Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 524 AM EDT Fri Apr 06 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 06/1200 UTC thru Apr 09/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 For the 12z issuance: little in the way of change to the 06z issuance. Small tweaks were made to the Southern Plains and Lower MS Valley in the first period (through 06/18z), based primarily on radar trends and the most recent HRRR. ...West Coast/Intermountain West... An atmospheric river transports deep Pacific moisture across portions of northern and central CA during Day 1. The increasing low level flow focuses the deep moisture on the higher terrain, and the resulting heavy rainfall over the higher terrain could produce flash flooding. For the most part, the WPC QPF was based on a blend of the 00z GFS/12z ECWMF, augmented by the 00z WRF ARW and the 00z HREF mean. Ahead of a short wave located near 38N 145W early on Day 1, an atmospheric river transports deep Pacific moisture toward the northern CA coast (where the most recent blended TPW product showed 1.75 inch precipitable water air sitting just off the northern CA coast). The low level flow ahead of the short wave (and associated surface front) increases to 25 to 35 knot from the west southwest between 06/18z and 07/00z, as the low level flow becomes orthogonal to the terrain over the northern CA coastal range, as well as the northern and central Sierra Nevada Range. The low level flow transports 1.50/1.75 inch precipitable water air (which is about five standard deviations above the mean) north of SFO in northern CA, with lower values the Sierra Nevada Range. During this time, the 00z HREF mean showed an area of 100/250 J/KG of MUCAPE coming ashore with the deep moisture plume in the 07/00z to 07/06z time frame. This is expected to add a convective element to the precipitation character during this time, as both the moisture and instability drop slowly south ahead of the surface frontal boundary. There was a multi model signal for 4.00 to 5.00 inches of qpf across the Coastal Range in Northern CA, as well as across the favored upslope flow regions of the northern and central Sierra Nevada range. Given the deep moisture and instability in the upslope flow, these amounts seem plausible. It should be noted that the 00z HREF mean and the 00z WRF ARW showed locations with 6+ inch rainfall amounts, but since the inflow is not as high as 50 knots for a longer period of time, these amounts may end up being maximum amounts in the higher terrain. The short wave forms a closed mid level low just off the Pacific Northwest coast by 07/12z, and the moisture plume drops south across central CA toward the end of the period. The highest rainfall amounts toward the end of the Day 1 are expected to drop south into the central Sierra Nevada range. Further east, some of the Pacific moisture survives the trip into the Great Basin and the Central and Southern Rockies. Upslope ahead of an arctic front stretched from the Northern Rockies into CO will be augmented by low level convergence, which is expected to produce areas of 0.75/1.25 inches of qpf over the CO Rockies during Day 1. ...Southern Plains across the Lower MS valley into the Mid Atlantic... Moisture and instability focused on a frontal boundary extending from the Southern Plains across the Lower MS Valley into the Mid Atlantic states will feed convection that produces heavy to excessive rainfall over portions of the Lower MS Valley during Day 1. The WPC QPF was based primarily on a blend of the 00z GFS/12z ECWMF (as well as the 00z WRF ARW and 00z HREF mean), as these models showed a qpf maximum further south, closer to the best instability. Moisture and elevated instability along a low level boundary extending from far northeast TX into southern AR will feed convection extending from eastern OK into central and northern AR and the western TN Valley during the first part of Day 1. There should be sufficient convective coverage to produce an axis of 1.50 to 2.50 inches of qpf along the aforementioned areas through 06/21z or so. As the convection drops south on an outflow boundary toward a developing cold front, a 35 knot low level southwest flow transports 1.25/1.50 inch precipitable water along the front from the Southern Plains into the Lower MS and western TN Valley. There is still some model spread concerning the placement of the qpf maximum ahead of the front, with the 00z NAM apparently too far north (and furthest away from the best instability). The 00z GFS/12z ECWMF were closer with the placement of the best instability, keeping it south of the AR/LA boundary after 07/00z, and the 00z HREF mean also keeps the best instability south of this line. Model soundings showed moderate instability near the front, and given the position of the best surface based instability, an axis of 3.00 to 4.00 inches of qpf was extended from northern LA across west central MS. Three hour flash flood guidance values here are generally between 2.00/3.00 inches, and the focuses instability along and south of the front could result in some training in the 07/00z to 07/06z time frame, before surface low pressure forms on the front over MS/AL, and drags the front south. Deep moisture and marginal instability could result in an axis of 1.50 inches of qpf into portions of eastern TN, but drier air to the north (supporting a growing model consensus) suggests that the northern extent of the qpf could be shunted south into central and southern VA by the end of day 1. ...Great Lakes/Northeast.... A short wave tracking out of the Upper Great Lakes early on Day crosses northern New England by the end of the period. Moisture focused on a surface low associated with the mid level system will produce a swath of qpf across the region. The WPC QPF was based mainly on a blend of the 00z GFS/12z ECWMF. Regional radars showed an increasing area of precipitation ahead of a short wave over WI early this morning (based on the GOES-16 mid level WV loop), and the mainly warm air advection induced precipitation shield is expected to expand across the Great Lakes into the eastern OH Valley during the first part of Day 1. Moisture ahead of the short wave (and its attendant surface low, which tracks across Quebec and northern New England into New Brunswick) is expected to extend into the Northeast after 06/18z, with the best mid level lift between 06/18z and 07/00z. For the most part, qpf amounts are expected to remain below 0.25 inches, with some local 0.35 inch amounts possible in the higher terrain of eastern NY state, central and southern VT into NH, as the broad upslope flow could enhance precipitation rates. Days 2/3 Pacific Northwest, California eastward through the Northern and Central Rockies: A deep and occluding cyclone will move to Vancouver Island Saturday before weakening in the western North American ridge Saturday night. The ongoing potent atmospheric river is directed south of the low and its cold front. This cold front comes ashore in OR Saturday morning and shifts down the CA coast through Saturday night as it weakens. The added forcing of the cold front into anomalously high PW in the 1.5-1.75 inch range (5-6 standard deviations above normal) will result in a notable Saturday round of heavy rain over the Sierra Nevada where snow levels will remain high in the prefrontal activity. A marginal risk for flash flooding was maintained for the Sierra. Farther north in western OR/WA, widespread precipitation is expected through much Saturday into Sunday, initially from the height falls and frontal passage early Saturday, then with persistent and strong onshore flow south of the cyclone bringing in 0.75 inch PW (which is 3 to 4 standard deviations above normal). Areal average amounts over the two-day period will range in the 2-4 inch range, with locally higher amounts expected over favored terrain in the Cascades and Olympics/Coastal Ranges. A marginal risk was expanded here given the enhanced rain rates on upslope areas. While the heaviest QPF will focus over California and from the Cascades westward through Washington/Oregon, a significant amount of Pacific moisture will continue to stream inland through the Northern and Central Great Basins/Rockies. As short wave energy rounds the base of the eastern Pacific closed low and ejects eastward...expect widespread precipitation to break out on Saturday, with the heaviest amounts across northern Washington and northern/central Idaho in response to enhanced lift from a combination of coupled jet dynamics and orographic lift. Drier conditions are likely on Sunday, though as the eastern Pacific closed low weakens and shears inland, it will act on remaining available moisture to support some light valley rain/mountain snow over the northern Intermountain region. A general model blend was used for QPF given good model agreement. Northern/Central Plains, Upper and Mid Mississippi Valleys: Shortwave energy from the west coast system Saturday will ride over the upper ridge and move out into the Plains into Sunday. A low will spin up on the arctic front set up on the lee side of the ridge over Montana Saturday evening and advance into the Upper Mississippi Valley by late Sunday. This will support a band of overrunning snowfall along and north of the low track, with liquid equivalent amounts ranging from about .25-.65". Farther south...trailing short wave energy will induce lee-side cyclogenesis early Sunday over the south-central high Plains. Moisture overrunning a front developing east of this low will result in a second band of light to locally moderate precipitation across the south-central Plains into the mid-Mississippi Valley during the day Sunday. Only minor timing differences were evident in global guidance. Therefore a blend of the 00Z GFS/ECMWF/NAM was used for WPC QPF. Southeast and Mid-Atlantic region: Global guidance held nearly steady after what had been southward trends for the Saturday QPF east from the central/southern Appalachians across the Mid-Atlantic states and Carolinas. A blend of the 00Z ECWMF/GFS/NAM over this region with the cool sector precipitation under upper level difluence, with the heaviest totals (around 1-1.75 inches) from central SC to eastern NC. Surface low pressure will develop as it moves off the coast Saturday evening with ensemble guidance generally keeping it offshore. The trailing cold front potentially spawning convection Saturday night where up to one inch average QPF was forecast. Hayes/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml