Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 559 PM EDT Tue May 15 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 16/0000 UTC thru May 19/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Upper Ohio Valley---Central Appalachians into the Mid-Atlantic... Much above average pw values 1.5 to 2.5+ standard deviations above the mean expected to persist day 1 along the west to east oriented stationary front forecast to lie from the Ohio Valley---through the Central Appalachians and into the Mid-Atlantic. Persistent low level southerly flow into this front and enhanced large scale accent from shortwave energy moving parallel to this boundary will support a good chance of widespread moderate to heavy precip totals across these areas. Lower confidence with some of the shorter term qpf details---but overall higher confidence on the widespread moderate to heavy totals given the overall favorable pattern. No changes made to the previous marginal to slight risk area aligned parallel to the front across these areas. ...Southern Mid-Atlantic---Southern Appalachians---Southeast into Florida... To the south of the above mentioned west to east front---pw values will also remain much above average through the southern Mid-Atlantic---Southern Appalachians--Southeast and across Florida. The slow moving closed low over the northeast Gulf of Mexico off the northwest Florida coast will continue to provide a broadly diffluent pattern to its northeast in this anomalous pw axis across Florida---the Southeast---Southern Appalachians and into the southern Mid-Atlantic. While there are the typical smaller scale qpf differences among the models---overall there is very good agreement on widespread moderate to heavy totals. The Southern Appalachians in the vicinity of northeast Georgia---the Upstate of South Carolina and into western North Carolina likely will have the greatest precipitation potential ahead of this upper low as uvvs will be enhanced by southerly upslope flow. ...Southern Plains toward the Lower MS Valley... The western portion of the stationary front across the Ohio Valley will remain across the Southern Plains into the Lower MS Valley. Shortwave energy rounding the top of the shortwave ridge over the Southern Plains tonight will push southeast toward the Lower MS Valley on Wednesday. Organized convection is possible along and north of this front---with model consensus of the qpf axis from the TX/OK panhandle region---eastward across much of OK and into western Arkansas. Moderate to heavy precip totals possible in this qpf axis---with the expected progressive nature of the convection being a limiting factor on very heavy totals---although no changes planned for the marginal flash flood risk area across this area. ...Northern Sierra---Oregon---Northern Sierra into the Northern Rockies... The closed low off the central California coast will be kicking northward the lead upper low over the Great Basin northward into the Northern Rockies day 1 as it presses inland into northern California Wednesday. A broad area of upper difluence ahead of each of these features will support a widespread area of scattered precipitation from the Northern Sierra---across much of Oregon---the Great Basin and into the Northern Rockies. Model consensus for the greatest totals is from the northern Sierra---northern California into the Cascades of Oregon to the north of the closed low moving into northern California Wednesday. No changes made to the marginal risk area to the north of this closed low where pw values will remain much above average. Days 2/3... ...Great Basin/Northern Rockies... A closed mid level low fills as it crosses the Great Basin Wednesday night through Thursday night. Day 3. CAPE of 1000 j/kg and up to one inch PW with the lift from the low should produce locally excessive rainfall, especially over the northern Rockies where snowmelt will factor into the flood risk. The marginal risk for excessive rainfall was maintained over OR/ID/MT for Day 2 (12Z Wed-12Z Thu) and along the ID/MT border for Day 3 (12Z Thu-12Z Fri). There was generally good model agreement concerning the evolution of the synoptic scale features, so the WPC QPF was based mainly on a blend of the 12Z ECMWF/GFS with some GEFS ensemble mean. ...Central and Northern Great Plains... Southwest flow aloft ahead of an upper trough over the western CONUS will allow Gulf moisture to spread up the extent of the Great Plains. 1.25 inch PW is 2 to 2.5 standard deviations above normal for the eastern Dakotas. A stationary front setting up near the SD/ND border will provide a focus for heavy rain Thursday night through Friday. This warrants adjusting the slight risk for excessive rainfall south to southern ND for Day 3 (12Z Thu-12Z Fri) despite higher FFG in this area than the previous area along the ND/MN border. An axis of heavy rain extends southwest to the NE/CO border which remains encompasses in a marginal risk for excessive rainfall on Day 3. Latitudinal differences in the placement of the front (and the associated QPF maximum) reduced with 12Z guidance, so WPC QPF on a blend of the 12Z ECMWF/GFS/GEFS mean. ...Lower/Mid MS Valley across the TN Valley and the Mid Atlantic... A Bermuda high and weak upper level trough axis over the southeastern CONUS will draw deep tropical moisture up the Eastern Seaboard. This moisture will combine with instability up to a stationary front over the central Mid-Atlantic to produce convection with heavy to excessive rainfall on both Days 2 and 3. Better forcing along the frontal boundary on Day 3 should result in a larger flash flood threat across portions of the Mid Atlantic during Day 3. The excessive rainfall risk areas where shifted slightly north per 12Z guidance. However, the majority of precip is non-convective according to the GFS, so the risk areas where not drawn to the northern side of the QPF maxes for both days. Model placement of the front and QPF max came into better agreement with the 12Z suite, so WPC QPF was based on a blend of the 12Z GFS/ECMWF/GEFS mean. ...Southeast/FL... A long wave trough axis protruding into the central Gulf of Mexico will feed deep moisture and instability over the region during Days 2/3. Slow storm motions in the deep moisture could result in heavy to locally excessive rainfall, especially during Day 2 where a marginal risk is in effect for the eastern Gulf Coast and north. Also, a marginal risk was raised for the greater Miami area (per consultation with WFO MFL) due to 2 inch PW which is 2.5 to 3 standard deviations above normal with onshore flow and sufficient instability. There was enough agreement with the synoptic scale features in the 12Z guidance to base the WPC QPF on a multi model blend. Jackson/Oravec Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml