Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 509 PM EDT Fri May 04 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 05/0000 UTC thru May 08/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Texas... In general the convection over Texas has continue to lose organization today...which is limiting the flash flood threat. As we continue to head through the day the environment does not appear to get any more favorable for flash flooding across much of the state. Note a general downward trend in 850 mb moisture transport into the outflow, and a decrease in upper level support as the jet streak quickly pulls off to the northeast out of the area. Thus generally anticipate convection will become less organized through the day, limiting the overall flash flood threat. Will however keep the Marginal risk across east Texas along/near the lingering outflow and prefrontal trough...as PWATs are high enough, and the boundaries slow enough, that some localized issues could arise. South Texas is however one part of the state where some more intense rainfall threat still appears to exist. RAP forecasts suggest this is where the better instability will remain as we head into the afternoon, and also note increasing low level convergence across this region. Thus some chance that convection intensifies this afternoon across the Slight Risk area, and shifts southward with time along the convergence axis. The 12z HREF members and recent HRRR runs do depict some higher QPF totals here as well, and thus think maintaining the Slight risk here is the way to go for now. The boundary over east Texas does not move much into the day Saturday. Thus will continue to see some threat for showers and thunderstorms through the period. Instability is forecast to remain marginal, inflow into the boundary weak, and synoptic support weak...thus not anticipating we will see any organized heavy rainfall threat. ...Lower MS Valley into the TN and OH Valley and western Mid Atlantic... Showers and embedded thunderstorms will track northeastward with time from the lower MS Valley towards the Mid Atlantic region on Saturday. Radar imagery is beginning to show the development of an MCV over northeast TX, which is also depicted in the high res model simulated reflectivities. This feature should track northeastward across the TN/OH Valley with time, bringing an increased convective threat ahead of it. With time, the larger scale shortwave will also move in from the west, with an increasingly favorable upper level jet structure noted. Thus rain chances will continue over the TN/OH Valley and into the western Mid Atlantic through Saturday. Instability will likely be a limiting factor for heavier rainfall rates. However do see enough MUCAPE forecast to suggest locally heavier convective cores are likely, especially on the southern edge of the expanding precipitation shield. Thus anticipate at least some locally heavy rains to spread across KY/TN and into WV and southwest VA into Saturday. Models were generally in good agreement with the evolution through Saturday. WPC QPF generally blended the GFS/ECMWF/UKMET with the HRW ARW and ARW2 for QPF. Did like the high res idea of some higher totals on the southern periphery of the rain shield over the Mid Atlantic (where instability will be a tad higher)...thus the WPC QPF axis is a bit south of the global consensus over WV/VA. ...New England... Overall convection here today and tonight should be progressive in nature...limiting areal averaged amounts. However across far northern New England, closer to the better mid/upper forcing, convection could be a bit more clustered, which could result in some higher totals potentially approaching FFG. Days 2/3... ...Ohio Valley into the Northern Mid-Atlantic... A baroclinic zone over the Mid-Atlantic into Southeast/Central Gulf Coast will move off most of the Eastern Seaboard by Monday morning. The associated short wave will have a short lived coupled upper-level jet that will enhanced the system's dynamics overnight Saturday. Moisture will also pool along the boundary with precipitable water values ranging from 1.00 to 1.25 inches. The system will produce maximum qpf amounts ranging from 0.75 to 1.10 inches over parts of the Eastern Ohio Valley into the Central Appalachians/Northern Mid-Atlantic on Saturday evening into Sunday evening. WPC used a blend of the GFS/UKMET/ECMWF as a starting point for the manual graphics. ...Southeast Coast into the Southern Mid-Atlantic Coast... Moisture circulating around an upper-level high over the Western Atlantic will stream moisture into the Outer Banks of North Carolina and Eastern Florida with precipitable water values up to 1.75 inches that will rapidly drop as a baroclinic zone sweeps the moisture off the coast by Sunday afternoon. The system will produce a maximum qpf amounts ranging from 1.50 to 2.00 inches from Saturday evening into Sunday evening over the Outer Banks of North Carolina. A modest secondary maximum will develop over parts of Central Florida Peninsula with maximum qpf amounts ranging from 0.25 to 0.35 inches also on Saturday evening into Sunday evening. WPC used a blend of the GFS/UKMET/ECMWF as a starting point for the manual guidance. ...Upper Midwest into Northeast/Ohio Valley... A second baroclinic zone extending from Northern Maine, Great Lakes into the Middle Mississippi Valley/Central Plains will advance off the Northeast Coast by Monday morning and southward to the Southeast/Southern Plains by Monday evening. The boundary intersects moisture with precipitable water values of 0.75 to 1.00 inches that will pool along the front. The system will produce a maximum qpf amounts ranging from 0.25 to 0.45 inches over parts of Upper Great Lakes Saturday evening into Sunday evening. On Sunday evening into Monday evening, the maximum qpf will develop over parts of Northern New England with maximum qpf amounts ranging from 0.25 to 0.40 inches. ...Northwest into Northern/Central High Plains... Weakening upper-level low off the Northwest Coast will eject a short wave eastward over the Northern Rockies by Monday evening. The energy will move into air with precipitable water values of 0.75 inches through Monday evening. The qpf amounts will be modest over the region on Saturday evening into Sunday evening with highest qpf amounts over the Cascades ranging from 0.25 to 0.40 inches. On Sunday evening into Monday evening as the short wave energy moves inland, the maximum qpf will develop over parts of the Northern Rockies with maximum qpf amounts ranging from 0.50 to 0.80 inches. Chenard/Ziegenfelder Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml