Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 626 AM EDT Sun Apr 22 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 22/1200 UTC thru Apr 25/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Southeast U.S... A closed low of moderate strength and maintaining itself along its slow eastward march - characteristically a very April system - will spread rain and thunderstorms throughout areas from the lower Ohio Valley to the Gulf Coast. In Florida easterly onshore flow, heating, and gradually increasing deep layer ascent will also promote widespread coverage of rainfall. Model guidance is in decent agreement, and the notion of widespread 1 to 3 inch rainfall is supported even by the GEFS Reforecast data based on analog events. The main question is whether rain rates will be intense enough or persistent enough over any given location to yield much in the way of flash flooding. Generally the most persistent forcing and better alignment of lower and upper level flow should occur during the day from northern Mississippi into western Tennessee. This is perhaps where training will be most prevalent, but instability will be more marginal. Areas farther south should see somewhat greater instability, but with mid level flow cutting across from west to east, the mean 0-6 km winds are at a sharp angle to Corfidi vectors, suggesting training will not be too common. Given flash flood guidance values are also much higher, and will be difficult to surpass with southward extent, WPC trimmed back a bit on the inherited Slight Risk area for the 12z Sun- 12z Mon period. Otherwise, Slight Risk is maintained in areas of lower FFG across northern MS/AL into southern TN, and including the Atlanta Metro to the east, as well as the more favorable training environment in western TN. Throughout these areas areal average precipitation is forecast at 1.5 to around 3.0 inches for the 24-hour period, and judging by moisture availability and hi-res model solutions, hourly rates could peak around 1.50 inches, leading to at least some risk of flash flooding. For QPF there was no ideal solution, but the models were well clustered. WPC began with a blend of the 00z GFS, WRF-NMM, and the non bias corrected version of our in-house ensemble (more generous with areal coverage). We expanded coverage northward, resulting in quite a bit more QPF over northern TN into KY compared to the previous cycle, and toned down QPF along the east coast of Florida, as low level convergence and cell motions appear erratic enough to spread the convection out. ...Northern Rockies... In the immediate wake of a lead shortwave - which deposited a frontal zone into the northern Rockies - a stronger shortwave trough will amplify Sunday over Idaho and Montana, bringing a period of strengthened frontogenesis and well defined upper jet forcing. Weakly convective rain/snow will likely result. The previous WPC forecast had a great handle on this event, and we made only minor tweaks using the 00z GFS and our in-house ensemble tools. Days 2/3... ...Tennessee and Ohio Valleys to the Eastern U.S.... A northern stream trough moving off of the Northeast coast Monday morning is forecast to give way to an amplifying ridge along the Eastern Seaboard. This will slow the forward progression of a compact mid-upper level low moving from the lower Mississippi into the Tennessee valley on Mon. This will set the stage for prolonged period of moderate to heavy rains along the Southeast coast back into the southern Appalachians Mon into early Tue. Heaviest accumulations during the Day 2 period (ending 12 UTC Tue) are expected to center from the South Carolina shores and the Cape Fear region of North Carolina back into the southern Blue Ridge. Deep moisture, on the order of 2 standard deviations above normal afforded by 30-40 kt southeasterly low level inflow, interacting with favorable upper jet forcing, is expected to support heavy totals on the order of 1 to 3 inches, with potentially heavier amounts, along the coast. Although removed from the deepest moisture and better instability, there remains a good model signal for a secondary max further west, where good orographic ascent should support heavier totals from the Piedmont back into the southern Blue Ridge. For the Day 2 Excessive Outlook, a 'slight' risk was maintained across these areas. For Day 3 (12 UTC Tue to 12 UTC Wed), the heavy rainfall threat is expected to wane some as the low begins to weaken and lift out to the northeast. Light to moderate rains are expected to spread north through the Mid-Atlantic region into the Northeast. Again, the potential for heavy amounts is expected to lessen as the onshore flow and the upper forcing weakens. WPC QPF gave significant weight to the GFS, NAM and recent runs of the ECMWF through the Day 2 period. By Day 3 however, less weight was given to the ECMWF, relying more heavily on the GFS and NAM as the 00 UTC ECMWF becomes a more southerly and a relative outlier with the position of the low. ...Northern Rockies to the Great Plains... A well-defined shortwave trough is expected to support light to moderate precipitation, including mountain snows, from the northern and central Rockies to the adjacent plains Mon into early Tue. While the models show fairly good clustering through Day 2, there remains significant spread thereafter, with the GFS showing more interaction with a shortwave trough moving across central Canada, carrying a more amplified trough further east into the central Great Plains and mid Mississippi valley Wed morning. With more separation in the flow, the ECMWF drops this energy further south along the central High Plains. With support from the UKMET and the Canadian Global, leaned closer to the ECMWF solution, with WPC QPF holding developing precipitation across the central and southern Great Plains further west late Tue into early Wed. Burke/Pereira Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml