214
FXUS02 KWBC 171900
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 PM EST SAT FEB 17 2024
VALID 12Z TUE FEB 20 2024 - 12Z SAT FEB 24 2024
...ATMOSPHERIC RIVER WILL PERSIST BUT START WINDING DOWN IN
CALIFORNIA TUESDAY-WEDNESDAY...
..OVERVIEW
AN ATMOSPHERIC RIVER TAKING AIM AT CALIFORNIA WILL BE ONGOING AT
THE START OF THE MEDIUM RANGE PERIOD TUESDAY AND COULD CAUSE SOME
HEAVY RAINFALL WITH FLOODING POTENTIAL AND HEAVY MOUNTAIN SNOW TO
LINGER, BUT SHOULD BE WEAKENING COMPARED TO SUNDAY-MONDAY. THE
WEAKENING SHORTWAVE ENERGY FROM THIS SYSTEM WILL QUICKLY EJECT
EASTWARD AFTER EARLY WEDNESDAY AND MAY COMBINE WITH AMPLIFYING
NORTHERN STREAM ENERGY TO DEVELOP A CENTRAL-EASTERN U.S. SURFACE
LOW/FRONTAL SYSTEM BY THE LATTER HALF OF NEXT WEEK. THIS FEATURE
SHOULD SPREAD RAIN AND NORTHERN TIER SNOW TO SOME AREAS AS A
LARGER SCALE UPPER TROUGH SETTLES OVER THE EAST. AHEAD OF THE
DEVELOPING SYSTEM, TEMPERATURES ARE FORECAST TO BE ABOVE AVERAGE
IN THE CENTRAL U.S. EARLY-MID NEXT WEEK AND SHIFT INTO THE EASTERN
U.S. MID-LATE NEXT WEEK. MEANWHILE GUIDANCE CONTINUES TO SHOW
ANOTHER UPPER LOW DROPPING INTO THE EASTERN PACIFIC BY LATE NEXT
WEEK, LIKELY STAYING OFFSHORE THROUGH NEXT SATURDAY, WHILE SOME
UPPER RIDGING BUILDS OVER PARTS OF THE WEST.
..GUIDANCE/PREDICTABILITY ASSESSMENT
AN OPERATIONAL MODEL COMPOSITE PROVIDES A REASONABLE STARTING
POINT FOR THE SYSTEM AFFECTING THE WEST INTO MIDWEEK. AMONG THE
00Z/06Z RUNS, THE 06Z GFS/GEFS WERE A BIT ON THE FASTER SIDE WITH
THE SOUTHERN PART OF THE UPPER TROUGH BY EARLY WEDNESDAY. RECENTLY
THE ECMWF-INITIALIZED MACHINE LEARNING (ML) MODELS WERE
CONSISTENTLY FASTER WITH THE TROUGH THAN MOST DYNAMICAL GUIDANCE
BUT TODAY'S ML MODELS HAVE ADJUSTED A LITTLE SLOWER, AT LEAST TO
THE GFS TIMING. THE 12Z GFS IS CONSISTENT WITH THE 06Z VERSION
WHILE THE NEW 12Z ECMWF/UKMET HAVE ADJUSTED A LITTLE FASTER.
GUIDANCE SPREAD/VARIABILITY OVER THE PAST FEW DAYS WOULD RECOMMEND
MAINTAINING AN INTERMEDIATE SOLUTION. ALSO OF NOTE, ECMWF RUNS
THROUGH THE 00Z CYCLE HAVE BEEN QUESTIONABLY LIGHT FOR SOUTHERN
CALIFORNIA QPF WITH THE AR/SURFACE FRONT. THE NEW 12Z RUN HAS AT
LEAST TRENDED HEAVIER IN THE DIRECTION OF OTHER GUIDANCE.
PRECISE TIMING OF THE EJECTING WEST COAST UPPER TROUGH AND DETAILS
OF EXACTLY HOW CANADIAN TROUGHING AMPLIFIES INTO THE EASTERN HALF
OF THE LOWER 48 CONTINUE TO BE A FORECAST CHALLENGE WITH RESPECT
TO SURFACE EVOLUTION OVER THE EASTERN HALF OF THE COUNTRY BY THE
LATTER HALF OF THE WEEK. ON THE GOOD SIDE, TODAY'S 00Z ML MODELS
HAVE TRENDED MORE IN THE DIRECTION OF RECENT DYNAMICAL GUIDANCE
THURSDAY INTO EARLY FRIDAY--SO NOW THERE IS BETTER AGREEMENT ON A
SOUTH-CENTRAL PLAINS INTO MID-ATLANTIC WAVE, WITH TYPICAL SPREAD
FOR DEPTH AND TRACK. BY EARLY FRIDAY THE 00Z ECMWF AND 12Z UKMET
ARE ON THE SOUTHERN SIDE OF THE SPREAD WITH THE NEW 12Z ECMWF
ADJUSTING NORTH BY A STATE TO ALIGN WITH THE 00Z ECENS MEAN.
GEFS/CMCENS MEANS ARE A LITTLE NORTH OF THAT. THE 00Z ML MODELS
FAVOR AN INTERMEDIATE TRACK NEAR THE NORTHERN MID-ATLANTIC WITH A
DEPTH AROUND THE MID 990S MB. ALREADY BY FRIDAY GFS RUNS OVER THE
PAST DAY HAVE BEEN QUESTIONABLY SHARP AND SLOW WITH AMPLIFYING
NORTHERN STREAM ENERGY. GOING INTO THE WEEKEND THIS RESULTS IN THE
00Z/06Z RUNS HOLDING LOW PRESSURE NEAR THE NEW ENGLAND COAST AS OF
LATE SATURDAY WHILE REMAINING CONSENSUS (AMONG BOTH DYNAMICAL AND
ML MODELS) IS ALREADY OVER NORTHERN NEW ENGLAND OR THE CANADIAN
MARITIMES BY EARLY SATURDAY. 12Z GFS DETAILS REMAIN A QUESTION
MARK, AND WHILE THE EARLY SATURDAY SURFACE DEPICTION IS A LOT
CLOSER TO THE MAJORITY CLUSTER, IT STILL LINGERS NEAR NEW ENGLAND
LONGER THAN CONSENSUS THEREAFTER. ON THE OTHER HAND, 12Z CMC/00Z
ECMWF RUNS TRACK THE SYSTEM FARTHER OFFSHORE NEW ENGLAND THAN MOST
OTHER SOLUTIONS.
SPECIFICS WITHIN GENERAL NORTHWESTERLY FLOW BEHIND THE EASTERN
U.S. UPPER TROUGH LATE IN THE PERIOD BECOME QUITE UNCERTAIN,
FAVORING A MOSTLY MEAN APPROACH. THE ECMWF SYSTEM REACHING THE
GREAT LAKES BY DAY 7 SATURDAY MAY BE ON THE STRONG SIDE BUT THERE
ARE HINTS IN OTHER MODELS/MEANS FOR AT LEAST A WEAK WAVE.
FOR THE UPPER LOW DROPPING INTO THE EASTERN PACIFIC LATER IN THE
WEEK, SOME GUIDANCE HAS BEEN REVERSING WHAT HAD BEEN A WESTWARD
TREND. THIS SHIFT IS MOST NOTABLE IN THE 00Z/12Z ECMWF PLUS ECENS
RUNS AND THE 12Z UKMET. THIS LEAVES THE GFS/GEFS AND 00Z CMCENS
MEAN ON THE WESTERN SIDE OF THE SPREAD. THE UPDATED FORECAST
RESOLVED THIS DIVERGENCE WITH A COMPROMISE POSITION THAT NUDGED
THE DETERMINISTIC FORECAST A LITTLE EASTWARD FROM CONTINUITY BUT
KEPT IT WILL WITHIN THE CURRENT SPREAD TO MAXIMIZE FLEXIBILITY
PENDING FUTURE TRENDS.
FORECAST CONSIDERATIONS LED TO STARTING WITH AN OPERATIONAL MODEL
COMPOSITE EARLY IN THE PERIOD, FOLLOWED BY A RAPID TRANSITION TO
MOSTLY THE 06Z GEFS/00Z ECENS MEANS LATE IN THE PERIOD. DAY 7
SATURDAY REACHED 80 PERCENT WEIGHT OF THE MEANS WITH ONLY
LINGERING INPUT FROM THE 00Z ECMWF/CMC AT THAT TIME.
..WEATHER/HAZARDS HIGHLIGHTS
AN ATMOSPHERIC RIVER WITH AMPLE SUBTROPICAL PACIFIC MOISTURE WILL
CAUSE ROUNDS OF PRECIPITATION THAT MAY CONTINUE TO BE HEAVY ACROSS
SOUTHERN CALIFORNIA INTO TUESDAY. A SLIGHT RISK OF EXCESSIVE
RAINFALL REMAINS IN PLACE 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 WEAKENING EASTERN PACIFIC LOW PRESSURE
HELPING TO ENHANCE FOCUS OVER THIS REGION. THE EXPECTED RAINFALL
SHOULD BE DECREASING BY TUESDAY COMPARED TO MONDAY, BUT
CONSIDERING THAT THE AREA WILL LIKELY BE SENSITIVE TO ADDITIONAL
RAIN AFTER HEAVY AMOUNTS IN THE SHORT RANGE, FELT IT PRUDENT TO
KEEP THE SLIGHT RISK GOING. A BROADER MARGINAL RISK COVERS 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 BY
MIDWEEK. BY WEDNESDAY PRECIPITATION TOTALS LOOK TO DECREASE
FURTHER AS THE UPPER TROUGH AXIS REACHES THE CALIFORNIA COAST
(MOST LIKELY EARLY IN THE DAY), WITH EXACT AMOUNTS UNCERTAIN DUE
TO TIMING DIFFERENCES WITH THE TROUGH. MOST 12Z GUIDANCE THUS FAR
HAS BEEN NUDGING LOWER WITH TOTALS OVER SOUTHERN CALIFORNIA WITH
SLIGHTLY FASTER UPPER TROUGH PROGRESSION. HOWEVER THERE IS A
RESIDUAL SIGNAL THAT THE BAND OF ENHANCED MOISTURE COULD STILL BE
OVER THE AREA VERY EARLY IN THE DAY BEFORE DEPARTING, AND GUIDANCE
HAS PERSISTENTLY VARIED WITH EXACT TIMING OF THE TROUGH OVER
RECENT DAYS. THUS THE UPDATED DAY 5 EXCESSIVE RAINFALL OUTLOOK
MAINTAINS THE MARGINAL RISK AREA OVER AND SOUTH OF THE LOS ANGELES
METRO/TRANSVERSE RANGES GIVEN THE LIKELY SENSITIVITY TO ADDITIONAL
RAIN. FARTHER EAST, MODERATE SNOW IS LIKELY IN THE HIGHER
ELEVATIONS OF THE INTERMOUNTAIN WEST AND ROCKIES, ESPECIALLY IN
CENTRAL/NORTH-CENTRAL AREAS THROUGH MIDWEEK BEFORE THE WEST DRIES
OUT FOR LATE WEEK. THE NEXT SYSTEM DROPPING INTO THE EAST PACIFIC
LATE IN THE WEEK SHOULD REMAIN FAR ENOUGH OFFSHORE TO KEEP MOST OF
CALIFORNIA DRY INTO SATURDAY, THOUGH THERE IS MODEST POTENTIAL FOR
A FARTHER EAST TRACK THAT WOULD BRING SOMEWHAT MORE MOISTURE INTO
THE STATE AT THAT TIME.
PRECIPITATION CHANCES ARE LIKELY TO INCREASE IN THE OHIO VALLEY TO
GREAT LAKES WEDNESDAY AND ESPECIALLY 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. RAINFALL
AMOUNTS OF AROUND AN INCH COULD BE WIDESPREAD IN THE LOWER OHIO
VALLEY, WITH LOCALLY HIGHER AMOUNTS. THE PRECIPITATION SHOULD
REACH THE EAST THURSDAY-FRIDAY WITH SNOW POSSIBLE IN PARTS OF THE
NORTHEAST. NORTHERN NEW ENGLAND HAS A RELATIVELY HIGHER
PROBABILITY OF SEEING NOTABLE SNOW ALONG WITH THE POTENTIAL FOR A
PERIOD OF BRISK WINDS. HOWEVER THE UNCERTAIN INTERACTION OF
SEPARATE FEATURES ULTIMATELY LEADS TO A WIDE RANGE OF
POSSIBILITIES FOR SENSIBLE WEATHER, INCLUDING A LOWER-PROBABILITY
STORM TRACK FAR ENOUGH OFFSHORE TO BRING MINIMAL
PRECIPITATION/WIND TO NEW ENGLAND. CONTINUE TO MONITOR FORECASTS
OVER THE COMING DAYS AS IMPORTANT DETAILS BECOME BETTER RESOLVED.
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 15-25F ABOVE
NORMAL WILL BE IN PLACE ACROSS THE PLAINS TO MIDWEST ON TUESDAY
AND WEDNESDAY. HIGHS ARE FORECAST TO BE INTO THE 80S FOR MUCH 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 OVER NORTHERN/CENTRAL PARTS OF THE REGION WHERE
READINGS MAY REBOUND ONCE AGAIN BY NEXT SATURDAY (UP TO PLUS
15-20F ANOMALIES). BY THURSDAY ABOVE NORMAL TEMPERATURES SHOULD
PUSH INTO THE EAST WITH PLUS 10-20F ANOMALIES, MODERATING THERE BY
FRIDAY. MEANWHILE THE WEST COAST SHOULD BE NEAR AVERAGE FOR HIGHS
THROUGH THE WEEK BUT SOMEWHAT ABOVE NORMAL FOR LOWS
TUESDAY-WEDNESDAY.
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
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page