850
FOUS30 KWBC 200032
QPFERD
EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
732 PM EST TUE NOV 19 2024
DAY 1
VALID 01Z WED NOV 20 2024 - 12Z WED NOV 20 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
NORTHWEST CALIFORNIA AND SOUTHWEST OREGON...
..01Z UPDATE
THE SLIGHT RISK ALONG THE GULF COAST WAS DOWNGRADED AS THE HEAVIEST
RAIN FOR MOST HAS SHIFTED OFF TO THE EAST. IN COORDINATION WITH
TAE/TALLAHASSEE, FL FORECAST OFFICE, THE MARGINAL RISK WAS
MAINTAINED ALONG AND SOUTH OF I-10 EAST OF PANAMA CITY FOR ANY
POTENTIAL ISOLATED FLASH FLOODING WITH THE SLOW MOVING AREA OF
HEAVY RAIN PUSHING EAST ACROSS THE AREA. ANY FLOODING CONCERNS ARE
RELEGATED MOSTLY TO URBAN AND FLOOD-PRONE AREAS OF THE EASTERN
FLORIDA PANHANDLE.
NO CHANGES WERE MADE TO THE SLIGHT AND MARGINAL RISKS ALONG THE
WEST COAST. THE MULTI-DAY RAIN EVENT FOR MUCH OF THE WASHINGTON AND
OREGON COASTS IS JUST GETTING STARTED AS THE PARENT LOW A COUPLE HUNDRED
MILES OFF THE COAST OF WASHINGTON DEEPENS AT AN ASTONISHING RATE.
THE LAST ESTIMATE WAS THE LOW DEEPENED 67 MB IN THE LAST 24 HOURS.
THIS IS A PRESSURE FALL ONLY MATCHED BY A VERY SELECT FEW STORMS
IN THE RECORDS, AND IS MORE THAN ANY NORTH PACIFIC LOW IN AT LEAST
THE LAST 50 YEARS!
WEGMAN
16Z UPDATE:
FOR THE CENTRAL GULF COAST, SURFACE WAVE ALONG THE COLD FRONT IS
LIFTING EAST-NORTHEAST SLOWLY UNDER RESPONSE OF STRENGTHENING
130KT JET ACROSS THE CENTRAL MS VALLEY AND IS EXPECTED TO SLOW A
BIT. PROGRESSIVE PRE-FRONTAL TROUGH LEAVING EASTERN LA INTO THE
NORTHERN GULF IS LIKELY TO INTERSECT BEST MOISTURE FLUX AND MAY
LIMIT NORTHWARD EXPANSION OF UNSTABLE AIR NORTH INTO S MS/S AL/W
FL, BUT IF IT CAN, STRONGER, SLOWER MOVING THUNDERSTORMS WILL
RETAIN AMPLE RISK OF PRODUCING LOCALIZED STREETS OF 3-5" ACROSS THE
AREA POTENTIALLY INTO SW GA TO MAINTAIN RISK AREAS WITH EROSION OF
THE WESTERN SIDE OF THE RISK AREAS AS THE FRONT HAS PASSED IN
CENTRAL LA.
FOR THE WEST COAST, RAPID CYCLOGENESIS IS ALREADY OCCURRING BASED
ON GOES-W SATELLITE SUITE. SHOWERS AND PERHAPS AN OCCASIONAL
SALLOW-TOPPED THUNDERSTORM WILL PROGRESS EASTWARD INTO THE WA/OR
AND N CA COAST INTO THIS EVENING/OVERNIGHT. PLACEMENT AND RISK
CATEGORIES REMAIN SOLID THAT ONLY COSMETIC CHANGES WERE MADE.
GALLINA
----PRIOR DISCUSSION----
..CENTRAL GULF COAST
LATEST SATELLITE IMAGERY IS SHOWING ENHANCING CONVECTION TO THE
SOUTH OF SOUTHEAST LA ASSOCIATED WITH THE ANOMALOUS TROPICAL
MOISTURE SURGE AHEAD OF THE COLD FRONT MOVING EAST ACROSS THE
CENTRAL GULF COAST. TRENDS IN THE LATEST HI RES GUIDANCE SUGGEST
THIS OFFSHORE ENHANCING CONVECTION WILL BECOME THE HEAVIEST
FRONTAL/PRE-FRONTAL RAINS AS ORGANIZED CONVECTION CURRENTLY ALONG
THE FRONT FARTHER TO THE NORTH BEGIN TO WEAKEN AFTER 1200 UTC. HI
RES GUIDANCE IS IN FAIRLY GOOD AGREEMENT THAT THE HEAVIEST RAINS
WITH THE OFFSHORE ENHANCING CONVECTION WILL REMAIN JUST TO THE
SOUTH OF FAR SOUTHERN MS, FAR SOUTHERN AL AND THE WESTERN FL
PANHANDLE, BUT STILL AFFECTING FAR SOUTHEAST LA. HREF NEIGHBORHOOD
PROBABILITIES SHOW THAT THE AXIS OF THE HIGHEST PROBABILITIES FOR 2
AND 3"+ AMOUNTS WILL REMAIN OFFSHORE WITH THE NORTHERN EDGE OF
THESE HIGHER PROBABILITIES AFFECTING AREAS FROM FAR SOUTHEAST LA
AND ALONG THE IMMEDIATE FAR SOUTHERN MS, FAR SOUTHERN AL AND
WESTERN FL PANHANDLE. CHANGES TO THE PREVIOUS OUTLOOK WERE TO TRIM
THE NORTHERN EXTENT OF THE SLIGHT RISK AREA BY APPROXIMATELY 20 NM
AND TO TRIM THE WESTERN EXTENT OF THE SLIGHT RISK BY APPROXIMATELY
40NM TO ACCOUNT FOR THE HI RES TRENDS. AT THE MOMENT, THE CONSENSUS
ON TIMING OF CONVECTION SUPPORTS REMOVING THE NEW ORLEANS METRO
AREA FROM THE SLIGHT RISK, WITH THE HEAVIEST RAINS LIKELY TO THE
EAST AND SOUTHEAST PRIOR TO 1200 UTC TUESDAY.
..COASTAL WASHINGTON, OREGON AND FAR NORTHWEST CALIFORNIA
LATEST MODELS CONTINUE TO BE IN GOOD AGREEMENT ON THE RAPID
CYCLOGENESIS EVENT ACROSS THE NORTHEAST PACIFIC OFF THE PACIFIC
NORTHWEST COAST, SIGNALING THE BEGINNING OF A MULTI DAY ATMOSPHERIC
RIVER EVENT FOR NORTHWEST CA INTO FAR SOUTHWEST OR. ANOMALOUS LOW
LEVEL SOUTHERLY FLOW EXPECTED ALONG AND AHEAD OF THE ASSOCIATED
COLD FRONT TUESDAY NIGHT INTO EARLY WEDNESDAY, WITH 850-700 MB
MOISTURE FLUX ANOMALIES OF 3-4 STANDARD DEVIATIONS ABOVE THE MEAN
IMPACTING THE COASTAL PAC NW, SOUTH INTO FAR NORTHWEST CA COAST. A
WELL DEFINED AREA OF FRONTAL PRECIP LIKELY TO PUSH INLAND ACROSS
COASTAL PAC NW/FAR NORTHWEST CA AFTER 0000 UTC WEDNESDAY. THE
STRONG SOUTHERLY LOW LEVEL FLOW WILL BE MOSTLY PARALLEL TO THE
TERRAIN ALONG THE PAC NW COAST, LIMITING HOURLY RAINFALL RATES TO
.10-.25"+. GREATER HOURLY RATES LIKELY ACROSS FAR SOUTHWEST OREGON
INTO COASTAL NW CA WHERE A MORE SIGNIFICANT UPSLOPE COMPONENT TO
THE LOW LEVEL FLOW IS LIKELY. HI RES GUIDANCE IS FAIRLY SIMILAR IN
SHOWING HOURLY RATES OF .25-.50"+ IN THE 0600 TO 1200 UTC WEDNESDAY
PERIOD ACROSS FAR SOUTHWEST OR INTO FAR NW CA. HREF .50"/HR
PROBABILITIES SUBSEQUENTLY ARE HIGH ACROSS THIS AREA, WITH 1"/HR
PROBABILITIES MUCH LOWER AND PRIMARILY OFFSHORE. NO CHANGES MADE TO
THE PREVIOUS SLIGHT RISK AREA, WHICH CORRESPONDS WELL TO THE ABOVE
MENTIONED AXIS OF HIGH .50"+/HR HREF NEIGHBORHOOD PROBABILITIES.
