Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 501 PM EDT Wed Apr 18 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 19/0000 UTC through Apr 22/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 The QPF pattern composed resembles a compromise of the 12z GFS/12z ECMWF guidance, which matches well with the 12z experimental in-house bias corrected QPF and WPC continuity. ...Lower Great Lakes---central Appalachians--northern Mid Atlantic into southern New England... A strong compact closed low producing late season heavy snow Wednesday afternoon over IA/MN/WI will move quickly eastward to the south of the Great Lakes and across the central Appalachians and Mid Atlantic Thursday. There is agreement on a narrow axis of moderate precip totals within its low- to mid-level frontogenetic band and within a well-defined comma head/deformation axis to the north of the closed low center, though spread remains concerning its latitude. See the latest QPFHSD/Heavy Snow and Icing Discussion for more winter weather detail here. ...Mid Atlantic---southeast---southern Appalachians---gulf coast and Lower MS Valley... A trailing frontal boundary is not expected to have much in the way of precipitation along and ahead of it. The inflow off the Gulf of Mexico ahead of this front is not expected to have a significant southerly component---limiting the potential for above average pw values being entrained. Along with the fast movement of the front and generally poor dynamics, precipitation is expected to be on the light side from the Mid Atlantic into the Southeast, southern Appalachians, Gulf coast, and the Lower MS Valley. ...Oregon---California into the Great Basin... The next in the series of strong closed lows to affect the western U.S. will push inland into northern to central California tonight and into the southern Great Basin Thursday. Precipitable water values are not expected to be anomalous---mostly at or below seasonal norms. This will limit precipitation potential with this system---with mostly light to moderate totals expected across Oregon, California, and into the Great Basin near the best 850-650 hPa frontogenesis at the mid-level cyclone's edge. Days 2/3 ...Great Basin... An upper-level closed low pushes east from the Great Basin Thursday evening to the OK panhandle (per guidance consensus) Friday night. Desert inflow with PW around 0.25 inch will limit precip over the Great Basin to light to locally moderate on preferred ridges through Thursday night. Guidance consensus was used for this portion of WPC QPF. ...South/Central Rockies and Texas to South Dakota... The upper-level low will draw moisture (PW 0.5 to 1.5 inch) from both the Pacific and the Gulf of Mexico across west Texas and up the High Plains into WY. Snow will be limited to the higher elevations of CO/NM Rockies late Thursday night into Friday night as the low passes. Surface low pressure passes the TX panhandle Friday night, shifting the focus to the southern into central Great Plains with a bulls eye from north TX into KS. This is merely one standard deviation above normal, but the slow nature of the closed upper low will result in heavy rainfall over this bull eyes of 2 areal averaged inches Saturday. The 12Z GFS remained slightly more progressive than the well agreed 12Z ECMWF/NAM/UKMET which were preferred for WPC QPF. ...Lower Rio Grande Valley... Showers and thunderstorms will develop on the Mexican side of the Lower Rio Grande Valley where moisture convergence sets up along the Mexican Plateau Thursday night into Friday. Flow from the Gulf will pool along the terrain, aiding in persistent convection through Friday. Little precipitation is expected in south TX from this system due to lack of lift and instability. ...Northeast... Surface low pressure shifts north across Nova Scotia Thursday night. The western edge of moderate precip around the low center looks to merely clip the eastern end of Maine Thursday night with light wrap around precip across northern New England and the Adirondacks through Friday per guidance consensus. ...Pacific Northwest... A northern stream trough shifts east into British Columbia Friday night with the vorticity maximum at the base of the trough crossing the Olympic Peninsula late in the night per both the 12Z GFS and ECMWF. The 0.75 inch PW ahead of the trough axis will allow moderate rainfall to the western slopes of the Olympics and northern WA Cascades late Friday night into Saturday morning. The progressive nature of the trough looks to limit the QPF max to 0.75 inch on areal average. Roth/Oravec/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml