Quantitative Precipitation Forecast
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400
FOUS30 KWBC 151453
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1052 AM EDT SAT JUL 15 2017

...VALID 15Z SAT JUL 15 2017 - 12Z SUN JUL 16 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
35 SSW NYL 35 ENE IPL 20 ENE BLH 60 W LUF GYR 30 SSW PRC
30 NW PRC 15 N INW 55 ESE SJN 35 S ABQ 20 W ROW 10 NNW PEQ
25 SSW FST 130 S E38 90 SSE MMCU.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 ENE SVN 20 SSW WRB 45 WSW MGM 40 SSE PQL KMDJ 10 SW KCRH
30 E KRP 45 ESE COT 25 ESE UVA 30 WNW HYI 20 NE SEP 25 WSW RPH
LTS 10 SSW HHF 20 ENE CAO 55 NW TCC 15 SW SAF 10 NNW FMN
20 NE 4BL 20 NE TEX 10 WNW DEN 30 ENE AKO 20 NNW HYS 25 WSW ICT
35 NNW BVO 20 ENE GCM 15 NW RUE 20 NNW AWM 15 WNW DYR 10 N CIR
20 SW BWG 25 SE EKQ LNP 20 N BUY 10 E RZZ 15 NE JGG 15 ESE MTN
15 NNE DOV 20 ENE OXB 60 SE MFV 90 ENE HSE 45 ESE MRH 25 S SUT
35 S SUT 15 ENE SVN.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 NNW CAV 10 SSE AUM 10 SSE RGK MDZ 15 E SUE 30 ENE MKE
10 WNW GYY 25 ENE GBG 20 SW CID 10 S BNW 15 NNW CAV.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
75 SW MMCS 25 NNE DMN 60 NE TCS 15 SE SRR 35 SE GDP 10 S MRF
70 SW MRF 65 NNE MMCU.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 SW RTN 35 SSW VTP 20 WSW PUB 15 W MNH 35 WSW AKO 40 NNW ITR
50 WSW HYS 20 SSW END 15 ENE HHF 10 WNW EHA 30 SW RTN.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 WNW TUP 20 NNW MSL 55 ENE MKL 40 SE PAH CKV 20 NW CSV
25 NE OQT 15 S UKF 10 N JNX 9W7 45 ESE MQI 25 S MRH 15 WNW SUT
30 ENE CHS 30 WSW OGB 10 SE AHN 20 SW ANB 20 S TCL 20 WNW MEI
25 S GWO 30 WNW TUP.


...15Z UPDATE...

WPC WENT AHEAD AND MADE SOME MODIFICATIONS FROM THE EARLIER 09Z
ISSUANCE, IN COMBINING THE TWO MARGINAL THREATS ACROSS THE
SOUTHERN TIER OF THE COUNTRY ALONG WITH COMBINING TWO SLIGHT AREAS
FROM THE SOUTHERN TN VALLEY TO THE EASTERN CAROLINAS. WPC ALSO
ENLARGED THE SLIGHT RISK OVER CO EXTENDING SOUTH AND EAST TO THE
OK PANHANDLE AND INTRODUCED A MARGINAL THREAT ACROSS THE UPPER MS
VALLEY.

BASED ON ONGOING CONVECTIVE HEAVY RAINFALL INVOF BNA AND 12Z NAM
CONEST/00Z HREF MEAN SUGGESTING HEAVY RAINFALL POTENTIAL
CONTINUING ACROSS EAST CENTRAL TN TOWARD THE SOUTHERN APPALACHIANS
AND PERHAPS EXTENDING TOWARD THE EASTERN CAROLINAS FOR LATER THIS
AFTERNOON INTO THE EVENING, WPC WENT AHEAD COMBINED THE TWO AREAS
INTO A LARGE SLIGHT RISK WITH WEAK WAVE OF LOW PRESSURE RIDING
ALONG WELL DEFINED QUASI-STATIONARY FRONT WITH DEEP TROPICAL
MOISTURE REMAINING ENTRENCHED FROM THE TN VALLEY INTO THE
SOUTH/SOUTHEAST. 12Z RAOBS SUGGESTING 850 MB FLOW BEING 10 KTS OR
LESS ACROSS THE AREA, MEANING SLOW MOVING CONVECTIVE RAINS AND
LOCALIZED RUNOFF ISSUES.

BASED ON HI-RES SUITE FROM OVERNIGHT AND MORNING SCATTERED
CONVECTION OVER EASTERN OK INTO NORTHERN TX, EXPECT THIS NORTHERN
ACTIVITY TO POSSIBLE INTERACT WITH EXPECTED SEA BREEZE ACTIVITY
ACROSS SERN TX FOR LOCALIZED HEAVY RAINFALL AND AN INTRODUCTION TO
A MARGINAL THREAT.

ALSO EXPANDED THE SLIGHT RISK OF EXCESSIVE FROM CO TO WESTERN OK
BASED ON HI-RES GUIDANCE AND NICE CONVERGENCE AXIS SETTING UP FROM
THE SMALL CONVECTIVE CLUSTER EAST OF APX TO LBL AND GAG.

FINALLY INTRODUCED A MARGINAL THREAT IN CONJUNCTION WITH SPCS
SLIGHT RISK OF SEVERE ACROSS THE UPPER MS VALLEY. FLASH FLOOD
GUIDANCE REMAINS RELATIVELY LOW ACROSS THIS AREA FROM HEAVY RAINS
EARLIER IN THE WEEK. DESPITE THE STRONG SYNOPTIC SCALE FORCING AND
LIKELY INSTABILITY LATER TODAY, MOISTURE CONTENT WILL BE SOMEWHAT
LIMITED AND NARROW FOR WIDESPREAD HEAVY RAINS. HOWEVER LOCALIZED
ISSUES WILL BE POSSIBLE FROM CONVECTIVE ELEMENTS TRACKING NW TO SE
ACROSS THE UPPER MS VALLEY.

MUSHER



...MID SOUTH / ALABAMA / MISSISSIPPI...

WE ARE STUCK IN A REPEATING PATTERN IN WHICH DIURNAL THUNDERSTORMS
FORM ALONG THE GULF COAST SEA BREEZE AND SEPARATELY ALONG A WAVY
FRONTAL BOUNDARY IN THE LOWER MS / TN VALLEY BEFORE MEETING IN THE
MIDDLE. USING MOISTURE IN THE 700-500 MB LAYER TO TRACE THE
GREATER COVERAGE OF CONVECTIVE RAINFALL POINTS US TOWARD A
SATURDAY FORECAST THAT FAVORS THE 00Z GFS AND NSSL WRF...WITH THE
18-21Z MAXIMUM IN LAYER RELATIVE HUMIDITY ORIENTED FROM SOUTHWEST
MISSISSIPPI TO NORTHERN
ALABAMA / FAR NORTHWEST GEORGIA. COLLISION OF OUTFLOWS WITHIN A
SEASONABLY MOIST AND UNSTABLE ENVIRONMENT WILL SUPPORT ISOLATED
HEAVY RAINFALL TOTALS AND A RISK OF FLASH FLOODING...ESPECIALLY
OVER PARTS OF MS/AL WHERE THE WPC MANUAL AREAL AVERAGE QPF IS
MAXIMIZED.


...CAROLINAS...

SIMILAR TO THE GULF COAST STATES...CONVECTION IS LIKELY TO FORM
BOTH ALONG SEA BREEZES AND IN THE ELEVATED TERRAIN FARTHER
INLAND...AND THEN MEET. THERE IS A BIT MORE WESTERLY MID LEVEL
FLOW AND STRONGER SOUTHWESTERLY LOW LEVEL INFLOW HERE...WHICH MAY
GIVE COLLIDING THE TRAINING THUNDERSTORMS MORE STAYING POWER /
LONGEVITY...ALLOWING FOR SOME EXCESSIVE LOCAL RAINFALL TOTALS.
MODELS SHOW A CONSISTENT HEAVY RAINFALL SIGNAL FROM MYRTLE BEACH
UP THROUGH THE OUTER BANKS...AND GENERALLY EAST OF I-95.


...SOUTHWEST U.S. / SOUTHWEST TEXAS...

THERE IS A MULTI-MODEL SIGNAL FOR HEAVY RAIN IN THE EL PASO AREA
PERHAPS AS EARLY AS THE MORNING HOURS. THE CIRA LAYER PW SATELLITE
PRODUCT FOR 700-500 MB SHOWS A SLUG OF MOISTURE MIGRATING INTO
THAT REGION ALONG THE CONVERGENT EASTERN PERIPHERY OF A BROAD
EASTERLY WAVE MIGRATING THROUGH NORTHERN MEXICO. ONCE
THUNDERSTORMS FORM THEY WILL EXPERIENCE SOME ENHANCED
SOUTHEASTERLY INFLOW AT 850-700 MB TO SUPPORT REGENERATION AND
SMALL SCALE TRAINING ALONG OUTFLOWS...RESULTING IN A RISK OF HEAVY
RAINFALL AND FLASH FLOODING. THE DISTRIBUTION OF MOISTURE WILL
ALSO CONTINUE TO FAVOR SOUTHERN
ARIZONA FOR RELATIVELY ORGANIZED CONVECTION AND RISK OF FLASH
FLOODING ASSOCIATED MORE WITH THE SHORT TERM RAIN RATE AND QUICK
RESPONDING TERRAIN. WIND PROFILES ARE MORE FAVORABLE FOR TRAINING
AND PERHAPS MORE EXCEPTIONAL EVENT TOTAL RAINFALL AMOUNTS OVER
SOUTHERN NEW MEXICO AND SOUTHWEST TEXAS.


...COLORADO / KANSAS / NEW MEXICO / OKLAHOMA...

NOCTURNAL VEERING OF LOW LEVEL WINDS...THEN INDUCING WEAK WARM
ADVECTION...COULD LEAD TO INITIATION OF ELEVATED THUNDERSTORMS
OVER EASTERN OKLAHOMA THIS MORNING. IF THIS OCCURS STORMS WILL BE
SLOW MOVING AND A CAPABLE OF HEAVY RAIN...ALTHOUGH THE WIND SPEEDS
/ FORCING WILL BE VERY WEAK AND THE HRRR WAS TENDING TO KEEP THE
AREA DRY.

OF MORE CONCERN IS EASTERN COLORADO AND WESTERN KANSAS. WE WILL
HAVE TO WATCH THIS AREA DILIGENTLY FOR HEAVY RAINFALL. THERE ARE
SEVERAL FAVORABLE INGREDIENTS IN PLACE WITH AMPLE SUMMERTIME
MOISTURE COINCIDENT BENEATH A PERSISTENT UPPER LOW / SHEAR AXIS.
THE DEEP LAYER WIND SPEEDS...HOWEVER...WERE SO WEAK THAT THE
DIURNAL THUNDERSTORM ACTIVITY IN RECENT DAYS HAD TENDED TO BE VERY
SMALL SCALE AND NOT SUPPORTED MUCH AFTER DARK. ALL IT WOULD TAKE
IS A TIMELY ARRIVAL OF GREATER UPPER DIVERGENCE TO ENHANCE THIS
SMALL WEATHER SYSTEM AND ITS RAINFALL PRODUCTION. ALTHOUGH SUCH AN
OCCURRENCE IS NOT EXPECTED ON THE SYNOPTIC SCALE...THE HI-RES
MODELS ARE EVEN MORE EMPHATIC IN PRODUCING ORGANIZED HEAVY RAIN
OVER THE REGION SATURDAY AND SATURDAY EVENING. THE MODELS HAVE
BEEN STRUGGLING...OVER-FORECASTING THE QPF PROBABLY BECAUSE OF THE
SMALL SCALE OF THE INDIVIDUAL THUNDERSTORMS. WPC QPF MAY STILL BE
A LITTLE ON THE HEAVY SIDE OVERALL...BUT THERE IS POTENTIAL FOR
LOCALLY VERY HEAVY RAINFALL WHEREVER THE MORE PERSISTENT CELLS ARE
ABLE TO FORM...WITHIN SOUTHERN/EASTERN CO AND WESTERN KS. MARGINAL
RISK EXTENDS AROUND THE SOUTHERN PERIPHERY OF THE UPPER LOW...INTO
NEW MEXICO WHERE HEAVY SHORT TERM RAINFALL IS POSSIBLE...THOUGH
WIND PROFILES WILL BE LESS FAVORABLE FOR TRAINING HERE COMPARED TO
AREAS EAST OF THE COLORADO FRONT RANGE.


...SOUTHEAST TEXAS / LOUISIANA...

ALTHOUGH THE FRONTAL BOUNDARY IN THE SOUTHERN PLAINS WILL MOVE
VERY LITTLE...NORTHERLY DEEP LAYER FLOW IS EXPECTED TO INCREASE IN
STRENGTH AS AN INVERTED TROUGH / EASTERLY WAVE CONTINUES TO
MIGRATE WESTWARD ALONG THE GULF COAST. THIS SHOULD PROMOTE
THUNDERSTORMS PROGRESSING SOUTHWARD AND PRODUCING ANOTHER RELATIVE
MAXIMUM SWATH OF RAINFALL OVER EAST TX AND WESTERN LA. AS IN OTHER
PARTS OF THE SOUTH...SOME COLLISIONS OR FOCUS ALONG THE SEA BREEZE
COULD LEAD TO HEAVIER LOCAL TOTALS...ESPECIALLY THE UPPER TX COAST
AND SOUTHWEST LOUISIANA.

BURKE
$$





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