Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 415 AM EDT Fri May 04 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 04/1200 UTC thru May 07/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Great Lakes---northern NY state into Northern New England... The elongated upper trof stretching northeast to southwest from the Upper MS Valley into the Central to Southern Plains is expected to separate into distinct northern and southern stream components day 1. The northern stream component will push a deepening surface wave across the Great Lakes this morning---through southern Ontario into southern QB tonight and across northern New England early Saturday morning. A well defined comma head/deformation precip band will affect the Great Lakes region Friday morning from northeast WI into the eastern U.P. of MI and the northern L.P. supporting moderate precip totals in the .25-.50" range. Farther to the east---the track of this system will likely keep the max precip axis across southeast Ontario---southern QB and far northern Maine with .25-.50"+ amount possible. Saturated soils and high stream flow across portions of northern New England may result in isolated runoff issues in the vicinity of northwest Maine into far northern NH and far northeast VT. A marginal risk area will be maintained across these areas in the day 1 excessive rainfall potential outlook. The trailing front from this low will be pushing rapidly east across the lower Great Lakes and northeast Friday into Friday night. The fast movement will limit precip potential--with mostly light totals expected to the south of the above mentioned max qpf axis. ...South central to eastern TX... The southern component to the initially elongated upper trof will be pushing across the Southern Plains today and into the Lower AR and Lower MS Valley region by Friday night-early Saturday. There is a model signal for heavy to potentially excessive rainfall amounts ahead of this trof in the axis of above average pw values stretching from south central to northeast TX. Confidence is low with qpf details across this region as guidance is already showing poor evolution of current activity. However---with overall favorable right entrance region jet dynamics and strong frontal convergence in this above average pw axis---confidence is greater on the heavy to isolated excessive rainfall amounts. Day 1 qpf leaned toward the href mean to mitigate the run to run variability in the hi res guidance---depicting areal average 1-2" totals across south central to eastern TX. While there is a large spread with the location of the heaviest amounts---there are multiple models that show max totals in excess of 5" across these areas. A broad marginal and slight risk areas were maintained from previous forecasts in the latest excessive rainfall potential outlook across these areas for the potential of locally very heavy rains and resultant runoff issues. ...Lower MS to Lower TN Valley... To the northeast of the potentially heavy rains over south central to eastern TX...convection may enhance Friday afternoon into Friday night/early Saturday from the Lower MS Valley into the Lower TN Valley along and ahead of the surface cold front moving across this region. The model signal is not as strong for heavy rains across this area---but with some of the same favorable large scale forcing upstream over TX also expected to push east northeast into the Lower MS to Lower TN Valley region---moderate to locally heavy rains are possible. ...Southeast Florida... The upper low across the central to southern Bahamas is expected to continue to press southwest day 1 across central Cuba. Shower activity is expected to increase across the Bahamas this afternoon and push westward toward the southeast Florida coast Friday night into early Saturday. This will support light to moderate precipitation totals in the .10-.25"+ range across southeast Florida. Days 2/3... ...East... A southern stream trough currently crossing NM/TX will slowly shift east through Sunday and move off the East Coast Sunday night. Gulf moisture streaming ahead of the trough will continue to bring 1.5 inch PW across the TN valley (1.5 standard deviations above normal) Saturday while a deeper plume of 2 inch PW approaches from the tropical Atlantic to the Carolinas (this PW is 2 to 3 standard deviations above normal). A swath of locally heavy rain is expected from Nashville to Cumberland, MD as the vort lobe crosses this area Saturday into Saturday night. Instability will be quite limited west of the Appalachians where 24hr areal averaged QPF is near the 6hr FFG which warrants a marginal risk for excessive rainfall on Day 2 (12Z Sat-12Z Sun). The Outer Banks will have quick hitting rain event Saturday night as the parent shortwave crosses the area with lingering instability and PW around 2 inches (2 to 3 standard deviations above normal). While the land in the Outer Banks is well-draining sand, flooding issues are possible, so a Day 2 (12Z Sat-12Z Sun) marginal risk for excessive rainfall was raised for the Outer Banks as well. ...Great Lakes to Northeast... Shortwave energy rounds a stalled upper trough over the eastern CONUS through the forecast period. Phasing between northern and southern stream troughs is progged for Sunday with the developing surface reflection developing along the New England coast and crossing near Maine before lifting into Canada. ...Northwest... A shortwave trough comes ashore over OR/northern CA Saturday night and reaches MT by Sunday night. Light precip mainly for preferred upslopes is expected through the Northwest for both days as continental air flows into the system. Oravec/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml