Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 433 PM EDT Thu May 31 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Jun 01/0000 UTC thru Jun 04/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr ...Lower-Mid Mississippi Valley into the lower Ohio and Tennessee Valleys... Ongoing convection across the lower Ohio valley will continue to move east toward the central Appalachians as a shortwave associated with overnight convection over the central Plains continues to translate east across the region. There remains a good model signal for additional development further west back across the mid Mississippi valley this afternoon as trailing energy moves out of the central Plains. As energy aloft interacts with anomalous moisture, these storms will have the potential to produce heavy amounts. The GFS show PWs remaining at or above 1.5 inches (1-2 standard deviations above normal) ahead of frontal boundary extending southwest from the Great Lakes back into the central Plains. These storms are expected to push east from the mid Mississippi into the lower Ohio valley. However the hi-res guidance shows a good signal for storms to develop and drop south along a trailing outflow boundary which is forecast to move into northern and central Arkansas overnight. Low level southwesterly inflow into the western flank of this boundary may support a period of back-building training convection, resulting in locally heavy totals across this region. WPC QPF reflects a blend of the HREF Mean with recent runs of the HRRR for the afternoon and even periods, with significant weighting given to the HREF Mean through the remainder of the period. ...Lower Great Lakes / Upper Ohio Valley / Central Appalachians / Mid Atlantic... A warm front extending from the Great Lakes through the Mid-Atlantic states will push north this period with backing flow supporting a surge of deeper moisture along the Mid Atlantic seaboard this afternoon/evening. This will support shower and thunderstorms moving north with a continued threat for additional moderate to heavy amounts extending to the south across the region. Given the anomalous moisture, there remains the threat for locally heavy amounts with these storms. ...Northern Intermountain West / Northern Rockies and Plains... A well-defined shortwave trough will continue to translate east, assuming a negative tilt as it pivots across the Intermountain West into the Rockies late Thu into early Fri. Favorable upper jet forcing along with low level convergence/frontogenesis is expected to support widespread light to moderate amounts from the northern Intermountain West into the northern Rockies and High Plains this evening and overnight. Then as the system pivots further east, guidance shows an amplifying southerly low level jet supporting increasing PWs of 1.5 inch or more across the central Dakotas tomorrow afternoon-evening. This along with approaching upper trough is expected to spur the threat for developing showers and thunderstorms with heavy rains across portions of the Dakotas and Nebraska. WPC QPF gave significant weight to the HREF Mean through the period. Days 2/3 ...Northern to Central Plains toward the Upper to Mid MS Valley... Convection should be active at the beginning of the day 2 period ahead of the well defined trof pushing east across the northern and central plains toward the Mid to Upper MS Valley. Given the overall strong upper dynamics ahead of this trof in an axis of pw values 1.5 to 2+ standard deviations above the mean---confidence is fairly high on the overall potential for widespread moderate to heavy precipitation totals stretching across large portions of the Northern to Central Plains toward the Upper to Mid MS Valley. There are some significant differences---however--on some of the placement details---especially at the beginning of the day 2 period with how far to the south the southern portion of this organized convection may be. The NAM conest and nmmb were on the farther south end compared to most of the remaining guidance. Can not rule out the southern solutions given the general model bias of being too far north with convection. Day 2 qpf leaned toward an in between solution offered by in house hi res mean---showing the southern max in the vicinity of southeast NE into northeast KS---while a second northern max was situated over the eastern Dakotas. These two areas should progress steadily eastward through the remainder of the day 2 period--possible weakening somewhat post 1200 utc Saturday. Moderate to heavy precip threat will continue day 3 for points eastward from the Mid to Upper MS Valley into the Great Lakes. Leaned closer to the GFS and ECMWF which were slightly faster than some of the ensemble means but seemed to fit better the eastward push of the strong height falls. Across the Southern Plains---our first impression is that models may be under done with convective potential toward the end of the day 2 period continuing into the day 3 period along and ahead of cold front pushing south through the Southern Plains and toward the Lower MS Valley. The NAM and GFS both show an expanding area of instability ahead of the boundary from northeast TX into southeast OK...northwest AR to southern MO late day 2 continuing into early day 3---with forecast MUCAPE values peaking shortly after 0000 utc Sun in the 3000-4000 j/kg across these areas. Precip was increased to areal average moderate totals day 3... .25-.50"+ from northeast TX---far southeast OK---central to southern AR---southeastward toward the lower MS Valley. Day 3 qpf was drawn to show potential for a fairly organized line of convection pushing southeastward. ...Southern to Central Appalachians---Mid-Atlantic into southern New England... Height falls in a northern and southern stream moving in tandem late day 1 from the Great Lakes into the OH Valley will begin to separate more day 2 with the northern stream height falls more progressive across New England---while southern stream height falls expected to be slower from the Central Appalachians into the Mid-Atlantic. The more progressive northern stream height falls will favor light to locally moderate totals--while moderate to heavy totals possible day 2 from the Central Appalachians into the Mid-Atlantic where pw values are expected to rise to 2"+. This slow motion of the mid level height falls and much above average pw values will persist into day 3 from the Central Appalachians into the Mid-Atlantic---with additional moderate to heavy precip likely across these areas. An excessive rainfall threat will persist day 2-3 across the Central Appalachians into the Mid-Atlantic---especially during the beginning of the day 2 time period when instability will be greatest along and ahead of the southward moving west to east oriented front moving across these regions. Increasing stable surface conditions likely day 3 across these areas with more of an overrunning regime likely. ...Central to Southern Rockies... Height falls moving slowly northeastward from the Southwest toward the Four corners region day 3 will draw higher pw values northward into the Southern to Central Rockies. This should support increasing scattered convection across these areas with light to moderate areal average totals depicted. ...Pacific Northwest into the Northern Rockies... Northeast Pacific height falls rotating around the base of the closed low off the Alaskan Panhandle/B.C. coast will push an associated frontal boundary toward the Pacific Northwest day 3. Day 3 WPC qpf did not deviate from the model consensus of light to moderate totals from the Pacific Northwest into the Northern Rockies. Oravec Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml