Quantitative Precipitation Forecast
Issued by NWS

Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
000
FOUS30 KWBC 051305
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
905 AM EDT THU MAY 05 2016

...VALID 15Z THU MAY 05 2016 - 12Z FRI MAY 06 2016...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 E ASJ 35 WNW HSE 10 E MRH 15 S NCA 10 ENE EYF INT 10 N BCB
30 N LWB EKN 30 SE EKN 15 NE LYH 10 ENE AVC 20 E ASJ.


...CENTRAL APPALACHIANS INTO VA/NC PIEDMONT/COASTAL PLAIN...TO
NEAR THE NC OUTER BANKS...

THE 546 DECAMETER CLOSED LOW DROPPING TOWARD THE SOUTHERN
APPALACHIANS REPRESENTS LARGE ANOMALIES...4 STANDARD DEVIATIONS
BELOW THE CLIMATOLOGICAL MEAN...BY THE TIME IT DIGS INTO THE
SOUTHEAST STATES. PERSISTENT DIFLUENT FLOW NEAR AND JUST
EAST/NORTHEAST OF THE CLOSED LOW WILL SUPPORT RELATIVELY FOCUSED
AND PERSISTENT RAINFALL FROM THE CENTRAL TO SOUTHERN APPALACHIANS
AND IN THE PIEDMONT OF VA/NC. WITH THE LARGER SCALE WARM CONVEYOR
BELT WELL OFF THE EAST COAST...HOWEVER...THIS SYSTEM WILL WORK
WITH VERY MODEST MOISTURE AND WEAK INSTABILITY...SUCH THAT VERY
HEAVY OR EXTREME RAINFALL IS UNLIKELY.

IN TERMS OF THE 00Z MODEL SUITE...THE GFS REMAINED A WET OUTLIER
WITH ITS 24-HOUR 3-4 INCH MAXIMUM... THIS TIME CENTERED OVER THE
NORTHERN NC COASTAL PLAIN. WPC QPF IN THIS REGION WAS COMPRISED
FROM A BLEND OF THE 00Z ECMWF AND PARALLEL GFS...WHICH NOTED A
MAXIMA BETWEEN 2-2.5"...COINCIDING TO WHERE THE BEST DIURNAL
INSTABILITY IS EXPECTED TO BE LATER THIS AFTN INTO THE EVENING
(MUCAPES 400-800 J/KG).

AS FAR AS THE EXCESSIVE RAINFALL POTENTIAL ON DAY 1...WPC WILL
MAINTAIN A SLIGHT RISK ALONG THE AXIS OF BEST DIFFLUENCE AND
DIVERGENCE ALOFT TO THE EAST OF THE UPPER LOW...AND ALONG THE WEAK
INSTABILITY AXIS WITH 700-500 MB LAPSE RATES ~7C/KM. PROBABILITIES
OF 3 AND 6 HOURLY QPF>FFG EXCEEDANCE ARE LOW PER THE LATEST SSEO
MEAN (30 PCT OR LESS)...WITH THE LOWER FFG VALUES BEING THE MAIN
FACTOR OVER THE NORTHERN HALF OF THE SLIGHT RISK AREA...WHILE THE
HIGHER QPF PER MODEL CONSENSUS IS WHAT DRIVES THE RISK OVER NC. IN
THE 15Z UPDATE WE TRIMMED AWAY AT THE NORTHERN EDGE OF THE RISK
AREA GIVEN THE MIGRATION OF THE UPPER LOW...BUT DID EXPAND THE
RISK TOWARD THE NC OUTER BANKS BASED ON TRENDS IN THE HRRR AND
OTHER HI-RES GUIDANCE. THERE WAS SOME QUESTION AS TO WHETHER A
HEAVY RAIN THREAT WILL DEVELOP INTO FAR SOUTHEAST VIRGINIA
OVERNIGHT...AS THE SYNOPTIC LOW PIVOTS AND LOW PRESSURE FORMS OFF
THE COAST. PREDICTED CELL MOTIONS ARE AGAIN VERY LIGHT OVER SE
VA...BUT THE GFS APPEARED AS AN OUTLIER WITH ITS SURFACE LOW AND
HEAVY PRECIP FARTHER WEST. OTHER GUIDANCE SUCH AS THE ECMWF/GEM
REGIONAL/UKMET KEEP THE SECONDARY SWATH OF HEAVY PRECIP MORE
OFFSHORE. GIVEN THIS PART OF VA HAS MISSED OUT ON HEAVY RAIN THE
PAST SEVEN DAYS...AND ANTECEDENT CONDITIONS WERE LACKING...WE DID
NOT EXPAND THE RISK AREA IN THAT DIRECTION.


...NORTHERN CALIFORNIA...

GIVEN A SUPPORTIVE MODEL QPF SIGNAL...THERE IS SOME CONCERN OVER
SLOW MOVING CONVECTIVE CELLS LATER TODAY WITHIN A LIGHT LOWER TO
MIDDLE LEVEL FLOW REGIME OVER NORTHERN CALIFORNIA. MEAN 0-6 KM
WINDS OF ZERO TO TEN KNOTS COINCIDE WITH MOST UNSTABLE CAPE VALUES
OF 500 J/KG OR MORE IN THE SACRAMENTO VALLEY AND ADJACENT MOUNTAIN
AREAS INCLUDING THE SIERRAS AND SHASTAS. HIGH RESOLUTION MODELS
GENERALLY PREDICT SPOTTY HALF INCH PER HOUR RATES...WHICH WOULD
FALL SHORT OF FLASH FLOOD GUIDANCE...BUT THE VERY SLOW CELL MOTION
AND POTENTIAL CELL MERGERS COULD EASILY RESULT IN OBSERVED
ACCUMULATIONS OVER ONE INCH IN AN HOUR GIVEN THE SOMEWHAT
ANOMALOUSLY HIGH PRECIPITABLE WATER VALUES IN THE NEIGHBORHOOD OF
0.80 INCHES. MANY OF THE HI-RES MODELS PREDICT SPOT EVENT TOTALS
GREATER THAN 1.5 INCHES THROUGHOUT NORTHERN CALIFORNIA THROUGH
THURSDAY NIGHT.


...EASTERN OREGON...

WITHIN DEEP SOUTHERLY MIDDLE AND UPPER LEVEL FLOW...A SHORTWAVE
RIDGE WILL PUSH DOWNSTREAM INTO MT/ID THIS AFTERNOON...OPENING UP
A MORE DIFLUENT PATTERN OVER EASTERN OREGON...AND OVERLAPPING WITH
A SLOPING BAROCLINIC ZONE TO THE WEST OF A SURFACE FRONT. WITH A
JET STREAK ROUNDING THE EAST SIDE OF THE UPPER LOW IN
CALIFORNIA...DEEP LAYER ASCENT WILL INCREASE ACROSS EASTERN
OREGON...WITH THE GFS DEPICTING PRONOUNCED 700 MB FRONTOGENESIS
AND INCREASING STRENGTH TO THE 850-700 MB SOUTHEASTERLY INFLOW.
THE MODEL QPF SIGNAL OVER OREGON IS SIMILAR TO THAT OVER NORTHERN
CALIFORNIA...SEE SECTION ABOVE. CELL MOTIONS WILL NOT BE AS
SLOW...BUT THE PATTERN IS SET UP FOR BRIEF SSE-NNW TRAINING OF
ECHOES...AND GREATER INFLOW INTO THIS REGION WILL AIDE GENERATION
OF NEW UPDRAFTS ALONG OUTFLOWS.

HURLEY/BURKE
$$





USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.