Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 459 PM EDT Sat May 05 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 06/0000 UTC thru May 09/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 For the 06/00z QPF issuance: little in the way of chance was made to the 05/18z QPF issuance. A few tweaks were made across KY/WV based on radar trends, as well as trends noted in the most recent HRRR. ...TN/OH Valley into the Central Appalachians... Showers and embedded thunderstorms continue to increase in coverage from eastern KY into the southern half of WV and western VA. Will be a rather prolonged period of favorable upper dynamics (both left exit and right entrance regions of upper jet) for rainfall over this area, resulting in the possibility of multiple rounds of locally heavy rains. 850 mb moisture transport, while not extreme, is forecast to gradually increase over this area through the day out ahead of a surface low currently over TN. This should aid in the development over showers/storms near and just north of the front draped across the area. Main limiting factor will probably end up being the degree of destabilization that is able to take place. Note a decent cloud shield over much of the area, which may tend to dampen instability and cap rainfall rates. Also working against a more organized heavy rainfall/flash flooding threat is an initial state of generally below normal stream flows, and while above normal, PWATs are not all that extreme. With that said, at least a localized flash flood risk exists into tonight across much of KT into the southern half of WV and into western VA. If convection this afternoon/evening is able to tap into some slightly better instability and become more intense, then it is possible that a bit more organized heavy rainfall/flash flooding risk could develop over portions of far eastern KY into southern WV and western VA. Noting an increase in activity over this area over the past few hours, although nothing too intense or organized as of 18z. Some breaks in the clouds ahead of the surface low is resulting in some uptick in instability. The question becomes how organized/intense does convection become from eastern KY into WV/VA through the afternoon. Am growing concerned that portions of southern WV and far western VA could see multiple rounds of convection, with some training. The first round ongoing now, with additional storms firing ahead of the low as 850 mb moisture transport increases into the evening. This could result in some 3"+ rainfall totals and an increasing flash flood threat with time. Will need to continue to monitor trends. In general, should see a weakening of storms and rates by later tonight into Sunday as rainfall moves out of the instability pool and into the Mid Atlantic and portions of the northeast. Will however continue to see showers underneath the main shortwave as it tracks across WV. By afternoon a signal for some decent storms to fire ahead of this shortwave across northeast NC into southeast VA. These storms could be locally intense and produce briefly heavy rains, but should generally be progressive in nature. Overall model clustering was pretty good with regards to QPF across this region. Thus WPC took a blend of the 12z HREF members, with some incorporation of the ECMWF/UKMET towards the end of day 1. Recent HRRR runs were also incorporated. ...Coastal NC/Outer Banks... The anomalous PWAT values off the coast will be drawn northward ahead of the above mentioned upper trough pushing into the eastern U.S. These anomalous PWAT values will impact coastal areas from the upper coast of SC, northeastward into eastern and coastal NC Saturday night into early Sunday. Heavy rains are possible in a region of enhanced upper difluence in the right entrance region of an upper jet over the Mid-Atlantic. There continues to be some spread with this rainfall axis. It looks like there will be some impressive rainfall totals focusing along a coastal front, with 12z HREF probabilities of localized 5"+ amounts quite high. Fortunately at this time it looks like the coastal boundary, and thus heaviest rains, will remain just offshore. However, will need to closely monitor trends into tonight. Either way, some moderate to locally heavy rains should skirt the coast, even if the most intense activity were to remain offshore. Generally followed a blend of the 12z HREF members for QPF here. ...Central to South Florida... Higher PWAT values will remain over FL into Sunday. Also, anticipate some instability to develop both this afternoon and Sunday afternoon. The approaching trough and jet to the north will produce a slightly more favorable environment for convection with time as well. Thus not surprising the 12z HREF shows storms both afternoons, with slightly more widespread storms on Sunday with the passage of the forcing and front. Days 2/3... ...Ohio and Tennessee Valleys to the Mid-Atlantic/Southeast... Previously noted storms developing late in the Day 1 period are expected to continue moving across southeast VA and northeast NC as a shortwave embedded with a broader scale trough lifts across the region Sunday evening. These storms could produce some locally heavy amounts before moving offshore. Meanwhile, the broader scale trough will drift slowly east from the Ohio and Tennessee valleys into the eastern U.S. Weak shortwave energy dropping into the backside of the trough, in addition to moisture pooling along an associated low level boundary, is expected to support showers and storms as they move across the Ohio valley toward the central and southern Appalachians Sunday evening; with some model signal for moderate to locally heavy accumulations. Storms are expected to dissipate overnight, redeveloping further east along the central and southern Appalachians into the Southeast as the upper trough shifts further east on Tue. Given the typical model spread with respect to the finer details, WPC QPF reflects a model blend that gave significant weight to the GFS and NAM. Less weight was given to the ECMWF, which despite being in generally good agreement with respect to the larger scale features, was a relatively dry outlier with regards to QPF. ...Northwest to the Northern Plains... A shortwave trough interacting with anomalous moisture is expected to produce moderate to locally heavy amounts as it moves from the Pacific Northwest into the northern Rockies Sunday evening. With PW anomalies increasing to 2-3 standard deviations above normal, there is good model signal for locally heavy amounts, especially across some of the favored terrain of northern Idaho into northwest Montana. These amounts combined with melting snows may result in rapid runoff concerns and therefore a 'Marginal' risk was introduced into the Day 2 Excessive Rainfall Outlook. The aforementioned shortwave is expected to continue moving further east into the northern High Plains by late Mon. Mon evening into early Tue, this system is expected to begin interacting with deepening moisture on the nose of an amplifying southerly low level jet. By Mon evening, the GFS shows PW anomalies of 1.5-2.5 standard deviations above normal extending from the High Plains across the Dakotas. This will support the development of showers and storms spreading east across the region, with moderate to locally heavy amounts possible. WPC QPF generally followed a blend of the GFS and the ECMWF through the period. Chenard/Pereira Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml