Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 452 PM EDT Thu May 03 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 04/0000 UTC thru May 07/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Central Plains into the mid/upper MS Valley, Great Lakes and Northeast... A strong shortwave ejecting into the Plains is resulting in an uptick in showers and storms over NE this afternoon...which will expand northeast into the mid and upper MS valley into tonight. The 12z HREF members are unanimous in showing a further north axis for convection this afternoon into tonight over IA/WI/MI and far southeast MN...and are in pretty close agreement with the global models as well (GFS,ECMWF,UKMET). This evolution and trend does seem to make sense, with the wave ejecting into the plains expected to push the current stationary front a bit northward with time. Also, note an impressive upper level divergence signature developing through the day over portions of MN/IA/WI with dual right entrance and left exit region support. Thus would anticipate the better coverage of activity to be where this upper level support combines with the strongest low level convergence. Instability may be a limiting factor here, although appears to be enough for embedded heavier convective cells...especially on the southern edge of activity. The 12z HREF members are all in agreement showing pockets of 3"+ across this area, which makes sense given the potential for training/repeat cells near the front. Will continue to monitor, but given the evolving synoptic pattern and the good agreement seen amongst the 12z HREF members, opted to shift the Slight risk north to include central and northern IA into southern WI. WPC QPF here utilized the 12z HREF members, recent HRRR runs and some of the ECMWF/UKMET. In general these models were pretty close, increasing confidence in the for axis of higher rainfall. A similar approach to QPF was utilized for the Great Lakes into New England. ....Texas into the lower MS Valley... Convection continues to move eastward across portions of northeast TX into eastern OK and western AR/MO. Earlier elevated convection expanded this morning across northern AR and southern MO on the nose of a strong corridor of 850 mb moisture transport. This activity has not been all that intense, but note some embedded heavier convective cores, and some repeat cell activity, resulting in locally heavy rains. Currently the northern edge of the squall line is moving across these same areas of northern AR, southern MO and adjacent areas of IL/KY. This additional rainfall, and at least briefly intense rates, may be enough to result in some flash flood concerns. Thus this Slight risk was maintained, and expanded slightly to the north and east based on latest trends. The convective line further south should generally remain progressive enough to prevent any organized flooding concerns...although isolated concerns could arise in any more susceptible areas. WPC QPF here started with a blend of the 12z HREF members, with some extra weight given to recent HRRR runs along with the ARW and ARW2...which all appear to be handling the eastward progression of this line best. Should see convection redevelop tonight over southern TX as height falls move into the dry line/cold front in place. Also note increasing upper level divergence as the region moves into the favorable right entrance region. This convection is expected to expand in coverage into the early morning hours, and progress eastward with time. Overall note decent model agreement here, although did slightly favor the more progressive solutions of the ARW and ARW2. Thus WPC QPF here was a global and high res model blend weighted more towards the aforementioned more progressive solutions. For the most part storms will be moving enough to limit the flash flood threat. However, PWAT values will be anomalous, and there is some signal for some brief cell training or repeat convection...thus some flash flood risk does seem to exist across portions of south central TX. Days 2/3... ...Western Gulf Coast/Lower Mississippi Valley to the Mid-Atlantic/Southern New England... A baroclinic zone extending from the Northeast to the Southern Plains will move off the Northeast Coast by Saturday morning while lingering from the Mid-Atlantic to parts of the Central Gulf Coast through Sunday afternoon. Moisture will pool along the boundary with precipitable water values ranging from 1.00 to 1.50 inches over the Tennessee and Southern Ohio Valleys into the Central Appalachians on Friday evening into Saturday evening. Dynamics associated with the system will be aided by a short lived coupled jet that will aid in producing maximum qpf amounts ranging from 1.00 to 1.75 inches over parts of the Tennessee and Southern Ohio Valleys from Friday evening into Saturday evening. Farther south along the front, precipitable water values over Western Gulf Coast into the Lower Mississippi Valley will range from 1.75 to 1.50 inches that will move into the Gulf of Mexico by Saturday evening. The heaviest rain will be over the Lower Rio Grande Valley with maximum qpf amounts ranging from 0.75 to 1.00 inches, also from Friday evening into Saturday evening. On Saturday evening into Sunday evening, the heavies qpf will be over parts of the Central Appalachians into parts of the Mid-Atlantic before Sunday afternoon. The maximum qpf amounts will range from 0.75 to 0.90 inches. WPC used a blend of the UKMET and the ECMWF for the starting point for the manual guidance. ...Southeast Coast to Southern Mid-Atlantic Coast... High pressure over Western Atlantic and upper-level low over Cuba will steer a plume of moisture into parts of the Southeast Coast and the Southern Mid-Atlantic Coast with precipitable water values increasing from 0.75 to 1.75 inches over Southeastern Florida on Saturday. Overnight Saturday, the plume of moisture will move northward to the Southern Mid-Atlantic Coast with the precipitable water values, likewise, increasing from 0.75 to 1.50/1.75 inches by Sunday morning before the moisture beginning to be shunted off the immediate coast by Sunday evening. The qpf amounts will be modest over Florida with maximum qpf amounts ranging from 0.10 to 0.25 inches on Friday evening into Saturday evening. The qpf amounts will be much heavier over the Outer Banks of North Carolina with maximum qpf amounts ranging from 2.50 to 3.00 inches on Saturday evening into Sunday evening. WPC used a blend of the UKMET and the ECMWF for the starting point for the manual qpf. ...Northern Rockies to Upper Great Lakes/Middle Mississippi Valley... Baroclinic zone over the Central U. S./Canadian border will move southeastward to the Northeast/Ohio Valley to the Central High Plains by Sunday evening. Moisture will pool along the boundary with precipitable water values of 0.75 inches that will increase to 1.00 inches by Sunday evening. Qpf amounts will be modest on Friday evening into Saturday evening with highest qpf amounts ranging from 0.10 to 0.25 inches over parts of the Upper Mississippi Valley. As the boundary intersects higher precipitable water values, the maximum qpf amounts will increase ranging from 0.25 to 0.45 inches on Saturday evening into Sunday evening over parts of the Upper Great Lakes. WPC used a blend of the UKMET and the ECMWF for the starting point for the manual qpf products. Chenard/Ziegenfelder Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml