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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1049Z Nov 21, 2017)
 
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Abbreviations and acronyms used in this product
 
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
549 AM EST TUE NOV 21 2017

...VALID 1049Z TUE NOV 21 2017 - 12Z WED NOV 22 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 S CWLY 20 WNW CWPR 30 SSW CWPR 25 NW KS52 20 WSW KS52
20 ENE SMP 45 WSW YKM 35 SE TTD 10 ESE KLS 15 SSE GRF RNT AWO
10 S CWWK CYYJ CWQK 10 ESE CLM 25 ESE CLM 10 WNW PWT 10 NE SHN
10 W OLM 20 SSW OLM 10 WSW TDO 10 WSW KLS 20 WNW SPB 15 W HIO
10 WNW MMV 20 SW MMV 20 NE ONP 10 N ONP 20 NW ONP 45 WNW ONP.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
45 NW P69 COE 10 S SZT 30 SE SZT 35 WSW MSO 45 SSW MSO 20 E P69
45 NW P69.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 SE KXIH 30 SSW BPT 30 N BPT 20 SSW DRI 10 ESE LCH 20 SW KVNP.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 ESE FLL FLL 25 WNW BCT 20 SW SUA 10 SW FPR 25 E VRB.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 WSW UIL 20 SW CWSP 10 W CLM 15 SSE CLM 20 NW PWT 15 N SHN
SHN 20 WSW SHN 10 ENE HQM HQM 25 W HQM 35 W HQM.


...PACIFIC NORTHWEST...

THE MOST RECENT IR IMAGES SHOWED A PLUME OF DEEP MOISTURE
STRETCHING FROM NEAR 20N 140W TO A POSITION OFF WELL WEST OF THE
NORTHERN CA COAST. THE BLENDED TPW PRODUCT SHOWED PRECIPITABLE 
WATER VALUES NEAR 2.00 INCHES IN THE HEART OF THE PLUME. AS MID
LEVEL RIDGING BUILDS ALONG CA COAST DURING DAY 1...THE DEEPEST
MOISTURE ASSOCIATED WITH THE ATMOSPHERIC RIVER IS SHUNTED TOWARD 
THE PACIFIC NORTHWEST COAST. A 45 TO 65 KNOT LOW LEVEL SOUTH
SOUTHWEST TRANSPORTS 1.25/1.50 INCH PRECIPITABLE WATER AIR (WHICH
IS BETWEEN TWO AND THREE STANDARD DEVIATIONS ABOVE THE MEAN) TO 
THE WA COAST...MAINLY BETWEEN 21/18Z AND 22/00Z (DURING THE BEST
DIVERGENCE IN THE LEFT FRONT QUADRANT OF A 110 KNOT JET STREAK
APPROACHING THE WA COAST TOWARD 22/00Z). 
 
THE LOW LEVEL FLOW TARGETS THE OLYMPIC RANGE IN WA DURING THIS
TIME PERIOD...AND THERE IS A MULTI MODEL SIGNAL FOR 5.00+ INCH
AMOUNTS OVER THE HIGHER TERRAIN. IN FACT...THE 00Z HREF MEAN AND 
00Z NBM SHOWED LOCAL 6.00+ INCH AMOUNTS...AND GIVEN THE STRENGTH
OF THE LOW LEVEL INFLOW...THESE AMOUNTS SEEM PLAUSIBLE. HOURLY
RAINFALL RATES OVER 0.50 INCHES ARE EXPECTED ACROSS THE OLYMPIC 
RANGE BETWEEN 21/18Z AND 22/03Z. A SLIGHT RISK AREA WAS PLACED
OVER THE OLYMPIC RANGE FOR DAY 1. 
 
FURTHER EAST...THE LOW LEVEL FLOW FOCUSES THE DEEPER MOISTURE ON
THE WA CASCADES...WITH LOCAL 2.50 TO 4.50 INCHES EXPECTED OVER THE
CENTRAL AND NORTHERN PORTIONS OF THE RANGE. SOME OF THE HIGH 
RESOLUTION GUIDANCE (SUCH AS THE 00Z NAM CONUS NEST...00Z HREF
MEAN AND THE 00Z NBM) SHOWED LOCAL 6.00+ INCH AMOUNTS OVER THE
SOUTHERN WA CASCADES...BUT THESE AMOUNTS SEEM HIGH...BASED ON THE 
ORIENTATION OF THE LOW LEVEL FLOW. SNOW LEVELS RISE DURING THE
STRONG SOUTHWEST FLOW...SO A MARGINAL RISK WAS PLACED OVER THE WA
CASCADES FOR DAY 1. 


...NORTHERN ROCKIES...

THE MOISTURE...WHILE NOT AS ROBUST AS IT IS EXPECTED TO BE OVER
WESTERN WA...REMAINS AT LEAST PARTIALLY INTACT ACROSS THE NORTHERN
ROCKIES DURING THE SECOND HALF OF DAY 1. UPSLOPE FLOW (AUGMENTED
BY LOW LEVEL CONVERGENCE ALONG A QUASI STATIONARY BOUNDARY OVER
EASTERN MT INTO WESTERN AND CENTRAL WY) AND SYNOPTIC SCALE ASCENT
MAKE THE MOST OF THE MOISTURE IN THE COLUMN TO PRODUCE LOCAL 1.25
TO 1.75 INCH QPF AMOUNTS ACROSS THE NORTHERN BITTERROOTS IN ID AND
THE ROCKY MOUNTAIN FRONT RANGE IN MT. SOME OF THE HIGH RESOLUTION
GUIDANCE...INCLUDING THE 00Z NAM CONUS NEST/00Z WRF ARW AND THE
00Z HREF MEAN...INDICATED QPF AMOUNTS BETWEEN 2.00 AND 3.50 INCHES
(HIGHEST IN THE HREF MEAN). WHILE THERE SHOULD BE MARGINAL
INSTABILITY IN THE COLUMN (DUE PRIMARILY TO STEEPENING LAPSE
RATES)...IT DOES NOT APPEAR AS THOUGH THE INSTABILITY WILL BE
SUFFICIENT TO SUPPORT 3.00+ INCH RAINFALL AMOUNTS.

SNOW LEVELS RISE IN THE WARM AIR ADVECTION IN THE SOUTHWEST
FLOW...SO MOST OF THE QPF IS EXPECTED TO FALL IN THE FORM OF RAIN.
THREE HOUR FLASH FLOOD GUIDANCE VALUES ARE AS LOW AS 1.00 INCHES
ACROSS THE NORTHERN BITTERROOTS...SO...AFTER COLLABORATING WITH
WFOS OTX/MSO...A MARGINAL RISK AREA HAS BEEN PLACED OVER THE
NORTHERN BITTERROOT RANGE FOR DAY 1. 


...TX/LA...

AS A COLD FRONT DROPS SOUTH ACROSS CENTRAL AND SOUTH TX DURING DAY
1...A LOW LEVEL SOUTH SOUTHWEST FLOW TRANSPORTS 1.25/1.50 INCH
PRECIPITABLE WATER TO UPPER TX COAST AND SOUTHWEST LA...WHERE 00Z
GFS MODEL SOUNDINGS SHOWED LOCAL 500-1000 J/KG OF MLCAPE NEAR THE
COAST. THE RESULTING CONVECTION IS EXPECTED TO PRODUCE 0.25 TO
0.50 INCHES OF QPF OVER THE UPPER TX COAST AND NEARBY SOUTHWEST 
LA...BEFORE THE FRONT MOVES OFFSHORE AFTER 22/00Z. WHILE THE
TIMING OF THE FRONT IS FAIRLY CONSISTENT AMONG 00Z MODEL
SOLUTIONS...THE AMOUNT OF INFLOW (AND ITS POSITION AHEAD OF THE
FRONT) VARIES BY A FAIR AMOUNT. SOME OF THE HIGH RESOLUTION
SOLUTIONS (SUCH AS THE 00Z NAM CONUS NEST...00Z NBM AND THE 00Z
WRF ARW) SHOWED AXES OF 2.00+ INCHES OF QPF SPANNING FROM THE
UPPER TX COAST TO WEST OF NEW ORLEANS BEFORE THE FRONT EXITS. 
 
THE COMBINATION OF MOISTURE AND INSTABILITY SUGGEST THESE ARE
PLAUSIBLE. THE PREPONDERANCE OF HIGH RESOLUTION GUIDANCE TARGETS
THE TX/LA BORDER. THREE HOUR FLASH FLOOD GUIDANCE VALUES ARE
FAIRLY HIGH (GENERALLY ABOVE 3.00 INCHES). HOWEVER...THE MOST
RECENT RUNS OF THE HRRR HAVE SHOWN AN UPTICK WITH RESPECT TO QPF
ALONG THE TX/LA BORDER...WITH THE LATEST RUN SHOWING THE POTENTIAL
FOR LOCAL 5.00+ INCH AMOUNTS. THIS RUN...IN CONJUNCTION WITH SOME
OF THE 00Z HIGH RESOLUTION GUIDANCE...SUGGESTS THERE IS A LOW END
FLASH FLOOD THREAT IN THIS AREA. BASED ON THE ABOVE...A MARGINAL
RISK WAS PLACED OVER THE FAR UPPER TX COAST AND NEARBY SOUTHWEST
LA FOR DAY 1.


...FL...

CONVECTION RIDING ALONG A SLOW MOVING WARM FRONT WILL MOVE NORTH
WITH THE BOUNDARY THROUGH THE MORNING HOURS. THE CONVECTION IS
TAPPING MUCAPE VALUES BETWEEN 1000-1500 J/KG (HIGHEST JUST
OFFSHORE) AND A PLUME OF 1.50 INCH PRECIPITABLE WATER AIR. HOURLY
RAINFALL RATES OF 1.50 INCHES WERE OBSERVED WITH THE CONVECTION
EARLIER THIS MORNING ACROSS MIAMI-DADE AND BROWARD COUNTIES. AS
THE CONVECTION MOVES NORTHWARD WITH THE FRONT...THE 850-300 MB
MEAN FLOW SHOULD KEEP THE DEEPEST CONVECTION NEAR THE COAST OR
JUST OFFSHORE...WHERE LOCAL 2.00 TO 3.00 INCHES OF RAINFALL IS
POSSIBLE THROUGH ABOUT 17Z. THIS SCENARIO IS SUPPORTED BY RECENT
HRRR RUNS...WHICH KEEP THE HIGHEST RAINFALL AMOUNTS CLOSE TO THE
FRONT...JUST OFFSHORE.

THESE RAINFALL RATES COULD RESULT IN URBAN FLOODING NEAR THE
INTERSTATE 95 CORRIDOR THROUGH THE LATE MORNING HOURS. SINCE THREE
HOUR FLASH FLOOD GUIDANCE VALUES ARE BETWEEN 4.00 AND 5.00
INCHES...A MORE ORGANIZED FLASH FLOODING WOULD REQUIRE TRAINING OF
THE CONVECTION WITH THE FRONT.
AS SUCH...A MARGINAL RISK AREA WAS PLACED HERE TO ACCOUNT FOR THE
CURRENT CONVECTION...AND ITS EXPECTED NEAR TERM MOVEMENT.

HAYES