MON FEB 19 2024 - 12Z FRI FEB 23 2024
...ATMOSPHERIC RIVER TO BRING HEAVY RAIN AND MOUNTAIN SNOW TO
CALIFORNIA EARLY NEXT WEEK...
..OVERVIEW
UPPER-LEVEL AND SURFACE LOWS OVER THE EASTERN PACIFIC EARLY NEXT
WEEK WILL STEER A STREAM OF PACIFIC MOISTURE INTO CALIFORNIA AND
PRODUCE HEAVY RAINFALL WITH FLOODING POTENTIAL AND HEAVY MOUNTAIN
SNOW. THESE FEATURES SHOULD WEAKEN BY MIDWEEK WHILE SHORTWAVE
ENERGY PUSHES EASTWARD. THIS SHORTWAVE LOOKS TO COMBINE WITH
AMPLIFYING NORTHERN STREAM ENERGY AND HELP FORM A LOW
PRESSURE/FRONTAL SYSTEM IN THE CENTRAL U.S. THAT PUSHES INTO THE
EASTERN U.S. BY THE LATTER PART OF NEXT WEEK, SPREADING RAIN AND
NORTHERN TIER SNOW TO SOME AREAS AS A LARGER SCALE UPPER TROUGH
SETTLES OVER THE EAST. AHEAD OF THESE FEATURES, TEMPERATURES ARE
FORECAST TO BE ABOVE AVERAGE IN THE CENTRAL U.S. EARLY-MID NEXT
WEEK AND SHIFTING INTO THE EASTERN U.S. MID-LATE NEXT WEEK.
MEANWHILE LATEST TRENDS ARE SUGGESTING THAT ANOTHER UPPER LOW
DROPPING INTO THE EASTERN PACIFIC BY LATE NEXT WEEK WILL STAY
FARTHER OFFSHORE FOR A TIME, WITH AT LEAST A WEAK UPPER RIDGE
BUILDING OVER PARTS OF THE WEST.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MOST GUIDANCE REFLECTS SIMILAR IDEAS FOR THE LARGE SCALE PATTERN
EVOLUTION THROUGH THE PERIOD BUT THERE ARE SOME SIGNIFICANT
EMBEDDED DIFFERENCES THAT WILL TAKE TIME TO BE RESOLVED. FOR THE
SYSTEM INITIALLY OVER THE EAST PACIFIC AND THE ASSOCIATED UPPER
TROUGH, A GUIDANCE AVERAGE PROVIDES A REASONABLE STARTING POINT IN
LIGHT OF SOME OFFSETTING CONSIDERATIONS. OVER RECENT DAYS THE
DYNAMICAL MODELS AND MEANS APPEAR TO HAVE TRENDED NORTHWARD FOR
THE OFFSHORE SURFACE LOW DURING THE FIRST HALF OF THE WEEK, AS
INITIALLY ADVERTISED BY THE ECMWF-INITIALIZED MACHINE LEARNING
MODELS. THERE IS STILL A FAIR AMOUNT OF NORTH-SOUTH SPREAD FOR THE
SURFACE LOW OFF THE PACIFIC NORTHWEST COAST BY MIDWEEK, WITH THE
12Z GFS TRENDING NORTH OF PRIOR RUNS BUT THE PREVIOUSLY NORTH CMC
NUDGING BACK A LITTLE SOUTHWARD. THE 00Z ML MODELS SUGGEST THE
NORTHERN SIDE OF THE SPREAD COULD BE IN PLAY THOUGH. ON THE OTHER
HAND, THE ML MODELS HAVE INSISTED THAT THE OPENING UPPER TROUGH
WILL BECOME A LITTLE MORE PROGRESSIVE THAN THE MODELS/MEANS BY
WEDNESDAY. THUS FAR THERE HAS BEEN NO EVIDENCE OF A FASTER TREND
IN THE CONSENSUS OF DYNAMICAL MODELS/MEANS.
THE TIMING OF THE EJECTING WEST COAST TROUGH AND DETAILS OF
EXACTLY HOW CANADIAN TROUGHING AMPLIFIES INTO THE EASTERN HALF OF
THE LOWER 48 WILL AFFECT THE SURFACE EVOLUTION OVER EAST BY THE
LATTER HALF OF THE WEEK. LATEST GFS/ECMWF/CMC RUNS SHOW WHAT COULD
BE FAIRLY VIGOROUS DEVELOPMENT OF A SOUTH-CENTRAL PLAINS THROUGH
MID-ATLANTIC SURFACE WAVE IN RESPONSE TO THE WESTERN SHORTWAVE,
WHILE THE AMPLIFYING NORTHERN STREAM FLOW SUPPORTS A LEADING COLD
FRONT WITH ONE OR MORE EMBEDDED WAVES. THE 00Z ECMWF WAS ON THE
SOUTHERN EDGE OF THE SPREAD FOR ITS PLAINS/MID-ATLANTIC WAVE
THOUGH, LEADING TO A FARTHER SOUTH THAN CONSENSUS RAIN-SNOW LINE
BY LATE IN THE WEEK. THE NEW 12Z GFS IS SLOWER THAN MOST OTHER
GUIDANCE IN AMPLIFYING THE EASTERN NORTH AMERICA TROUGH LATE IN
THE WEEK. BY EARLY NEXT FRIDAY THE ENSEMBLE MEANS SHOW A MORE EVEN
BALANCE FOR STRENGTH AMONG THE MID-ATLANTIC WAVE AND THE SEPARATE
NORTHERN STREAM FRONTAL WAVE. ON THE OTHER HAND, SLIGHTLY FASTER
PROGRESSION OF THE WESTERN SHORTWAVE IN THE ML MODELS ULTIMATELY
LEADS TO MORE NORTHERN STREAM DOMINANCE FOR DEVELOPING LOW
PRESSURE REACHING THE NORTHEAST. AN OPERATIONAL MODEL/MEAN BLEND
MAINTAINS REASONABLE CONTINUITY AND TILTS SLIGHTLY TO THE
MID-ATLANTIC WAVE FOR STRENGTH.
AFTER THE WESTERN SHORTWAVE DEPARTS, SOME MODEL RUNS HAVE BEEN
SIGNALING POTENTIAL FOR LINGERING ENERGY FROM THE NORTHERN PART OF
THE FEATURE AND/OR A SHORTWAVE DROPPING THROUGH WESTERN CANADA TO
SETTLE OVER OR NEAR THE NORTHWEST. THE GFS HAS TRENDED WEAKER THAN
ITS 00Z/06Z RUNS BUT THE NEW 12Z ECMWF BRINGS ITS FEATURE FARTHER
SOUTH THAN ITS PREVIOUS RUNS OR OTHER GUIDANCE. THE ENSEMBLE MEANS
AND LARGE SCALE PATTERN DO NOT APPEAR TOO FAVORABLE FOR A DEFINED
SHORTWAVE/LOW, SO BY DAY 7 FRIDAY THE FAVORED BLEND WEAKENS WHAT
REFLECTION COULD LINGER INTO THURSDAY.
MULTI-RUN CONTINUITY FROM THE GFS/ECMWF AND THEIR ENSEMBLE MEANS
SHOWS A NOTICEABLE WESTWARD TREND FOR THE UPPER LOW FORECAST TO
DROP INTO THE EASTERN PACIFIC LATE NEXT WEEK. NEW 12Z GUIDANCE IS
SIMILAR TO OR EVEN WESTWARD OF THE PRIOR CYCLE (IN THE CASE OF THE
12Z ECMWF). INTERESTINGLY, THE ML MODEL AVERAGE IS SOMEWHAT EAST
OF THE LATEST DYNAMICAL GUIDANCE CONSENSUS.
BASED ON 00Z/06Z GUIDANCE COMPARISONS, THE UPDATED FORECAST USED A
00Z/06Z MODEL COMPOSITE FOR ABOUT THE FIRST HALF OF THE PERIOD AND
THEN TRANSITIONED TOWARD HALF MODELS AND HALF 06Z GEFS/00Z ECENS
MEANS BY THE END OF THE PERIOD NEXT FRIDAY.
..WEATHER/HAZARDS HIGHLIGHTS
AN ATMOSPHERIC RIVER WITH AMPLE PACIFIC MOISTURE WILL CAUSE ROUNDS
OF PRECIPITATION THAT WILL BE HEAVY ACROSS CALIFORNIA INTO MONDAY
AND TUESDAY. FOR COASTAL/LOWER ELEVATION AREAS, THE DAY 4 AND 5
EXCESSIVE RAINFALL OUTLOOKS INDICATE SLIGHT RISKS ACROSS CENTRAL
TO SOUTHERN PARTS OF CALIFORNIA FOR THE FLOODING POTENTIAL.
