Quantitative Precipitation Forecast
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FOUS30 KWBC 020757
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
256 AM EST FRI DEC 02 2016

...VALID 12Z FRI DEC 02 2016 - 12Z SAT DEC 03 2016...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 S KMZG 20 SSW KMZG 30 S NGP 25 NE LRD 20 SW UVA 25 ENE JCT
10 WNW 05F 25 SSW CRS 20 S PSN 20 ENE CXO 25 ENE BPT 15 N KCMB
20 SSE KCMB 20 WNW KEHC.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 NNW KMZG 2R8 25 NNW RBO 45 S SSF 15 SSE RND 15 E HYI
15 NE 62H 11R 30 WNW SGR 15 NNE EFD 15 SSE BPT 20 ESE KVBS KCRH
25 WSW KCRH.


...WESTERN GULF COAST TOWARD CENTRAL TX...

AMPLIFYING UPPER TROUGH DIGGING ACROSS THE SOUTHWEST REGION TODAY
WILL LEAD TO A MID-UPPER LEVEL CLOSED LOW OVER NORTHWEST MEXICO
LATER TONIGHT. BECAUSE OF THE MERIDIONAL AND EVENTUAL CUTOFF
NATURE OF THIS SOUTHERN STREAM TROUGH...THE EASTWARD PROGRESSION
OF THIS SYSTEM WILL BE QUITE SLOW THROUGH THE WEEKEND...THUS
SETTING THE STAGE FOR A FAIRLY SIGNIFICANT MULTI-DAY RAINFALL
EVENT ALONG THE WESTERN GULF COAST AND (POTENTIALLY) FARTHER
INLAND.

AS FOR THE DAY 1 FORECAST (12Z FRI-12Z SAT)...INCREASING UPPER
DIFFLUENCE AHEAD OF THE AMPLIFYING UPPER TROUGH WILL COUPLE WITH
THE RIGHT ENTRANCE REGION UPPER DIVERGENCE LATE IN THE PERIOD
(TONIGHT) ACROSS THE WESTERN GULF COAST...THOUGH GIVEN THE BREADTH
OF THE UPPER JET ACROSS THE CENTRAL U.S....THE MODELS ARE NOT WELL
CORRELATED IN TERMS OF THE PINPOINTING THE AREA THAT WILL BENEFIT
THE MOST FROM THE COMBINED FAVORABLE NORTHERN AND SOUTHERN STREAM
DYNAMICS. THIS IS PART OF THE REASON WHY WE CONTINUE TO SEE A
DISPARITY IN MODEL QPFS...INCLUDING HIGH RESOLUTION CAM
GUIDANCE...THOUGH SUSPECT THE OTHER PERHAPS MORE SIGNIFICANT CAUSE
IS DUE TO THE VARYING DISTRIBUTION/GRADIENT OF AVAILABLE
INSTABILITY PER THE MODELS FROM THE WESTERN GULF OF MEXICO INTO
SOUTH TEXAS. THE NAM AND GFS ARE FARTHER INLAND WITH MODEST
DEEP-LAYER CAPES (AOA 1000 J/KG)...AND AS SUCH IS WETTER WITH THE
QPF NW FROM THE COAST INTO THE TEXAS HILL COUNTRY. THE ECMWF
MEANWHILE IS HEAVIER OVER SOUTH-CENTRAL TX...MATCHING ITS MORE E-W
ORIENTED INSTABILITY AXIS. IN ANY EVENT...A ROBUST WARM CONVEYOR
BELT WILL ULTIMATELY ENSUE...TAPPING THE SUBTROPICAL PW PLUME AT
MID AND UPPER LEVELS WHILE ALSO DRAWING IN COPIOUS LOW LEVEL
MOISTURE FROM THE WESTERN GULF OF MEXICO. SOME OF THE HIGH-RES
GUIDANCE (ESPECIALLY NSSL-WRF) IN FACT DEPICT MULTIPLE CONVECTIVE
BANDS IN A SE-NW FASHION...INDICATIVE OF STRENGTHENING
FRONTOGENESIS WITH THE TRANSITION FROM UPRIGHT CONVECTION (HIGHER
CAPES AND NEGATIVE THETA-E LAPSE RATES) ALONG THE COAST TO
POTENTIALLY MORE SLANTWISE CONVECTION FARTHER INLAND. TOWARD THE
END OF THE PERIOD (12Z SAT)...PW ANOMALIES WILL HAVE INCREASED TO
2.5-3.0 STANDARD DEVIATIONS ABOVE NORMAL ACROSS SOUTHERN AND
CENTRAL TX...WHILE 850 MB MOISTURE FLUX ANOMALIES CLIMB BETWEEN +3
TO +4 OVER THE SAME AREAS AS THE 850 MB SOUTHERLY WINDS STRENGTHEN
TO 40+ KTS.

IF NOT FOR THE INSTABILITY...THE STRENGTHENING...DEEPENING
ISENTROPIC ASCENT WOULD FAVOR A SIGNIFICANT INLAND TRANSPORT OF
HEAVY RAINFALL. HOWEVER...BECAUSE OF THE TIGHT CAPE GRADIENT
ALONG-JUST OFFSHORE THE GULF COAST...WILL CONTINUE TO BE MORE
CONSERVATIVE WITH THE INLAND EXTENT OF THE >1.00 INCH 24 HOUR
AMOUNTS...AS EXPECT SOME DEGREE OF OBSTRUCTION. TO THAT END...WPC
FAVORED A BLEND OF CONTINUITY (PREVIOUS FORECAST) WITH A
COMBINATION OF THE 00Z HREF MEAN AND IN-HOUSE HIGH-RES ENSEMBLE
MEAN. THE RESULT WAS A SLIGHT INLAND EXTENSION OF THE HIGHER
AMOUNTS (0.50-1.00+ INCH) ACROSS SOUTH TX...THOUGH NOT TO THE
EXTENT OF THE GLOBAL MODEL CONSENSUS...WHILE MAINTAINING AN AXIS
OF HIGHEST TOTALS (1.5-2.0+ INCHES) ALONG THE CENTRAL AND UPPER TX
COAST.

IN TERMS OF THE DAY 1 EXCESSIVE RAINFALL OUTLOOK...WPC BROADENED
THE "MARGINAL" OR ISOLATED RISK AREA TO CAPTURE THE THE
DISTRIBUTION OF HIGHER TOTALS (2-3+ INCHES) FARTHER INLAND PER THE
NAM-CONEST...PARALLEL NAM-CONEST...WRF-ARW...AND NSSL-WRF. THE
"SLIGHT RISK" AREA WAS ALSO EXPANDED INLAND (THOUGH NOT TO THE
EXTENT AS THE MARGINAL RISK AREA)...ALONG THE AXIS OF HIGHER
DEEP-LAYER INSTABILITY AND ESSENTIALLY CAPTURING THE HIGH-RES
ENSEMBLE MEAN AMOUNTS ABOVE 1.5 INCHES (PER BOTH HREF AND SSEO
MEANS). THE LIMITING FACTOR...AT LEAST INITIALLY...WILL BE THE
RELATIVELY DRY ANTECEDENT SOILS WITH 3 HOURLY FLASH FLOOD GUIDANCE
VALUES OF AROUND 3 INCHES. EXPECT THE FFG VALUES TO COME DOWN
FOLLOWING THE INITIAL BATCH OF MOD-HEAVY RAINS LATER TODAY...AS
WOULD EXPECT THE MAIN RISK FOR SHORT-TERM RUNOFF ISSUES TO BE LATE
IN THE PERIOD (I.E. 00-12Z SAT).

HURLEY
$$





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