Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 341 PM EDT Tue Apr 17 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 18/0000 UTC thru Apr 21/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Northern to Central Plains---Upper MS Valley toward the lower Great Lakes... The next in the series of mid-level shortwaves moves through the Rockies-Great Basin into the northern/central high plains late this afternoon and evening engulfing the region tonight before moving into the Upper MS Valley on Wednesday. Precipitation concentrated along the meridionally-oriented inverted trough over western Northern Plains early on will shift southeast early Wednesday across the lower portions of the MO River Valley/Upper MS Valley as the shortwave strengthens into a mid-level closed low. The enhancing area of overrunning spreads east-northeast across the Upper MS Valley toward the lower Great Lakes Wednesday afternoon within a region of low- to mid-level frontogenesis. Snow will be the primary precipitation type across these areas---with the best chance of late season heavy snows from northeast NE...far southeast SD---central to northern Iowa---far northwest IL into southern WI. The precipitation forecast relied upon the in house experimental bias corrected ensemble mean and 12z ECMWF to mitigate recent model spread. See the latest QPFHSD/Heavy snow and icing discussion for additional winter weather details. ...Northern NY state into northern New England... The deep layer cyclone over northern New England-southeast Canada lifts slowly northeast into Atlantic Canada. Scattered rain and snow showers possible underneath the upper trough, with the best chances of any measurable precip from northern NY state into northern New England. ...Northern California into the Pacific Northwest... The third in the series of strong systems to affect the western U.S. will push from the northeast Pacific and toward Northern California Wednesday afternoon. The expected timing of this system should keep the most organized precipitation offshore until Wednesday night when a greater inland push is likely across California into the Great Basin. Mostly light precipitation amounts are expected from the terrain of western OR and WA into northern California. The precipitation forecast here trended towards a 12z GFS/12z ECMWF compromise. Days 2/3 ...California to Desert Southwest... An upper-level trough with a closing low off the northern CA coast Wednesday evening will deepen and shift southeast through the Great Basin the turn east over the Four Corners by Friday evening. Inflow from northern Mexico into this system will limit moisture with PW around 0.25 inch. Light shower activity is expected over CA/NV/UT/AZ Wednesday night through Thursday night with max QPF around a quarter inch for preferred higher elevations in the cyclonic flow. The slower 12Z ECMWF was preferred in WPC QPF with some 12Z NAM and a little 12Z GFS (which seems too fast given the closed low) included. ...South-Central Rockies and South-Central High Plains... A southerly fetch ahead of the closing low moving east from CA Thursday night will draw a mix of Pacific and western Gulf moisture into the southern high plains. PW of 0.5 to 1.25 inches will wrap into the system, producing light to moderate qpf along the Rockies in CO/NM late Thursday night, spreading into the adjacent high plains (which are quite parched) Friday. Despite the depth of the low, thicknesses remain great enough to limit snow to high elevations. The slower 12Z ECMWF was preferred in WPC QPF with some 12Z NAM and a little 12Z GFS (which seems too fast given the closed low) included. ...Lower Rio Grande Valley... Deep tropical moisture from the western Gulf flows across the lower Rio Grande Thursday with terrain enhanced precip west of the river both Days 2 and 3. Little precip is expected in south TX from this system due to lack of lift. ...Great Lakes to the Northeast/Northern Mid-Atlantic... A quick moving low pushes through the mid-CONUS ridge Wednesday which weakens the system over the upper Midwest until it merges with a lobe of the exiting low over the eastern Great Lakes Thursday and redevelops over the northeast through Friday. Expect diminishing QPF rates Thursday evening as the low moves from WI to MI then increasing to moderate in places over NY state and New England Thursday into Thursday evening as 0.75 inch PW air flows into the system. Wrap around northwesterly flow will also promote lake effect precip off the eastern Great Lakes. Good model agreement with this low with a non-CMC blend used for WPC QPF that favored the 12Z ECMWF which came in a little lighter than the 00Z run. Oravec/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml