Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 429 PM EDT Mon Apr 23 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 24/0000 UTC thru Apr 27/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Eastern / Southeastern U.S... A closed deep layer low continues to slowly move east across the TN and OH Valley this afternoon....bringing with it a broad area of showers and embedded thunderstorms. In general, there continues to be a displacement of the better instability and the stronger low level moisture transport axis. Thus the more organized area of precipitation across SC/NC is more stratiform in nature...with just localized heavier convective cores on the southwest flank closer to the instability pool. Meanwhile, further southwest within the instability pool weaker low level moisture transport, lower PWATs, and less of a low level focus, is resulting in less organized convection (although still locally and briefly heavy). Thus not the best setup for flash flooding across the Southeast and eastern Mid Atlantic...although some isolated issues could still develop where any heavier convective cores are able to briefly train. Closer to the closed low we are seeing steep lapse rates and diurnal heating result in the development of scattered thunderstorms. PWATs are lower here, however cells will likely be slow moving underneath the low, and storm motions are conducive for some repeat south to north cell activity east of the low. Localized 1-3" amounts are possible in the Marginal risk area that was introduced here, and with FFGs lowered over portions of the area due to recent rainfall, some localized flooding concerns could arise. Across the southern Appalachians, the favored upslope regions in western SC/NC/VA will experience an extended period of strong upslope southeasterly low level flow beneath difluent upper flow. Rainfall rates will be lower here given little to no instability, however the persistent strong southeasterly flow into the terrain supports some 0.5" in an hour amounts in the more favored terrain, which given the duration, could result in localized 5"+ totals. Totals of this magnitude should result in some flooding concerns developing with time. The day 1 Slight was trimmed some, as the main threat should primarily be focused into and just east of the terrain Showers and embedded thunderstorms will continue into the day Tuesday across portions of the OH/TN Valley into the Mid Atlantic. Again, instability should be limited within the corridor of best moisture transport and higher PWATS over the Mid Atlantic, thus in general not expecting rates to be all that impressive. Some chance we see instability work into coastal NC which could result in heavier rates here, although also possible this instability and heavier convective activity remains offshore. Over portions of eastern TN/KY should see another round of diurnally driven convection underneath the upper low. Could be some locally heavy rain here, although not expecting much of a flash flood threat at this point. WPC QPF followed a multi model blend here, and ends up pretty close to the 12z HREF mean. In general this results in a modest increase in QPF amounts, with the biggest increases over the eastern slope of the Appalachians, where orographic impacts will be maximized, and underneath the closed low. ...Central and Northern Plains... A wave will move out of the Rockies and into the Plains on Tuesday, bringing a swath of rain/snow into the Plains. Again, generally good model agreement allowed for a multi model blend to be used, with the new WPC QPF looking similar to the 12z HREF mean. Days 2/3... ...Mid-Atlantic and Ohio Valley to the Northeast/Great Lakes... The mid-upper level low moving slowly through the Tennessee Valley today and Tuesday will weaken as it ejects off the Mid-Atlantic coast on Wednesday and interacts with a northern stream trough amplifying over the Great Lakes. Ongoing heavy rainfall along and ahead of the attendant cold front on Day 1, will continue into the very early periods of Day 2, but as the low level inflow weakens along with the upper level forcing, widespread heavy rains should also lesson. Given the day 2 excessive period (valid 12z Tues to 12z Wed) spans the last half of the day 1 qpf period, a marginal risk across the Mid-Atlantic was left basically untouched from the previous shift. On Wednesday, a surface low develops along the Mid-Atlantic coast which should still promote the continuation of mainly moderate to locally heavy rains from the northern Mid-Atlantic to the Northeast Wednesday and into Thursday. Light to moderate precipitation should also develop farther north and west of the low in a deformation zone which is forecast to set up over the Great Lakes. Models continue to diverge somewhat on this, with the NCEP models slightly faster with the northern stream trough than the non-NCEP models. Although the newest model runs seem to be trending faster, so the WPC QPF was based on a compromise of the GFS/ECMWF, with a slight lean towards the faster solutions. The result is a QPF depiction very close to that of the previous shift. ...Great Plains... Upper troughing moving out of the Rockies on Tuesday, will drop southeastward across the central Great Plains on Wednesday and into the lower Mississippi Valley on Thursday. Moisture along the associated frontal boundary, in combination with favorable upper dynamics, should allow for light to moderate rainfall across the central and southern Plains on Tuesday into Wednesday, which moves into the lower Mississippi Valley and Deep South by Thursday. The slightly faster previous run of the ECMWF has come into better agreement with the slower GFS, so the WPC QPF was based on a blend of these two models. Chenard/Santorelli Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml