Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 330 AM EDT Sat Mar 24 2018 Prelim Day 1 QPF Discussion Valid Mar 24/1200 UTC thru Mar 25/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Mid MS Valley to the lower OH Valley, TN Valley, Southern Appalachians, and lower mid Atlantic Region... A vigorous compact shortwave and attendant southern stream upper jet streak will slide e-se from the central Plains into the lower OH by late Saturday night. This southern stream feature, which impacted central/southern CA earlier this week with torrential rains, will continue to foster a surface low from eastern KS early Saturday morning into the eastern TN Valley by early Sunday. Ahead of this system, a considerable amount of moisture return from the Gulf of Mexico within the compact warm conveyor belt (wcb) -- transported via robust southwesterly low-level inflow (40-50kts at 850 mb) -- will generate 850 mb moisture flux anomalies between 3-4 standard deviations above normal per the SREF and GEFS. Meanwhile, another upper shortwave across western Quebec will dive south and reinforce the northern stream trough across the northeast. This in turn will sharpen the confluence and thus strengthen the jet at upper levels within the trough base (into the mid-Atlantic region), providing an uptick in northern stream forcing (upper divergence and low-level frontogenesis) within the right-entrance region of the jet streak. The coupled dynamical forcing from both the southern and northern streams will allow widespread mod-heavy pcpn to blossom in a nw-se fashion north of the surface warm front -- while the latter (ensuing strengthened low-level frontogenesis) helps to narrow the corridor of steep isentropic lift, leading to a tightening moisture cutoff to the north. The 00Z models were more tightly clustered with the axis of heavier pcpn compared to yesterday, though WPC continued to rely more heavily on the members that were situated on the southern edges of the guidance spread (including the ECMWF). This given the weaker static stability farther south, toward the instability axis (MUCAPES at least ~200 j/kg) where 700-500 mb theta-e lapse rates become negative according to the GFS and ECMWF. In this scenario, the heaviest qpf from 12Z Sat-12Z Sun would set up across south-central to southeast KY into southwest VA, eastern TN, far western NC, and far northern GA (western slopes of the Smoky Mtns and southern Appalachians). Highest areal-average totals of 1.5 to 2.0 inches were noted, some of which expected to fall as snow (please refer to the latest QPFHSD for further details). Given the lack of deep-layer instability, evidenced by the limited variation with the max qpf among the high-res CAMs, the risk that the short-term rainfall rates would reach excessive thresholds at this point remains below the marginal threshold. ...Western U.S... A very active and wet regime will persist across the West on Sat. A large 500mb closed low and deep upper trough offshore of the PAC NW will sweep into the region. The main mid to upper circulation will dig south and east toward southern OR/northern CA, while the main thermal zone pushes into the inter-mountain West. Overall the onshore flow will remain intact but maybe elevated in one location. A dynamic upper jet/jet streak will focus strong PAC moisture transport into swrn OR and northern CA for possible 1-2 inch areal avg qpf amounts, especially from the coast to terrain. Overall WPC followed a blend of the gfs and ecmwf with the hi-res suite for qpf amounts. Hurley/Musher