Quantitative Precipitation Forecast
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942
FOUS30 KWBC 232058
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
457 PM EDT FRI SEP 23 2016

...VALID 2057Z FRI SEP 23 2016 - 12Z SAT SEP 24 2016...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
CWZE 10 SSW CWTX 20 ENE OEO 10 SW GYL 25 SSW KBAC 25 E DIK
45 N RCA 25 SSW IEN SNY 25 S IBM 30 WSW IBM 30 W IBM BRX
10 WSW SIB 30 NNE DRC 20 NNE TMH 15 W CPR HLD 10 SSE WRL
50 S COD LND 40 SW BVR 10 E BPI 45 N FIR 40 SSW 77M BYI IDA
40 E DLN 30 NNE BZN 3HT 40 SE LWT 35 NE MLS 30 W ISN 25 NNE CWDJ.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
80 SSE CHS 10 WNW CHS 15 ESE FLO ISO 20 SSE EDE 10 SSW HSE
70 SE MRH.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 ESE 2R8 25 SSW VCT 3T5 30 WNW UTS 20 SW JAS 15 NNW KCRH.


21 UTC SPECIAL UPDATE...

UPDATES TO THE PREVIOUS OUTLOOK INCLUDED THE REMOVAL OF THE SLIGHT
RISK AREA ACROSS IOWA.  RAINFALL CONTINUES TO LIFT NORTH OF THE
STATE WITH NO ADDITIONAL SIGNIFICANT ACCUMULATIONS EXPECTED
THROUGH THE REMAINDER OF THE PERIOD.  ALSO INTRODUCED A MARGINAL
RISK ALONG THE UPPER AND MID TEXAS COAST INTO SOUTHEAST TEXAS.
SLOW MOVING CELLS/HIGHLY EFFICIENT RAINFALL PROCESSES MAY PRODUCE
LOCALLY HEAVY AMOUNTS...RESULTING IN LOCAL RUNOFF CONCERNS.
ELSEWHERE...MADE SOME MINOR ADJUSTMENTS BASED IN PART ON RECENT
RUNS OF THE HRRR AND THE 12Z SSEO MEAN.

PEREIRA

PREVIOUS DISCUSSION...

...IOWA AND ADJACENT SOUTHWEST MINNESOTA AND NORTHWEST ILLINOIS...

WHILE MEAN UPPER SHORTWAVE RIDGING WAS BECOMING
ESTABLISHED...FORCING HAD REMAINED SUFFICIENT OVERNIGHT TO SUPPORT
THUNDERSTORMS WITHIN RELATIVELY WEAK YET CONVERGENCE LOWER TO
MIDDLE LEVEL FLOW CROSSING A SHARP BAROCLINIC ZONE THAT HAD BEEN
REINFORCED BY PREVIOUS OUTFLOWS. THE AXIS OF ASCENT HAD SLID JUST
SOUTH OF THE AREAS IMPACTED BY SEVERE FLOODING IN NORTHEAST
IOWA...BUT ORGANIZED SLOW MOVING ECHOES HAD PERSISTED ACROSS
CENTRAL IOWA. OUR THINKING FOLLOWS CLOSELY TO THE 00Z
WRF-ARW...AND IS CONSISTENT WITH ISENTROPIC PLOTS BASED ON THE
NAM...WHICH INDICATE ASCENT ON THE 305 K SURFACE SLIDING OUT OF
CENTRAL IA TOWARD NW IL THIS MORNING...WHILE ADDITIONAL ASCENT
BECOMES FOCUSED BACK UP TOWARD THE EASTERN DAKOTAS AND SOUTHWEST
MINNESOTA AS SOUTHERLY FLOW FORMS IN RESPONSE TO HEIGHT FALLS
AHEAD OF THE WESTERN U.S. TROUGH. MUCH OF THIS WOULD BE CONSIDERED
A MARGINAL THREAT...BUT GIVEN ONGOING FLOODING IN IOWA...WE HAVE
INCLUDED A SLIGHT RISK. THAT SLIGHT RISK MAY BE RELATIVELY SHORT
LIVED AS SHORTWAVE RIDGING WILL BECOME MORE FIRMLY ESTABLISHED
OVER TIME.


...WESTERN U.S. INTO THE NORTHERN PLAINS...

AT 15Z WE UPDATED TO TRIM BACK THE MARGINAL RISK OVER NORTHERN
UTAH...WHERE RAINFALL HAD BECOME FULLY STRATIFORM WITH OBSERVED
RATES WELL UNDER A HALF INCH PER HOUR. MODELS INDICATE VERY LITTLE
CHANCE OF ANY SUBSTANTIAL INSTABILITY REFORMING HERE THIS
AFTERNOON AS THE UPPER LOW MOVES ON TOWARD THE EAST-NORTHEAST AND
NORTHERN UTAH EXPERIENCES SOME WRAP-AROUND MID LEVEL MOISTURE /
CLOUDINESS. FARTHER SOUTH AND EAST...SOME EMBEDDED CONVECTION WILL
BE POSSIBLE OVER SOUTHERN UTAH AND PARTS OF COLORADO. THIS AREA ON
THE SOUTHEAST SIDE OF THE WESTERN U.S. CYCLONE WILL EXPERIENCE AN
INCREASE OF WESTERLY WIND COMPONENT AND DRIER MID LEVEL
AIR...WHICH SHOULD PROMOTE FORWARD PROGRESSION OF ANY
CONVECTION...AND GENERALLY TOLERABLE RAIN RATES...ALTHOUGH RAPID
RUNOFF IN ANY PARTICULARLY SUSCEPTIBLE MOUNTAIN LOCATIONS CANNOT
BE RULED OUT.

ELSEWHERE...MARGINAL RISK IS MAINTAINED AS DEEP LAYER ASCENT
EXPANDS INTO MORE OF WY/MT AND ADJACENT PARTS OF THE HIGH PLAINS.
PREVIOUS DISCUSSION FOLLOWS...

AN IMPRESSIVE GREAT BASIN / ROCKIES STORM WILL CONTINUE TO UNFOLD
THROUGH FRIDAY NIGHT. NEGATIVE 500 MB HEIGHT ANOMALIES OF UP TO 3
STANDARD DEVIATIONS WILL SLIDE EAST WITH THE CENTER OF THE
DEVELOPING CLOSED CIRCULATION. THE SYSTEM REMAINS
PROGRESSIVE...BUT THE PROGRESSION IS EXPECTED TO BE SO SLOW...AND
WITH A PIVOT OCCURRING OVER THE CENTRAL GREAT BASIN AND CENTRAL
ROCKIES TODAY...ADDITIONAL MOD-HVY AMOUNTS OF RAIN AND HIGH
ELEVATION SNOW ARE EXPECTED. PRECIPITATION EFFICIENCY IS ENHANCED
BY THE INVOLVEMENT OF ANOMALOUS DEEP LAYER MOISTURE HAVING BEEN
PUMPED INTO THE REGION OUT OF THE SUB-TROPICS PRIOR TO ARRIVAL OF
THE NORTHERN STREAM TROUGH...AND STEEP LAPSE RATES OWING TO THE
DEPTH OF THE TROUGH / STRENGTH OF THE DEEP LAYER LIFT. PW
ANOMALIES OF 4+ STANDARD DEVIATIONS ABOVE NORMAL WILL SHIFT INTO
THE NORTHERN HIGH PLAINS DURING THE PERIOD...ALONG WITH THE REGION
OF FAVORABLE UPPER DIFFLUENCE AHEAD OF THE HIGHLY-AMPLIFIED UPPER
TROUGH/CLOSED UPPER LOW. THE MAIN INHIBITOR FOR EXCESSIVE
SHORT-TERM (1-3 HOURLY) RAINFALL RATES...IN ADDITION TO THE UPPER
SYSTEM BECOMING MORE PROGRESSIVE WITH TIME...WILL BE THE LACK OF
DEEP LAYER INSTABILITY AS MUCAPES AVERAGE LESS THAN 1000 J/KG.
MAXIMUM AREAL-AVERAGE QPF OF 1-1.25" IS NOTED... HOWEVER PER THE
HIGH-RES CAM GUIDANCE...LOCALIZED ADDITIONAL AMOUNTS BETWEEN
1.5-2.5" WERE NOTED. THEREFORE...WILL MAINTAIN A MARGINAL RISK FOR
EXCESSIVE RAINFALL IN THE DAY 1 EXCESSIVE RAINFALL OUTLOOK (ERO)
AS EXPECT ANY SHORT-TERM RUNOFF ISSUES TO BE BE ISOLATED.


...LOWER MID ATLANTIC COAST...

BROAD LOW-MID LEVEL CYCLONIC (ONSHORE) FLOW WILL MAINTAIN DEEP
MOISTURE THROUGH THE FIRST HALF OF THE DAY 1 PERIOD (FRIDAY)...AS
PWS REMAIN BETWEEN 1.75-2.00 INCHES. THE WEAK UPPER LEVEL SUPPORT
(MID LEVEL DEFORMATION) AND INFLUX OF ADDITIONAL SUBTROPICAL BANDS
OFF THE SOUTHEAST COAST WILL BRING ADDITIONAL LOCALLY HEAVY
RAINFALL TO AREAS ALONG THE NORTHERN SC/SOUTHERN NC COASTS THAT
HAVE SEEN 3-6" OF RAIN OVER THE PAST FEW DAYS. WPC UTILIZED MORE
OF THE HIGH-RESOLUTION MODELS OVER THIS REGION EARLY IN THE PERIOD
(ESPECIALLY THROUGH 18Z FRI)...GIVEN THE BETTER HANDLE ON THE
MOIST CONVECTIVE ENVIRONMENT. THE MODEL SIGNAL SUPPORTS A MARGINAL
RISK OF EXCESSIVE RAINFALL...WITH LOCALIZED TOTALS UP TO 1.5 TO
2.0 INCHES.

BURKE/HURLEY
$$





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