Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 232 PM EDT Tue May 01 2018 Prelim Day 1 QPF Discussion Valid May 02/0000 UTC thru May 03/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Central Plains into the Mid/Upper MS Valley and Great Lakes... Convection will expand in coverage this afternoon/evening across the Central Plains and MS Valley. In general this convection will be progressive in nature today, however continue to focus on two areas where locally heavier rainfall and a greater flash flood risk may exist. The southern area is from northern KS into northwest MO and far southeast NE, and will account for the potential of slow moving and/or training supercells that should develop this afternoon and evening. This activity may train along a strengthening stationary front in the area, and will have the benefit of stronger CAPE and 850 mb moisture transport into the boundary. Limiting factors here are the displacement from the better synoptic forcing and the presence of a CAP. Thus unclear if this activity will indeed become organized through the evening...or if it will tend to remain more isolated in nature and weaken after sunset. Generally see a mixed signal amongst the 12z HREF members...although recent runs of the HRRR and Experimental HRRR are more aggressive with QPF and organization. Confidence is only medium...but given the impressive 850 mb moisture transport and presence of a stationary boundary...tend to think convection will indeed become more organized in nature. Thus leaned the WPC QPF towards the more aggressive solutions, such as the Experimental HRRR and HRW ARW. The consensus amongst solutions is for a further south axis than our previous forecast, thus adjusted in that direction. The northern area across northeast IA into far southeast MN and southern WI will have the benefit of stronger synoptic forcing in the right entrance region of a strengthening upper jet. Limiting factors here will be CAPE and the potential for the convection further south robbing some of the inflow further north. However, think CAPE will be sufficient enough for convection...and 850 mb moisture transport vectors suggest enough of a separation from the southern activity to potentially limit the aforementioned robbing of inflow. Thus, while not a sure bet, looks pretty good for a second convective maximum near the MN/IA border into southern WI tonight, with some training possible along the front in the area. Have noted a southward trend in this axis in recent HRRR runs. Thus WPC shifted in this direction as well...with our forecast weighted towards the 16z HRRR, with some of the HRW ARW and ARW2 blended in as well. Convection should be on a downward trend Wednesday morning. However anticipate another round of convection to develop Wednesday afternoon as height falls begin to overtake the dry line over the Central Plains. Some model differences on exactly where this convection initiates during the afternoon hours. The 0z ECMWF and 12z UKMET were in good agreement focusing on central KS. Given some consistency with this location, WPC QPF is weighted towards these solutions Wednesday afternoon. The high res models can sometimes struggle with this more synoptically driven initiation at longer lead times...thus leaning towards the global UKMET/ECMWF is sometimes better in these situations...with the high res becoming more useful as we get closer to the event and/or convection is being driven by more mesoscale processes. ...Western U.S.... The amplifying upper trough will generate a broad area of generally light-moderate QPF -- which given the degree of forcing (broad area of favorable upper level difluence) -- will not be linked exclusively to the elevated terrain. Some locally heavy rain is possible across the southwest, although limited instability should generally cap the rates. WPC generally followed a multi model blend weighted towards the 12z HREF mean across this area. Chenard