Quantitative Precipitation Forecast
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858
FOUS30 KWBC 230745
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
245 AM EST MON JAN 23 2017

...VALID 12Z MON JAN 23 2017 - 12Z TUE JAN 24 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
105 SW XVW 65 WSW VBG 10 E LPC 15 SW PMD 35 NNE ONT 15 W NXP
20 NW NJK 60 S NJK 95 SSE MMTJ 120 SSW NRS 140 SW NRS.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 WNW SAD TUS 55 SSW CGZ 30 SSW CGZ PHX 25 NNW LUF 60 WSW PRC
45 E IGM 20 ENE 40G 30 S INW 25 SSE SJN 50 NNE SAD 20 WNW SAD.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
65 NE FFA 50 NE FFA 15 ESE NTU 15 NNE LFI 20 S NHK 10 S RJD
15 NE ABE 10 S MGJ 30 NNW OXC FIT SFM 30 SE PWM 70 SE PWM
55 ENE CQX.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
65 W NSI 65 S LPC 40 S LPC 25 WSW SBA 20 ENE SBA 3A6 10 NE POC
25 NW PSP 15 SSW PSP 30 E RNM 15 ENE CZZ 25 SE CZZ 45 S CZZ
55 S MMTJ.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 NNW NEL VAY PHL 10 NNE DYL 12N 20 S MGJ HPN JFK BLM
10 NNW NEL.


...SOUTHERN CALIFORNIA...

THE PLUME OF ENHANCED MOISTURE AND VERY STRONG ONSHORE FLOW THAT
HAD BROUGHT FLOODING TO SOUTHERN CALIFORNIA ON SUNDAY...WILL
PROGRESS INTO THE DESERT SOUTHWEST ALONG WITH A PRONOUNCED FRONTAL
PASSAGE. SOME OF THE HEAVIER PRECIPITATION
TODAY...THEREFORE...WILL OCCUR EARLY MONDAY IN THE MOUNTAINS EAST
OF SAN DIEGO...AND ALSO DOWNSTREAM IN THE FAVORED AREAS OF ARIZONA
AND SOUTHWEST UTAH. PRECIPITATION DOES NOT QUITE END OVER
CALIFORNIA... HOWEVER...AS CONTINUED HEIGHT FALLS AND INSOLATION
WILL STEEPEN LAPSE RATES WHILE PRECIPITABLE WATER VALUES REMAIN
SUFFICIENT...AROUND A HALF INCH...FOR CONVECTIVE RAIN SHOWERS TO
WANDER THROUGH COASTAL SECTIONS...INCLUDING SANTA BARBARA TO LOS
ANGELES TO SAN DIEGO. HOURLY OBSERVED RAINFALL MAY STRUGGLE TO
EXCEED A FEW TENTHS OF AN INCH...BUT GIVEN THE WET ANTECEDENT
CONDITIONS AND REPORTS OF SIGNIFICANT FLOOD IMPACTS ON SUNDAY...WE
WILL MAINTAIN A SLIGHT RISK OF EXCESSIVE RAINFALL THROUGH MONDAY
AFTERNOON...NOTING ALSO THAT FLASH FLOOD WATCHES REMAIN IN EFFECT
FROM THE LOCAL WEATHER OFFICES.

AS THE SYSTEM PROGRESSES THROUGH THE SOUTHWEST STATES...ADDITIONAL
RAINFALL OVER 1.5 INCHES IS POSSIBLE IN THE HIGH TERRAIN ALONG
FAVORED SLOPES...WHILE 0.50 TO 1.25 INCH AMOUNTS WILL BE MORE
COMMON. PW ANOMALIES OF ONLY ABOUT 1.5 STANDARD DEVIATIONS
PENETRATE INTO ARIZONA...AND GIVEN THE PROGRESSIVE NATURE OF THE
SYSTEM AT THIS POINT IN TIME...THE RISK OF EXCESSIVE RAINFALL OVER
ARIZONA IS MARGINAL...BUT HOURLY RATES APPROACHING 0.50 INCHES AT
TIMES MAY RESULT IN RAPID RUNOFF.


...MID ATLANTIC AND NORTHEAST...

AN UPPER LOW CENTERED IN NORTHWEST GEORGIA WAS IMPRESSIVELY WOUND
UP AT 06Z...WITH A SPIRAL PATTERN TO RADAR REFLECTIVITY. THE LOW
WAS EMBEDDED IN A GENERALLY PROGRESSIVE PATTERN...BUT AT SUCH
INTENSITY AND BEING OF MEDIUM WAVELENGTH...PROGRESS TOWARD THE
EAST AND NORTHEAST WILL BE SOMEWHAT SLOW...LEADING TO A WIDESPREAD
PRECIPITATION SHIELD AFFECTING THE EASTERN / NORTHEASTERN U.S.
THROUGH DAY 1 AND INTO DAY 2. AT THE SURFACE THE OCCLUDED AND
STACKED LOW BENEATH THE TIGHT UPPER VORTEX WILL GRADUALLY HAND OFF
TO COASTAL LOW DEVELOPMENT OFF NORTH CAROLINA/VIRGINIA BY MONDAY
AFTERNOON. A SWATH OF PARTICULARLY HEAVY RAINFALL WILL LIKELY
AFFECT THE COASTAL MID ATLANTIC AND NEW ENGLAND FROM MARYLAND TO
MASSACHUSETTS...AS THE COASTAL CYCLONE IS FORECAST TO UNDERGO A
PERIOD OF INTENSIFICATION WHEN THE UPPER TROUGH SWINGS INTO A
SLIGHT NEGATIVE TILT MONDAY NIGHT AND TUESDAY MORNING. THIS WILL
PRODUCE A STRONG EASTERLY FETCH OFF THE ATLANTIC DIRECTED INTO
NJ/NY/CT/RI/MA...WITH LOW LEVEL MOISTURE FLUX ANOMALIES SPIKING AT
3.0 TO 4.0 STANDARD DEVIATIONS ALONG THE IMMEDIATE COAST. THE
HI-RES GUIDANCE SUGGESTS 1-HOUR PRECIPITATION RATES OVER 0.75
INCHES DURING THE HEIGHT OF THE EVENT...WITH 3-HOUR ACCUMULATIONS
OVER 2 INCHES. THIS WILL APPROACH FLASH FLOOD GUIDANCE
VALUES...ESPECIALLY IN THE URBAN CORRIDOR FROM PHILADELPHIA TO NEW
YORK.

BURKE
$$





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