Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 442 AM EDT Mon Apr 23 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 23/1200 UTC thru Apr 26/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Eastern / Southeastern U.S... A well defined, slowly-moving closed low will bring a wet day to parts of the southern and eastern United States. Models have done well handling the spatial distribution of rainfall with this system, but they have tended to over-forecast the magnitude. The low itself, at 564 decameters at 500 mb, is barely 1.0 standard deviation from climatology. Cloud cover hampering daytime heating, and a mis-match between areas of strong forcing and areas of larger CAPE, has also toned down the rainfall production. On Monday there is seemingly a little greater chance for heavy and excessive rainfall to occur, as the favored upslope regions in western SC/NC will experience an extended period of strong southeasterly low level flow beneath difluent upper flow. Models also signal a heavy rainfall event along the Carolina coastline where Gulf and Atlantic moisture streams will come into phase. There are, however, still some uncertainties regarding instability and event evolution. The NAM forecasts CAPE of greater than a few hundred J/kg to develop not much farther north and west than Greer and Charlotte. Sustained upslope will certainly boost rainfall totals farther west, but the event in the mountains may be only weakly convective. Meanwhile, at the coast, predicted cell motions and model simulated reflectivity loops suggest that supercells will move very slowly, and could contribute to locally intense rainfall, but with widespred forcing leading to cell mergers - and a slow but steady progression of the synoptic system may limit duration of the heaviest rainfall at any given location. For QPF, WPC favored the HREF Mean and NAM CONUS Nest which were more tempered than the aggressive WRF-ARW solutions. The 03z National Blend of Models also looked very good. Our forecast trended upwards along the coast and in the mountains, still maintaining a wet day, but also a relative minimum in the area between. Ultimately, we may trim back amounts for the final issuance, seeing as downscaling / post processing tends to go a little too heavy in the Appalachians. ...Northern Rockies / Northern High Plains... A relatively vigorous system with well defined mid level forcing mechanisms had amplified over Idaho last Sunday / early Monday, and will translate eastward today. The trough is forecast to maintain a tight, closed mid level circulation along with a sense of jet coupling and strong 700 mb frontogenesis, producing an aggregate swath of forcing and associated rain/snow across southern Montana and northern Wyoming - spilling into portions of the Dakotas. Steep lapse rates hanging back in association with the tail end of the trough will support some coverage of rain/snow showers over areas farther west and south, and the arrival of deep layer lift and a difluent upper level pattern onto the High Plains should promote scattered convection from southeast Wyoming / northeast Colorado into western Nebraska. Model QPFs are clustered quite well. Therefore, WPC leaned toward consensus and bias corrected tools such as our in-house ensemble and the National Blend of Models. The previous forecast had a good handle on this system, so overall change was minimal, but we did increase amounts slightly along the swath of greatest deep layer forcing affecting MT/WY/ND/SD. Days 2/3... ...Mid-Atlantic and Ohio Valley to the Northeast/Great Lakes... The mid-upper level low that is expected to enter and slow over the Tennessee valley on Monday is forecast to gradually weaken and begin ejecting out to the northeast ahead of a northern stream trough amplifying over the Great Lakes early Wed. Ongoing heavy precipitation developing along the Southeast to southern Mid-Atlantic region on Day 1, is expected to continue through Tue morning, with 40-50 kt southeasterly low level inflow along with strong upper forcing, supporting heavy precipitation across eastern North Carolina into southeastern Virginia. Axis of organized precipitation is expected to shift further north into the northern Mid-Atlantic and Northeast late Tue into early Wed. However, the more progressive nature of the system, along with lessening onshore moisture advection and weakening upper dynamics is expected to hamper the threat for widespread heavy amounts. Remnant energy associated with the weakening low is expected to continue lifting along the Mid-Atlantic to Northeast coast Wed to early Thu, phasing with the previously noted northern stream trough moving into the Great Lakes. Surface low pressure developing along the Mid-Atlantic coast on Wed is forecast to track into the Northeast by early Thu. Moderate to locally heavy rains are expected to push east ahead of the low, as light to moderate precipitation develops further west in a deformation zone over the Great Lakes. At this point the models begin to diverge, showing significant differences with respect to the interaction of the phasing systems over the Great Lakes. The 00 UTC guidance generally falls into two camps, with the NCEP guidance more progressive than the non-NCEP camp - which close off a new low further west, in turn spreading precipitation further west into the upper Great Lakes region late Wed into early Thu. Despite the clustering of the ECMWF, UKMET and Canadian, there was not a lot of ensemble support for a solution quite as far west. Therefore, WPC QPF followed a compromise approach, but with a lean toward the faster solutions. ...Great Plains... A positively-tilted upper trough moving across the northern Rockies Mon into early Tue is forecast to drop southeast across the central Great Plains into the lower MS valley Tue to early Thu. Pooling moisture along the associated low to mid level frontal zone, in combination with the favorable upper forcing, is expected to support some light to moderate amounts dropping south from the central into the southern plains on Tue, before spreading east from the southern plains into the lower MS valley on Wed into early Thu. WPC QPF gave significant weight to the ECMWF through the period. Burke/Pereira Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml