Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 429 AM EDT Sat May 05 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 05/1200 UTC thru May 08/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...TN Valley into the Central Appalachians,,, There is good model agreement with the evolution of the well defined southern stream trof pushing into the Lower MS/Lower TN Valley region Saturday---east northeastward to the south of the OH Valley and toward the Central Appalachians Saturday night into early Sunday. Strongly difluent upper flow ahead of this trof in an axis of above average pw values---1 to 1.5+ standard deviations above the mean---will support widespread moderate to heavy precip totals from the TN Valley northeastward into the Central Appalachians and Mid-Atlantic. Stream flow is initially near or slightly below normal across these areas but is expected to rise quickly to much above to high flow as per the national water model from hourly rainfall rates of 1"+ as the area of organized precip spreads east northeast across these areas. No significant changes made to the previous excessive rainfall potential outlook across these regions with a marginal risk area stretching from north central TN into eastern KY--much of WV into Southwest VA. ...Central to South Florida... The closed upper low moving into central Cuba is expected to become nearly stationary day 1. Higher pw values expected to rotate west northwestward to the north of this closed low increasing pw values to much above average values across much of central to south Florida day 1. The increasing pw values will also be accompanied by increasing shower activity for central to South Florida. Low confidence on the details of qpf across these areas with model consensus for moderate to locally heavy totals. ...Coastal NC---Outer Banks... The anomalous pw values will also be drawn northward ahead of the above mentioned upper trof pushing east northeast from the Lower MS Valley---TN Valley into the Central Appalachians. These anomalous pw values will impact coastal areas from the upper coast of SC---northeastward into eastern and coastal NC Saturday night into early Sunday. Heavy rains are possible in a region of enhanced upper difluence in the right entrance region of an upper jet over the Mid-Atlantic. There is some spread with the exact axis of the heaviest precip---with some models showing it just along the coast of NC---especially across the Outer Banks---while others are just offshore. The day 1 qpf leaned toward the href mean depicting the heaviest axis just offshore of the Outer Banks but still having heavy precip affecting the Outer Banks. No changes planned to the small marginal risk area on the excessive rainfall potential outlook over the Outer Banks. This will be for potential ponding of water on roads. ...Upper MS Valley---Upper Great Lakes... A surface cold front expected to drop southward day 1 across the Northern Plains---Upper MS Valley---Upper Great Lakes region. Scattered shower activity possible along and ahead of this front. Low confidence in any qpf details here---with light to locally moderate totals depicted from southern MN into central to northern WI and the U.P. of MI. Days 2/3... ...East Coast... A southern stream trough shifting east across the southeastern CONUS reaches the southern Appalachians by Sunday morning. Northern stream troughs over eastern Canada draw the trough axis northeast with surface low pressure developing along the coast north from Cape Hatteras. Deep tropical moisture associated with the amplified trough will remain just offshore through given the low track through Sunday night. Mere wrap around rain is expected up the eastern seaboard from NC through this time with max average QPF generally a half inch. One exception could be southeastern VA where secondary surface low pressure may be farther inland per the 00Z NAM and ECMWF. QPF was based on a blend of the 00Z GFS/NAM/ECMWF/UKMET. ...Northwest to the Northern High Plains... A weakening upper-level low off the Northwest Coast will eject a short wave eastward from the CA/OR border Sunday morning and across the Northern Rockies Monday. QPF will be modest over the northwest with only continental air to work with. The highest QPF Sunday will be south facing slops of the Northern Rockies with 0.50 to 0.80 inches. The system taps Gulf moisture over the northern Plains Monday night with moderate rainfall and QPF around 0.75 inches over western SD/ND. Oravec/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml