Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 446 PM EDT Sun May 06 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 07/0000 UTC thru May 10/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...East... Showers and thunderstorms will exit the VA/NC coasts this evening, with the activity over the OH/TN/MS Valley gradually weakening with the loss of daytime heating. The mid/upper level trough will remain overhead into the day Monday. The cooler temperatures aloft underneath this trough should allow for scattered convective development with daytime heating along a low level convergence axis forecast from the western Carolinas into WV and western VA. The flow will be rather weak along this corridor, thus anticipate some slow moving pulse activity. Briefly heavy rains will be possible, but any individual cell should be short lived, and PWATs will only be around average. Also may see some shower activity along another convergence axis closer to the coast. Less instability forecast here, thus looking more like generally light shower activity. WPC QPF generally followed a multi model blend, weighted more towards the 12z HREF guidance. ...Florida... Convection will continue to expand in coverage this afternoon ahead of a southward dropping cold front. The heaviest totals should focus across the central and eastern portion of the state, with the intensity decreasing with the loss of heating tonight. ...Pacific Northwest---Northern Great Basin into the Northern Rockies... Showers and embedded thunderstorms will expand in coverage this afternoon across northern ID into western MT ahead of a well defined shortwave. Storms should be quick moving, however instability increasing to around or above 1000 J/KG will support at least a few more intense storms, capable of producing briefly heavy rainfall rates. The 12z HREF does indeed show moderate probabilities of localized amounts exceeding 1" in an hour. This rainfall potential along with melting snows will result in rising stream flows and potential for runoff issues from northern ID into western MT. The best window for these higher rates is from around 21z - 04z. During the day Monday isolated to scattered convection should again break out near this shortwave as it tracks east across MT and towards the western Plains. Most cells should be quick moving, and not expecting much organization. Thus areal averaged rainfall will generally be light, with just some localized heavier totals. Days 2/3... ...Northern Plains and Upper Midwest... Showers and storms are expected to develop and spread east from the northern High Plains as a shortwave trough moves east of the northern Rockies late Mon. Models show the wave amplifying as it moves east of the mean ridge centered over the western U.S. and into the northern plains Mon night into Tue morning. This shortwave is expected to interact with a plume of deepening moisture, pooling on the nose of a strengthening low level jet reaching into the Dakotas. By early Tue, the GFS show PW anomalies of 1.5-2.5 standard deviations above normal extending across much of the Dakotas - supporting the potential for moderate to locally heavy amounts across the region. WPC QPF through into late Tue reflects a compromise between the GFS and ECMWF - which are fairly well-clustered with respect to the timing and amplitude of the larger scale features. However, in comparison to the other deterministic solutions, the GFS amounts appear to be overdone, particularly across eastern South Dakota on Tue. This may be attributed to some feedback issues that the GFS incurs Mon night into Tue morning across the Dakotas. The ECMWF however, given the available moisture and forcing, appears to be displaying its more typical dry bias. Late Tue into Wed, models show the initial wave continuing to move east into the upper Mississippi valley as it phases with an upstream wave sliding southeast off of the ridge out of western Canada. Again, continued to follow a compromise between the GFS and ECMWF, with generally light to moderate amounts moving east ahead of a deepening surface wave tracking across Minnesota on Wed. ...Central Plains to the Mid Mississippi valley... Shortwave energy moving east from the central Rockies Mon night is expected to help extend the upper trough further south across the central Plains into the Mid Mississippi valley Tue night to Wed morning. This is expected to spur the development of showers and storms, with locally heavy amounts possible across portions of Missouri and southern Iowa, as it interacts with a deepening moisture pool centered along a surface to low level warm front lifting across the region. There remains some model signal for slow-moving back-building storms - which may result in local runoff concerns Tue evening. Therefore, a 'Marginal' risk, centered along the Iowa-Missouri border into northern Missouri, remains in the Day 3 Excessive Rainfall Outlook. Here also, WPC QPF largely reflects a compromise between the GFS and ECMWF. ...Pacific Northwest... A closed upper low is expected to transition to negatively-tilted open wave as it approaches Vancouver Island Tue night. Southwesterly inflow ahead of the trough will support PW anomalies of 2-3 standard deviations above normal across northern Oregon into Washington late Tue into early Wed, fueling the potential for moderate amounts across the Olympics and northern Cascades. WPC amounts largely reflect a blend of the GFS and ECMWF through the period. Chenard/Pereira Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml