Quantitative Precipitation Forecast
Issued by NWS

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FOUS30 KWBC 211918
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
317 PM EDT FRI AUG 21 2015

...VALID 1917Z FRI AUG 21 2015 - 12Z SAT AUG 22 2015...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...



...GULF COAST TO THE SOUTHEAST...

MULTICELLULAR CLUSTERED CONVECTION WILL CONTINUE TO RESULT IN
LOCALLY HEAVY RAINFALL ALONG THE TX/LA GULF COAST INTO THIS
EVENING...WITH MORE SCATTERED CONVECTION FARTHER EAST (EASTERN
GULF COAST INTO THE SOUTHEAST) WHERE THE UPPER LEVEL FORCING
(CONVECTIVELY-ENHANCED VORT/SHEAR AXIS) IS NOT AS ROBUST AS WITH
ALONG THE WESTERN GULF COAST. MODERATE INSTABILITY REMAINS
AVAILABLE ALONG THE COAST INTO THE SOUTHEAST...WHERE MIXED-LAYER
CAPES ARE NOW BETWEEN 2000-3000 J/KG. HIGH PWS (2.00-2.25
INCHES)...DEEP-LAYER INSTABILITY (EQUILIBRIUM LEVELS CLOSE TO 50
KFT) AND FREEZING LEVELS OVER 15 KFT WILL MAKE FOR HIGHLY
EFFICIENT WARM RAIN RATES...WITH THE MCV/VORT LOBE HELPING TO KEEP
THE CONVECTION MORE ORGANIZED (NUMEROUS CLUSTERS) OVER SOUTHERN LA
INTO EARLY THIS EVENING. AT THE SAME TOKEN...0-6KM BULK SHEAR
BETWEEN 15-20 KTS WITH WEAKENING/LESS ORGANIZED LOW-LEVEL INFLOW
WITH THE DIURNAL (DIFFERENTIAL) HEATING SHOULD LEAD TO MORE
PULSE-TYPE ACTIVITY WHICH WOULD KEEP THE INTENSE RAINFALL RATES
SHORT-LIVED. NEVERTHELESS...GIVEN THE CURRENT FFG (AROUND 3 TO 4
INCHES IN 3 HOURS)...BASED ON SOME OF THE LATEST
CONVECTIVE-ALLOWING MODEL (CAM) OUTPUT SUCH AS THE
WRF-ARW/NMMB...SPC-WRF...AND HRRR AS WELL AS EXPERIMENTAL
HRRR...COULD SEE ISOLATED AREAS OF EXCEEDANCE AND THUS LOCALIZED
RUNOFF ISSUES INTO THE AFTERNOON.


...SOUTHERN PLAINS EAST-NORTHEAST TOWARD THE MID MS VALLEY...

THE MODELS CONTINUE TO SHOW A DECENT MOISTURE RETURN OVERNIGHT
ALONG THE TAIL END OF TRAILING SURFACE FRONTAL BOUNDARY...WHICH
WILL BEGIN TO MAKE A JOG NORTHWARD AS A WARM FRONT. THE SPREAD
WITH THE MODEL QPFS REMAIN HIGH... GIVEN THE LESS-THAN-ROBUST
UPPER LEVEL FORCING AND MODEST THERMODYNAMIC PARAMETERS. THE
GUIDANCE DOES SHOW AN UPTICK IN THE S-SW LLJ OVERNIGHT (TO 30+
KTS) INTO THIS AREA...WITH ELEVATED MUCAPES ON THE ORDER OF
1000-2000 J/KG AND PW VALUES INCREASING TO 1.50-1.75 INCHES.
MEANWHILE THERE IS SOME UPPER LEVEL ENERGY TO WORK WITH...WITH A
SHORTWAVE CURRENTLY MOVING EASTWARD THROUGH THE CENTRAL/SOUTHERN
HIGH PLAINS. THIS WAVE SHOULD HELP TO FOCUS THE LOW LEVEL
INFLOW/MOISTURE CONVERGENCE ALONG AND NORTH OF THE SURFACE
BOUNDARY...HOWEVER THE COMPACT/LOW AMPLITUDE CONFIGURATION OF THIS
SHORTWAVE SHOULD LEAD TO A SMALLER-SCALE MCS. GIVEN THE
ANTICIPATED SCALE ALONG WITH THE SPREAD IN THE CAM GUIDANCE...HAVE
NOTED A "SEE TEXT" IN THE EXCESSIVE RAINFALL OUTLOOK OVER THIS
REGION...AS EXPECT ANY AREAS OF FFG EXCEEDANCE (ANY RUNOFF ISSUES)
TO BE ISOLATED AT MOST.

HURLEY
$$





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