Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 542 AM EDT Thu May 24 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 24/1200 UTC thru May 27/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Overview... The pattern looks like May, but overall, considering the full CONUS, it is quite a bit warmer than climatology and the mid level flow is generally weaker than usual, at least where it is precipitating. A shear axis and embedded circulation centered around 700 mb will provide some level of organization to diurnal thunderstorms over Georgia and South Carolina. A better defined northern stream trough will bite into the ridge over the northern Plains and upper Midwest today, producing thunderstorm clusters from the Dakotas to Kansas and eastward...with more widely scattered activity over the southern Plains and the rest of the Southeast. In much of the central and southeast United States the convection will be driven as much or more by the available instability / strong May sun angle and plentiful surface moisture as it will be driven by the mid level flow regime. Meanwhile an upper low poking into California will provide lift enough to yield organized thunderstorms from the Sierras to southern Oregon. ...Southern / Southeast U.S... With slow cell movement but also a lack of deep layer shear this afternoon, the greatest areal average rainfall should occur where CAPE and low level inflow are maximized. This especially describes central to eastern Georgia and western South Carolina, where low level inflow will be slightly enhanced by the presence of the 700 mb circulation over central GA. Models also signal heavy rain around the southern periphery of the low in somewhat a repeat of what occurred Wednesday. Precipitable water values of 1.50 inches and greater provided a pretty good trace of where organized diurnal convection took place on Wednesday. For Thursday this threshold led us to favor the WRF-ARW2 and the HREF blended mean. We combined these sources with 30 to 40 percent of continuity to derive the updated forecast. The ECMWF ensemble QPF probabilities were also very useful at separating some of the mesoscale areas of precipitation on the 6-hour time scale. ...Plains / Upper Midwest... This is a pretty standard issue Great Plains convective forecast. Expect a couple areas of concentrated upglide early in the day, mainly Minnesota and near Omaha. By afternoon clusters of organized storms will form in a near-surface convergence axis / pre-frontal - from eastern Colorado to parts of the Dakotas and Minnesota. Predicting each mesoscale cluster of rainfall will be a low confidence endeavor, but we did our best at combining the available model signal with known tendencies. Our QPF favored the NAM CONUS Nest and HREF Blended Mean, while using our in-house ensemble as a more broad scale base. The result gave us areal average quarter inch or greater amounts over a large area, but relatively little 1.00 inch areal average based on uncertainty as to where to place the heavier amounts. There is no clear focus, however, for a heavy rain event. Flash flood potential will depend more on storm scale outflows and interactions not very well knowable with much lead time. Certainly parts of eastern Nebraska and also parts of Iowa and southern Minnesota are a little more susceptible right now, per Flash Flood Guidance values. ...West... Dewpoints remained in the upper 40s and low 50s from northern California into the northern Great Basin during peak heating Wednesday. Expect similar values today, and the moisture through the column - PW - is forecast at around 2.0 standard deviations above climatology. When combined with the development of an upper low just off California and difluent upper flow extending inland, we attempted to lean heavier in terms of convective coverage. We favored the NAM CONUS Nest and HREF blended mean, while also preserving some percentage of continuity. The degree of lift could support cells regenerating quickly along outflow boundaries so as to lead to some training and flash flood concerns in the mountains terrain of northern California and adjacent NV/OR. ...Maine... Warm advection becomes focused ahead of a fast-moving, low amplitude shortwave trough this evening. Model QFPs are in good agreement, so we kept with a consensus, with the forecast bringing a tenth to quarter inch liquid into northern Maine. The heavier rain may stay just outside the state to the north and east. Days 2/3... ...Southeast/eastern to central Gulf Coast/FL... Deep tropical moisture within confluent flow combines with diurnal heating/destabilization to produce widespread showers/storms in eastern GA/southern SC east of a remnant 850 mb circulation. Above normal moisture leads to a continuation of a Marginal risk of excessive rain. The models also depicted persistent, deep flow of moisture from the Bahamas into southeast FL with peak precipitable water values near 2 inches so showery conditions are expected along the southeast coast. The pattern favors inland penetration of the sea breeze from the east coast so peak afternoon convection should occur over interior portions of the FL peninsula to the west coast. Lapse rates are not as high as usual due to warm mid level temps, so bands of larger scale lift are needed to trigger FL convection. Convective activity into southern FL warrants a Marginal risk for excessive rainfall for Day 2 and 3 given recent wet conditions. On day 3 the models diverge as to the track and intensity of a low forecast to develop over the Gulf of Mexico. The GFS. Canadian global,and UKMET are east of the ECMWF/NAM/ECMWF Ensemble Mean, which have better continuity. With the GFS and Canadian global trending west, much more weighting was given to the 12-00z ECMWF/ECMWF Ensemble mean and 18-00z NAM QPF. ...Pacific Northwest/Great Basin/Northern Rockies... A closed mid-upper level low drifts onshore and across CA Fri to NV through Saturday. Southerly flow ahead of the circulation increases moisture. On Friday the deep layer circulation drifts onshore in CA with the upper trough drifting east across OR. Well defined coupled 700 mb convergence and 300 mb divergence cross from northern Ca and NV into OR, with better than average cluster of higher QPF across southern to northeast OR as a result. A Marginal risk for excessive rainfall is in effect for impacted areas of OR. Good agreement among global guidance led a consensus based approach of the 00z ECMWF/NAM/UKMET. Less weighting was given to the lighter GFS and heavier 12z ECMWF. On Sat., the 700 mb circulation moves northeast out of NV towards the WY/UT border. Continuing 700 mb convergence along and northwest of the low track promotes showers across northeast NV across southern ID and much of MT. With low pressure developing on the northern high Plains, low level convergence leads to showers developing in the northern Plains Sat night. The 00z NAM shows heavier amounts than the other models, with what might be a spurious maxima in 700 mb convergence near the MT/ND border centered on 06z Sun. A more conservative approach using the multi-model consensus was used. ...Central Plains to the mid-upper MS Valley, Great Lakes, Ohio Valley, New York and New England... Activity on Fri will focus ahead of a low level front and prefrontal moisture plume with low-mid level convergence on the nose of the low level jet from the central Plains towards the mid-upper MS Valley. The 00z NAM depicts what appears to be a case of convective/grid scale feedback, showing anomalous low pressure and forced convergence and strong lift from southeast KS across OK. Less weighting was given to the 00z NAM as a result of low confidence in its high QPF amounts. Activity on Sat-Sat night continues to move east across the Great Lakes, Ohio Valley, TN Valley, and central Appalachians. The parent circulation crosses Ontario to the Quebec border and downstream 700 mb warm/moist convergence produces rain crossing western and northern New York. A weak frontal wave crossing the mid Atlantic into the nearby coastal waters is predicted by the ECMWF/GFS/NAM to produce rain Sat night in New York City across Long Island, NY. Manual QPF was derived by blending the 00Z GFS/ECMWF/NAM/UKMET. Burke/Petersen Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml