Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 414 AM EDT Sun Apr 15 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 15/1200 UTC thru Apr 18/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Southeast into the Mid Atlantic... Convection is ongoing as of 06z across eastern MS into the FL Panhandle. This convection should maintain as it moves across the southeast today. As we head through the day instability should increase out ahead of the line...with the trough to the west also taking on more of a negative tilt with time. The combination of this increasing instability and increasing upper level divergence ahead of the trough...should allow for an intensification of the line by mid to late morning into the evening hours. In general the convective line will remain progressive in nature...likely limiting the potential for any extreme rainfall amounts. However, deep layer mean winds parallel to the line supports at least some south to north training of cells embedded within the line. Also, current convection is actually forming ahead of the true cold front, thus the potential exists for additional storms to fire on the true cold front as well...which would result in multiple rounds of locally heavy rainfall in any one location. Overall, this looks like a widespread 1-3" rainfall from the southeast into the Mid Atlantic region...with the aforementioned training and repeat cell potential allowing for localized 3-5" amounts. Would anticipate the line will remain most progressive over the southeast...with the orientation of the line likely more favorable for some training further north closer to the better mid/upper level forcing. Thus kept our highest QPF amounts from the western Carolinas northeast into portions of the Mid Atlantic. Strong southerly flow ahead of the low should help enhance rainfall totals some into the eastern slopes of the Blue Ridge and Appalachians as well. Instability will be greater across SC/NC...so certainly some chance the convective line really gets organized there this afternoon...which could limit amounts a bit further northwest over southwest VA. However, tough to get into those details much ahead of time...but something to monitor. WPC QPF leaned exclusively on the high res guidance for this warm sector convection. Most of these solutions already seemed a bit slow compared to the current radar...thus leaned more towards the quicker high res solutions. Also incorporated some of the 0z HRRR experimental, which was more robust with amounts over SC/NC than the HREF component members were. As mentioned above, given the better instability here, there is certainly a possibility that the higher amounts do verify here. ...Upper MS Valley---Great Lakes into northern NY and northern New England... A surface low will continue to slowly track east across the OH Valley today. To the north and northwest of this low will see continued wintry precipitation across MN/WI/MI. Did trend WPC QPF a bit slower, with the closed low not as progressive as previously forecast. This increases QPF amounts over portions of MN/WI. The GFS may still be a tad quick with progressing precipitation eastward, with the new 0z HREF closer in line with the 12z ECMWF and 0z UKMET. WPC QPF is thus higher than the GFS solution, and ends up pretty much in line with the HREF mean. Across northern NY into northern New England saw better consistency with QPF amounts, thus changes were generally minor here. Days 2/3 ...West Coast/Intermountain West... A long wave trough crosses the West Coast during the first part of Day 2, reaching the Rockies by the end of the period. The long wave trough takes on a negative tilt as it heads for the Plains during Day 3, as a short wave approaches the Pacific Coast late in the period. For the most part, there was good model agreement with the evolution of the mid level systems. so the WPC QPF was based on a blend of the 00z ECMWF/GFS. Some 00z NBM was used to better delineate higher terrain qpf amounts during Day 2. Day 2... As the long wave trough tracks from the West Coast to the Northern and Central Rockies during Day 2, the dynamic lift associated with the mid level feature, combined with upslope flow, is expected to make the most of the moisture in the column. Closer to the coast over WA/OR, the upslope flow is expected to focus the 0.50 inch precipitable water air on the WA/OR Cascades, where there was a multi model signal for 1.00/1.50 inches of qpf, with the highest amounts over the northern WA Cascades. Further south across CA and the Great Basin, the moisture is not quite as robust (with precipitable water values generally between 0.25/0.50 inches). However the synoptic scale lift and upslope flow over the Sierra Nevada range is expected to produce an axis of 0.50/1.00 inches of qpf. The highest amounts are expected over the central part of the range, where the low level flow is most orthogonal to the terrain. Over east central NV and the Wasatch Mountains in UT, local 0.25 inch qpf amounts are expected, especially over the higher terrain. Finally, some of the Pacific moisture survives to the Northern Rockies during the first part of Day 2. Focused by an approaching front, the upslope flow targets the higher terrain of the northern Bitterroot Mountains in ID and the Rocky Mountain Front range in MT. Local 0.75 inch qpf amounts were placed here, with lesser amounts (generally else than 0.50 inches) over the Blue Mountains in OR, the Sawtooth Mountains in ID, and the northern Absaroka Mountains in MT. Day 3... The long wave trough moving from the Northern and Central Rockies during the first part of Day 3 supplies sufficient synoptic scale ascent for qpf across the region. Ahead of a frontal boundary extending from western MT across WY into central and eastern CO, the upslope flow makes the most of the dwindling moisture in the column to produce local 0.75 inch qpf amounts across the northern Bitterroot Mountains in ID and the Rocky Mountain Front range in MT. Lesser amounts (between 0.25/0.50 inches of qpf) were placed over the Grand Tetons and Wind River Mountains in WY, as well as the Front Range in CO. A short wave approaches the Pacific Northwest during the last part of Day 3. Ahead of the short wave, a low level southwest flow transports 0.50 inch precipitable water air to the WA/OR coast. The upslope flow produces 0.25/0.50 inch qpf amounts across the WA Cascades. Heavy snowfall is possible across the higher terrain of the West Coast and Intermountain West during Days 2 and 3. Please refer to the latest suite of WPC Winter Weather products for more information. ...Northern and Central Plains/Upper MS Valley... A negatively tilted long wave trough approaching from the Rockies on Day 3 forms a closed mid level low over eastern NE/western IA by the end of the period, as surface low pressure tracks from the northern Rockies to the Central Plains. The 00z GFS continues that models tendency to be too fast and too far north with the mid level and surface systems. With that in mind, the WPC QPF was based on a blend of the 00z ECMWF/NAM. The southern end of a negatively tilted long wave trough tracking from the Rockies to the Northern and Central Plains evolves into a closed mid level low that reaches eastern NE/western IA by the end of Day 3. Surface low pressure over WY early in the period becomes better organized as the mid level system closes off, moving from western KS into southwest IA between 18/00z to 18/12z. Ahead of the surface and mid level systems, a 45 knot low level south southeast flow transports 0.75 inch precipitable water air north of a surface front over northeast NE into northwest IA. Model soundings in the the increasing low level flow showed 500/1000 J/KG of MUCAPE in the same area, so elevated convection is possible between 18/06z and 18/12z. A qpf bulls eye of around an inch was placed over far northeast NE/far southeast SD/northwest IA in associated with the increasing low level moisture. At this time, it appears as though some of the qpf could fall in the form of snow, which would affect the potential for local runoff issues. For now, no excessive area was placed here due to the precipitation type issues. ...Great Lakes/Northeast... An elongated negatively tilted closed mid level tracks across the Great Lakes and Northeast during Days 2 and 3, with surface lows at each end of an occluded front that crosses the same area. Strong inflow and deep moisture ix expected to produce heavy to locally excessive rainfall across portions of southeast NY state into New England during Day 2. Moisture wrapping around the surface and mid level systems affects NY state and New England during Day 3. The 00z GFS was again too fast and too far north with its surface and mid level systems, so the WPC QPF was based primarily on a blend of the 00z ECMWF/GFS. Some 00z NBM was used to better delineate higher terrain qpf amounts during Day 2. Day 2... As the elongated closed low wobbles over northern NY state and the Northeast on Day 2, a 65 knot low level southeast flow transports 1.00/1.25 inch precipitable water air (which is between two and three standard deviations above the mean) from southeast NY state across much of southern and eastern New England. The very strong upslope flow focuses the moisture on the southern Green Mountains in VT and the Berkshire Mountains between 17/12z and 17/18z, producing an axis of 2.00 inches of qpf here. Some elevated instability could increase rainfall rates briefly across eastern NY state into southern and central New England, so a Marginal Risk was placed over this area for Day 2. As the occluded front associated with the closed mid level system crosses eastern New England after 17/00z, the low level flow strengthens to 80 knots, which produces a strong upslope flow across the higher terrain of western ME, where there was a multi model signal for 2.50 inches of qpf. Instability is nearly non-existent as the low level flow reaches ME, so the heavy rain is more likely going to result in a flood vs flash flood threat. However, if rainfall rates increase enough, a low level flash flood threat could develop in the presence of the strong upslope flow. With this in min, the Marginal Risk for Day 2 was extended across much of western and southern ME. Day 3... The elongated closed mid level low continues to wobble in place over New England on Day 3. Moisture extends around the surface and mid level systems across northern New England and northern NY state. The highest amounts are expected across the Adirondacks in NY state (with local 0.50 inch amounts). Elsewhere over northern New England, qpf amounts are expected to remains between 0.10 to 0.25 inches. The exception could be over northern ME, where local 0.50 inch qpf amounts are possible over the rooftop of ME. Chenard/Hayes Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml