732
FXUS02 KWBC 170717
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
216 AM 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.
SHORTWAVE ENERGY PUSHING EASTWARD INTO THE WEST 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
OFFSHORE THROUGH THE PERIOD, WITH AT LEAST A WEAK UPPER RIDGE
BUILDING OVER PARTS OF THE WEST.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE HAS COME INTO BETTER AGREEMENT OVER THE PAST DAY
WITH THE SURFACE LOW AND ASSOCIATED UPPER TROUGH ATOP THE EASTERN
PACIFIC TO WEST COAST TUESDAY-WEDNESDAY. MODELS HAVE TRENDED
TOWARD A NORTHERN POSITION WITH THE SURFACE LOW AND INDICATE IT
WEAKENING AS IT REACHES THE PACIFIC NORTHWEST WEDNESDAY. MOST
12/18Z DETERMINISTIC GUIDANCE WAS WITHIN REASON AND ABLE TO BE
USED FOR THESE FEATURES, WITH THE EXCEPTION OF THE 12Z UKMET WHICH
HELD THE UPPER AND SURFACE LOW FARTHER SOUTHWEST. THE QPF OF THE
ECMWF RUNS FARTHER SOUTH ACROSS SOUTHERN CALIFORNIA WITH THE
AR/FRONT HAVE BEEN QUESTIONABLE THOUGH.
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. FORTUNATELY MOST MODELS SHOW FAIRLY GOOD
AGREEMENT WITH A SURFACE LOW CONSOLIDATING ATOP OKLAHOMA OR SO
EARLY THURSDAY, BUT UNFORTUNATELY MODEL SPREAD INCREASES AS THE
LOW TRACKS QUICKLY EAST BY FRIDAY. THE 12/18Z SUITE SHOWED SOME
SPREAD WITH THE SURFACE LOW'S STRENGTH AND POSITION BUT ALL HAD A
GENERAL THEME OF THE LOW BEING ATOP THE MID-ATLANTIC, FITTING WELL
WITH THE ENSEMBLE MEANS. 00Z MODELS HAVE ACTUALLY COME IN WITH
INCREASING SPREAD IN THE LOW POSITION WITH THE ECMWF SHOWING A
MUCH SLOWER TREND AND THE GFS FASTER AND TO THE NORTH. A NORTHWARD
TURN OF THE LOW IS LIKELY BY SATURDAY BUT THE DIFFERENCES IN
PLACEMENT AND STRENGTH CONTINUE AND WILL CERTAINLY HAVE SENSIBLE
WEATHER EFFECTS, INCLUDING POTENTIAL SNOW IN THE NORTHEAST.
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 12Z ECMWF BRINGS ITS
FEATURE FARTHER SOUTH THAN ITS PREVIOUS RUNS OR OTHER GUIDANCE.
THE EC-BASED AI MODELS DID NOT SHOW THIS EVOLUTION, WHICH GAVE
MORE CONFIDENCE TO LEAN AWAY FROM THE DETERMINISTIC ECMWF
SOLUTION. THE NEW 00Z ECMWF SEEMS IN MORE REASONABLE ALIGNMENT.
REGARDING ANOTHER UPPER LOW OVER THE EASTERN PACIFIC, GUIDANCE HAD
BEEN TRENDING WESTWARD WITH ITS POSITION AND THE 12Z ECMWF SEEMED
EXCEPTIONALLY FAR WEST. MEANWHILE THE 12Z CMC WAS FARTHER EAST
THAN THE BEST CONSENSUS. THE MOST RECENT 00Z GUIDANCE HAS SHOWN A
BIT OF AN EASTWARD JOG, SO THIS WAFFLING MAKES THE POSITIONING OF
THE UPPER LOW UNCERTAIN.
THUS THE WPC FORECAST UTILIZED THE 18Z GFS AND 12Z ECMWF/CMC/UKMET
AT THE START OF THE PERIOD, BUT ELIMINATED THE UKMET BY DAY 5 AND
REDUCED THE PROPORTIONS OF ALL DETERMINISTIC MODELS, MORE QUICKLY
WITH THE CMC AND ECMWF, TO FAVOR THE EC/GEFS ENSEMBLE MEANS. THE
ENSEMBLE MEAN PROPORTION WAS ABOUT HALF BY DAY 5 BUT INCREASED
RAPIDLY DAYS 6-7 GIVEN THE ISSUES WITH THE DETERMINISTIC RUNS IN
VARIOUS LOCATIONS.
..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, BUT THE AMOUNT OF REMAINING RAIN IS UNCERTAIN. WITH MOST
00Z MODELS COMING IN WITH NEAR/OVER AN INCH OF RAIN THOUGH, HAVE A
MARGINAL RISK FOR THE LOS ANGELES METRO TO THE 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.
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 SEEMS MOST LIKELY TO GET NOTABLE
SNOW, BUT 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 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 FOR LOWS. IN FACT, TEMPERATURES LOOK TO WARM ONCE AGAIN
IN THE CENTRAL AND NORTHERN PLAINS FOR LATE WEEK. 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.
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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