009
FXUS02 KWBC 180717
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
217 AM EST SUN FEB 18 2024
VALID 12Z WED FEB 21 2024 - 12Z SUN FEB 25 2024
..OVERVIEW
UPPER TROUGHING COMING QUICKLY INTO THE WEST COAST AS THE PERIOD
BEGINS MIDWEEK WILL SPREAD SOME LINGERING PRECIPITATION TO
CALIFORNIA AND SNOW INTO THE INTERMOUNTAIN WEST. THE WEAKENING
SHORTWAVE ENERGY FROM THIS SYSTEM WILL THEN QUICKLY EJECT EASTWARD
AND MAY COMBINE WITH AMPLIFYING NORTHERN STREAM ENERGY TO DEVELOP
A CENTRAL-EASTERN U.S. SURFACE LOW/FRONTAL SYSTEM BY THE LATTER
HALF OF THE WEEK. THIS FEATURE SHOULD SPREAD RAIN AND NORTHERN
TIER SNOW TO PARTS OF THE EASTERN THIRD OF THE COUNTRY AS A LARGER
SCALE UPPER TROUGH SETTLES OVER THE EAST. MEANWHILE, PERIODS OF
UPPER RIDGING ACROSS THE WEST-CENTRAL U.S. WILL LEAD TO MUCH ABOVE
NORMAL TEMPERATURES ON AVERAGE THROUGH THIS WEEK INTO NEXT
WEEKEND. UPPER AND SURFACE LOWS MAY APPROACH THE WEST COAST AGAIN
BY NEXT WEEKEND BUT WITH UNCERTAINTY IN THEIR PLACEMENT AND TIMING.
..GUIDANCE/PREDICTABILITY ASSESSMENT
AN OPERATIONAL MODEL BLEND PROVIDES A REASONABLE STARTING POINT
FOR THE SYSTEM AFFECTING THE WEST MIDWEEK. SHORTWAVE ENERGIES IN
THE NORTHERN AND SOUTHERN STREAM THEN TRACK QUICKLY EAST INTO THE
CENTRAL U.S. BY EARLY THURSDAY AND CAUSE SOME DIFFERENCES WITH THE
UPPER TROUGH FORMING ACROSS THE EAST-CENTRAL U.S. THAT AFFECT THE
ASSOCIATED SURFACE LOW. GFS RUNS HAVE BEEN QUESTIONABLY SHARP AND
SLOW WITH AMPLIFYING NORTHERN STREAM ENERGY, CAUSING THE SURFACE
LOW TRACK TO BE FARTHER NORTHWEST INTO THE OHIO VALLEY/NORTHEAST
RATHER THAN THE FLATTER WEST-EAST TRACK THAT THE NON-NCEP GUIDANCE
SHOWS. BY SATURDAY THE NON-NCEP GUIDANCE DIVERGES SOMEWHAT TOO,
WITH THE 12Z ECMWF TRACKING SLOWER NORTH (THE NEWER 00Z RUN SPED
UP) AND THE 12Z CMC FARTHER OFFSHORE THAN THE BULK OF OTHER
GUIDANCE, THOUGH MANY CMC ENSEMBLE MEMBERS AGREED WITH AN OFFSHORE
TRACK. OVERALL A MODEL AND GEFS/EC ENSEMBLE MEAN BLEND SEEMED THE
BEST INTERMEDIATE SOLUTION.
MODELS ARE PRETTY CONSISTENT SHOWING RIDGING BEHIND THE EASTERN
TROUGHING, BUT COULD BE DISRUPTED BY UNCERTAIN SHORTWAVES COMING
THROUGH THE NORTHWEST FLOW. ON THE LARGER SCALE, AN UPPER LOW
DROPPING SOUTHEASTWARD IN THE EASTERN PACIFIC HAS BEEN WAFFLING IN
THE MODELS WITH ITS POSITION ON HOW NEAR TO THE WEST COAST IT WILL
BE. ECMWF RUNS HAVE BEEN THE FARTHEST EAST WITH GFS RUNS ON THE
WESTERN SIDE, PARTICULARLY THE NEW 00Z GFS. MANY ECMWF ENSEMBLE
MEMBERS WERE FARTHER EAST AND PULLED THE EC MEAN EAST OF THE GEFS
MEAN. THIS REMAINS UNCERTAIN FOR NOW AND A BLENDED POSITION OF THE
MEANS SEEMED OKAY.
THE WPC FORECAST WAS THUS BASED ON A BLEND OF DETERMINISTIC
GUIDANCE EARLY IN THE PERIOD BUT GRADUALLY REDUCED THE PROPORTION
OF THE OPERATIONAL MODELS IN FAVOR OF THE MEANS. REDUCED THE GFS
PROPORTION MOST QUICKLY DUE TO THE DIFFERENCES IT HAD IN THE EAST.
ENSEMBLE MEANS REACHED HALF THE BLEND DAY 6 AND OVER HALF DAY 7.
..WEATHER/HAZARDS HIGHLIGHTS
AN ATMOSPHERIC RIVER EVENT MAINLY IN THE SHORT RANGE PERIOD WILL
BE WINDING DOWN ACROSS CALIFORNIA BY WEDNESDAY. THERE REMAINS
SOMEWHAT LOW CONFIDENCE IN HOW MUCH PRECIPITATION MAY CONTINUE
ACROSS SOUTHERN CALIFORNIA WEDNESDAY BEHIND THE MAIN COLD FRONT.
OVERALL, TOTALS SHOULD BE REDUCED SIGNIFICANTLY FROM EARLIER DAYS,
BUT AS THE UPPER TROUGH COMES THROUGH, THE LOWERING HEIGHTS ALOFT
CAN STEEPEN LAPSE RATES AND ALLOW FOR SOME INSTABILITY TO PRODUCE
LOCALLY HIGHER RAIN RATES. HAVE MAINTAINED A MARGINAL RISK
COVERING SOUTHERN CALIFORNIA FOR EARLY WEDNESDAY GIVEN THE LIKELY
SENSITIVITY TO ADDITIONAL RAIN AT THE TAIL END OF THE EVENT. SNOW
COULD ALSO CONTINUE ACROSS THE SIERRA NEVADA WEDNESDAY. FARTHER
EAST, MODERATE SNOW IS LIKELY IN THE HIGHER ELEVATIONS OF THE
INTERMOUNTAIN WEST AND ROCKIES, PARTICULARLY IN THE WASATCH TO
CENTRAL ROCKIES BEFORE THE WEST DRIES OUT FOR LATE WEEK. THE NEXT
SYSTEM DROPPING INTO THE EAST PACIFIC LATE IN THE WEEK MAY START
TO BRING RETURNING PRECIPITATION CHANCES TO CALIFORNIA BY NEXT
WEEKEND DEPENDENT ON HOW CLOSE THE UPPER/SURFACE LOWS GET TO THE
COAST.
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 AN INCH OR TWO ARE LIKELY IN THE OHIO VALLEY ON
THURSDAY, WITH LOCALLY HIGHER AMOUNTS. THE SURFACE LOW AND
SHORTWAVE HELPING TO FORCE THE PRECIPITATION SHOULD BE MOVING
RATHER QUICKLY, BUT THE WEST TO EAST TRACK MEANS THAT RAIN HAS THE
POTENTIAL TO TRAIN WEST TO EAST. SOME INSTABILITY COMBINED WITH
THE GOOD DYNAMICAL SUPPORT (LEFT EXIT REGION OF THE JET) MAY LEAD
TO HIGH RAINFALL RATES. HOWEVER, STREAMFLOWS AND SOIL MOISTURE ARE
QUITE DRY ACROSS THE UPPER OHIO VALLEY BUT CLOSER TO AVERAGE IN
THE LOWER OHIO VALLEY. THERE IS ALSO STILL MODEL SPREAD WITH THE
LOW TRACK AND HEAVIEST QPF. THIS FORECAST SHIFTED THE AXIS OF
HEAVIEST RAIN NORTH FROM THE PREVIOUS ISSUANCE PER MODEL TRENDS,
WHICH PUTS THE HEAVIEST QPF IN AN AREA WITH DRIER ANTECEDENT
CONDITIONS, BUT THE NEWER MODELS COMING IN HAVE SHOWN A SLIGHT
SOUTHWARD SHIFT AGAIN. CONSIDERING ALL THESE COMPETING FACTORS,
OPTED TO ISSUE A SOMEWHAT BROAD MARGINAL RISK IN THE OHIO VALLEY
WITH THE EXPECTATION THAT THE AREA COULD NARROW OR CHANGE WITH
TIME. 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 PUSHING FROM THE CENTRAL U.S. WEDNESDAY TOWARD THE
EASTERN U.S. THURSDAY WILL PROMOTE ABOVE NORMAL TEMPERATURES IN
THOSE AREAS. TEMPERATURES AROUND 15-25F ABOVE NORMAL WILL BE IN
PLACE ACROSS THE PLAINS TO MIDWEST ON WEDNESDAY, AND 10-20F
ANOMALIES ARE LIKELY INTO THE EAST BY THURSDAY. 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, AND RENEWED RIDGING OVER THE WEEKEND WILL LIKELY
RAISE TEMPERATURES AGAIN TO 15-25F ABOVE NORMAL THERE. MEANWHILE
THE WEST COAST SHOULD BE NEAR AVERAGE FOR HIGHS THROUGH THE WEEK
BUT A FEW DEGREES ABOVE NORMAL FOR LOWS AT TIMES.
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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