522
FXUS04 KWBC 020957
QPFPFD
QUANTITATIVE PRECIPITATION FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
557 AM EDT SAT JUN 02 2018
FINAL DAY 1, DAY 2 AND DAY 3 QPF DISCUSSION
VALID JUN 02/1200 UTC THRU JUN 05/1200 UTC
REFERENCE AWIPS GRAPHICS UNDER...PRECIP ACCUM - 24HR
..EASTERN U.S.
SLOW-MOVING UPPER TROUGH WILL NUDGE EASTWARD TOWARD THE ATLANTIC
COAST TODAY. WILL CONTINUE TO SEE LITTLE CHANGE IN THE OVERALL
PATTERN UNTIL THIS DEEP, COMPACT TROUGH PUSHES OFF THE COAST.
GIVEN THE ORIENTATION, THE MODELS CONTINUE TO INDICATE THE MORE
FOCUSED AREA OF FORCING (DPVA, UPPER DIVERGENCE) ACROSS THE MID
ATLANTIC REGION DURING THE PEAK HEATING HOURS THIS AFTERNOON INTO
THE EVENING. THUS EXPECT MORE WIDESPREAD CONVECTIVE COVERAGE TODAY
COMPARED TO YESTERDAY ACROSS THIS REGION, WHERE MUCH RAIN HAS
FALLEN OVER THE PAST SEVERAL WEEKS AND FFG VALUES ARE QUITE LOW AS
A RESULT OF THE WET ANTECEDENT CONDITIONS. LIGHT DEEP-LAYER
FLOW/LOW 0-6 KM BULK SHEAR (GENERALLY AOB 10 KTS) WOULD AGAIN BE A
HUGE DETERRENT FOR MORE ORGANIZED, PROLONGED CONVECTION; HOWEVER,
GIVEN THE LOW FFG VALUES (ESPECIALLY WEST OF I-95), PERSISTENT
BROAD-SCALE FORCING, AMPLE/DEEP SUBTROPICAL MOISTURE, AND
SURFACE-BASED CAPES LIKELY PEAKING BETWEEN 1000-2000 J/KG PRIOR TO
THE ONSET OF CONVECTION, BELIEVE THE HOURLY OR EVEN SUB-HOURLY
RAINFALL RATES WILL EXCEED THE 1 HOURLY FFGS OVER MANY AREAS, AS
DEPICTED BY THE SSEO AND HREF PROBABILITIES. THUS PER
COLLABORATION WITH WFOS AKQ, LWX, AND PHI, HAVE HOISTED A MODERATE
RISK FOR FLASH FLOODING ACROSS A LARGE PORTION OF THE MID
ATLANTIC, ENCOMPASSED BY SLIGHT AND MARGINAL AREAS. IN TERMS OF
THE QPF, WPC AGAIN RELIED HEAVILY ON THE HIGH-RES MEANS IN THIS
AREA, ESPECIALLY THE HREF.
..CENTRAL-NORTHERN PLAINS AND UPPER MIDWEST
MID/UPPER LEVEL TROUGH ACROSS THE NORTHERN PLAINS EARLY THIS
MORNING WILL BECOME MORE NEGATIVELY TILTED DURING THE DAY 1
PERIOD, AS THE MORE CONFLUENT UPPER FLOW ACROSS THE CENTRAL
PLAINS-MID MS VALLEY CAUSES THE SOUTHERN PERIPHERY OF THE VORT
LOBE (TROUGH BASE) TO TAKE OFF A BIT FASTER COMPARED TO THE
REMAINDER OF THE TROUGH. UPPER LEVEL FORCING WILL BE BOLSTERED BY
THE NEGATIVE TILT (INCREASED DIVERGENCE ALOFT AND MORE FOCUSED
LOW-MID LAYER QS CONVERGENCE), HOWEVER THE MODEST-AT-BEST DEEP
LAYER INSTABILITY WILL PRECLUDE THE EXCESSIVE RAINFALL THREAT TO
SOME DEGREE OVER AREAS THAT ARE EXPECTED TO BENEFIT FROM THE
STRONGEST DYNAMICS (INCLUDING MN-WI). FARTHER SOUTH, ONGOING MCS
ACROSS THE LOWER MO VALLEY WILL REMAIN PROGRESSIVE EARLY THIS
MORNING, THE UNDERGO DECAY AFTER 12Z FOLLOWING THE DIMINISHING
NOCTURNAL LLJ. RAINFALL AMOUNTS INTO THE MID MS VALLEY AND WESTERN
OH VALLEY WILL BE MORE MODEST ON AVERAGE, I.E. AVERAGING UNDER
1.0", THOUGH PER THE INDIVIDUAL HIGH-RES CAMS, LOCALIZED TOTALS OF
2-3+ INCHES WILL REMAIN POSSIBLE AS THE AXIS OF FAVORABLE
THERMODYNAMICS (CAPES OF AT LEAST 1000-2000 J/KG AND ANOMALOUS PWS
OF 1.5-1.75") ALONG AND AHEAD OF THE SURFACE FRONT PUSHES W-E
ACROSS THE AREA. WPC QPF WAS BASED LARGELY FROM THE HIGH-RES
MEANS, ESPECIALLY THE HREF PROBABILITY-MATCHED MEAN IN PROVIDING
MORE DETAIL WITH THE AREAS MORE LIKELY TO SEE HEAVIER AMOUNTS.
..LOWER MISSISSIPPI VALLEY
ANOTHER MORE FOCUSED AREA OF MODERATE-HEAVY RAINFALL WILL ALIGN
GENERALLY N-S/NNE-SSW ALONG THE PROGRESSIVE COLD FRONT, WITH
VIGOROUS DEEP-LAYER INSTABILITY OWING TO THE STEEP MID-LEVEL LAPSE
RATES AND PW VALUES MAXING OUT BETWEEN 1.75-2.0". WPC ALSO
UTILIZED A BLEND OF THE HIGH-RES CAM GUIDANCE IN THIS REGION WITH
THE QPF, ESPECIALLY WITH THE HREF MEAN, IN GENERATING
AREAL-AVERAGE TOTALS UNDER 1.0" FOR MOST. WOULD ALSO EXPECT
LOCALIZED TOTALS OF 2-3+ INCHES IN THIS AREA AS WELL; HOWEVER,
GIVEN THE SWIFT PROGRESSION (AIDED BY THE STRONG DOWNDRAFT CAPE
AND RESULTANT CONVECTIVE OUTFLOWS), THE EXCESSIVE RAINFALL THREAT
WILL REMAIN SOMEWHAT MITIGATED (I.E. MARGINAL).
..SOUTHEAST
THE MODELS, ESPECIALLY THE HIGH-RES CAMS, SHOW A HEAVY RAINFALL
SIGNAL ACROSS PORTIONS OF THE GULF AND SOUTHEAST COASTS FROM
SOUTHEAST MS EASTWARD ALONG AND NORTH OF I-10 THROUGH FAR NORTHERN
FL, SOUTHERN AL-GA, AND FAR SOUTHERN SC. THIS AREA WILL LIE WITHIN
A COL REGION TODAY, AS A MID LEVEL SHEAR AXIS (FAR SOUTHERN
PERIPHERY OF THE EAST COAST TROUGH) TRAVERSES EVER SO GRADUALLY
ACROSS THE AREA IN BETWEEN THE UPPER RIDGE AXES W AND E. THE HIGH
CAPE/LOW SHEAR ENVIRONMENT WILL ONCE AGAIN FAVOR PULSE-TYPE
CONVECTION DURING THE PEAK HEATING HOURS THIS AFTERNOON INTO EARLY
EVENING; HOWEVER THE WEAK UPPER LEVEL FORCING DOES IMPLY MORE
COVERAGE EARLIER AND LATER IN THE DAY COMPARED TO CLIMATOLOGY,
ESPECIALLY GIVEN THE LOW CONVECTIVE TEMPERATURES. SIMULATED
REFLECTIVITY FORECASTS PER THE CAMS SHOW THE MOST WIDESPREAD
CONVECTIVE EXTENT BETWEEN 17-23Z, BOLSTERED BY THE DAYTIME HEATING
ALONG WITH DEVELOPING GULF AND ATLANTIC BREEZES WHICH WILL HELP
FOCUS THE LOW-LEVEL MOISTURE CONVERGENCE. ONE-HOURLY FFG ACROSS
THIS REGION IS BETWEEN 2.5-3.0"; GIVEN THE DEEP SURFACE-BASED
INSTABILITY (CAPES IN UNTAPPED AIRMASS AVERAGING 2500-3500 J/KG
DURING PEAK HEATING) ALONG WITH THE SUBTROPICAL MOISTURE (PWS
1.75-2.0"), LOCALIZED HOURLY RATES APPROACHING OR EVEN EXCEEDING
FFG ARE ANTICIPATED. AS A RESULT, A MARGINAL RISK WAS INCLUDING
OVER THIS REGION IN THE DAY 1 ERO.
DAY 2
..GREAT LAKES TO CENTRAL APPALACHIANS
STRENGTH AND TIMING OF A NEGATIVELY TILTED TROUGH ARE SLIGHTLY OFF
AMONG THE 00Z GUIDANCE, THOUGH CLOSE ENOUGH TO ALLOW QPF TO BE
BASED ON THE 00Z GFS/NAM/ECMWF. THE HIGHEST DAY 2 QPF IS OVER
ONTARIO.
..GULF COAST
THE FRONT PROGRESSES SOUTH SUNDAY UNTIL REACHING THE GULF OF
MEXICO. DYNAMICS ARE NOT AS STRONG ALONG THIS PORTION OF THE FRONT
BUT INSTABILITY IS MUCH GREATER. QPF WAS RAISED FOR COASTAL GULF
WITH ISOLATED HEAVY AMOUNTS STILL POSSIBLE WHERE CONVECTION
BECOMES ORGANIZED PER A BLEND OF THE 00Z GFS/NAM/ECMWF.
...SOUTHERN TO CENTRAL ROCKIES..
AN UPPER TROUGH MOVES EAST FROM THE FOUR CORNERS TO THE CO/KS
BORDER SUNDAY. THIS SYSTEM WILL HELP DRAW INCREASING PW VALUES
NORTHWARD THROUGH THE SOUTHERN TO CENTRAL ROCKIES AND SOUTHERN
HIGH PLAINS. THIS WILL SUPPORT INCREASING CONVECTION ACROSS THESE
AREA SUNDAY AFTERNOON IN A REGION OF FAIRLY WELL DEFINED UPPER
DIFLUENCE AHEAD OF THESE HEIGHT FALLS. QPF WAS BASED ON A BLEND OF
THE 00Z NAM/GFS/ECMWF.
..MID-ATLANTIC
A MID-LEVEL SHORTWAVE TROUGH CROSSING THE SOUTHERN GREAT LAKES
SUNDAY WILL DRAW IN A REMNANT TROUGH OVER THE MID-ATLANTIC. THIS
WILL MAINTAIN SURFACE LOW PRESSURE OVER EASTERN NC AND MAINTAIN
ONSHORE FLOW OVER THE CENTRAL MID-ATLANTIC UNTIL THE COLD FRONT
FROM THE GREAT LAKES CROSSES SUNDAY NIGHT. STABLE ONSHORE FLOW
WILL BE IN A TROPICAL PLUME OF MOISTURE MAKING FOR LOCALLY HEAVY
RAIN. QPF BASED A BLEND OF THE 00Z NAM/GFS/ECMWF WHICH IS A LITTLE
FASTER THAN THE FRIDAY EVENING GUIDANCE. THE MARGINAL RISK OF
EXCESSIVE RAINFALL WAS MAINTAINED OVER THE ENTIRE MID-ATLANTIC.
DAY 3
..SOUTHERN NEW ENGLAND
MID-LEVEL LOW PRESSURE SPINS OVER ONTARIO/QUEBEC MONDAY. TROPICAL
MOISTURE IS LIFTED INTO NEW ENGLAND WITH GOOD DYNAMICS. LITTLE TO
NO INSTABILITY LIMITS EXCESSIVE RAIN RISK TO A DAY 3 MARGINAL RISK
OF EXCESSIVE RAINFALL. QPF BASED ON A BLEND OF THE 00Z
NAM/GFS/ECMWF WHICH WERE IN GOOD AGREEMENT FOR SOUTHERN NEW
ENGLAND.
HURLEY/JACKSON
GRAPHICS AVAILABLE AT WWW.WPC.NCEP.NOAA.GOV/QPF/QPF2.SHTML
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