Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 450 AM EDT Thu May 10 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 10/1200 UTC thru May 13/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Central Plains to the Upper Mississippi Valley... As deepening moisture interacts with a shortwave trough emerging from the northern Rockies, there remains a good model signal for moderate to locally heavy, and potentially excessive rains, from the high plains of eastern Wyoming and western Nebraska to southern Minnesota and northern Iowa Thu night into Fri morning. As a mid-upper level shortwave trough moves across the northern Rockies Thu afternoon, Overall models are in good agreement indicating surface low development over eastern Colorado This will result in deepening moisture across eastern Wyoming and western Nebraska as increasing southeasterly flow north of the low draws moisture into an east-west oriented boundary stretched across the region. This moisture, along with afternoon heating and the larger scale lift supported by the approaching shortwave, is expected to spur convection developing from southeastern Wyoming across western and central Nebraska Thu evening. As the low level jet continues to amplify, focus for convection and the heavy rain potential is expected to shift further north during the overnight hours to along the 850mb boundary extending from the eastern Nebraska-South Dakota border to the Minnesota-Iowa border. Sustained southerly low level inflow into the western flank of the boundary may support redeveloping convection as it interacts with the approaching shortwave and right-entrance region upper jet dynamics -- raising the threat for heavy amounts across the region through the end of the period. Given the overall good agreement with respect to the larger scale features, WPC QPF was weighted largely toward the HREF Mean. ...Northeast/Mid-Atlantic... A shortwave trough currently centered over the Great Lakes will de-amplify and move progressively east ahead of an amplifying trough over Ontario on Thu. Increasing instability afforded by daytime heating along with moisture pooling ahead of the associated cold front is expected to support renewed convection as the boundary moves across the Northeast and Mid-Atlantic later today. This may produce some brief periods of heavy rainfall, however give the progressive nature of the system, widespread heavy amounts are not expected. Here too, WPC QPF gave significant weight to the HREF Mean. ...Northern Rockies... A well-defined shortwave will continue to dig southeast from the northeast Pacific, with an upper low closing off over the Pacific Northwest-northern Rockies late Thu-early Fri. Modest moisture is expected to support only light amounts west of the continental divide, however pooling moisture along a lingering low level boundary draped east of the Rockies may support some locally heavier totals across central and eastern Montana as interacts with the favorable forcing aloft. WPC QPF reflects a blend of the HREF Mean, along with the NAM, GFS and ECMWF. Days 2/3... ...Intermountain West, Northern/Central Rockies, and High Plains... A closed upper low will persist over the Great Basin Friday through Saturday night (before weakening in place through the first half of next week). The positive tilt to this low and surrounding trough will wrap both Pacific and Gulf moisture into the Intermountain West. Favorable upper jet dynamics and upslope flow will support moderate to heavy precipitation with this system, particularly over the northern Rockies of WY/ID. Ongoing snow melt will raise flood concerns for this area (already under a hydrologic outlook from WFOs in the area), but lowering snow levels Friday (as heights fall from the trough amplification) will limit the flood potential to areal rather than flash. Two to three inches of precip is expected over preferred terrain (particularly WY and ID) Friday through Saturday night. ...North-central Great Plains to the Middle/Upper Mississippi Valley and Northeast... Broad warm advection ahead of the amplifying western U.S. trough/closed low will bring an increasing risk for heavy rainfall from the north-central Great Plains to the Middle/Upper Mississippi Valley and Northeast late this week. Two main threats for excessive rainfall will be with weak shortwave energy streaking out of the western U.S. and across a frontal boundary over the Midwest both Friday and Saturday. The Friday activity will mainly be on the cold side of the front with little instability limiting convective potential. Therefore the marginal risk for Day 2 (12Z Fri-12Z Sat) was maintained. A slight risk for excessive rainfall was raised for Day 3 (12Z Sat-12Z Sun) for northern IL/IN where 1.5 inch PW (2 standard deviations above normal), moderate instability, and the front which will have sagged south from Friday will interact. 00Z ECMWF/UKMET both continue to prog 2 inch areal average rainfall over this area. The 00Z GFS actually continues a trend for less rainfall Saturday into Sunday despite it all being considered convective precip (a forecast element available from the GFS) and higher moisture content than the wetter ECMWF. The consistency in the ECMWF and UKMET over the past few runs warrants the higher risk at this time. ...Southern Florida... The majority of model guidance is in agreement that an axis of highly anomalous moisture and a weakening frontal boundary should focus periods of heavy rain just south of the Florida peninsula on Friday, but then there is a growing model consensus for the heavy rain to spread northward into the greater Miami area Saturday as a weakness aloft progresses through the Gulf of Mexico and across Florida. Localized particularly heavy rainfall is possible, so a marginal risk for excessive rainfall is warranted for Day 3 (12Z Sat-12Z Sun). Pereira/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml