Quantitative Precipitation Forecast
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689
FOUS30 KWBC 271452
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
952 AM EST MON FEB 27 2017

...VALID 15Z MON FEB 27 2017 - 12Z TUE FEB 28 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 SW CHD 35 W GYR 60 WNW LUF 20 SSE PRC 30 SW SOW 50 SE SOW
70 SSE SJN 50 NNW SVC 45 NW SVC 30 NE SAD 20 WNW SAD 40 W SAD
40 E CGZ 15 SW CHD.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 S NUC 30 E NUC 25 W NFG 15 S RIV 10 E RIV 15 WNW PSP
15 SSE PSP 10 S TRM 20 SSE TRM 25 S TRM 30 S TRM 30 NE CZZ
20 SW NJK 70 S NJK 130 SSE CZZ 155 S MMTJ 155 SSW NRS
140 SSW NRS 115 SW NRS.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 SSE JAN 25 WSW MEI 35 SE NMM 50 W MGM 15 NE TOI 25 SSW OZR
15 SW HRT 25 SE PQL 30 S GPT 10 SSW AUD 30 WNW MSY 10 NE BTR
40 SE ESF 15 WNW HEZ 35 SSW HKS 30 SSE JAN.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 SW NRS 10 SW NZY NKX 10 NW RNM 20 NE RNM 25 E RNM 20 NE CZZ
45 SE CZZ 75 SSE MMTJ 80 S MMTJ 65 SSW NRS.


15Z UPDATE...

CHANGES TO THE EXCESSIVE RAINFALL OUTLOOK FOR THIS UPDATE WERE TO
SHIFT THE MARGINAL RISK AREA A BIT FARTHER SOUTH TO INCLUDE PARTS
OF THE CENTRAL GULF COAST STATES...WITH MORE OF A FOCUS ON
SOUTHERN MS/SOUTHERN AL AND THE WESTERN FL PANHANDLE. THIS IS
MAINLY BASED ON A COMBINATION OF THE LATEST RADAR TRENDS WHICH
SHOW A COLD-POOL DRIVEN AXIS OF CONVECTION CONTINUING TO ADVANCE
EAST-SOUTHEAST ACROSS SOUTH-CENTRAL MS AND CENTRAL AL...BUT ALSO
REPEATED SIGNALS FROM THE HRRR AND HRRR-EXP WHICH SHOW A SEPARATE
AXIS OF HEAVY CONVECTIVE RAINS DEVELOPING AND IMPACTING AREAS OF
FAR SOUTHERN MS AND POSSIBLY SOUTHERN AL THIS AFTERNOON. IT IS A
BIT UNCLEAR IF THERE WILL BE SUFFICIENT LOW LEVEL
FORCING/CONVERGENCE TO DRIVE THE HEAVY RAINFALL THAT THE HRRR IS
SUGGESTING...BUT THE 00Z ARW SHOWED THIS SIGNAL IN A WEST/EAST
FASHION WITH SOME TRAINING POTENTIAL AS WELL. SUPPORT FOR THIS
APPEARS TO BE PREDICATED ON THE COLD-POOL DRIVEN OUTFLOW BOUNDARY
SETTLING FARTHER SOUTH AND EAST OVER THE NEXT SEVERAL HOURS WHILE
INTERACTING WITH A GRADUALLY VEERING LOW LEVEL WIND FIELD...AND A
DESTABILIZATION AND CONTINUED MOISTENING OF THE BOUNDARY LAYER.
THE ENVIRONMENT DOES APPEAR CONDUCIVE FOR DEVELOPING CONVECTION TO
BACK-BUILD AND POSSIBLY TRAIN OVER THE SAME AREA...AND COINCIDING
WITH AT LEAST SOME WEAK UPPER JET FORCING. SO...EXPECT THE
POTENTIAL FOR AS MUCH AS 2 TO 4+ INCHES OF RAIN LOCALLY...AND THUS
SOME CONCERNS FOR RUNOFF PROBLEMS.

ORRISON

PREVIOUS DISCUSSION....

...SOUTHERN CALIFORNIA...

THE AIR MASS WAS INITIALLY VERY DRY...WITH PW BELOW A HALF INCH
AND IN MANY CASES BELOW A QUARTER INCH. THIS WILL
CHANGE...PARTICULARLY FROM SOUTHERN CALIFORNIA TO CENTRAL
ARIZONA...AS A PLUME OF PACIFIC MOISTURE WORKS ITS WAY ONSHORE.
OVERALL...QPF HAS BEEN INCREASING IN THE MODELS. THE TREND WAS
MOST NOTABLE OVER SOUTHERN CALIFORNIA / SAN DIEGO COUNTY.
SYNOPTICALLY THE EVENT WILL CONSIST OF A STEADILY PROGRESSING FULL
LATITUDE TROUGH...WITH SLIGHT POSITIVE TILT...BUT THE TROUGH IS
PICKING UP A SHEARING SUB-TROPICAL WAVE AND ASSOCIATED PLUME OF
ENHANCED MOISTURE AROUND 29 DEGREES LATITUDE. WHILE THE MODEL MASS
FIELDS AND SATELLITE LOOPS WOULD SUGGEST PLACING MAXIMUM QPF SOUTH
OF THE U.S. BORDER...THERE IS INCREASING CONFIDENCE PER THE MODEL
CONSENSUS...THAT THIS MAXIMUM WILL BE SOMEWHAT BROAD...AND WILL
CATCH SAN DIEGO AND THE LAGUNA MOUNTAINS WITH RAPIDLY INCREASING
PW VALUES AND ENHANCED ONSHORE FLOW STRENGTH...LEADING TO AT LEAST
1.00 INCH 24-HOUR TOTALS AT THE COAST AND 2.00 INCH TOTALS IN THE
MOUNTAINS.

THIS EVENT IS NOT LIKELY TO MEET ATMOSPHERIC RIVER CRITERIA...BUT
STRONG ONSHORE FLOW COULD NONETHELESS LEAD TO HEAVY PRECIPITATION
IF THE COLUMN CAN MOISTEN SUFFICIENTLY...AND IT LOOKS LIKELY THAT
MOISTENING WILL OCCUR GIVEN TRENDS IN THE RAP AND MUCH OF THE
OTHER GUIDANCE. HOURLY RAIN RATES APPROACHING 0.50 INCHES IN THE
NAM CONUS NEST AND WRF-ARW WILL NECESSITATE AT LEAST A MARGINAL
RISK OF EXCESSIVE RAINFALL TODAY. PER COORDINATION WITH WFO
SGX...WE ALSO EMBEDDED A SMALL SLIGHT RISK AREA ALONG THE COAST
FROM SAN DIEGO SOUTHWARD...AND INLAND. THERE IS STILL SOME
UNCERTAINTY AS TO THE MAGNITUDE OF THE EVENT...EVEN AT THIS CLOSE
RANGE. THE MORE EXTREME SOLUTIONS...3 TO 5 INCHES LIQUID PER THE
00Z WRF-ARW, NMM...SEEM LESS LIKELY GIVEN THE SYNOPTICS AND LAST
MINUTE TAP INTO SUB-TROPICAL MOISTURE...BUT AMOUNTS OF 2 TO 3...OR
EVEN ISOLATED 4 INCHES...SEEM MORE LIKELY NOW THAN IN PREVIOUS
FORECAST CYCLES...AND WILL CARRY AN ASSOCIATED RISK OF FLOODING /
FLASH FLOODING. SEE MESOSCALE PRECIPITATION DISCUSSION NUMBER 64
FOR DETAILS.


...DESERT SOUTHWEST...

PERSISTENT RAIN RATES OF AROUND A QUARTER INCH...AND EVENT TOTALS
OF 1 TO 2 INCHES MAY CAUSE A FEW RUNOFF ISSUES ALONG THE MOGOLLON
RIM...ANTECEDENT STREAM FLOWS WERE ALREADY NEAR OR ABOVE NORMAL IN
SOME CASES PER THE NATIONAL WATER MODEL.


...LOWER MISSISSIPPI VALLEY...

A BROAD REGION OF RETURNING GULF MOISTURE AND ISENTROPIC UPGLIDE
WILL BE SUPPORTIVE OF MESOSCALE AREAS OF PRECIPITATION...MORE
FAVORED WHERE LAPSE RATES ARE STEEPER AND MOISTURE DEPTH
GREATER...GENERALLY WEST OF THE APPALACHIANS AND ESPECIALLY DOWN
TOWARD THE SOUTHERN STATES. THE RISK OF EXCESSIVE RAIN RATES WILL
BE TIED TO AREAS OF GREATER INSTABILITY OVER THE LOWER MISSISSIPPI
VALLEY...ALTHOUGH CONVECTION WILL FIGHT AGAINST INITIALLY DRY
AIR...AND DEEP LAYER ASCENT WILL WANE OVER THIS REGION BY
AFTERNOON/EVENING AS LARGE SCALE TROUGHING REORGANIZES FARTHER
NORTH. THUS...THE MAIN CONCERN IS FOR CELL MERGERS AND PERHAPS THE
EVOLUTION OF A SMALL MCS EARLY IN THE DAY WHEN WARM ADVECTION WILL
BE BEST COUPLED WITH DEEP LAYER ASCENT FROM A SHORTWAVE WHIPPING
ACROSS THE LOWER/MIDDLE MISSISSIPPI VALLEY.

BURKE
$$





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