Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 501 AM EDT Sat Jun 02 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Jun 02/1200 UTC thru Jun 05/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr ...Central-Northern Plains and Upper Midwest... Mid/upper level trough across the northern Plains early this morning will become more negatively tilted during the day 1 period, as the more confluent upper flow across the central Plains-mid MS Valley causes the southern periphery of the vort lobe (trough base) to take off a bit faster compared to the remainder of the trough. Upper level forcing will be bolstered by the negative tilt (increased divergence aloft and more focused low-mid layer Qs convergence), however the modest-at-best deep layer instability will preclude the excessive rainfall threat to some degree over areas that are expected to benefit from the strongest dynamics (including MN-WI). Farther south, ongoing MCS across the lower MO Valley will remain progressive early this morning, the undergo decay after 12Z following the diminishing nocturnal LLJ. Rainfall amounts into the mid MS Valley and western OH Valley will be more modest on average, i.e. averaging under 1.0", though per the individual high-res CAMs, localized totals of 2-3+ inches will remain possible as the axis of favorable thermodynamics (CAPES of at least 1000-2000 j/kg and anomalous PWs of 1.5-1.75") along and ahead of the surface front pushes w-e across the area. WPC QPF was based largely from the high-res means, especially the HREF probability-matched mean in providing more detail with the areas more likely to see heavier amounts. ...Lower Mississippi Valley... Another more focused area of moderate-heavy rainfall will align generally n-s/nne-ssw along the progressive cold front, with vigorous deep-layer instability owing to the steep mid-level lapse rates and PW values maxing out between 1.75-2.0". WPC also utilized a blend of the high-res CAM guidance in this region with the QPF, especially with the HREF mean, in generating areal-average totals under 1.0" for most. Would also expect localized totals of 2-3+ inches in this area as well; however, given the swift progression (aided by the strong downdraft CAPE and resultant convective outflows), the excessive rainfall threat will remain somewhat mitigated (i.e. Marginal). ...Eastern U.S.... Slow-moving upper trough will nudge eastward toward the Atlantic coast today. Will continue to see little change in the overall pattern until this deep, compact trough pushes off the coast. Given the orientation, the models continue to indicate the more focused area of forcing (DPVA, upper divergence) across the mid Atlantic region during the peak heating hours this afternoon into the evening. Thus expect more widespread convective coverage today compared to yesterday across this region, where much rain has fallen over the past several weeks and FFG values are quite low as a result of the wet antecedent conditions. Light deep-layer flow/low 0-6 km bulk shear (generally aob 10 kts) would again be a huge deterrent for more organized, prolonged convection; however, given the low FFG values (especially west of I-95), persistent broad-scale forcing, ample/deep subtropical moisture, and surface-based CAPES likely peaking between 1000-2000 j/kg prior to the onset of convection, believe the hourly or even sub-hourly rainfall rates will exceed the 1 hourly FFGs over many areas, as depicted by the SSEO and HREF probabilities. Thus per collaboration with WFOs AKQ, LWX, and PHI, have hoisted a Moderate risk for flash flooding across a large portion of the mid Atlantic, encompassed by Slight and Marginal areas. In terms of the QPF, WPC again relied heavily on the high-res means in this area, especially the HREF. Day 2 ...Great Lakes to Central Appalachians... Strength and timing of a negatively tilted trough are slightly off among the 00Z guidance, though close enough to allow QPF to be based on the 00Z GFS/NAM/ECMWF. The highest Day 2 QPF is over Ontario. ...Gulf Coast... The front progresses south Sunday until reaching the Gulf of Mexico. Dynamics are not as strong along this portion of the front but instability is much greater. QPF was raised for coastal Gulf with isolated heavy amounts still possible where convection becomes organized per a blend of the 00Z GFS/NAM/ECMWF. ...Southern to Central Rockies.. An upper trough moves east from the Four Corners to the CO/KS border Sunday. This system will help draw increasing pw values northward through the Southern to Central Rockies and Southern High Plains. This will support increasing convection across these area Sunday afternoon in a region of fairly well defined upper difluence ahead of these height falls. QPF was based on a blend of the 00Z NAM/GFS/ECMWF. ...Mid-Atlantic... A mid-level shortwave trough crossing the southern Great Lakes Sunday will draw in a remnant trough over the Mid-Atlantic. This will maintain surface low pressure over eastern NC and maintain onshore flow over the central Mid-Atlantic until the cold front from the Great Lakes crosses Sunday night. Stable onshore flow will be in a tropical plume of moisture making for locally heavy rain. QPF based a blend of the 00Z NAM/GFS/ECMWF which is a little faster than the Friday evening guidance. The Marginal Risk of excessive rainfall was maintained over the entire Mid-Atlantic. Day 3 ...Southern New England... Mid-level low pressure spins over Ontario/Quebec Monday. Tropical moisture is lifted into New England with good dynamics. Little to no instability limits excessive rain risk to a Day 3 Marginal Risk of excessive rainfall. QPF based on a blend of the 00Z NAM/GFS/ECMWF which were in good agreement for southern New England. Hurley/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml