Quantitative Precipitation Forecast
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FOUS30 KWBC 021455
QPFERD

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

...VALID 15Z 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
70 WSW KHHV 40 SE KOPM 30 SE BKS 25 NE LRD 20 W UVA 30 ENE JCT
10 ENE 05F 30 S CRS 35 NNW UTS 10 N JAS 20 NNE CWF 15 N KVNP
30 NNE KVQT 15 W KGHB 25 SW KGBK.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 SE KMZG 15 NNW KMIU 15 NW RBO 45 SSE SSF 15 SSE HYI 62H
15 SE 11R 10 WSW IAH 30 ENE IAH 15 SE BPT 15 SE KVBS 25 SSW KCRH
25 NE KEMK.


...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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