Quantitative Precipitation Forecast
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FOUS30 KWBC 231424
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1023 AM EDT MON OCT 23 2017

...VALID 15Z MON OCT 23 2017 - 12Z TUE OCT 24 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 SE DNV IGQ 15 NW FKS 20 N CYAM 50 ENE ANJ 20 NW CYZE
35 NE OSC 10 SW VLL 10 SE MFD 15 W BFD 35 NW IPT 10 NNW MRB
10 WNW PTB 35 SE ILM 40 SSE CHS 10 ENE AGS 20 WNW EZM 10 NE CSG
10 WNW CSV 25 NNW LOU 20 SE DNV.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
WDR RYY DNN 30 WNW RHP 40 SW TRI 10 S 6V3 10 SW BKW 15 SSE 48I
10 S EKN 30 WNW SHD 15 ENE ROA 20 N INT 10 SSE SVH GSP WDR.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 ENE RQB AMN 15 SW FPK 10 E EKM 15 SSW BEH 20 SSW MKG
20 SE LDM 15 SW CAD 25 ENE RQB.


1500 UTC UPDATE...

ONLY MINOR...MAINLY COSMETIC CHANGES MADE TO THE DAY 1 ERO BASED
ON THE LATEST COMPOSITE RADAR LOOPS...LATEST HRRR/EXPERIMENTAL
HRRR TRENDS...AND MOST RECENT NATIONAL WATER MODEL SOIL MOISTURE
ANALYSIS AND SHORT-TERM FORECAST.

HURLEY

PREVIOUS DISCUSSION BELOW...


...GREAT LAKES...

AN IMPRESSIVE SYNOPTIC SETUP AND ABOVE NORMAL MOISTURE WILL RESULT
IN A SWATH OF HEAVY RAINFALL OVER THE GREAT LAKES TODAY INTO
TONIGHT. THE UPPER LEVELS WILL BECOME INCREASINGLY DIVERGENT
THROUGH THE PERIOD...WITH A PERSISTENT AXIS OF SURFACE CONVERGENCE
PRESENT AS WELL AS RAPID CYCLOGENESIS OCCURS THIS AFTERNOON INTO
TONIGHT. A LACK OF INSTABILITY WILL CAP THE UPPER LIMIT OF
RAINFALL RATES...WITH AMOUNTS EXPECTING TO PEAK IN THE 0.25"-0.5"
IN AN HOUR RANGE. HOWEVER WILL BE A PROLONGED PERIOD OF THIS
STEADY RAINFALL...AND AS SUCH EXPECTING A CORRIDOR OF 2-4" OVER
PORTIONS OF WESTERN MI. MODELS CONTINUE TO TREND WEST WITH THIS
AXIS OF HEAVIER RAINFALL...AND SOME UNCERTAINTY STILL EXIST ON ITS
EXACT PLACEMENT. BUT FOR NOW FOLLOWED A MULTI MODEL BLEND...WITH
THE MOST WEIGHTING GIVEN TO THE VARIOUS HIGH RES CAM SOLUTIONS.
THIS RESULTS IN A PRETTY SIGNIFICANT WESTWARD SHIFT FROM OUR
PREVIOUS FORECAST. CONFIDENCE IS HIGH IN THIS
ADJUSTMENT...ALTHOUGH AS MENTIONED ABOVE STILL LINGERING
UNCERTAINTY ON THE EXACT AXIS. PORTIONS OF SOUTHWEST MI HAVE SEEN
WELL ABOVE NORMAL RAINFALL RECENTLY...WHICH COULD RESULT IN SOME
FLOODING CONCERNS DEVELOPING DESPITE THE RELATIVELY LOW RATES
ANTICIPATED...AND FOR THIS REASON WILL ISSUE A SLIGHT RISK FOR
PORTIONS OF SOUTHWEST MI FOR THE POTENTIAL OF LONGER DURATION
FLOODING CONCERNS.


...OHIO VALLEY SOUTHEAST INTO THE MID ATLANTIC AND SOUTHEAST...

A BROAD SWATH OF MODERATE TO LOCALLY HEAVY RAINFALL AMOUNTS IS
EXPECTED FROM THE OHIO VALLEY INTO THE SOUTHEAST. A DEEPENING LOW
OVER THE GREAT LAKES WILL PUSH A COLD FRONT ACROSS THE AREA
THROUGH THE DAY TODAY. THE DYNAMICS WILL BE PRETTY IMPRESSIVE WITH
A NEGATIVELY TILTED TROUGH AND DIVERGENT FLOW ALOFT. WILL ALSO
HAVE STRONG LOW LEVEL SOUTH TO SOUTHEASTERLY FLOW...WHICH WILL
RESULT IN AN OROGRAPHIC ENHANCEMENT TO THE RAINFALL TOTALS ACROSS
THE SOUTHERN APPALACHIANS. INSTABILITY IS MARGINAL...BUT GIVEN THE
INCREASED NEGATIVE TILT...ANTICIPATE WE WILL BE ABLE TO REALIZE
ENOUGH INSTABILITY FOR CONVECTION AND LOCALLY ENHANCED RAINFALL
RATES. THE DEPICTION SHOWN BY THE VARIOUS HIGH RES CAM SOLUTIONS
SEEM REASONABLE...WITH THE BROAD AREA OF CONVECTION EVOLVING INTO
A NARROW SQUALL LINE THIS AFTERNOON...WHICH WILL QUICKLY PUSH OFF
TO THE NORTHEAST.

IN GENERAL THE PROGRESSIVE NATURE OF THE SYSTEM AND THE EXPECTED
CONVECTIVE LINE SHOULD CAP THE UPPER LIMIT OF RAINFALL
TOTALS...WITH AMOUNTS GENERALLY FALLING INTO THE 1-2" RANGE. THUS
IN GENERAL THE FLASH FLOOD THREAT FROM THIS SETUP SHOULD BE
MARGINAL...WITH 1 HOUR RAINFALL TOTALS GENERALLY MAXING AT AROUND
1-1.5". THE EXCEPTION COULD BE ACROSS PORTIONS OF THE SOUTHERN
APPALACHIANS...WHERE OROGRAPHICALLY ENHANCED RAINFALL WILL PRECEDE
THE CONVECTIVE LINE...RESULTING IN A LONGER DURATION OF STEADY TO
HEAVY RAINFALL. EXPECTING A SWATH OF 2-4" ALONG THE UPSLOPE
FAVORED PORTIONS OF THE SOUTHERN APPALACHIANS...WITH MORE
LOCALIZED 4-6" AMOUNTS A POSSIBILITY IN THE MOST PRONE AREAS. THE
DRY ANTECEDENT CONDITIONS OVER THIS AREA SHOULD LIMIT THE OVERALL
FLOOD THREAT...ALTHOUGH AMOUNTS OF THIS MAGNITUDE COMBINED WITH
THE POSSIBILITY OF RATES APPROACHING 1.5" IN AN HOUR...SUGGESTS
THAT AT LEAST SOME FLASH FLOOD RISK DOES EXIST. FOR THAT REASON
WILL CARRY A SLIGHT RISK ACROSS PORTIONS OF THE SOUTHERN
APPALACHIANS FROM NORTHEAST GA INTO WESTERN SC/NC...SOUTHWEST VA
AND SOUTHEAST WV.

CHENARD
$$




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