Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 503 PM EDT Mon May 07 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 08/0000 UTC thru May 11/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 For the 08/00z QPF issuance: no significant changes were made to the 07/18z QPF issuance. A few tweaks were made across the Northern Plains for the first forecast period (08/00z to 08/06z), based mainly on radar trends and the most recent HRRR runs. ...Northern to Central Plains... A 700 mb wave and potential embedded closed circulation cross the northern Plains tonight into Tuesday. Return warm/moisture advection develops across the northern Plains. As well defined upper difluence moves across the northern Plains, convection should increase through the region with lift in the positive 850-700 mb theta-e advection zone and pre-frontal convergence. Increasing pw values--up to 1.25-1.5 inches Tuesday afternoon...2 to 2.5 standard deviations above the mean....supports increasing amounts of rainfall with showers/storms Tuesday. Multiple models indicate potential for a strip of 1 inch rainfall in portions of the eastern Dakotas and MN. Difference persist because there are multiple spokes of embedded cyclonic shear axes within the 700 mb circulation, so rainfall maxima are forecast with each cyclonic vorticity maxima rotating around the common circulation. Manual progs placed the rainfall in the middle of the cluster of solutions, with less weighting on the maxims Tue in eastern ND and northern MN owing to typical moisture intercept from convection further south. consequently, more weighting was given to the 12z NAM/UKMET and less on the GEM regional and GFS. ...Central to Southern Appalachians/coastal NC to southeast VA ... The mean mid to upper level trough drifts southeast across the central to southern Appalachians later today into this evening and then east of the mountains Tuesday. Diurnal instability underneath this trof---mu cape values 500+ j/kg-- will support scattered diurnal showers through the central to southern Appalachians this afternoon and evening. Light to moderate precip totals in the .10-.25" range depicted from western NC/northeast TN---Southwest VA---central to eastern WV into far western MD and far southwest PA. High resolution model forecasts indicate locally heavier totals are possible, which could produce some runoff issues where ffg values are low. Neighborhood probabilities from the hi res guidance...ARW...NMMB and NAM Conest all show a chance of exceeding 1 and 3 hour ffg across these areas. A marginal risk area continues on the day 1 excessive rainfall potential outlook across much of WV into Southwest Virginia for this afternoon into early evening. On Tuesday the upper trough drifting east with the upper low drifting across central VA and NC takes the peak upper divergence maxima out of the mountains in tandem with the movement of the upper low. The models forecast less rain on Tuesday as a result in the mountains. With a separate upper difluence/embedded upper divergence maxima/700 mb convergence maxima along the coast of NC to adjacent VA, the models indicate showers in this area as well. ...Pacific Northwest/Northwest California... The models indicate a deep layer circulation over the northeast Pacific that heads in the direction of Vancouver Island late Tuesday-Tuesday night. A trailing frontal boundary move towards the coastal of the Pacific northwest and northwest CA late Tuesday. Pre-frontal convergence an and enhanced band of moisture with 700 mb relative humidity increasing to 90 percent leads to showers across these areas. Amounts are light with good agreement among the model solutions, so a consensus approach was taken for QPF. Days 2/3... ...Middle/Upper Mississippi Valley to the Great Lakes, Ohio Valley, and Northeast... Merging shortwave energy interacting with a deepening plume of moisture will trigger widespread precipitation while crossing the Middle/Upper Mississippi Valley Tuesday night into Wednesday. The overall progressive nature of the system should minimize the large scale threat for excessive rainfall. However, anomalous moisture across the region (PW values 1.5 to 2.0 standard deviations above normal) could support periods of locally heavy rainfall. A marginal risk of excessive rainfall was maintained over northern Missouri, although it was expanded and shifted slightly southeastward to account for some of the latest model guidance. Also, a marginal risk of excessive rainfall was added over southern Minnesota and northwestern Iowa, where low level convergence ahead of a weak surface wave lifting out of South Dakota could focus some heavier totals. The chance for precipitation will shift into the Great Lakes, Ohio Valley, and Northeast Wednesday night into Thursday, but no risk for excessive rainfall was added to the Day 3 period since the shortwave will be quick to lift northeastward and get absorbed into a large scale upper trough swinging into southeastern Canada. ...Pacific Northwest to the northern Rockies and adjacent Great Plains... An active pattern is expected across the Pacific Northwest and northern Rockies as a closed upper low transitions to a negatively-tilted open wave while approaching Vancouver Island Tuesday night, with further shortwave activity swinging into BC through Wednesday night. Southwesterly inflow ahead of the trough will bring one inch PW air (2-3 standard deviations above normal) across northern Oregon into Washington Tuesday afternoon and evening, fueling the potential for moderate rain across the Olympics and northern Cascades. Activity should spread eastward into the northern Rockies and adjacent Great Plains as height falls progress eastward Wednesday into Thursday. Petersen/Gerhardt Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml