Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 552 AM EDT Sat Mar 24 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 24/1200 UTC thru Mar 27/1200 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 early today 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 this 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) -- borne by 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 latest 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/strengthen the jet at upper levels within the trough base across 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 per the GFS and ECMWF. As such, the heaviest 24 hour qpf (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, short term rainfall rates will be considerably hampered. Therefore, the risk of excessive rainfall over this region at this point remains below the marginal category. ...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. Days 2/3 ...Pacific Northwest/Upper Intermountain West/Northern to Central Rockies... A myriad of shortwaves swinging from the Pacific Northwest eastward into the northern/central Rockies will focus a persistent period of precipitation, with snow across the higher elevations. On Day 2, the best focus should be over northeastern Wyoming into the Black Hills of South Dakota given favorable moist upslope flow. A consensus of solutions favors locally heavier amounts over this region while lighter activity prevails over the remainder of the Intermountain West/Rocky Mountains. By Day 3, a 850-mb low swinging through the central Rockies should spread a separate axis of precipitation over much of central Colorado. While much of the action shifts toward the middle of the nation, heights will build over the eastern Pacific while sufficient moisture transport takes aim at the Pacific Northwest coast. This should maintain a steady period of lower elevation rain/higher elevation snow over the western half of Washington down into northwestern Oregon. A three-way blend of the 00Z NAM/GFS and 12Z ECMWF was utilized throughout the western U.S. ...Great Plains/Ozarks/Middle to Upper Mississippi Valley/Great Lakes/Ohio Valley... An active period will unfold across the center of the country, particularly by Day 3 as an increasing threat for flash flooding builds from central Oklahoma northeastward into the southern half of Missouri. Given the increasing signal noted in the 00Z guidance, decided to introduce a slight risk area from the Oklahoma City metro area northeastward across southwestern Missouri along I-70 into St. Louis. On Day 2, precipitation across the middle of the country will be focused in two primary areas. Across the Dakotas, a north/south oriented frontal zone extending from a wave over Manitoba will spread a narrow axis of precipitation over the north-central U.S. The guidance has remained in decent agreement favoring the best action over eastern North Dakota where the strongest low-level convergence should set up. Otherwise, a broad shield of light to moderate precipitation is likely from Nebraska up to the Canadian border given broad ascent with the approaching longwave trough. Looking toward the Arklatex and Ozarks regions, loosely organized convection should continue in close proximity to a poleward lifting warm front. Weak upstream height falls may lead to slightly better organization nearby the eastward ejecting surface low and attendant frontal boundaries. Additionally, some dryline thunderstorms are possible across central Texas although the 00Z/18Z GFS are on their own with the more robust QPF output. Comparing instability parameters, it appears the GFS shows much greater surface-based CAPE numbers suggesting much more pronounced vertical motions along the dryline. Decided to downplay this scenario favoring broad, lighter amounts. A combination of the 18Z GFS/12Z ECMWF/18Z in-house pseudo-ensemble bias-corrected model was utilized with more emphasis on the latter two solutions. By Day 3, much strong forcing arrives within the southern branch of the jet stream. 500-mb winds pick up significantly as a shortwave rounds the base of the trough across the Four Corners region. By Monday evening, 50 to 60 knot mid-level winds overspread a rapidly moistening boundary layer across the Southern Plains. The upstream trough continues to re-load over Arizona/New Mexico which favors a continued period of southwesterly flow aloft across the center of the country. Unidirectional low/mid-level flow is expected to unfold across Oklahoma northeastward into Missouri which should focus a steady period of organized, heavy rainfall, particularly Monday evening into the overnight hours. 6-hour rainfall totals may reach the 1 to 1.50 inch range during this period which will enhance the risk for flash flooding. Models have come into much better agreement with many 00Z solutions increasing amounts over the region. Decided to favor a combination of the 00Z ECMWF/GFS/UKMET/CMC given the solid consensus noted. To the north, additional shortwave energy is forecast to slip through the Northern Plains/Upper Midwest. It should be a quick mover as the north/south convergence zone accelerates downstream. ...Southeastern U.S.... An exiting frontal zone across the southeastern U.S. will favor a period of moderate rainfall over the region early on Day 2. A wave of low pressure is expected to push off the coast with residual activity lingering across southern Georgia. However, with surface ridging extending down into the eastern Gulf of Mexico, much of the remaining precipitation would become more elevated and less concentrated in nature. A multi-model consensus was utilized which agreed on the heaviest amounts ending 26/0600Z. Hurley/Rubin-Oster Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml