600
FXUS04 KWBC 291057
QPFPFD
QUANTITATIVE PRECIPITATION FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
656 AM EDT TUE MAY 29 2018
FINAL DAY 1, DAY 2 AND DAY 3 QPF DISCUSSION
VALID MAY 29/1200 UTC THRU JUN 01/1200 UTC
REFERENCE AWIPS GRAPHICS UNDER...PRECIP ACCUM - 24HR
DAY 1
..ALBERTO / SOUTHEAST U.S. / OHIO-MISSISSIPPI CONFLUENCE
THE ALBERTO TRACK FORECAST WAS GENERALLY DOING WELL, AND NOT
CHANGING IN ANY DRAMATIC WAY. THE INHERITED QPF HAD THIS SYSTEM
HANDLED PRETTY WELL - WE MERELY ENHANCED AMOUNTS JUST A BIT NEAR
THE CENTER AS IT TRACKS TOWARD WESTERN TENNESSEE, BUT WE ALSO DID
TREND A LITTLE WESTWARD WITH THE QPF SWATH THERE. THE HI-RES
MODELS, IN PARTICULAR, WANT TO DEFLECT THE MORE INTENSE RAINFALL
WEST OF THE CIRCULATION CENTER. THIS SEEMS A LITTLE
COUNTER-INTUITIVE, ALTHOUGH WITH SOUTHERLY SHEAR THE STRONGER
ECHOES CAN ADVECT NORTH OF THE CENTER AND GET DRAWN BACK TO THE W
OR SW. WE HANDLED THE TREND BY USING 40 PERCENT CONTINUITY AND 20
PERCENT EACH OF THE HREF BLENDED MEAN, 00Z NAM, AND WRF-NMMB. THE
NMMB HAS DONE WELL THIS SPRING, AND REPRESENTED A LESS STARK
CONTRAST TO THE CONTINUITY FORECAST.
ELSEWHERE THERE IS CONCERN THAT THE CONFLUENT MOIST INFLOW SOUTH
OF THE CIRCULATION WILL OVERLAP WITH AREAS IN ALABAMA WHICH
EXPERIENCED THE CORE RAINS ON MONDAY, LEADING TO PARTICULARLY HIGH
2-DAY TOTALS WHICH MAY WARRANT MAINTENANCE OF A MODERATE RISK OF
EXCESSIVE RAINFALL THERE. MODERATE RISK WILL LIKELY BE INCLUDED UP
ALONG THE CENTER TRACK - PENDING COLLABORATION WITH THE FIELD, AND
ALSO MAINTAINED OVER THROUGH THE UPSLOPE AREAS IN THE CAROLINAS.
THE QPF IN THE HIGHER TERRAIN DID NOT CHANGE MUCH, BUT WE DID
SPLIT THE PRECIPITATION OVER CENTRAL AND EASTERN NORTH CAROLINA AS
THE CONVEYOR BELT IS SHED FROM THE CYCLONE, AND MODEL QPFS TRENDED
DOWNWARD.
..CENTRAL U.S
THE MCS IN NORTHWEST OKLAHOMA WILL HAVE A HARD TIME MAKING
EASTWARD PROGRESS INTO AN AREA OF LARGE INHIBITION THIS MORNING.
EXPECT THE NEXT ROUND TO HAVE MORE SUCCESS TUESDAY AFTERNOON AS
THE WESTERN U.S. TROUGH PUNCHES OUT INTO A NEGATIVE TILT AND ACTS
TO COOL THE MID LEVELS NORTH OF I-40. WPC EXPECTS AN MCS TRACK
SOMEWHERE ACROSS CENTRAL TO SOUTHEAST KS AND NORTHERN OK TUE
NIGHT, LEANING ON THE GFS, ECMWF, AND WRF-ARW. WE SPREAD GENEROUS
COVERAGE OF CONVECTIVE QPF NORTHWARD ALONG THE INSTABILITY AXIS TO
MINNESOTA, AND ALSO WITHIN THE SECONDARY FRONTAL ZONE THAT HANGS
BACK OVER THE HIGH PLAINS AND DAKOTAS. OVERALL THE QPF IN THIS
REGION LEANED TOWARD THE GFS, WRF-ARW, AND WRF-NMMB. THE CANADIAN
GEM REGIONAL ALSO GAVE SUPPORT.
