Quantitative Precipitation Forecast
Issued by NWS

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013
FOUS30 KWBC 030101
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
800 PM EST FRI DEC 02 2016

...VALID 01Z SAT DEC 03 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 NW KMIU 25 SE NQI 10 W BKS 50 SSE MMNL 45 SSW MMNL
25 NW MMNL 45 NW MMNL 25 SW MMPG 35 SSW DRT 30 N DLF 15 ENE JCT
10 ESE 05F 35 SW PSN UTS 15 WSW JAS 20 NE CWF 15 W 7R4 25 S KVNP.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 E 2R8 2R8 15 NNE CRP 25 NNW RKP 10 SSE 5R5 10 NW HOU
35 ENE IAH 20 N BPT 20 WSW LCH 7R5 20 SW KCMB.


01Z UPDATE...

GIVEN THE TRENDS OVER THE LAST SEVERAL HOURS IN THE HRRR
GUIDANCE...WHICH SHOWS NOT AS MUCH WIDESPREAD HEAVY RAIN AT LEAST
OVERNIGHT FOR SOUTHEAST TX...THE SLIGHT RISK AREA WAS ADJUSTED AND
PRIMARILY CONFINED TO THE IMMEDIATE CENTRAL/UPPER TX COAST REGION.
THIS AREA AFTER 06Z SHOULD SEEN A DEVELOPMENT/EXPANSION OF
CONVECTION ALONG AND JUST NORTH OF A STRENGTHENING FRONTAL ZONE.
AN INCREASING SOUTHEAST LOW LEVEL JET WILL INCREASE MOISTURE
TRANSPORT/FLUX TOWARD THE COAST AND ALLOW A STRONGER INSTABILITY
GRADIENT WHICH IS WELL OFFSHORE TO BEGIN ENCROACHING ON THE AREA.
HRRR-TLE GUIDANCE STRONGLY SUGGESTS A NOTABLE UPTICK IN CONVECTION
AFTER 06Z...WITH RAINFALL RATES OF 1 TO 2 INCHES/HR POSSIBLE AS
CONVECTION FINALLY ORGANIZES. SOME POSSIBLE TRAINING OF CONVECTION
NEAR AND JUST INLAND OF THE COAST MAY CAUSE SOME ENHANCE RUNOFF
POTENTIAL.

MEANWHILE...MADE SOME MODEST ADJUSTMENTS TO THE MARGINAL RISK AREA
AND ESPECIALLY FROM NEAR THE TX HILL COUNTRY SOUTH TO THE RIO
GRANDE WHERE SOME EXPANSION OF THE COVERAGE AREA WAS MADE. THE
LATEST HRRR/HRRR-TLE GUIDANCE SUPPORTS AN EXPANSION OF HEAVIER AND
MORE DYNAMICALLY ENHANCED RAINS GIVEN THE COLLOCATION OF
STRENGTHENING 250 MB RIGHT-ENTRANCE REGION JET DYNAMICS AHEAD OF
THE MAIN UPPER TROUGH OVER NORTHWEST MEXICO WITH MUCH STRONGER LOW
LEVEL MOISTURE TRANSPORT AIMING UP THE LOWER RIO GRANDE VALLEY.
SOME ISOLATED RUNOFF PROBLEMS WILL BE POSSIBLE.

ORRISON

PREVIOUS DISCUSSION...

...TEXAS AND SOUTHWEST LOUISIANA...

AN 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 WILL COMBINE 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 CLUSTERED IN TERMS OF THE
PINPOINTING THE AREA THAT WILL BENEFIT THE MOST FROM THE COMBINED
NORTHERN AND SOUTHERN STREAM DYNAMICS. THIS IS PART OF THE REASON
WHY WE CONTINUE TO SEE A DISPARITY IN MODEL QPFS...INCLUDING HIGH
RESOLUTION GUIDANCE. WE SUSPECT THE OTHER...PERHAPS MORE
SIGNIFICANT...CAUSE IS 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 THEY FEATURE
WETTER SOLUTIONS NORTHWARD FROM THE COAST INTO THE TEXAS HILL
COUNTRY. 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. TRENDS IN THE HRRR...HOWEVER...INDICATE
THAT THE NSSL WRF AND OTHER HI-RES MODELS ARE NOT TO BE TRUSTED
IMPLICITLY WITH THE TIMING AND PLACEMENT DETAILS. THE EARLY
CONVECTIVE BANDS MAY OCCUR FARTHER OFFSHORE THROUGH THIS
AFTERNOON...WITH GREATER COVERAGE OF CONVECTION DEVELOPING NEAR
THE COAST AND INLAND DURING THE OVERNIGHT PERIOD. DEEP LAYER
ASCENT WILL BECOME MAXIMIZED WELL INLAND...WHERE MODERATELY HEAVY
RAIN RATES ARE EXPECTED. MORE INTENSE RAIN RATES ARE POSSIBLE
WITHIN THE INSTABILITY GRADIENT NEAR THE COAST...BUT IT REMAINS TO
BE SEEN WHETHER CONVECTION CAN GROW UPSCALE THERE...IF THE MAIN
FORCING MECHANISM IS BROAD WARM ADVECTION...REMAINING SOMEWHAT
DISPLACED FROM GREATER MID AND UPPER LEVEL SUPPORT FARTHER NORTH.
WHEREVER ORGANIZED CONVECTION TAKES HOLD...HOWEVER...THE
ENVIRONMENT WILL FAVOR INTENSE RAIN RATES...AS 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.

THE SLIGHT RISK CAPTURES AREAS IN WHICH THE HIGH-RES ENSEMBLE MEAN
AMOUNTS ARE GREATER THAN 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/BURKE
$$





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