Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 558 PM EDT Sat May 12 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 13/0000 UTC thru May 16/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Midwest to Mid Atlantic... Rain and thunderstorms are expected to occur all along a quasi-stationary frontal zone from Iowa to the New Jersey coast and the DELMARVA this weekend. The difficulty is in deciphering the mesoscale evolution of each precipitation area, as well as getting the north to south placement and expanse correct. We used observational trends and clues from the mesoscale guidance to stitch together the 6-hour periods as best we could, and have average or above average confidence in some of the details given overall model agreement. The narrow corridor of usable instability also helps matters. Most of the convection in this pattern has been to the cool side of the surface boundary. Certainly some of the storm this evening in the Mid-Atlantic states and perhaps the Ohio Valley may sneak out into the warm sector, but expect further upstream redevelopment to take place in the 850-700 mb frontal zone this evening across Iowa and Illinois. At 15Z we introduced a Slight Risk of excessive rainfall from eastern Ohio through northern West Virginia into southern Pennsylvania, valid through 12Z Sunday. Another Slight Risk area remains valid back across eastern Iowa to western Ohio. In the eastern Ohio Valley and Mid-Atlantic states deep layer forcing will come into phase with the peak heating cycle, and a round of strong to severe storms is expected this afternoon and evening (see also Storm Prediction Center Day 1 Outlook). Although the event will be overall characterized by progression, the early development phase could easily see west to east training as deep layer flow is nearly parallel to the stalled frontal zone in the area. With moderate CAPE, greater than 1000 J/kg depicted in all the hi-res models, and sufficient moisture around 1.00 to 1.25 inch precipitable water, it would not take more than a couple of mature thunderstorms rolling through any given basin to produce quick 1.50-inch or more rainfall. Flash Flood Guidance in portions of this region is also fairly low, with only inch per hour or 1.50 inch per 3 hours needed to exceed. Elsewhere, Marginal Risk is maintained within the narrow frontal zone extending through the rest of PA and southtward a bit into MD/VA, although the storms should be more progressive, with greater cold pool development by the time they head south. Differential heating owing to low cloud cover near the coast and extending into eastern PA may enhance the upglide contribution to convective coverage later on, and the 12Z hi-res models are favoring locally heavy rain over eastern PA into NJ. WPC preferred many of the details seen in the 12Z WRF-ARW. This includes the potential for an old MCV and associated forcing in southern Ohio to spill over and spawn a few storms or a small organized line of storms overnight in Virginia, separate from the main event up across PA/MD/NJ. The WPC QPF through 00Z Monday was based on a blend that included the WRF-ARW, HREF Mean, previous WPC (continuity), and a small percentage 12z GFS. ...Florida... From a QPF standpoint the Florida forecast is particularly difficult because the QPF magnitudes vary so widely among the models. It appears which produce heavier rainfall inland over the southern half of the peninsula are relying on increased deep layer ascent, especially mid and upper level, as a broad low continues to take shape. The surface pattern and CAPE forecasts, however, suggest troughing and rain-reinforced outflow will continue to hug the coast of south and southeast Florida, with only gradual advection of deeper low level moisture and instability inland. The upper jet configuration should produce better lift just off the east coast of the state, and also near the developing low center near and north of Key West. In between, we tried to keep the heaviest downpours / QPF near the surface trough and near the coast, favoring solutions more like the GFS and the previous 00z WRF-ARW2, along with a little bit of the 12z WRF-ARW and HREF Means to capture the trend of increased QPF. So overall we played it conservatively, but did raise amounts in the Keys and near Miami. ...Western U.S... The upper low over the Great Basin had produced heavy snow at high elevations in the northern Rockies, and widespread coverage of light to moderate rain at lower elevations. Some heavy snowfall is still yet to come in the mountains in northern Wyoming and southern Montana. The intensity should start to dwindle overnight and Sunday as the system becomes increasingly stacked, with upper jet speeds weakening during that time. Precipitation, nonetheless, will continue, especially in the deeply moistened vertical profile near the upper low center, from central/northern Nevada northeastward, and especially tonight as some upper difluence remains. Expect the precipitation to become more fractured on Sunday except near the inner core of the upper low over Nevada where some of the more consistent rains will occur. For QPF we used our in-house ensemble minus the bias correction (which preserves greater coverage of measurable precipitation) along with the HREF Mean to incorporate higher resolution without the high magnitude bias of the individual models. The resulting QPF was somewhat heavier in Nevada, with more areal average half inch and greater - and somewhat drier in Wyoming where the trend has been to adjust precipitation westward with little to support areas that try to spill out into the High Plains. Days 2/3... ...Great Basin, Northern/Central Rockies and High Plains... A closed mid level low over the Great Basin with a northeastward elongation of it up across the northern and central Rockies Sunday night and early Monday will begin to gradually weaken through late Monday and Tuesday. The heaviest precipitation will be distributed generally in two areas. One area will be across the Great Basin where proximity of the closed low will foster steeper mid level lapse rates which in conjunction with diurnal heating and at least modest Pacific moisture should foster some moderate to locally heavy showers. Adjacent areas of the northern Sierra-Nevada and the northern Rockies will see some more orographically enhanced shower activity as well which will be aided by the deep layer cyclonic flow and embedded shortwave energy. These areas will see more shower activity through Tuesday, but with the core of the closed low beginning to weaken, areas of the Great Basin will tend to see the activity become less organized and likely not quite as heavy. A second focused area of heavier precipitation will be in the immediate High Plains region of southeast WY, northeast CO, western NE and southwest SD where relatively moist low level east to southeast flow coupled with ejecting vort energy from the base of the Great Basin closed low will foster clusters of showers and thunderstorms. Some locally organized small scale clusters with the aide of diurnally enhanced instability will favor some locally heavy rainfall amounts that could exceed 1 inch. The best coverage should be Sunday evening with the convection through the remainder of the period generally a little more disorganized. WPC felt the ECMWF was overall a little too dry, and so more weight was given to the GFS and even the WRF-ARW for the first part of the day 2 period. ...Central and Southern Plains... Shortwave energy tracking out of the Southwest through the period around the based of the Great Basin closed low will interact with deepening moisture and strong instability pooling north up along a very slow moving front situated from the CO High Plains east toward the lower MO River Valley. The result will be numerous clusters of heavy showers and thunderstorms which will be aided by a southerly low level jet of 30 to 40 kts overrunning the front. Precipitable water values are forecast to rise to between 1.25 and 1.5 inches which will enhanced rainfall rates and given the potential for repeating convective clusters, there will be at least a low-end concern for flash flooding. The heaviest rainfall threat will tend to be Sunday night, and especially over central and southeast NE and northern KS, but some additional convection may tend to get organized Monday afternoon through Monday night as the front gradually settles south in the wake of a wave of low pressure advancing toward the middle MS Valley. Farther south across the southern Plains, there will be a dryline that extends south from the wave of low pressure over the central Plains and the attendant front. The dryline will also be a focus for convection across central and western OK and down across western TX. Some locally organized clusters of convection will be possible with heavy rainfall amounts of up to a couple inches not out of the question. WPC QPF tended to lean toward a multi-model consensus led by the GFS and ECMWF along with some WRF-ARW for Sunday night. ...Mid and Upper MS Valley east to the OH Valley, Mid-Atlantic and Northeast... Shortwaves embedded within fast mid level flow around the southern edge of the westerlies will be interacting with a quasi-stationary front and a pool of relatively deep moisture and instability along it to foster numerous convective clusters in vicinity of this boundary through the period. The heaviest rainfall threat should tend to be from areas of southern IA and northern MO east across portions of central IL and IN which will be a result of stronger vort energy interacting with a stronger southwest low level jet in vicinity of the front. Somewhat lesser amounts of rain are expected farther east toward the upper OH Valley, central Appalachians and the Mid-Atlantic region where forcing will not be as strong, but there will still be at least moderately favorable thermodynamics pooled along the front. Overall, the greatest concern for any flash flooding will tend to be over southern IA, northern MO and into west-central IL where multi-day totals of as much 2 to 4 inches of rain will be possible. ...FL/Southeast... An inverted surface trough over the east coast of FL Sunday night and Monday will will focus instability and a rather deep pool of tropical moisture to produce convection with heavy to possibly excessive rainfall. A developing closed low off the FL west coast through Tuesday will provide a low level southeast Atlantic fetch that will sustain the inflow of favorable moisture and instability through the period for much of FL. In fact, precipitable water values are expected rise to near 2 inches along the east coast of FL which will be indicative of a highly tropical environment. The synoptic scale set-up favors some locally organized convective bands of particularly intense rainfall setting up, with rates possibly reaching 2 to 3 inches/hr. This coupled with low level frictional convergence along the coast may result in some highly focused areas of very heavy rainfall, with multi-day totals possibly exceeding 5 inches. Will need to monitor some isolated totals much higher than this. This will certainly drive a concern for runoff problems and flooding. On Tuesday, the influx of tropical moisture will also be spreading in across the interior of the Southeast and closer to the central Gulf Coast region. Locally heavy showers and thunderstorms with the aid of diurnal heating will occur across these areas as well. For now, WPC generally leaned toward a multi-model consensus for QPF. Burke/Orrison Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml