Quantitative Precipitation Forecast
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FOUS30 KWBC 202256
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
555 PM EST MON FEB 20 2017

...VALID 2255Z MON FEB 20 2017 - 12Z TUE FEB 21 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
125 W CEC 125 WNW CEC 80 WSW OTH 40 SSW OTH 20 WNW SXT
35 SSW SXT 30 ESE CEC 25 ESE ACV 20 WSW O54 15 ESE O54
20 NNW RDD 15 NNE RDD 10 ESE RDD 15 ENE RBL 30 NNE CIC 40 NE CIC
50 NE CIC 45 NNW BLU 10 NNW BLU 30 W TVL 40 SSW TVL 55 WNW MMH
30 ENE FAT 30 ENE VIS 35 ENE PTV 30 WNW IYK 35 ESE PTV
10 ENE VIS 10 NNE FAT 15 NNE MAE 10 ESE MCE 15 SW MCE 35 SSW MCE
30 WNW NLC 25 SW NLC 30 ENE PRB 15 ESE SMX 35 SSW LPC 105 SW XVW
155 SW VBG 190 SW 87Q.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 ENE KGHB 20 SW SRN 20 NNE LFT 30 NNE LFT 15 SSE MLU
30 SW PBF 30 NNW LRF 30 S UNO 20 NNE BVX 40 SSE M19 40 E GWO
15 S MEI 15 ESE HBG 20 ESE NBG 10 WSW KDLP 10 NW KMYT
15 ESE KGRY.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
55 SW O87 10 NW O87 35 NE O87 30 S O54 15 WNW RBL 15 NW CIC
20 NNE CIC 35 ENE CIC 30 NNW BLU BLU 20 S BLU 30 SW TVL
50 SSW TVL 50 NE MCE 45 NNE MAE 30 NNE FAT 20 ENE FAT 15 E FAT
20 NNE FAT 20 NE MAE 20 NE MCE 20 NE MOD 15 N MOD 10 W MOD
35 SW MCE 35 W NLC 25 NNE PRB 15 N SMX 30 WSW VBG 80 W VBG
110 SW 87Q.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
65 W SFO 40 SW STS 10 W STS 20 NE STS 10 W VCB 10 ENE VCB BAB
15 NE OVE 20 NE OVE 25 ENE OVE 20 WNW BLU 15 E AUN 40 SW TVL
45 NE MCE 30 ENE MCE 35 ENE MOD 25 ENE SCK SCK 25 E RHV
25 NNW PRB 15 SSE 87Q 40 WSW 87Q 70 W 87Q 90 SW MRY.


...CALIFORNIA AND FAR SOUTHWEST OREGON...

THE LATEST IN THE SERIES OF BONAFIDE ATMOSPHERIC RIVER EVENTS WAS
NEARING ITS PEAK INTENSITY LATE THIS AFTERNOON...AFFECTING
PRIMARILY NORTHERN AND CENTRAL CALIFORNIA. ALL OF THIS WAS
OCCURRING ON THE EAST SIDE OF AN ESTABLISHED LARGE SCALE
TROUGH...WITH ONE PRONOUNCED SHORTWAVE HAVING PULLED THE RIVER
ONSHORE THIS MORNING...AND A SECOND PRONOUNCED SHORTWAVE FOCUSING
DIFLUENCE / VIGOROUS ASCENT OVER NORTHERN CA THIS AFTERNOON.
SATELLITE IMAGERY CLEARLY DEPICTED A DIFLUENT UPPER AIR
PATTERN...AND COULD BE DESCRIBED AS SHOWING A BROAD WEDGE FEATURE
WITH ITS ORIGIN POINT AROUND 35 N / 125 W TO THE SOUTHWEST OF SAN
FRANCISCO. PRECIPITATION FANNED OUT WITHIN THE WEDGE...DELIVERING
WIDESPREAD RAINFALL OF ABOUT 0.30 TO 0.50 INCHES PER THREE
HOURS...BUT WITH LOCALLY ENHANCED AMOUNTS IN FAVORED TERRAIN. IT
APPEARED THE STRONGLY FORCED PRECIPITATION WILL OCCUR NEAR AND
NORTH OF THE LATITUDE OF THE BAY AREA TO STOCKTON AND SONORA THIS
EVENING...SIMILAR TO THE NSSL WRF FORECAST...AND THUS WE MADE SOME
NORTHWARD EXPANSION TO THE SLIGHT AND MODERATE RISK AREAS FOR
EXCESSIVE RAINFALL.

THE SHORTWAVE WILL REMAIN OF LOW AMPLITUDE AS IT SWINGS THROUGH
NORTHERN CA THIS EVENING...RESULTING IN SLIGHT VEERING OF WINDS
THROUGHOUT THE COLUMN. BY 03Z LARGE SCALE ASCENT MAY BECOME LESS
ORGANIZED...AND THE ATMOSPHERIC RIVER WILL BROADEN...WITH PW
VALUES FALLING OFF TO AROUND 1.OO INCH RATHER THAN THE 1.20-PLUS
INCHES SEEN EARLIER IN THE DAY. STILL...ADDITIONAL TERRAIN
ENHNACED HEAVY RAINFALL WILL OCCUR OVERNIGHT...PARTICULARLY IN THE
COASTAL RANGES SOUTH OF SAN FRANCISCO AND IN THE UPSLOPE REGION OF
THE SIERRAS...AS THE ONSHORE COMPONENT OF THE WINDS INCREASES AND
MOISTURE REMAINS MORE THAN ADEQUATE TO SUPPORT HOURLY RATES
GREATER THAN 0.25 INCHES. PER FLASH FLOOD WARNINGS FROM WFO
MONTEREY...NUMEROUS STREAM GAUGES HAD SHOWN RAPIDLY RISING WATER
ALREADY DURING THIS EVENT...AND A HEIGHTENED FLOOD / FLASH FLOOD
POTENTIAL WILL CONTINUE THROUGH TONIGHT IN MANY AREAS. REFERENCE
MESOSCALE PRECIPITATION DISCUSSIONS FOR FURTHER DETAILS.


...ARKANSAS / LOUISIANA / MISSISSIPPI...

A LINE OF DEEP CONVECTION HAD REMAINED FORWARD OF THE NEUTRALLY
TILTED UPPER TROUGH OVER TEXAS...WITH LOCALLY HEAVY RAINFALL
PUSHING SLOWLY BUT STEADILY EASTWARD THROUGH LOUISIANA AT 2230Z.
CONVERGENCE WAS INCREASING UP INTO CENTRAL/EASTERN ARKANSAS AS
WELL...AND A BROKEN LINE OF CONVECTION THERE MAY FILL IN AS LARGE
SCALE ASCENT OVERTAKES THE REGION. THE UPPER TROUGH IS FORECAST TO
CLOSE OFF A LOW CENTER OVERNIGHT...WHICH MAY PROLONG THE DURATION
OF SYNOPTIC ASCENT AROUND THE PERIPHERY OF THE SYSTEM ACROSS PARTS
OF LA/MS/AR...BUT AT THE SAME TIME THE SYSTEM DOES MAINTAIN AN
EASTWARD PROGRESSION...WHICH WILL LIMIT THE POTENTIAL FOR
EXCESSIVE RAINFALL. THE GREATEST CONCERN WILL BE THE NEARLY TWO
INCH IN ONE HOUR RAIN RATES BEING ESTIMATED BY DUAL-POL RADARS
THIS AFTERNOON IN LOUISIANA...ALTHOUGH EVEN THESE VALUES ARE
SLIGHTLY BELOW FLASH FLOOD GUIDANCE. WITH LOSS OF DIURNAL HEATING
THERE SHOULD BE SOME REDUCTION IN INTENSITY OVERNIGHT...BUT HI-RES
GUIDANCE DOES MAINTAIN AT LEAST ONE INCH PER HOUR RAIN RATES ON
SMALLER SCALES...OWING TO STRONG DEEP LAYER ASCENT AND MODERATE
INFLOW OF UNSEASONABLY WARM/MOIST AIR. THE MORE SUSCEPTIBLE
LOCATIONS IN THIS REGION COULD EXPERIENCE SOME HIGH WATER ISSUES.

BURKE
$$





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