DAYS 2/3...
..GREAT LAKES, OHIO VALLEY, AND MID ATLANTIC
WHATEVER SURFACE LOW THAT REMAINS FROM ALBERTO WILL ADVANCE FROM
THE OHIO VALLEY INTO THE GREAT LAKES ON WEDNESDAY, EVENTUALLY
GETTING ABSORBED IN BROADER PRESSURE FALLS ASSOCIATED WITH A
TROUGH ADVANCING EAST FROM THE NORTHERN PLAINS. THIS SHOULD ALLOW
THE QPF PATTERN TO TRANSITION FROM A MORE FOCUSED CORRIDOR OF
HEAVIER RAINFALL DURING THE DAY ON WEDNESDAY, TO BROADER MODERATE
AMOUNTS BY WEDNESDAY NIGHT BEFORE THE LOW CONTINUES PUSHING
NORTHEAST INTO CANADA ON THURSDAY. QPF GENERALLY FOLLOWED A BROAD
MODEL BLEND WITH ALBERTO AND ITS EVENTUAL REMNANTS, WITH GREATEST
WEIGHT PLACED ON THE GFS AND ECMWF.
AWAY FROM THE TRACK OF THE CENTER OF ALBERTO, RAIN SHOULD BE FAR
MORE CONVECTIVE IN NATURE. WITH A LACK OF A STRONG FOCUSING
MECHANISM, BUT A BROAD AREA OF TROPICAL MOISTURE, THERE SHOULD BE
SCATTERED CONVECTION OVER MOST OF THE REGION. THE FOCUS WAS
PUTTING IN A BROAD AREAL AVERAGE OF ABOUT 0.25 TO 0.50 INCHES OF
RAIN OVER MOST OF THE REGION, AND A BROAD MODEL BLEND GENERALLY
DELIVERED THOSE AMOUNTS WITH SOME MINOR ADJUSTMENTS.
WITH A LACK OF A STRONG LOW-LEVEL FOCUSING MECHANISM (OTHER THAN
ALBERTO'S CENTER), THE EXCESSIVE RAINFALL OUTLOOK FOR DAY 2 SHOWS
A BROAD SLIGHT RISK FOR MOST OF THE REGION. ALTHOUGH RAIN MAY BE
FOCUSED MORE ALONG THE TRACK OF THE LOW, FLASH FLOOD GUIDANCE IS
HIGHER IN THOSE AREAS SUGGESTING THAT ANTECEDENT CONDITIONS MAY
PREVENT MORE WIDESPREAD FLOODING. THERE IS CONCERN ABOUT FLASH
FLOOD POTENTIAL IN THE BROAD WARM SECTOR TO THE EAST (OH, WV, VA)
WHERE CONVECTIVE INSTABILITY SHOULD BUILD THROUGH THE DAY ON
WEDNESDAY WITH ANOMALOUSLY HIGH PRECIPITABLE WATER VALUES AROUND
OR ABOVE THE 99TH PERCENTILE FOR LATE MAY AND EARLY JUNE. SUCH
ENVIRONMENTS TYPICALLY SUPPORT VERY HEAVY RAIN RATES WITH
CONVECTION, AND THE POTENTIAL FOR MORE SIGNIFICANT EPISODES OF
FLASH FLOODING. HOWEVER, GIVEN THE LACK OF DISTINCT FOCUSING
MECHANISMS, PINPOINTING THE HIGHEST THREAT AREAS IS SOMEWHAT
UNCERTAIN AT THE MOMENT. THEREFORE, A SLIGHT RISK IS MAINTAINED
ACROSS THIS REGION WITH THE POSSIBILITY OF AN UPGRADE AT A LATER
TIME IF A MORE CONCENTRATED AREA OF CONVECTION SEEMS LIKELY.
PLEASE REFER TO THE NATIONAL HURRICANE CENTER FORECASTS FOR THE
LATEST INFORMATION ON ALBERTO.
..SOUTHEAST AND FLORIDA PENINSULA
THE PORTION OF THE SOUTHEAST TO THE EAST OF 86-87W LONGITUDE
SHOULD REMAIN WITHIN A PLUME OF DEEPER TROPICAL MOISTURE WITH
PWATS GENERALLY ABOVE 1.8 INCHES. THESE ARE LESS ANOMALOUS
RELATIVE TO CLIMATOLOGY THAN POINTS FURTHER NORTH, BUT SHOULD
STILL BE SUPPORTIVE OF HEAVY RAIN RATES AND LOCALIZED FLASH
FLOODING. A MARGINAL RISK WAS EXTENDED DOWN INTO PARTS OF THE
REGION FOR THE DAY 2 EXCESSIVE RAINFALL OUTLOOK. RAINFALL AMOUNTS
ARE GENERALLY CLOSE TO THE NBM, BUT WERE ENHANCED IN THE 18-06Z
TIME FRAME DOWN THE SPINE OF THE FLORIDA PENINSULA AND UP INTO
SOUTH GEORGIA WHERE CONVERGING SEA BREEZE BOUNDARIES IN THE VERY
MOIST ENVIRONMENT MAY SUPPORT A GREATER CONCENTRATION OF
CONVECTION.
..CENTRAL PLAINS INTO THE OZARKS
MODELS SHOW AN EXPANDING LOW-MID LEVEL THERMAL RIDGE INTO THE
SOUTHERN PLAINS OVER THE NEXT SEVERAL DAYS, WITH SPORADIC QPF
ALONG THE PERIPHERY OF THE ELEVATED MIXED LAYER FROM EASTERN
COLORADO, THROUGH KANSAS, AND INTO THE OZARKS AND MID MISSISSIPPI
VALLEY REGIONS. FROM A PATTERN RECOGNITION STANDPOINT, THIS SEEMS
REASONABLE AND ALSO PROVIDES GREATER THAN NORMAL UNCERTAINTY IN
THE QPF. MODELS CAN STRUGGLE WITH THE TIMING OF MCSS AS THEY ROLL
IN A GENERALLY EASTERLY DIRECTION AROUND THE PERIPHERY OF A RIDGE,
AND ALSO STRUGGLE WITH HOW MUCH LATITUDE THEY CAN LOSE AS THEY
MATURE AND PROPAGATE FURTHER INTO THE HIGHER THETAE AIR MASS.
GENERALLY FOLLOWED A COMPROMISE OF THE ECMWF AND GFS IN TERMS OF
TIMING AND POSITION OF QPF MAXIMA IN THIS REGION, WITH A MARKED
INCREASE IN NOCTURNAL QPF RELATIVE TO THE NBM.
..NORTHERN ROCKIES
AN AMPLIFIED TROUGH PUSHING INTO THE NORTHWEST AND EVENTUALLY THE
NORTHERN ROCKIES SHOULD LEAD TO INCREASINGLY WIDESPREAD
CONVECTION, ESPECIALLY BY THE DAY 3 PERIOD FROM IDAHO INTO
MONTANA. SLIGHTLY DIFFERENT TIMING PRODUCES DIFFERENT RESULTS IN
THE QPF WITH CONVECTION INITIATING ON TERRAIN FEATURES AND
LOW-LEVEL BOUNDARIES AT DIFFERENT TIMES. GENERALLY FOLLOWED A
BLEND OF THE GFS AND ECMWF IN THIS REGION, AND WEIGHTED THE QPF
MAXIMA TOWARD THE GFS IN MAGNITUDE GIVEN THE ECMWF TYPICAL DRY
BIAS. GIVEN THE VERY WET ANTECEDENT CONDITIONS ACROSS THE REGION,
FLASH FLOODING WILL BE A THREAT, AND A SLIGHT RISK WAS INTRODUCED
ON DAY 3 ACROSS MONTANA.
BURKE/LAMERS
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