Moderate to heavy rainfall continues to move from south to north across the Houston metro this morning with storm total accumulations approaching 30 inches. An additional 5 to 10 inches are probable by early afternoon – exceeding totals and flooding seen in Allison. This is becoming a biblical-level event and may become one of – if not the most devastating flood event the United States has seen in modern times. While Houston has seen catastrophic flooding this morning it is by far not the only location dealing with flooding. Record flooding on most streams will continue and only get worse. We may see a break in the heavy precipitation at times this afternoon in Houston, but heavy rains will return tonight. Off and on very heavy rains are expected through Wednesday – bringing an additional 20 to 30 inches of rain on top of what has already fallen – which is 25 to 30 inches. By the time this event is concluded it is probably some locations will have event totals of 55 to 60+ inches of rain. Very heavy rainfall and flooding is also expected across Southeast Texas, the Coastal Plains, Brazos Valley, and eastern sections of South-Central Texas over the next four days. We’ll have a more detailed forecast a bit later this morning when I have some relief arrive.

MESOSCALE PRECIPITATION DISCUSSION 0742
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
809 AM EDT SUN AUG 27 2017

AREAS AFFECTED…SOUTHEAST TX & SOUTHWEST LA

CONCERNING…HEAVY RAINFALL…FLASH FLOODING LIKELY

VALID 271208Z – 271808Z

SUMMARY…CATASTROPHIC FLOODING IN THE HOUSTON METROPOLITAN AREA
IS EXPECTED TO WORSEN AND COULD BECOME HISTORIC IN ASSOCIATION
WITH HARVEY, WITH POTENTIALLY SIGNIFICANT FLOODING ALSO EXPECTED
IN OTHER SATURATED AREAS OF SOUTHEAST TX. HOURLY RAIN TOTALS UP
TO 3″ WITH LOCAL AMOUNTS TO 7″ ARE EXPECTED, BRINGING LOCAL STORM
TOTAL ACCUMULATIONS TOWARD 35″ LOCALLY BY 18Z.

DISCUSSION…A COMBINATION OF DRIER/STABLE LOW LEVEL AIR HAS
INFILTRATED THE CENTER AND FORCED THE CONVECTIVE PATTERN AROUND
THE SYSTEM TO RESEMBLE A CONVECTIVE T-BONE STRUCTURE. A
SIGNIFICANT DEW POINT GRADIENT IS SHOWING UP SOUTH OF ITS CENTER
NEAR THE COAST OF MAINLAND TX — WEST OF PADRE ISLAND, EXTENDING
RIGHT UP THE COAST BEFORE CURLING NORTHWARD WEST OF ANGLETON TX.
MID-LEVEL DRY AIR IS IMPINGING ON ITS WESTERN SIDE — THERE IS THE
APPEARANCE OF SOME WESTERLY VERTICAL WIND SHEAR OVER HARVEY.
BANDS OF DEEP CONVECTION CONTINUE TO ROTATE INTO GALVESTON,
HOUSTON, AND AREAS NORTHEAST OF PALACIOS TX OVER 100 MILES FROM
THE CENTER OF HARVEY, WHOSE CENTER IS SLOWLY EDGING AWAY FROM THE
AREA. HOURLY RAIN TOTALS PER RECENT RADAR ESTIMATES HAVE FALLEN
TO ABOUT 3″, WHICH CORRESPONDS TO HARRIS COUNTY EMERGENCY
MANAGEMENT RAIN GAGE TOTALS. RAIN TOTALS OVER THE PAST 48 HOURS
HAVE REACHED 27″ IN SOUTHEAST HARRIS COUNTY, WITH AMOUNTS CLOSER
TO 12″ ACROSS NORTHERN AND WESTERN HARRIS COUNTY. VOLUME-WISE,
THIS HAS LIKELY REACHED THE RAINFALL THAT FELL DURING ALLISON IN
JUNE 2001, AND IT CONTINUES TO RAIN. BOTH THE ML CAPES AND MU
CAPES HAVE BEEN SLOWLY DECLINING, WHICH LIKELY EXPLAINS THE
REDUCING HOURLY RAIN RATES, WITH VALUES NEAR 1000 J/KG ALONG THE
UPPER TX COAST. INFLOW AT 850 HPA IS CYCLONIC, OUT OF THE
SOUTHEAST AT 35-45 KTS PER VAD WIND PROFILES. PRECIPITABLE WATER
VALUES ARE ~ 2.75″.

SPIRAL BANDS TEND TO MOVE AWAY FROM THE CENTER OF CONVECTIVE LOWS,
SO IT IS POSSIBLE THAT THE BANDING IN THE GALVESTON/HOUSTON AREA
COULD CONTINUE FOR SEVERAL MORE HOURS — FOR AS LONG AS THE STORM
IS DRIFTING SOUTHWARD. ANY EASTWARD DRIFT WOULD POTENTIALLY MOVE
THE HEAVIEST RAIN AXIS EASTWARD. MEANWHILE, TO THE NORTHEAST,
ISSUES ARE EXPECTED ACROSS THE GOLDEN TRIANGLE OF SOUTHEAST TX AND
IMPERIAL CALCASIEU AS RECENT RAINS HAVE LOWERED FLASH FLOOD
GUIDANCE VALUES AND CONVECTION FROM THE SLOWLY INCOMING BAND
OCCASIONALLY MERGES IN WITH THUNDERSTORMS ALONG AN EFFECTIVE
STATIONARY FRONT TO ITS EAST-NORTHEAST. RAINFALL HAS BEEN WELL
ABOVE AVERAGE IN SOUTHWEST LA AND DAM ISSUES REMAIN IN PORT ARTHUR
TX. THE MESOSCALE GUIDANCE SHOWS A SLOW RETREAT IN RAINFALL
RATES/AMOUNTS WHEN COMPARED TO OVERNIGHT AND EARLIER THIS MORNING,
BUT THE DAMAGE HAS BEEN DONE. THE MESOSCALE GUIDANCE INDICATES
LOCAL AMOUNTS TO 7″, WITH HOURLY RAIN TOTALS UP TO 3″ EXPECTED.
THIS WOULD ELEVATE THE CATASTROPHIC FLOOD EVENT INTO RECORD
TERRITORY FOR THE HOUSTON METROPOLITAN AREA.