Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 501 PM EDT Mon May 28 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 29/0000 UTC thru Jun 01/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Southeast/Southern Appalachians/southern mid Atlantic... There is expected heavy rainfall of 4-8 inches along and to the right of the track of Alberto, with the main core of heavy rain expected in the Florida panhandle, Alabama, and western Georgia. A secondary maxima develops in the upslope regions of the Blue Ridge / southern Appalachians over the western Carolinas and northeast Georgia. Models indicate some build of up instability just upwind, and low level flow steadily increases into the mountains. The flash flood threat will likely increase in this area tonight into Tuesday. Farther east along the coast, broad upper difluence and warm advection will continue tonight in the eastern Carolinas with lift aided by an inverted trough. Showers/storms persist into the evening near the front in VA, and should redevelop in the mountains of western VA/WV Tue. The NHC Alberto forecast track is closer to the NAM, with the GFS and Hurricane WRF tracking the system west of the NHC track, leading to their QPF axes further west. Consequently, manual QPF gave the last amounts of weighting to the GFS and Hurricane WRF, and more to the HMON hurricane model and the 12z NAM/High Res Ensemble mean QPF. ...Rockies / central to northern Plains/upper MS Valley... The long-lived upper low over the Great Basin will eject toward the Plains during this period. Instability and upslope flow will continue to favor eastern Wyoming and southeast Montana for showers and thunderstorms tonight, with locally heavy rain possible. An inverted low level trough persists in this area this afternoon/evening, and convergence along the trough combines with upslope flow to produce rainfall. A shortwave rotating around the base of the upper trough emerges over the high Plains of CO. A strong upper divergence max will generate shower/storms and a potential MCS, feeding off deep southerly flow that advects GFS forecast 1.5 inch PW values combined with strong low level convergence this evening over western KS. The GFS has backed off some of the prolific 10-11 inch bulls eyes of prior runs but continues showing heavy rain potential in western KS. The GFS axis/orientation is similar to the 12z high res windows and 12z NAM/UKMET, so the GFS QPF can now be included as part of the overall blend. A very warm boundary layer will promote cold outflows, but the enhanced inflow above the surface could support a slow moving system producing very heavy rain. Much of the hi-res guidance does paint heavy rain over western Kansas and Nebraska. Expect another active convective cycle from northwest Kansas to southeast Wyoming and western South Dakota, as the airmass has already moistened and the ground has as well from prior rainfall. WPC QPF blended the High Res Ensemble Mean with the NAM/UKMET/GFS for the details, with the primary adjustment higher amounts tomorrow afternoon in central KS to eastern NE as the next diurnal destabilization occurs in the moist airmass in an area of low-mid level moisture convergence. Return warm/moisture advection develops with bands of 850 mb convergence extending northeast out of the central Plains to and across the upper MS Valley overnight and Tue. During Tue lapse rates are favorable in the 7-8 degrees c/km range in NE to MN along with deep southerly flow, so diurnal destabilization leads to increasing shower/storm development. Manual QPF amounts were increased from continuity to match the high res ensemble and global model mean QPF. ...New England... The models show showers occurring ahead of a cold front this evening/overnight. Sufficient low/mid level moisture and cold air aloft is forecast to support a few hundred J/Kg of CAPE over northern Maine and northern New York / New England. Showers are also supported by frontal circulation-driven ascent. With the frontal passage, cooling and drying occurs Tue with the loss of ascent, so no additional rain is expected during the day in northern New England. Modest, barely measurable amounts are possible in southern New England. Days 2/3... ...Southeast, Ohio Valley, and Florida Peninsula... *** MODERATE RISK of Excessive Rainfall on Tuesday and Tuesday Night for the southern Blue Ridge Mountains of western North Carolina, Upstate South Carolina, and northeast Georgia *** Remnants of Subtropical Storm Alberto will move north from AL into the Ohio Valley late Tuesday into Wednesday, and into the Great Lakes through Thursday. Heavy rainfall is expected in a relatively narrow swath along the track of the center per 12Z model consensus. In addition to the near-track rainfall, moderate to heavy convective rain is likely to continue across much of the region east of the track through the Day 2-3 period. A broad area of PWATs 1.5 to 2 inches will have been drawn north with Alberto, and the lack of significant synoptic features to drive cooler or drier air in should allow the deep tropical moisture to be maintained (with some gradual modification of the air mass). WPC QPF was based on a blend of the 12Z NAM/GFS/ECMWF/UKMET, which were in good agreement on the progression of the remnant low center into the Midwest. With the scattered convection in the broad cyclonic flow away from the center of Alberto, localized areas will receive much higher values than depicted in the QPF. One area of greater certainty on high QPF is the eastern slopes of the southern Appalachians of NC/SC/GA where moist low-level flow topographically lifted will result in heavy rain. A Moderate Risk of excessive rainfall was maintained for this area with a noted threat for landslides and mudslides given the wet soils and terrain. From Day 2 into Day 3, a Slight Risk of excessive rainfall was maintained along the track of Alberto until it reaches the general vicinity of Lake Michigan. For most of this swath -- from northeast Alabama to up the Wabash Valley on the IL/IN border -- rainfall over the past 30-60 days has been below normal and FFG values are currently higher than some of the surrounding areas. Given these dry antecedent conditions and the low accelerating as it interacts with a trough in the Upper Midwest lessens the flash flooding threat. The one exception is the Chicago metro area since antecedent conditions are not as important to vulnerable urban areas such as this. Otherwise, convective rain bands should continue away from Alberto's center with less dense cloud cover allowing for some instability to build each day. The combination of moderate instability with anomalously high PWATs will promote heavy rain rates and a risk of flash flooding across much of the region. Some concern exists for the area to the southeast of Alberto on Day 3 (Ohio into West Virginia and western Virginia), where PWATs are forecast to be above the 99th percentile and near record levels for late May and early June with storm motions around 10 knots. A Day 3 (12Z Wed-12Z Thu) Slight Risk of excessive rain remains for this area. Please refer to the National Hurricane Center forecasts for the latest information on Alberto. ...Northern Great Plains... An ejecting trough will be providing synoptic scale support for rainfall with Gulf moisture spreading east from ND to MN. Uncertainties regarding this trough interaction with Alberto continue to lead the models to place the narrow swath of heavy rainfall in slightly different trajectories over the northern tier of the CONUS. The Marginal Risk was continued for North Dakota into northern Minnesota given 2 to 3 standard deviations on the moisture anomaly and relatively low FFG across the area. ...Central Plains... The ongoing MCS in KS/NE will continue be fed by the open Gulf and should allow slow progress to the east-southeast on the nose of a low-mid level thermal ridge and low-level jet. Mesoscale processes are not handled well in the global guidance, so this area will need to continue to be monitored. For now, QPF was spread south to OK per 12Z model consensus. The Marginal Risk stretched north-south with noted drought conditions over KS/OK. Petersen/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml