Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 448 PM EDT Thu Apr 19 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 20/0000 UTC through Apr 23/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 The precipitation pattern forecast for tonight and Friday generally resembles a compromise of the 12z GFS, 12z ECMWF, and 18z in-house experimental bias-corrected QPF. This led to slight increases from continuity mainly across the High Plains, WY/UT border, and across southwest UT. ...Great Basin into the central to southern Rockies The strong closed mid-level low will move steadily eastward through the Great Basin this evening/overnight and into the central to southern Rockies on Friday. Precipitable water (PW) values ahead of this closed low across the Great Basin will be near or slightly above the mean. This should be a limiting factor for widespread heavy precipitation totals with model consensus of .10-.25" areal average totals across the Great Basin and into the southern to central Rockies. Locally heavier totals likely in the favored upslope regions of the Wasatch of UT and into the central to southern Rockies from south central WY---through CO and into north central AZ. Heavy early spring snows likely through the southern Wasatch and through the southern to central Rockies. See the latest QPFHSD/Heavy Snow and Icing Discussion for additional winter weather details here. ...Southern to Central High Plains... PW values expected to become more anomalous to the east of the Rockies into the central to southern High Plains where low-level south-southeasterly flow is expected to strengthen significantly Friday ahead of the upstream closed low. Increasing upper difluence ahead of the Great Basin/Rockies closed low within the increasingly anomalous PW axis will support increasing precip coverage Friday through large portions of the central to southern High Plains. Areal average amounts of .25 to .50"+ with locally heavier totals are possible where meridionally-oriented precipitation bands train for a short period of time. ...Western to northern NY state into northern New England... Surface low pressure moving to the south of New England this afternoon will be pushing into the Canadian Maritimes tonight into Friday with the western peripheries of the associated comma head precipitation area possibly affecting far eastern Maine tonight. In the wake of this low, deep layer cold cyclonic flow will persist from the eastern Great Lakes into New England. This supports scattered snow showers for portions of western to northern NY state and into northern New England. Light to locally moderate precipitation/snowfall totals are possible across these areas. See the latest QPFHSD/Heavy Snow and Icing Discussion for additional winter weather details here. Days 2/3 ...Central-Southern Plains...Lower Mississippi Valley...Western Tennessee Valley... Compact, closed mid-upper level low will migrate slowly eastward through the southern Plains and lower MS Valley during days 2-3 (Friday night-Sunday). This feature is essentially cut off from the northern stream, i.e. within an otherwise broad northern stream ridge, and as such given the Rex Block characteristics, believe the eastern progression will be hindered somewhat. Exit region upper level dynamics ahead of the southern stream trough and associated upper jet streak approaching the trough base will be fairly robust, albeit focused given the compact nature of the low, owing to the stream curvature and degree of upper difluence. Moreover, this will also favor a strong southerly ageostrophic response at low levels, bolstering the deep-layer warm air advection/theta-e advection, evidenced by the uptick in PW and 850-700 mb moisture flux/transport (albeit not overly anomalous). At the same time, limiting factors for a widespread heavy (potentially excessive) rainfall event will be the compact nature of the upper low and the absence of upper jet forcing to the north (which would enhance low-level frontogenesis). Deep-layer instability within the warm conveyor belt (WCB) will be modest at best, with MUCAPES peaking around 1000-1500 j/kg per the GFS/ECMWF during the day on Sat-Sun. In terms of the QPF, WPC stayed fairly close to a blend of the ECMWF-GFS along with the NBM and WPC in-house bias corrected ensemble QPF, which yielded a broad area of 1-1.5+ inches across portions of the southern Plains in to the western TN Valley. In terms of the day 3 excessive rainfall outlook, WPC opted to pull the MARGINAL outlook farther south toward the TX-LA border coast, while extending the SLIGHT a little farther west to northwest to encompass the pivoting/back-building WCB across northeast TX and southeast OK. Oravec/Roth/Hurley Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml