Quantitative Precipitation Forecast
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FOUS30 KWBC 180900
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
500 AM EDT Fri May 18 2018

Valid 12Z Fri May 18 2018 - 12Z Sat May 19 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
90 SE HST 85 SE HST 25 SW HST 20 E APF 30 S LAL 10 W VVG
30 SW SGJ 55 E SGJ 80 ENE DAB.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
40 ESE HXD 30 ESE OGB 25 W 45J 10 E RUQ 10 N IPJ 10 SSW AND
20 N RHP 10 ESE TRI 10 SSW BKW 20 SSE 48I 15 NNE EKN 15 SSW JST
20 N AOO 20 SW SEG 10 SSW PHL 10 SE ACY 70 SE MJX.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
35 SW PUM 30 SE PAT 20 WNW CPR 10 NW BFU 30 NNE IKA 10 ENE RCA
45 ENE IEN 20 ENE ICR 25 NE YKN 15 NNW OLU 20 SSE BIE 25 W FOE
EWK 25 NW AVK 45 E LBL 45 WNW GCK 35 S ITR 25 E MNH 35 WSW FNL
35 SW PUM.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 WNW SUS 10 SSE PPQ 20 S PIA 10 NW LAF AOH 10 SSW MNN
15 ENE VTA 15 WSW PKB 25 WNW I16 25 WNW LNP 20 SE SME
25 WNW K8A3 15 NW BNA 30 NNW MSL 30 SSW MKL 25 NNW BYH 10 W POF
30 SSW FAM 20 W FAM 25 WNW SUS.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
40 ESE CHS 35 ENE CHS 15 SSW LBT CTZ 10 SSE GSB 20 NE OAJ
45 SSE NCA 55 ESE SUT 60 SE SUT.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
65 E NTU 30 SSE MFV 10 WSW NGU 15 NNE SFQ AVC 10 SSW DAN
25 W INT 10 NE HKY 15 NW FQD 35 NNE AVL 25 SSE TRI 15 WNW GEV
MKJ 15 ESE BLF 25 SW LWB 20 NW HSP 30 SSE EKN 30 E W99 MRB DMH
10 NNE W29 10 SE DOV 20 SE WWD 40 ESE WWD.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 SSE SIB 50 ENE DGW CDR 15 SE AIA 30 WNW LBF 15 NNE LBF
15 WSW ODX 20 NNE AUH HJH 30 N RSL 50 NNE GCK 50 SE GLD 35 N ITR
35 N LIC GXY 10 N CYS 20 SSE SIB.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 S FVX 15 NNE DAN 10 ESE MTV MWK 10 ESE HLX 15 SSE BCB
20 NNE BCB 20 N ROA 20 S HSP 30 ESE HSP 20 SSW CHO LKU 15 SE NYG
20 S NAK 10 SE ESN SBY 15 W SBY 20 S NHK 20 NNW AKQ 25 S FVX.


Day 1...

...MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN
AND CENTRAL VA INTO SOUTHERN MD...


...Mid Atlantic and Southeast...

Another wet day with locally excessive rainfall expected across
the area. The mid and upper levels remain broadly diffluent ahead
of the large scale trough, with multiple weak embedded shortwaves
continuing to progress northward in the southerly flow. PWATs
remain in the well above normal to near climatological record
range, supportive of efficient rainfall processes. A nearly
stationary west to east boundary will remain draped across central
VA through much of Friday. This boundary will once again act as a
focus for backbuilding/training convection, with continued 850 mb
moisture transport into the boundary. Modest instability should
again develop along this front, allowing for an uptick in rainfall
rate potential this afternoon/evening. North of the front,
rainfall will be more stratiform in nature, resulting in generally
lower rates. South of the front, convection will generally be less
organized, but still capable of briefly intense rates. Thus the
best focus for heaviest rains and an increasing flash flood threat
will be along/near the front, and into the northwest NC and
southwest VA terrain, where differential heating across the higher
terrain and some upslope component to the flow may enhance
convective coverage and rainfall magnitudes. Overall a good
consensus amongst the 0z HREF members to focus higher amounts near
this front, with high probabilities of exceeding 3" and low to
moderate probabilities of localized 5"+ amounts. Overall appears
like a very similar setup as yesterday. Given this additional
rainfall potential is over increasingly saturated grounds, a more
widespread and significant flash flood threat appears to exist
today. Thus have opted to go with a MODERATE risk of excessive
rainfall across portions of southern and central VA into southern
MD.

North of the front, rainfall will generally be more stratiform in
nature, keeping rates lower. However could still see some
localized heavier rates, especially tonight as the whole system
lifts northward. This northward shift should dissipate the
stationary boundary over VA, and thus think by this time we should
see an end to the better backbuilding/training setup over
southern/central VA. However should still see showers and embedded
thunderstorms shifting north across the Mid Atlantic, which may
focus into a narrower north/south axis. Instability should be
decreased by this point, tempering rates, although localized
heavier amounts are possible where any south to north training is
able to occur. Given the lower FFG here, will maintain a Slight
risk across northern VA into portions of MD and DE.

Will also will need to watch portions of eastern NC/SC through the
day. Looks like a low level convergence axis will try to set up
here, with a long fetch of moisture streaming north out of the
Caribbean as well. Thus possible we see south to north streamers
of convection through the day, capable of producing very heavy
rainfall rates. FFG is higher here, and not certain we will see
convection organize enough to pose a more widespread flooding
threat. However, the 0z HREF does indeed shower some higher
probabilities of exceeding 3" and 5" here, and do think the threat
is a bit more elevated than it was yesterday. Thus see no strong
reason to remove the Slight risk that was in place, and thus just
adjusted it a bit to better fit the newer guidance.

...Central Plains...

West to east training of convection will continue across NE into
the morning hours on the nose of strong 850 mb moisture transport.
Anticipate this convection will weaken through the morning as we
see the diurnal weakening of the low level jet. By afternoon
another round of storms will fire across CO/WY. Will be an even
more favorable synoptic pattern today compared to yesterday, with
the broad mid/upper level trough and embedded shortwaves moving
closer. PWAT values will remain well above normal to near
climatological record values as well. Anticipate a complex
evolution to convection across this area with multiple factors at
play. Will have a very strong gradient in place over the western
Plains, with moderate/high instability forecast. An 850 mb low
over southern CO will strengthen southerly and easterly flow ahead
of the instability gradient feeding into convection. Will also
heave a northern stream cold front progressing south with time
across the area. In general anticipate we will see widespread
convective development, with some chaotic storm motions resulting
in areas of training and repeat convection. Will likely end up
seeing all this convection result in a well defined MCV taking
shape as well, adding even more complexity to the evolution. Thus
for QPF, WPC generally followed a blend of the 0z HREF members.
Tough to pin down exactly where the heaviest amounts will fall,
but think at least some flash flood risk will likely evolve with
time into tonight from northeast CO into southeast WY, western and
central NE, and western KS. Storms further south into OK/TX should
generally be less organized and more progressive being displaced
further form the better deep layer forcing and better 850 mb
moisture convergence.

...Mid MS Valley into the OH/TN Valley...

Scattered to widespread convection is likely across this region
today underneath the mid level shear axis and embedded shortwave
features. This setup supports slow moving convection, and periodic
cell mergers, and with PWATs running well above normal, locally
excessive rainfall will be possible. WPC generally followed close
to the 0z HREF. As was the case in previous days underneath the
shear axis, amounts will likely be higher than the global models
suggest, and thus not surprising that this blend results in a
pretty significant increase in amounts from our previous forecast.
Will go with a Marginal risk to account for the localized flash
flooding threat.

Chenard




$$





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