Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 456 AM EDT Tue Apr 24 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 24/1200 UTC thru Apr 27/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Eastern U.S... The slow moving, stacked cyclone affecting the eastern United States will bring a wet, showery day to areas from Ohio to eastern Tennessee and toward North Carolina and the Mid-Atlantic states. Rain will spread northward into New York and southern New England Tuesday night. The model biases with this system have been very consistent. The ECWMF is too confined with its QPF output, especially around the northern periphery. The hi-res models are generally over-estimating the precipitation magnitude, and also failing to pick up on diurnally driven swaths of convection that occur in the narrow instability axis near / south of the warm front. The GFS, while not having an ideal deep layer solution for the mass fields, at least does a better job of spreading synoptically forced precipitation downstream to the east and north. All of these biases are readily apparent again this morning. Perhaps the good news is that the hi-res models were less bullish with amounts today, and they are more unanimous in keeping the largest swath of deep convection just offshore or grazing the Outer Banks. So sources like the WRF-ARW and HREF Mean looked more usable. WPC leaned toward a version of our in-house ensemble which incorporates the hi-res models, melding their output with the global model consensus - which was advantageous given what was described above about the complimentary GFS/ECMWF strengths and weaknesses for this system. Our manually derived QPF ends up looking also very similar to the National Blend of Models, which has been performing well the past couple of days. A Marginal Risk of excessive rainfall will likely be justifiable in over some part of the Carolinas and Mid-Atlantic states, although any truly heavy rainfall will probably occur on small scales, with no real stand-out signal on the large scale. Our primary concern is with parts of North Carolina where antecedent rain this morning will lower flash flood guidance values before late afternoon convection ramps up. And upslope-enhnaced rainfall over Virginia. The big cities / I-95 corridor is more of a question mark, being located north of the surface low and occlusion. The literal FFG numbers requiring 1.50 to 2.0 inches in 3 hours will have a low chance of being exceeded, but there may be enough local convective enhancement, especially Tuesday night as the upper low approaches, that the combined effects of daytime and nighttime rainfall could eventually lead to some isolated issues with high water. ...Central U.S... A fairly vigorous mid level low will remain well defined as it drops south to southeastward through the Plains / High Plains today. Given relatively scant moisture, most precipitation will be slightly post-frontal and rooted in strong mid level ascent from South Dakota down through Kansas, and eventually parts of TX/OK. Some additional convection may act as surface-based during peak heating over Colorado and west Texas. There may be quite a bit of mesoscale influence to the precipitation pattern, given the steep lapse rates, marginal low level moisture, and interplay between the synoptics and terrain influences. That being said, however, the model guidance was in unusually good agreement for such an environment, and WPC took a consensus approach while leaning also on the SREF 6-hour QPF probabilities and deterministic output from the WRF-ARW and WRF-ARW2. Days 2/3... ...Northern Mid-Atlantic to the Northeast/Great Lakes... A negatively-tilted shortwave trough, associated with the remnants of the upper low presently centered over the Tennessee valley, is forecast to continue lifting north along the Mid-Atlantic to the Northeast coast Wed into early Thu. There it is expected to phase with a northern stream trough pivoting across the Great Lakes. A newly formed mid-upper level low closing off over the Great Lakes is expected to move into the Northeast on Thu, continuing into the Canadian Maritimes by early Fri. Surface low pressure organizing along the Mid-Atlantic coast on Wed is forecast to track north into New England Thu morning. Strong, moist southerly flow ahead of the low, along with favorable forcing aloft and marginal instability, is expected to support some moderate to heavy amounts across New England, particularly along coastal Maine. These resulting amounts, along with snow and ice melt, may result in local runoff concerns. Therefore, a 'marginal' risk for excessive was introduced to the Day 2 Excessive Rainfall Outlook. Meanwhile, a developing deformation zone is expected to support light to moderate precipitation shifting east from the Great Lakes into the Northeast. By late Thu, with the low beginning to lift out to the northeast, expect precipitation to begin to wane - limited to some light precipitation wrapping around the backside of the departing low. Guidance have been moving into better agreement with the evolution and timing of this system. While differences remain, WPC QPF followed a compromise solution, represented by a blend of the ECMWF and NAM. ...Central and Southern Great Plains to the Southeast... A positively-tilted shortwave trough/compact mid-upper level low is expected to pivot from the central plains to the lower Mississippi valley Wed into early Thu. Deepening moisture along a low-mid level wave/frontal band, interacting with the favorable forcing aloft, is expected to support areas of precipitation moving southeast from the plains into the lower Mississippi valley late Wed into early Thu. This shortwave is expected to swing progressively from the lower Mississippi valley into the Southeast on Thu, before lifting further north into the Mid-Atlantic ahead of an upstream trough digging across the Mississippi valley on Fri. This is expected support light to moderate amounts across the region. WPC QPF generally followed a blend of the GFS and ECMWF through the period. Burke/Pereira Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml