Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 403 PM EDT Mon Apr 02 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 03/0000 UTC thru Apr 06/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 Northern Plains---Upper MS Valley---Great Lakes---Ohio Valley As deep low pressure remains firmly entrenched across Hudson Bay, a vigorous shortwave over the northern intermountain west early this afternoon will push ese into the mid-upper MS Valley by late Tuesday. Ensuing lee-side cyclogenesis across the Central Plains-Mid MS Valley-Ohio Valley during the same time will allow for an expanding baroclinic leaf/area of deepening moist isentropic ascent. Dynamical support will be optimized across the OH Valley -- given the coupled favorable forcing via the approaching shortwave and from the right entrance region of the upper jet streak to the north (owing to the strengthened upper level confluence south of the Hudson Bay low). Not coincidentally, low-mid layer moisture transport is highest over this region per the guidance, evidenced by the 850-700 mb moisture flux anomalies between 4-5 standard deviations above normal per the latest GEFS. Aiding to the favorable thermodynamical profile is the relatively steep lapse rates aloft (nearing 7 c/km between 850-500 mb ahead of the surface warm front and dry slot/warm sector south), resulting in modest elevated instability (400-800 mb mucapes) distributed in a relatively deep layer between 06-12Z Tuesday. In terms of the QPF, the WPCQPF was decided wetter over the previous forecast along and north of the I-64 corridor between STL and CVG, owing to the high-res CAMs (WRF ARW, ARW2, and NAM CONUS nest in particular) that along with recent HRRR runs, appear to be doing a better job capturing the latest short-term thermodynamic trends. WPC also hoisted a MARGINAL risk area in an updated Day 1 excessive rainfall outlook (ERO), based on the aforementioned thermodynamic factors noted above along with the antecedent wet soils/relatively low FFG values. In addition, within the MARGINAL risk area, the wsw low-level inflow increases to 50-55 kts at 850 mb overnight, i.e. approaching the magnitude of (while aligning parallel to) the mean 850-300 mb flow. This will favor increased upwind propagation and cell training/repetitive convection overnight into early Tuesday, especially between 06-12Z as per the high-res hourly and 3-hourly rainfall rates. The risk area encompasses the highest HREF probabilities of 3 hourly rainfall totals exceeding 1 inch (40-60%). Farther north, WPC noted another QPF maxima (of lower liquid-equivalent totals) between 0.50-0.75+ inch across portions of the upper Midwest (southern MN, much of WI, and into northern Lower MI) -- associated with a robust cold conveyor belt (ccb) that will produce a swath of late-season heavy snow. For further details, please refer to the latest QPFHSD. Days 2/3 ...Pacific Northwest/Northern Rockies... Short wave energy approaching the Pacific Northwest late on Day 2 provides synoptic scale lift over a warm front just offshore. A deepening southwest flow of Pacific moisture ahead of additional short wave energy approaching the Pacific Northwest could result in locally heavy precipitation over the higher terrain during Day 3. There are some timing differences with respect to the short wave energy, so in an attempt to mitigate the differences, the WPC QPF was based on a blend of the 12z ECMWF/GFS. Day 2... As mid level ridging breaks down over the Pacific Northwest and Northern Rockies early on Day 2, short wave energy approaches the Pacific Northwest coast. A 20 knot low level southwest flow in conjunction with a surface warm front focuses 0.05/0.75 inch precipitable water air on the higher terrain over western WA. There was a multi model signal for 1.25 inches of qpf across the Olympic Range in WA, with lesser amounts (generally between 0.50/1.00 inches) over the higher terrain of the WA Cascades. Further east, upslope flow becomes aligned ahead of an arctic front dropping into the Northern Rockies late. While moisture is scant, the upslope flow makes the most of the moisture in the column to produce local 0.35 inch qpf amounts over the Rocky Mountain Front Range in MT, as well as the Absaroka Range in WY/MT. Day 3... The arctic front becomes banked up against the Northern Rockies from western MT into central WY and far western NE during Day 3. While moisture remains a limiting factor for qpf (model soundings showed precipitable water values of 0.25 inches or less), upslope flow (augmented by low level convergence along the arctic front) and short wave energy from the Pacific Northwest makes the most of the moisture in the column. There was a multi model signal for 0.50 inches of qpf stretched across the Absaroka Range in MT/WY, with lesser amounts (between 0.25/0.50 inches) over the northern Bitterroot Range in ID and the Rocky Mountain Front range in MT. The next piece of short wave energy in the mid level flow approaches the Pacific Northwest late on Day 3. Ahead of the short wave (and behind the surface warm front which moves into southwest British Columbia), a 25/35 knot low level southwest flow transports 0.75/1.00 inch precipitable water air across northwest OR and western WA. The best mid level lift occurs between 05/18z and 06/00z, and the combination of moisture and lift is expected to produce axes of 1.00/1.50 inches of qpf over the Olympics and Cascades in WA. ...OH Valley/Great Lakes/Northeast... Deepening moisture and instability ahead of a frontal boundary crossing the OH Valley into the Northeast on Day 2 could support heavy to locally excessive rainfall, especially over the OH Valley. A broad cyclonic mid level flow between closed mid level lows could produce mainly light QPF amounts over the Great Lakes and the Northeast during Day 3. There was generally good model agreement with the synoptic scale setup, so the WPC QPF was based on a blend of the 12z ECMWF/GFS. Day 2... As a short wave tracks from the Northern Plains into the Upper Great Lakes during Day 2, surface low pressure moves across the OH Valley. The cold front associated with the surface low becomes the focus for moisture and instability during the first half of Day 2. Ahead of the surface front, a 40/50 knot low level southwest flow transports 1.00 inch precipitable water air and marginal instability (some of which is elevated, based on model soundings across IN/OH) are expected to fuel low topped convection over IN/OH/WV. There is a solid model signal for 1.00/1.25 inches of qpf with the convection. Higher amounts are possible, especially early in the period, as the 850-300 mb mean wind becomes better aligned with propagation vectors, which will promote training. Three hour flash flood guidance along the aforementioned axis is as low as 1.00 inches, and the combination of wet antecedent conditions and the potential for training necessitated the placing of a Marginal Risk here on Day 2. The short wave takes on a negative tilt as it crosses the Upper Great Lakes and southern Quebec, and the surface low tracks along the same path. The low level flow increases as the surface low tracks down the St Lawrence River valley, and the flow transports 1.00+ inch precipitable water air into northern NY and northern New England. Instability is fairly limited (outside of northern NY state, where model soundings indicated the potential for 100-250 J/KG of MUCAPE just ahead of the front), so much of the qpf is expected in the form of moderate to heavy mainly stratiform rainfall. There was a good consensus for 1.50/2.00 inches of QPF from far northern NY state into far northern ME (as the moisture is initially on a warm front extending from the surface low), with lesser amounts to the south and west. While there may be flooding issues associated with the heavy rainfall (especially in conjunction with snow melt), there does not appear to be sufficient instability for a Marginal Risk of flash flooding in these areas on Day 2. Day 3... The negatively tilted short wave over Quebec forms a closed mid level circulation on Day 3, as another closed mid level flow enters the northern stream just northern of MN. The flow between the two system is broadly cyclonic, supportive of broad and weak mid level lift. The low level flow becomes more conducive for lake enhanced precipitation, especially downwind of Lakes Superior and Ontario, where local 0.25 inch qpf amounts were placed. ...Lower MS Valley/Southeast into the Mid Atlantic... Deep moisture and instability ahead of the frontal boundary crossing the Lower MS Valley into the Southeast states during Day 2 is expected to support convection producing heavy to locally excessive rainfall, especially over the Lower MS Valley. As the front moves off the FL coast during Day 3, convection could produce locally heavy rainfall over the peninsula. The timing of the front is similar among 12z model solutions, so the WPC QPF was based on a multi model blend. Day 2... Short wave energy tracking from MO into the eastern OH Valley early on Day 2 provides synoptic scale ascent over the southwest extent of a cold front extending from the TN Valley across the Lower OH Valley. Ahead of the front, a 25/30 knot low level southwest flow transports 1.50/1.75 inch precipitable water air across far eastern TX, northern and central LA and central MS/northern AL. Model soundings showed marginal to locally moderate instability across east TX into central LA, and the combination of moisture and instability is expected to feed organizing convection (which is supported by divergence in the right rear quadrant of a 130 knot jet streak crossing the OH Valley between 04/06z and 04/12z). The resulting convection is expected to produce local 1.50 inch qpf amounts over northern LA, with locally higher amounts possible in training as the 850-300 mb flow briefly becomes better aligned with the propagation vectors. The high moisture content air and the potential for training required keeping the Day 2 Marginal Risk already in place. Day 3... The mid level support for the the front shears out to the northeast during Day 3, allowing the front to slow as it crosses FL. A low level southwest flow continues to supply 1.50+ inch precipitable water air ahead of the front, but model soundings suggest instability remains capped at marginal through the frontal passage (which is expected before 05/12z). Given the early timing of the frontal boundary, qpf amounts across south Fl were capped at 0.50 inches. Hurley/Hayes Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml