058
FXUS04 KWBC 300831
QPFPFD
QUANTITATIVE PRECIPITATION FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
431 AM EDT WED MAY 30 2018
PRELIM DAY 1, DAY 2 AND DAY 3 QPF DISCUSSION
VALID MAY 30/1200 UTC THRU JUN 02/1200 UTC
REFERENCE AWIPS GRAPHICS UNDER...PRECIP ACCUM - 24HR
DAY 1
..OVERVIEW
GUIDANCE WAS ALL GENERALLY WELL CLUSTERED WITH QPF OUTPUT FOR
LARGE SCALE SYNOPTIC SYSTEMS IN THE MIDWEST (ABSORBING TROPICAL
DEPRESSION ALBERTO) AND IN THE GREAT BASIN / NORTHERN ROCKIES.
SOMEWHAT MORE TEDIOUS CONVECTIVE FORECASTS WERE SEEN OVER THE
SOUTHERN PLAINS INTO THE SOUTHEAST, INCLUDING FLORIDA. WPC QPF
LEANED TOWARD DETAILS IN THE WRF-NMMB, WHICH HAS PERFORMED BETTER
THIS YEAR THAN IN ANY PREVIOUS YEAR, AND SEEMS A BETTER CHOICE
THAN THE OTHER HI-RES OPTIONS MUCH OF THE LAST FEW MONTHS. WE ALSO
USED THE HREF PROBABILITY MATCHED MEAN - WHICH BOOSTS THE SIGNAL
FOR EMBEDDED HEAVIER AMOUNTS. OVERALL THIS APPROACH LED TO AN
INCREASE OF QPF ALONG THE PAST OF T.D. ALBERTO, AND ALSO IN
ASSOCIATION WITH THE NEXT MCS EXPECTED TO TOP THE THERMAL RIDGE
OVER OKLAHOMA/KANSAS WEDNESDAY EVENING.
PLEASE SEE ALSO WPC TROPICAL CYCLONE ADVISORIES AND STORM
SUMMARIES FOR ALBERTO.
DAYS 2/3...
..EASTERN SEABOARD FROM THE MID ATLANTIC SOUTH TO FLORIDA
THE DAY 2 PERIOD (THURSDAY) WILL BEGIN WITH ABUNDANT MOISTURE IN
PLACE FOR MOST OF THE EAST COAST EXTENDING WEST TO THE
APPALACHIANS. PRECIPITABLE WATER VALUES SHOULD BE ABOVE 1.6 TO 1.7
INCHES IN MOST PLACES, WITH SOME VALUES APPROACHING 2 INCHES
LIKELY. AN UPPER LEVEL TROUGH OVER THE GREAT LAKES ON THURSDAY
WILL STEADILY ADVANCE TO THE EAST, AND PERHAPS DIG SLIGHTLY,
THROUGH THE DAY 2-3 PERIOD WITH AN ASSOCIATED LOW-LEVEL COLD FRONT
ALSO PUSHING INTO NORTHERN PORTIONS OF THE REGION. THE ANOMALOUSLY
HIGH PRECIPITABLE WATER VALUES ALONG THE ENTIRE EAST COAST AND
GENERAL UPPER LEVEL SUPPORT FOR VERTICAL MOTION SHOULD CONTRIBUTE
TO SCATTERED CONVECTION THROUGH MUCH OF THE REGION.
WPC QPF WAS BASED ON A BLEND OF THE 00Z ECMWF, UKMET, AND GFS,
WITH THE GREATEST WEIGHT PLACED ON THE ECMWF. THE GFS APPEARED TO
BE ADVANCING THE FRONT TOO QUICKLY, AND THIS AFFECTS ITS
DISTRIBUTION OF RAINFALL, PARTICULARLY IN THE MID ATLANTIC. THE
DIFFERENCES BETWEEN THE NBM AND WPC QPF ARE NOT TOO SIGNIFICANT
OVERALL, BUT MAY BE LARGER IN AREAS WHERE THE NAM IS INDICATING
HEAVY RAINFALL. THE NAM WAS GENERALLY EXCLUDED FROM WPC QPF DUE TO
POTENTIAL ISSUES WITH CONVECTIVELY INDUCED VORTICITY MAXIMA OVER
THE PLAINS PROPAGATING DOWNSTREAM.
A SLIGHT RISK WAS CONTINUED ON DAY 2 OVER THE MID ATLANTIC, AND
INTRODUCED ON DAY 3 FOR SIMILAR AREAS. SEVERAL ROUNDS OF SCATTERED
CONVECTION MAY NOT PRODUCE SIGNIFICANT AREAL AVERAGED RAINFALL
TOTALS, BUT THE CONVECTIVE NATURE OF THE RAIN SHOULD LEAD TO MORE
SUBSTANTIAL LOCALIZED TOTALS. WITH PWATS AROUND THE 99TH
PERCENTILE ON THURSDAY (SLIGHTLY LOWER BUT STILL ABOVE AVERAGE ON
FRIDAY), THE ENVIRONMENT WOULD BE SUPPORTIVE OF RAIN RATES AS HIGH
AS 2 IN/HR. MANY PARTS OF THE AREA ALSO HAVE ABOVE NORMAL
STREAMFLOW GIVEN A WET PATTERN IN RECENT MONTHS, SO SOME ISOLATED
TO SCATTERED EPISODES OF FLASH FLOODING APPEAR POSSIBLE BOTH DAYS.
..OZARKS INTO THE TENNESSEE VALLEY
THIS IS THE REGION WITH THE GREATEST FORECAST UNCERTAINTY. THE
INFLUENCE OF THE TROUGH SHOULD BE RESTRICTED TO THE NORTH, AND AS
SUCH THE DOMINANT FORCING FOR CONVECTION MAY BE CONVECTIVE
PROCESSES (OUTFLOW, COLD POOLS) AS WELL AS LOW-LEVEL BOUNDARIES
AND THE LOW-LEVEL JET. THUS, THE PRECIPITATION FORECAST IS HIGHLY
LIKELY TO BE INFLUENCED BY CONVECTIVE EVOLUTION IN PREVIOUS DAYS,
AND THIS LEADS TO UNCERTAINTY IN PLACING THE SWATH OF HEAVIEST
RAINFALL. IT ALSO APPEARS LIKELY THAT ONE OR MORE MCSS MAY EVOLVE
GIVEN THAT THE REGION WILL BE SITUATED ON THE N/NE PERIPHERY OF A
LOW-LEVEL THERMAL RIDGE AND ELEVATED MIXED LAYER. ONE MCS MAY BE
ONGOING AT THE BEGINNING OF THE PERIOD IN THE OZARKS, WITH ANOTHER
ONE POTENTIALLY INITIATING THURSDAY EVENING WHERE LOW-LEVEL INFLOW
INTERCEPTS A RESIDUAL OUTFLOW BOUNDARY. FOR NOW, A BLEND OF THE
ECMWF AND GFS WAS GENERALLY FOLLOWED, BUT SEVERAL CHANGES WERE
MADE: (1) TO NUDGE THE QPF AXIS SOUTH, AS MCS PROPAGATION TENDS TO
BEND A BIT MORE TOWARD THE HIGHER THETAE VALUES THAN MODELS
INDICATE; (2) SHOW MORE OF A NATURAL PROGRESSION, AS THE MODELS
TEND TO SHOW THE HIGHER QPF MOVING SLOWLY OR SITTING IN SIMILAR
LOCATIONS FOR CONSECUTIVE 6-HR PERIODS; AND (3) TO INCREASE THE
QPF MAXIMA ABOVE WHAT A SIMPLE BLEND WOULD YIELD. SLIGHT RISKS
WERE INCLUDED ON THE DAY 2 AND 3 EXCESSIVE RAINFALL OUTLOOKS TO
ACCOUNT FOR THE POTENTIAL FOR ORGANIZED CONVECTIVE CLUSTERS/LINES
TO PRODUCE FLASH FLOODING.
..NORTHERN ROCKIES INTO NORTHERN PLAINS
A POTENT SHORTWAVE WILL FOCUS DCVA AND MID-UPPER LEVEL HEIGHT
FALLS ACROSS THE REGION, SPREADING FROM THE NORTHERN ROCKIES ON
THURSDAY, INTO THE NORTHERN PLAINS ON FRIDAY. GIVEN THE STRONG
FORCING AND ABOVE AVERAGE PWATS, IT IS UNSURPRISING THAT THE
MODELS ALSO SHOW A CLEAR SIGNAL FOR HEAVY RAIN. THIS IS
PARTICULARLY TRUE ON FRIDAY (DAY 3), AS THE HEIGHT FALLS ARRIVE IN
THE PLAINS AND REACH A CORRIDOR OF SIGNIFICANT CONVECTIVE
INSTABILITY. MODELS DEVELOP WIDESPREAD CONVECTION DURING THE
AFTERNOON AND THEN SPREAD IT EAST, LIKELY IN THE FORM OF ONE OR
MORE CONVECTIVE LINES. FOLLOWED A GENERAL BLEND OF THE ECMWF, GFS,
AND UKMET, BUT QPF MAXIMA WERE INCREASED TO MATCH OUR INTERNAL
BIAS-CORRECTED ENSEMBLE. GLOBAL MODELS CAN TEND TO UNDERESTIMATE
QPF IN CASES OF WIDESPREAD CONVECTION OR SIGNIFICANT CONVECTIVE
LINES. THE GFS AND ECMWF SHOW A LARGE QPF MAXIMUM IN NEBRASKA, AND
THIS MAKES SENSE. ON THE SOUTHERN PERIPHERY OF THE ADVANCING
TROUGH, FLOW AND STORM MOTIONS WILL BE WEAKER. ANY CONVECTIVE
CLUSTERS OR LINES MAY PROPAGATE MORE SLOWLY THAN FURTHER NORTH,
AND THIS MAY ALSO ALLOW THE CONVECTION TO TAKE ON AN INCREASING
WEST-EAST ORIENTATION WITH TIME (WHICH WOULD FAVOR HEAVIER RAIN).
A SLIGHT RISK WAS MAINTAINED IN MONTANA ON DAY 2, AND INTRODUCED
IN THE PLAINS ON DAY 3.
BURKE/LAMERS
GRAPHICS AVAILABLE AT WWW.WPC.NCEP.NOAA.GOV/QPF/QPF2.SHTML
CLICK HERE TO GO TO PREVIOUS BULLETINS.
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