ORAVEC
DAY 2
VALID 12Z WED NOV 20 2024 - 12Z THU NOV 21 2024
...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF NORTHWEST CALIFORNIA...
..1930 UTC UPDATE
SLIGHT CHANGES WERE MADE TO THE DAY 2 ERO OUT IN CA AND FAR
SOUTHWEST OR -- MAINLY TO TRIM THE NORTHERN PORTION OF THE MODERATE
RISK, WHILE ALSO PULLING THE SOUTHERN PERIPHERIES OF THE
MODERATE/SLIGHT/MARGINAL RISK AREAS SLIGHTLY SOUTHWARD BASED ON THE
LATEST GUIDANCE TRENDS. THE MODERATE IS NOW CLOSER (THOUGH STILL
NORTH OF) THE BAY AREA, WHERE THE LATEST (12Z) HREF 24
PROBABILITIES OF >8" IN 24HRS IS NOW 60-80+ PERCENT.
HURLEY
..PREVIOUS DISCUSSION
THE MULTI DAY ATMOSPHERIC RIVER EVENT THE BEGAN LATE DAY 1 FOR FAR
SOUTHWEST OREGON INTO NORTHWEST CALIFORNIA WILL CONTINUE DAY 2,
PRIMARILY ACROSS NORTHWEST CALIFORNIA. THE RAPIDLY DEEPENING LOW
OVER THE NORTHEAST PACIFIC DAY 1 WILL ANCHOR A NEARLY STATIONARY
UPPER VORTEX OFF THE PAC NW COAST DAY 2. STRONG PERSISTENT DEEP
LAYERED WEST SOUTHWEST FLOW ON THE SOUTH SIDE OF THIS VORTEX WILL
IMPACT NORTHWEST CALIFORNIA DAY 2, WITH IVT VALUES OF 500-800
KM/M-1 S-2. AND 850-700 MB MOISTURE FLUX ANOMALIES OF 2-4+ STANDARD
DEVIATIONS ABOVE THE MEAN. THERE IS FAIRLY GOOD MODEL AGREEMENT
WITH HEAVY PRECIP TOTALS OF 3-5" PRIMARILY OVER NORTHWEST
CALIFORNIA, WITH ISOLATED TOTALS OF 6-8" POSSIBLE. THE PREVIOUS
MODERATE RISK AREA WAS ADJUSTED SLIGHTLY SOUTHWARD BY APPROXIMATELY
10-40 NM TO ACCOUNT FOR MODEL MAX AXIS CONSENSUS. HOURLY RAINFALL
AMOUNTS OF .25-.50"+ LIKELY THROUGH THE DAY 2 TIME PERIOD ACROSS
NORTHWEST CA WITH INCREASING FLOODING RISKS WITH TIME AS SOILS
BECOME SATURATED AND STREAMS AND RIVER LEVELS RISE. THE HREF 1 HOUR
PROBABILITIES FOR .50"+ TOTALS FOR THE FIRST HALF OF DAY 2 WHEN
THE DATA IS AVAILABLE, REMAIN HIGH ACROSS NORTHWEST CA.
ORAVEC
DAY 3
VALID 12Z THU NOV 21 2024 - 12Z FRI NOV 22 2024
...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
NORTHWEST CALIFORNIA...
..1930 UTC UPDATE
NOT TOO MANY CHANGES MADE TO THE DAY 3 ERO, ESPECIALLY WITH
RESPECT TO THE WELL-COLLABORATED HIGH RISK ISSUED OVERNIGHT. LATEST
TRENDS IN THE GUIDANCE DO SUPPORT NUDGING THE HIGH A LITTLE FARTHER
SOUTH, PERHAPS EVEN SOUTH OF EKA. FOR NOW, HAVE BEGUN THAT SHIFT,
BUT NOT DRASTICALLY SO. WILL EVALUATE FURTHER WITH THE OVERNIGHT
PACKAGE TO SEE IF MORE ADJUSTMENTS ARE NECESSARY (INCLUDING MOVING
THE HIGH RISK SOUTH OF EKA).
..PREVIOUS DISCUSSION BELOW
THE DEEP NORTHEAST PACIFIC VORTEX
REMAINS NEARLY STATIONARY DAY 3 OFF THE PACIFIC NORTHWEST COAST,
SUPPORTING THE CONTINUATION OF THE MULTI DAY ATMOSPHERIC RIVER
EVENT FOR NORTHWEST CA INTO FAR SOUTHWEST OR. ANOMALOUS DEEP
LAYERED SOUTHWESTERLY FLOW WILL CONTINUE ON THE SOUTH SIDE OF THIS
VORTEX. 850 TO 700 MB MOISTURE FLUX ANOMALIES WILL CONTINUE TO BE 2
TO 4+ STANDARD DEVIATIONS ABOVE THE MEAN WITH IVT VALUES ALSO
MAINTAINED IN THE 500 TO 800 KM/M-1 S-2 RANGE ACROSS NORTHWEST CA
INTO FAR SOUTHWEST OR. THERE IS A LIKELIHOOD OF VERY HEAVY RAINS
AGAIN FALLING DAY 3 OVER SIMILAR REGIONS FROM THE DAY 2 PERIOD.
HOURLY RATES OF .25-.50"+ LIKELY TO CONTINUE AT TIMES SUPPORTING
ADDITIONAL 24 HOUR RAINFALL AMOUNTS OF 3 TO 6 INCHES AND ISOLATED
ADDITIONAL 24 HOUR TOTALS OF 6-8" ACROSS NORTHWEST CA INTO
SOUTHWEST OR. STORM TOTAL AMOUNTS OF 12 TO 16+ INCHES POSSIBLE
ACROSS PORTIONS OF NORTHWEST CALIFORNIA. GIVEN THIS DAY 2-3 HEAVY
RAIN OVERLAP POTENTIAL, A SMALL HIGH RISK AREA WAS INTRODUCED OVER
NORTHWEST CA, ACROSS THE WEST CENTRAL PORTIONS OF THE EKA'S CWA.
FLOOD RISKS WILL CONTINUE TO INCREASE DAY 3 ALONG WITH ROCK AND
LAND SLIDES AS SOILS BECOME SATURATED AND STREAMS AND RIVERS
CONTINUE TO RISE AND OVERFLOW. THERE WILL BE A NORTHWARD SHIFT BACK
INTO FAR SOUTHWEST OREGON DAY 3 FOR THE HEAVY RAIN POTENTIAL AS
THE UPPER FLOW BACK AND HEIGHTS RISE. THE RISING HEIGHTS WILL ALSO
KEEP MUCH OF THE DAY 3 PRECIP AS RAIN, INCREASING OVERALL RUNOFF.
ORAVEC
DAY 1 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/94EPOINTS.TXT
DAY 2 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/98EPOINTS.TXT
DAY 3 THREAT AREA: HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/99EPOINTS.TXT
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