Quantitative Precipitation Forecast
Issued by NWS

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FOUS30 KWBC 200832
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
431 AM EDT SUN AUG 20 2017

...VALID 12Z SUN AUG 20 2017 - 12Z MON AUG 21 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 N IMT 25 NNE ISQ 20 NW PLN 15 NNW TVC 10 NNE RFD UIN
15 NNE AIZ 30 SSW SZL MCI 25 SW AFK ORC FKA 15 ENE STE 30 N IMT.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
40 N MMH 30 SSE BIH 30 NW IYK 30 SE PTV 30 NE VIS 45 ENE MCE
45 ESE MHR 10 W BLU 15 E TVL 40 N MMH.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
60 NNE MMHO 25 S OLS 45 ENE DMA 40 W SOW 25 NE SJN 35 SSE FMN
30 SW TEX 30 SW GUC 35 SSE MYP 30 E CCU 10 SW MNH 25 S PUB
35 NW CAO 15 W DHT 45 ENE TCC 25 ESE ATS 30 N MRF 100 SSW MRF.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 WSW BIH 45 S BIH 35 ENE VIS 35 SW MMH 35 W MMH 55 WNW MMH
45 SSE TVL 40 SSE TVL 30 NNW MMH 15 WSW BIH.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 SSW ICL 30 W DSM 10 ENE IOW MLI 10 WSW GBG 30 W UIN
15 ENE DMO 25 NE LXT 20 SSW ICL.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
90 NE MMHO 55 SSW DUG 25 N DUG 10 S SAD 40 S SOW 10 S SOW
30 SE SJN 40 SSW GNT 25 NNE GNT 35 NNW 4SL 20 ESE E33 25 NE SKX
45 N TCC 10 E TCC 25 N ROW 15 ENE GDP 65 ENE MMCU.


LOWER MO VALLEY INTO THE MID AND UPPER MS VALLEY AND UPPER GREAT
LAKES

MCS/EMERGING MCV ACROSS EASTERN NE-WESTERN IA DURING THE PRE-DAWN
HOURS SUNDAY WAS NOT CAPTURED WELL BY THE ECMWF...AS THE CONSENSUS
AMONG THE OTHER MODELS IS FOR A EASTERLY TRACK OF THE MCV ACROSS
NORTHERN IL BY SUN EVENING BEFORE DAMPENING CONSIDERABLY AS IT
MIGRATES INTO THE UPPER OH VALLEY. WPC UTILIZED PREDOMINATELY
NSSL-WRF AND WRF-ARW2 HIGH-RES CAM GUIDANCE WITH THE QPF FOR THE
INITIAL MCS...WITH THE HEAVIEST ACTIVITY MOVING SOUTHEAST INTO
NORTHWEST MO SUNDAY MORNING -- PARALLEL TO THE 850-300 MB
THICKNESS AND ALONG THE ELEVATED INSTABILITY GRADIENT.

THE MODELS ONCE AGAIN ADVERTISE ADDITIONAL MCS DEVELOPMENT SUNDAY
NIGHT-EARLY MONDAY...HOWEVER ALSO DEPICT DISTINCT LATITUDINAL
DIFFERENCES WITH THE QPF MAXIMA...WITH THE HIGH-RES CONSENSUS
GENERALLY FARTHER SOUTH OF THE GLOBAL MODELS (AS IS
TYPICAL...THOUGH SOME WITH A BI-MODEL DISTRIBUTION OF HEAVIER
AMOUNTS). SUBTLE RIGHT-ENTRANCE REGION UPPER JET FORCING WOULD
FAVOR A NORTHERN GENESIS REGION BETWEEN 00-06Z MON (I.E. IA INTO
NORTHERN MO)...HOWEVER GIVEN THE STRENGTHENING SW-WSW LOW LEVEL
JET/INCREASING UPWIND PROPAGATION OVERNIGHT...THE MODELS SHOW
MBE/CORFIDI VECTORS VEERING MORE NORTHERLY ~10-15 KTS OVERNIGHT.
AS A RESULT...WPC OPTED TO REMAIN SOUTH OF THE GLOBAL CONSENSUS
WITH THE STRIPE OF HEAVIEST QPF FROM 00-12Z MON...CLOSER TO A
BLEND OF THE WRF-ARW/PARALLEL ARW RUNS ALONG WITH THE PARALLEL
HREF MEAN. ONE CAVEAT: THE NSSL-WRF AND WRF-ARW2 ARE EVEN FARTHER
SOUTH (INTO KC METRO)...WHICH IS CERTAINLY PLAUSIBLE GIVEN THE
SOUTHWARD PROPAGATION POTENTIAL AND POOL OF UNTAPPED DEEP-LAYER
INSTABILITY...AND WILL AT LEAST TO SOME DEGREE DEPEND ON HOW FAR
SOUTH THE EFFECTIVE SURFACE FRONTAL BOUNDARY GETS WITH THE WEAK
~700 MB CAPPING IN PLACE. AS A RESULT...SUBSEQUENT FORECASTS MAY
CONTINUE A SOUTHWARD TREND. THE "SLIGHT" RISK AREA IN THE DAY 1
ERO REFLECTS AREAS THAT WILL MOST LIKELY BE IMPACTED BY BOTH
MCSs... I.E. SOUTHERN IA INTO NORTHERN MO.


SIERRA NEVADA / AZ-NM INTO SOUTHERN CO

A SHORTWAVE TROUGH WILL REMAIN ACROSS CA SUN-SUN NIGHT...WHILE THE
COMPACT WARM CONVEYOR BELT (WCB) ON THE EASTERN FLANK WILL
CONTINUE TO IMPORT MONSOONAL MOISTURE ACROSS EASTERN AZ AND THE
SOUTHERN ROCKIES. HIGHEST MOISTURE ANOMALIES WILL BE WITHIN THIS
QUASI-STATIONARY WCB...WITH PWAT VALUES AVERAGING 2-2.5 STANDARD
DEVIATIONS ABOVE NORMAL ACROSS MUCH OF NM. MEANWHILE...THE MODELS
CONTINUE TO DEPICT A MESO/CONVECTIVELY-ENHANCED VORT MAX ALONG THE
SOUTHERN AZ-NM BORDER EARLY SUNDAY LIFTING SLOWLY N DURING THE
DAY...THEN MORE NORTHEAST ACROSS NORTH-CENTRAL AND NORTHEAST NM
INTO SOUTHEAST CO SUNDAY NIGHT. GIVEN THE FAVORABLE DYNAMICAL AND
THERMODYNAMICAL SETUP...THERE CONTINUES TO BE A MULTI-MODEL SIGNAL
FOR OROGRAPHICALLY-ENHANCED HEAVY RAINFALL ACROSS FAR WESTERN TX
AND SOUTHERN NM INTO THE SOUTHERN ROCKIES. THE GUIDANCE CONSENSUS
(INCLUDING THE HIGH-RES MEANS) SHOW A FAIRLY WIDESPREAD...SOMEWHAT
ATYPICAL SWATH OF 0.25-0.50" OF AREAL-AVERAGE RAINFALL
TOTALS...WITH SOME OF THE CAMS INDICATING FAIRLY PRODIGIOUS
SMALLER-SCALE TOTALS (2-4"). AS A RESULT...WPC WILL CONTINUE TO
HIGHLIGHT A MORE ENHANCED (I.E. "SLIGHT") RISK OF EXCESSIVE
RAINFALL OVER MUCH OF NM INTO SOUTHERN CO WITH THE NEW DAY 1 ERO.

ACROSS THE SIERRA NEVADA...MODEST DEEP-LAYER INSTABILITY (MUCAPES
1000-1500 J/KG) ALONG WITH ANOMALOUS MOISTURE (PWAT VALUES ~2
STANDARD DEVIATIONS ABOVE NORMAL)...DIFLUENT UPPER FLOW/PERIODIC
DPVA...AND OROGRAPHIC ENHANCEMENT WILL FAVOR MORE WIDESPREAD
DIURNAL CONVECTION ALONG THE SIERRA...PARTICULARLY THE WESTERN
SLOPES. PER THE HIGH-RES GUIDANCE...LOCAL AMOUNTS BETWEEN 1-2+
INCHES ARE ANTICIPATED.

HURLEY

$$





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