RAINFALL RATES IN THIS REGION HAVE THE POTENTIAL TO REACH 0.5-1"
PER HOUR, WITH OVER 1"/HOUR RATES POSSIBLE, AND SOME AREAS ARE
LIKELY STILL SENSITIVE FROM AR EVENTS EARLY IN THE MONTH/COOL
SEASON. THE SLIGHT RISK MONDAY IS LIKELY MORE ON THE HIGHER END OF
THE PROBABILITY RANGE FOR A SLIGHT ACROSS THE LOS ANGELES METRO
AND SOUTHWARD-FACING TERRAIN OF THE TRANSVERSE RANGES, WITH
GUIDANCE AGREEING ON AN AXIS OF ANOMALOUS DEEP MOISTURE AND AN
ADVANCING FRONT EXTENDING FROM INITIALLY STRONG EASTERN PACIFIC
LOW PRESSURE HELPING TO ENHANCE FOCUS OVER THIS REGION. TUESDAY'S
SLIGHT IS LOWER-END AS THE EXPECTED RAINFALL DECREASES, WITH SOME
SHIFT SOUTH AS WELL. BROADER MARGINAL RISKS COVER MUCH OF THE
STATE FARTHER NORTH AND EAST, UNTIL THE HIGHER ELEVATIONS OF THE
SIERRA NEVADA ARE REACHED WHERE THE PRECIPITATION WILL BE IN THE
FORM OF HEAVY SNOW, POSSIBLY REACHING MULTIPLE FEET. LOCATIONS
OVER NORTHERN CALIFORNIA AND ALONG THE SIERRA NEVADA MAY ALSO SEE
HIGH WINDS ON MONDAY. FARTHER EAST, MODERATE SNOW IS LIKELY IN THE
HIGHER ELEVATIONS OF THE INTERMOUNTAIN WEST AND ROCKIES,
ESPECIALLY IN CENTRAL/NORTH-CENTRAL AREAS.
PRECIPITATION CHANCES ARE LIKELY TO INCREASE IN THE OHIO VALLEY TO
GREAT LAKES WEDNESDAY-THURSDAY UNDER THE INFLUENCE OF A COUPLE OF
LOW PRESSURE/FRONTAL SYSTEMS. PRECIPITATION IS MOST LIKELY TO BE
RAIN EXCEPT IN THE FAR NORTHERN TIER. THE PRECIPITATION SHOULD
REACH THE EAST THURSDAY-FRIDAY WITH SOME SNOW POSSIBLE IN PARTS OF
THE NORTHEAST. CONTINUE TO MONITOR FORECASTS AS THE DETAILS ARE
YET TO BE DETERMINED, GIVEN THE UNCERTAIN INTERACTION OF SEPARATE
FEATURES THAT WILL INFLUENCE PRECIPITATION AMOUNTS/COVERAGE/TYPE.
UPPER RIDGING TRAVERSING THE INTERIOR WEST TO THE CENTRAL AND THEN
EASTERN U.S. THROUGH NEXT WEEK WILL PROMOTE ABOVE NORMAL
TEMPERATURES IN THOSE AREAS. TEMPERATURES AROUND 10-20F ABOVE
NORMAL WILL RISE TO 15-25F ABOVE NORMAL ACROSS THE PLAINS TO
MIDWEST ON TUESDAY AND WEDNESDAY. HIGHS ARE FORECAST TO BE INTO
THE 80S FOR PARTS OF TEXAS WITH 70S STRETCHING INTO THE SOUTHEAST.
TEMPERATURES SHOULD MODERATE CLOSER TO NORMAL AFTER THE COLD FRONT
COMES THROUGH AFTER MIDWEEK BUT MAY STILL REMAIN ABOVE NORMAL IN
THE CENTRAL U.S., ESPECIALLY FOR LOWS. BY THURSDAY THE ABOVE
NORMAL TEMPERATURES SHOULD PUSH INTO THE EAST WITH PLUS 10-20F
ANOMALIES, ALSO MODERATING THERE BY FRIDAY. MEANWHILE THE WEST
COAST SHOULD BE NEAR AVERAGE FOR HIGHS BUT SOMEWHAT ABOVE NORMAL
FOR LOWS.
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, 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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