Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 455 AM EDT Wed Mar 28 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 28/1200 UTC thru Mar 31/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Central and Eastern U.S... The northern stream jet streak and associated upper vort lobe will continue to dive se down the upper ridge and into the western periphery of the southern stream trough. This will result in a more progressive tough and surface front by Thursday, in-turn reducing the excessive rainfall threat. Until then however, a well established, slow moving warm conveyor belt (wbc) is continuing to fuel the "Atmospheric River" of deep-layer moisture into the southern Plains northeast across the MS Valley and Ohio Valley, as noted from the ESRL AR detection tool. The continued subtropical connection aloft (from south of Baja) along with the strong low-mid layer moisture transport/flux off the western GOMEX are maintaining anomalous PW values into these regions -- about 2.5-3.0 standard deviations above normal per the SREF and GEFS. Convection should be undergoing a weakening trend later this morning as it outruns the better forcing/instability, as depicted by the trends from the high-res CAMs, although some locally heavy rains should continue from portions of east TX into northern LA, southern AR, and western TN. By late morning into the afternoon, we should see convection regenerate across east TX into western LA, with the short-term rainfall rates bolstered by the uptick in deep-layer instability (mucapes 1000-2000 j/kg). This convection will have strong support synoptically as the southwestern U.S. shortwave finally begins to eject east. Some uncertainty whether this convection forms along the morning outflow closer to the TX Gulf Coast or farther northwest along the actual cold front, or whether both boundaries eventually become active. Given these uncertainties we preferred to stay close to a consensus of the high-res ARW/ARW2 (though not quite the same degree of QPF per the ARW, which noted a fairly broad swath of 5-7+ inch totals from eastern TX into portions of northern LA). The risk for training/repetitive convection will increase once again overnight Wed into Thu morning given the nocturnal uptick in the LLJ, which as the night progresses, will veer and align closer to the mean 850-300 mb southwesterly flow. WPC noted a fairly broad MODERATE risk in the new day 1 Excessive Rainfall Outlook (ERO), from the Upper Tx Coast and eastern TX through northern LA and western MS, encompassing the heavier rain areas moving in during the day Wed along with the potential for additional, slow-moving convection overnight into early Thursday. The moderate risk also incorporates the areas with the highest probs of receiving >2 inches of rain per hour/>3 inches within 3 hours. Days 2 & 3 ...Lower MS Valley/Gulf Coast/TN & OH Valleys/Eastern Seaboard... The period starts on Thursday morning with an ejecting shortwave lifting out of the Arklatex region. Unlike the preceding day, the heavy rainfall event should be on the move as a broad swath of convection sweeps along the Gulf Coast and up into the OH Valley on Thursday. While the attendant mid-level wave will be in the process of being sheared/stretched out on Thursday evening, the surface cyclone is forecast to strengthen a bit on approach into New England. Locally heavy rainfall is likely in close proximity to this wave of low pressure given favorable upper right entrance region jet support coupled with ample moisture feed. While instability may be lacking over the northern extent of the precipitation shield, it will more than make up for it with sufficient dynamics. To the south, ongoing convection from the Day 1 period will likely be sweeping across the upper Texas coast and Lower MS Valley. This should complicate matters a bit given future heavy rainfall will depend on the timing of the likely MCS, position of convective-driven boundaries, and areas where instability remains vs. has been exhausted. With that said, a broad cluster of showers and thunderstorms will progress downstream along and ahead of the cold front with both a severe and flash flood potential. The latter may be somewhat offset by the progressive nature of the convective elements with additional limitation by a lack of high CAPE values. The 00Z GFS suggests numbers possibly reaching 1000 J/kg along the central Gulf Coast midday Thursday. In spite of limiting factors over both mentioned areas, a slight risk was maintained through 30/1200Z. Some local flash flood guidance values across the lower Ohio Valley indicates some inherent risk for hydrologic issues where convection hangs up/repeats. As the system accelerates into New England, a broad swath of moderate rainfall should extend across the lower Great Lakes into the interior northeastern U.S. This would generally focus along the parent low track with areal-average amounts staying in the 0.50 to 1.00 inch range through Saturday morning. Overall, took an equal weighting of the 00Z GFS/12Z ECMWF throughout the period which seemed to offset differences each solution showed. Once this system moves into the Day 1 realm, the convective-allowing models should offer better support for activity which is more instability induced along the southern tier of the country. ...Upper Intermountain West/Northern to Central Rockies/Northern Plains/Upper Midwest... Underneath a closed upper anticyclone across the Yukon Territory, a series of perturbations in the flow will slide southeastward across British Columbia and into the northern Rockies. Vertical motions attached to these waves in conjunction with local upslope flow will spread light precipitation across sections of the northern/central Rockies and into the adjacent Great Plains. A stronger feature crossing the Dakotas late Friday/early Saturday will a better impetus for wintry precipitation over the Northern Plains into the Upper Midwest. Models remain at odds with one another in the handling of the deepening surface low as it races toward the upper Great Lakes. Consistency is not great while QPF solutions sit several hundred miles apart from one another. For instance, the 00Z/12Z ECMWF favors a heavier stripe across ND into northern MN while the 00Z/18Z GFS is down over SD and southern MN/northern IA. Sitting in the middle are the 00Z NAM/UKMET which was the preferred route here. Did offset numbers with ensemble mean use given the uncertain placement of this heavier band of snowfall. ...Western WA... The guidance continues to show a steady plume of Pacific moisture streaming toward the western coast of WA. The grazing blow of the mentioned shortwaves combined with this onshore flow regime will focus periods of precipitation over the Olympics and WA Cascades. Models form a decent consensus here so took a combination of the 00Z GFS/NAM and 12Z ECMWF here. Hurley/Rubin-Oster Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml