Quantitative Precipitation Forecast
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FOUS30 KWBC 210059
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
758 PM EST FRI JAN 20 2017

...VALID 01Z SAT JAN 21 2017 - 12Z SAT JAN 21 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 N NSI 10 WSW VNY 20 ESE PMD 25 NNW PSP 30 NE CZZ 60 SSE CZZ
85 S MMTJ 105 SSW NRS 90 SSW NUC.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 NW KMZG IAH 15 N POE 35 SE GTR 30 NW MXF 15 ENE GZH
25 SE PQL 10 W S58.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 NE NSI 10 NNE EMT 15 NE RIV 20 SSW PSP 15 N CZZ 25 SSW CZZ
35 SSW NRS 60 WSW NZY.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 N KBQX 10 ESE IAH 10 NW HEZ 25 E NMM 45 NNW GZH 30 NNE BFM
25 SSW HSA 25 W KEIR.


0100 UTC UPDATE...

TWO NOTABLE CHANGES WERE MADE TO THE PREVIOUS DAY 1 EXCESSIVE
RAINFALL OUTLOOK. FIRST...WAS ABLE TO TRIM THE NORTHERN TWO-THIRDS
OF THE MARGINAL AND SLIGHT RISK AREAS ACROSS CA...BASED ON THE
LATEST RADAR AND SATELLITE TRENDS...ALONG WITH THE FORECAST GOING
FORWARD AS THE NARROWING ATMOSPHERIC RIVER CONTINUES TO WEAKEN AS
IT AND THE COLD FRONT IN ITS WAKE PUSH SOUTH OF THE STATE LATER
THIS EVENING.

ELSEWHERE...HAVE EXPANDED THE MARGINAL AND SLIGHT RISK AREAS
FARTHER WEST THROUGH WESTERN LA AND THE UPPER TX COAST...AGAIN
ALSO BASED ON THE OBSERVATIONAL TRENDS ALONG WITH THE LATEST HRRR
TRENDS.

HURLEY

...CALIFORNIA...

BROAD SCALE CYCLONIC FLOW EXISTS OVER THE WESTERN U.S...ANCHORED
UPSTREAM BY A POWERFUL 160-170 KNOT JET STREAK. A BETTER DEFINED /
DEEPER TROUGH WILL DEVELOP AND MIGRATE INTO CALIFORNIA EARLY TODAY
ALONG THE CYCLONIC SHEAR SIDE OF THIS RATHER STRAIGHT AND RATHER
STRONG JET STREAK. THIS WAVE WILL TAP INTO SUB-TROPICAL MOISTURE
AND PRODUCE DEEP ONSHORE FLOW THROUGH THE COLUMN...WITH 700 MB
U-WIND COMPONENT ANOMALIES OF 3 TO 5 STANDARD DEVIATIONS FROM
CLIMATOLOGY ALONG THE COAST. THIS ALONG WITH STRONG HEIGHT FALLS
AS THE FLOW AMPLIFIES A BIT...WILL PROMOTE SOME HEAVY RAINFALL FOR
COASTAL CALIFORNIA...PROGRESSING NORTH TO SOUTH DURING THE DAY.
RAINFALL RATES WILL LIKELY EXCEED A HALF INCH PER HOUR AT
TIMES...LEADING TO EVENT TOTALS OF 2.0 TO 3.5 INCHES IN THE
FAVORED TERRAIN ALONG THE COAST...AS WELL AS SIMILAR LIQUID
EQUIVALENTS OVER THE SOUTHERN END OF THE SIERRAS. ALONG THE COAST
THE RAINFALL WILL BE FAIRLY EVENLY DISTRIBUTED FROM MONTEREY TO
SAN DIEGO...AS THE SYSTEM WILL REMAIN POTENT BUT PROGRESSIVE.
FLOODING / FLASH FLOODING WILL BE POSSIBLE.


...CENTRAL GULF COAST STATES...

THE NEXT IN A SERIES OF ENERGETIC AND PROGRESSIVE SHORTWAVE
TROUGHS WILL PULL OUT IN THE PLAINS TODAY...ACTING TO TIGHTEN
GRADIENTS AND STRENGTHEN A JET STREAK WITHIN SSW - NNE FLOW OVER
THE WESTERN GULF STATES. THE LOW LEVEL
RESPONSE WILL DRAW MOISTURE AND INSTABILITY RAPIDLY BACK NORTHWARD
OUT OF THE GULF INTO LOUISIANA AND ADJACENT PARTS OF TX / MS. THE
GREATEST MOISTURE FLUX SHOULD BE ORIENTED INTO SOUTHERN LOUISIANA
BY 00-06Z
THE 21ST...WITH WINDS THEN BEGINNING TO VEER AS THE SYNOPTIC FLOW
REGIME REMAINS PROGRESSIVE. THEREFORE...WE FAVORED SOLUTIONS SUCH
AS THE NSSL WRF AND GEM REGIONAL WHICH DEPICT HEAVIER SWATHS OF
RAIN ORIGINATING IN SOUTHERN LOUISIANA AND EXPANDING EASTWARD.
THIS ORIENTATION...AS DEPICTED IN THE DETERMINISTIC WPC
FORECAST...WOULD OVERLAP WITH AREAS THAT SAW HEAVY RAIN ON
THURSDAY...AND WHERE FLASH FLOOD GUIDANCE VALUES HAVE DECREASED.
WE ENCOMPASSED MUCH OF SOUTHERN LOUISIANA AND SOUTHERN MISSISSIPPI
IN A SLIGHT RISK OF EXCESSIVE RAINFALL...BUT WERE ALSO GENEROUS
WITH THE NORTHERN EXPANSE OF THE SLIGHT AND MARGINAL RISK
AREAS...ACCOUNTING FOR UNCERTAINTY AND ACCOUNTING FOR A SECONDARY
SWATH OF HEAVIER RAIN RATES SEEN IN MUCH OF THE HI-RES GUIDANCE
OVER NORTHERN LA / CENTRAL MS. THIS SEEMS TO CORRESPOND TO MID
LEVEL FRONTOGENESIS AND DEEP LAYER Q-VECTOR CONVERGENCE PER THE
GFS AND ECMWF.

BURKE
$$





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