Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 642 PM EDT Wed May 02 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 03/0000 UTC thru May 06/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Central Plains into the Mid Mississippi Valley and Upper Midwest... Complex convective evolution today leads to below average confidence on placement of Slight Risk areas and QPF. The greatest flash flood threat through tonight will be from central KS into far southeast NE, southwestern IA, and western MO. Progressive, yet robust, convection, will produce heavy rain rates this evening, in some cases crossing paths with wet antecedent conditions from yesterday's activity. Later in time the associated outflow could stall against the low level flow - enhanced speeds in the favorable 03-09z time frame - resulting in more training across southern/eastern Kansas into parts of Missouri. There was quite a bit of model spread noted amongst the 12z high res guidance. In general we prefer the ARW cores during events such as today...thus will stick with mentioning only the HRW ARW, ARW2 and the Experimental HRRR. Both the 12z ARW and ARW2 move convection rather quickly off the front, which ends up resulting in a QPF maximum farther south. Meanwhile, the 12z and again 18z Experimental HRRR runs keep convection along the boundary longer, before eventually propagating it off to the east...resulting in a QPF axis farther north, from central KS into central MO. Confidence is not high, but as of 22z observational trends seemed to support our choice to lean toward the Experimental HRRR and an eventual axis from central KS to central MO. The uncertainty continues downstream into portions of IL/IN/WI. Both the ARW and ARW2 show some pretty impressive totals across portions of central IL/IN, as multiple rounds of convection move across this area. However, we note that the 12z Experimental HRRR was much less aggressive, and more timely afternoon HRRR runs depict relatively progressive convection overall. A small Slight Risk of excessive rainfall was added to the Day 1 Outlook at 20z, based on jet entrance region dynamics / frontogenesis which may organize the early activity into training lines before being overtaken by the MCS coming out of the lower Missouri Valley. The flash flood risk is somewhat contingent on the early activity becoming organized, otherwise issues with rapid runoff may be more isolated. Convection will again expand in coverage Thursday afternoon over portions of IA/MO and vicinity. Again some disagreement was noted in the models by this time. Given the longer lead time and synoptic nature of the system by this time...we did incorporate more of the global guidance (GFS,ECWMF,UKMET) into this portion of the forecast. Did blend in some of the 12z ARW and Experimental HRRR as well, as they generally seemed in pretty close agreement to the global consensus, and added in some more convective detail. ...Northeast CO into southeast WY and western NE... The approach of the closed mid/upper level low will result in increased synoptic ascent over this area into tonight. Instability will be limited, but continued cooling aloft should be able to support some embedded heavier convective cores. Will be a long duration of showers and these heavier embedded cores, with repeat and slow moving cells a concern. Given the good high res agreement for isolated amounts exceeding 1 inch over this area, an isolated flash flood risk could develop. ...Central TX... Dryline convection from the Concho Valley northward will eventually spread into central TX. WPC generally followed a blend of the 12z HREF members and recent HRRR runs for QPF here. In general convection should be progressive in nature, keeping the flash flood threat low. There could, however, still be some briefly intense rates which could cause issues in any more susceptible areas. Something to watch will be whether any storms get anchored to the dryline over south central TX...which is suggested by the HRW ARW and ARW2. If this were to occur then a greater flash flood risk could develop, so something to monitor. Days 2/3... ...Great Lakes/Northeast and Southern Plains to the Mid-Atlantic/Tennessee Valley/Lower Mississippi Valley... Baroclinic zone extending from the Northeast to the Great Lakes then southwestward to the Southern Plains will move eastward off the Northeast Coast by Friday evening as the southern end of the boundary will extend from the Mid-Atlantic to the Western/Central Gulf Coast by Saturday evening. Moisture will be pooling along the front with precipitable water values ranging from 1.75 to 1.50 inches that will drop off to 1.00 to 1.25 inches over parts of the Mid-Atlantic to parts of the Central Appalachians by Saturday evening. The moisture axis will move southward to the Central/Western Gulf Coast with precipitable water values ranging from 1.75 to 1.50 inches by Saturday evening, too. The maximum qpf will develop over parts of the Southern Plains with maximum qpf amounts ranging from 1.50 to 1.75 inches from Thursday evening to Friday evening. A secondary maximum will develop over parts of the Upper Great Lakes with maximum qpf amounts ranging from 1.00 to 1.20 inches. A tertiary maximum will also develop over parts of the Eastern Ohio Valley with maximum qpf amounts ranging from 0.75 to 1.00 inches on Thursday evening to Friday evening. From Friday evening to Saturday evening, maximum qpf amounts will be over the Lower Rio Grande Valley with maximum qpf amounts ranging from 1.00 to 1.25 inches. A secondary maximum will develop over the Central Appalachians will maximum qpf amounts ranging from 0.75 to 1.10 inches. WPC used a blend of the GFS and the ECMWF as a starting point for the manual graphic. ...Northern Intermountain Region to the Upper Great Lakes... A second baroclinic zone over West-Central Canada, on Friday morning, will move southward into the Northern Plains into the Upper Great Lakes by Saturday evening. Moisture will be pooling along the front with precipitable water values of 0.75 inches. The system will produce maximum qpf over parts of the Upper Mississippi Valley/Upper Great Lakes with maximum qpf amounts ranging from 0.25 to 0.55 inches on Friday evening into Saturday evening. A secondary maximum will develop over parts of the Northern Rockies with modest qpf amount ranging from 0.10 to 0.25 inches. WPC used a blend of the GFS/ECMWF/UKMET for the starting point of the manual guidance. The 12Z UKMET was added to the blend as the ECMWF was too dry over the Upper Mississippi Valley/Upper Great Lakes and the GFS was a little too wet over the area. Chenard/Ziegenfelder Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml