Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 500 AM EDT Fri Apr 20 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 20/1200 UTC thru Apr 23/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Central U.S... A closed low over the Southwest will progress slowly, in mid April fashion, producing mountain snows and increasing coverage of rainfall / some convective / on the high Plains and Plains. Certainly the precipitation up through Thursday evening had been sparse, with the system having no connection to sub-tropical moisture. It will entrain Gulf moisture and should produce an expanding coverage of rain and snow on Friday. Still, moisture return is confined and hampered somewhat by extensive surface ridging extending from the Great Lakes to the northern Gulf of Mexico. The greatest intensity / longevity of precipitation should occur along the inverted trough axis north of the low center (generally northern Colorado and adjacent WY/NE), also in the zone of max height falls downstream of the low center over western Kansas, and finally in the marginally unstable environment developing near / north of an advancing warm front into the Texas panhandle, Texas South Plains, and western/southern Oklahoma. Given this is a well defined, closed, medium wavelength system, model differences are minimal, and a consensus approach to the QPF should perform well. The inherited WPC forecast was already in good shape, and we made minor adjustments, favoring the WRF-ARW2 and some of our in-house ensemble / consensus tools. ...Pacific Northwest... A progressive, open trough will move onshore Friday evening and overnight. Forcing will be strong, with 12-hour height falls of 150 meters, but also short-lived for this quick moving system. Precipitable water values in the GFS just briefly spike above a half inch. These factors will minimize areal average rainfall, but given the time of year and intensity of the cold air aloft, one could imagine some weak convective enhancement for a time this evening over western Washington. Even with this occurring, showers will be moving quickly, and any given location is unlikely to exceed an inch of liquid for the event. WPC QPF is in line with consensus, and changed very little since the previous model cycle. ...Elsewhere... Height rises in the Northeast should tend to suppress showery activity relative to recent days, although there may be enough residual low level moisture to shake out additional showers early in the day over northern New England. Easterly onshore flow will begin to promote similarly shallow rain showers along the east coast of Florida, but based on the dry and stable look of the NAM forecast soundings WPC took a conservative approach to coverage and intensity. Days 2/3... ...Central-Southern Plains to the Southeastern U.S.... While there are plenty of differences in the details, there remains a good model signal for widespread moderate to heavy and potentially excessive rains to develop and move east across the region as a compact mid-upper level low tracks east-southeast from the central Great Plains on Saturday to the lower Mississippi and Tennessee valleys by Monday morning. Moderate to locally heavy rains are expected to continue developing Saturday morning across the Red River into Oklahoma as strong southerly low level flow draws moisture from the western Gulf of Mexico into a region of strong lift supported in part my left-exit region upper jet dynamics. Overall, models are in good agreement showing precipitation translating southeast with the better moisture advection and dynamics into the lower Mississippi valley Saturday night into Sunday morning. Models suggest a period of training convection which will help bolster the potential for heavy to excessive amounts from Northeast Texas through southern Arkansas and northern Louisiana into the ArkLaMiss region. A slight risk for excessive was maintained across this region for the Day 2 period (ending 12 UTC Sun). From Sunday morning to early Monday, the heavy rain threat will continue to shift further east. Moisture anomalies are forecast to further increase as southerly flow begins to tap deeper moisture from the Caribbean and western Atlantic. By late Sunday, the GFS shows anomalies increasing to over 1.5 standard deviations above normal across the Southeast ahead of the low. This deep moisture along with the strong lift supported by the low is expected to produce widespread moderate to heavy rainfall accumulations across portions of the Southeast and Tennessee valley on Day 3. Despite the antecedent dry conditions across portions of the region, these amounts may prove problematic. Therefore, a slight risk was introduced in the Day 3 excessive (ending 12 UTC Monday) - focused mainly across portions of northern Alabama and northern/central Georgia - where some of the heaviest totals are forecast to occur and flash flood guidance values are relatively lower. Overall, WPC QPF followed a blend of the GFS, with recent runs of the ECMWF and ensemble guidance. Less weighting was given to the NAM, particularly late in the period when it begins to show a broader upper low and a significantly slower northern stream shortwave over the northeastern U.S. - which helps to draw heavier amounts further north along the southern Appalachians. Burke/Pereira Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml