Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 255 PM EDT Wed May 09 2018 Prelim Day 1 QPF Discussion Valid May 10/0000 UTC thru May 11/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Great Lakes into the OH Valley/Northeast/Mid-Atlantic... A mid-level shortwave will track through the Great Lakes tonight and into the Northeast for Thursday, with a progressive cold front at the surface moving through the same regions. Precipitable water values ahead of the frontal boundary will range from about 1.25 to 1.50 inches, with weak to locally moderate CAPE values in place. Convection is expected to develop later this evening across the Ohio Valley, out ahead of the cold front beneath a broadly diffluent pattern and track eastward in a somewhat disorganized fashion. These showers/thunderstorms should dissipate by Thursday morning with a renewed round expected early Thursday afternoon ahead of the cold front. While confidence remains low with the QPF details, the marginally anomalous precipitable water values (+1 to +2 standardized anomalies) and progressive nature of the boundary should preclude maximum rainfall totals in the 1-2 inch range. Given good mass field agreement, QPF was weighted toward the HREF mean with adjustments locally based on other available guidance, including the 12Z GFS and 00Z ECMWF. ...Southeast New England... A surface low in the western Atlantic will track north as a closed low, currently just east of the Mid-Atlantic coast, moves north-northeast to just east of Nova Scotia by 00Z/11. Ensemble scatter low tracks support a track between the farther west 12Z CMC and farther east 00Z ECMWF/12Z UKMET. This "middle of the road" track brings light rain to Cape Cod, Martha's Vineyard and Nantucket later tonight, with .01 to .10+ over land areas and much heavier rainfall just east over open water depicted by a tight gradient. Outside of the western CMC, there was good agreement with the QPF with WPC following a consensus approach. ...Central Plains... Convection is expected to develop late this evening near the tail end of a front across northwestern KS, with a mid-upper level shortwave likely helping to break CIN in place via forecast soundings across the region. An increase in southerly low level flow beyond 00Z/10 and westerly 850-300 mb flow should allow any organized convection to propagate toward the southeast in a progressive fashion, weakening by early Thursday morning with additional scattered convection near the frontal boundary farther east near MO and the Mississippi valley. Farther north, by Thursday evening, convection is again expected to break out, in advance of a shortwave over the higher terrain of WY and into the High Plains. There were some differences regarding capping, with the 12Z GFS more aggressive with convective development in the 18Z/10 - 00Z/11 period. The NAM, remaining global guidance (outside of the GFS) and hi-res guidance did not support the GFS, so this was the approach taken, focusing nearer to the HREF mean. ...Pacific Northwest---Northern Rockies into the Northern High Plains... A lead area of height falls will be pushing eastward tonight across Montana, out ahead of a larger closed low to the west through Thursday morning. A concentrated area of large scale ascent along and ahead of the associated frontal boundary and between an area of low pressure at the surface over northern WY and a strong ridge over south-central Canada supporting an upslope component into Montana. The model consensus is for .25-50"+ totals from central and eastern MT along with spotty 0.25+ totals farther west nearer to the ID/MT border. Light onshore flow beneath the developing closed low over the Pacific Northwest will support 0.10 to 0.25+ areal average totals into the Coastal Ranges and Cascades with a consensus approach taken for QPF given good agreement. Otto