Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 151 PM EDT Thu May 17 2018 Prelim Day 1 QPF Discussion Valid May 18/0000 UTC thru May 19/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Great Plains/Northern to Central Rockies... At the start of the forecast, a mean mid-level ridge will be anchored over the Great Plains while an upstream longwave trough remains in place over the western U.S. Models continue to show erosion of this mentioned ridge as strengthening southwesterly flow draws a series of perturbations into the Northern High Plains. This will all occur in accordance with a persistent southerly influx of Gulf Moisture as shown by 12Z RAOBs as well as current VAD wind profilers. The low-level flow should continue to strengthen toward 18/0000Z with many moisture transport vectors suggesting an upslope component into the eastward extension of the northern/central Rockies. As the evolving convection pushes off the higher terrain, it will likely become better organized within the high theta-e environment over the Northern Plains. Recent solutions depict such convection becoming particularly focused over the Dakotas in close proximity to a slow-moving surface cyclone and along the attendant baroclinic zones. By the following day, convection should take on a more progressive storm mode once the parent cold front begins to accelerate downstream. Based on the latest forecast models, the 18/0000Z-0600Z window should contain the heaviest rainfall which may lead to some flash flood concerns with a slight risk indicated over central North Dakota. The forecast was heavily driven by the overnight continuity with some minor/modest adjustments via the latest CAM data as well as the global guidance. Farther south, the central U.S. ridge should also buckle somewhat across the Southern Plains as a mid-level speed max sweeps through the TX/OK panhandles by the late afternoon/early evening hours. In addition to diurnally forced dry line convection across west TX, some further development is likely for locations downstream as the shortwaves traverse a moisture-laden environment with appreciable buoyancy profiles. Areal coverage of convection will be much lower given weaker support aloft as compared to locations farther north. ...Southeastern U.S./Tennessee and Ohio Valleys/Mid-Atlantic... Continued heavy rainfall and an associated flood/flash flood risk is in place over vast sections of the mid-Atlantic back toward the southern/central Appalachians. A wavy stalled frontal zone remains generally over the mid-Atlantic, just south of the D.C. metro area, south-southwestward back toward the lower Ohio/upper Tennessee valleys. Aloft, a persistent elongated upper trough continues to sit over the north-central Gulf of Mexico which remains a player in drawing copious sub-tropical moisture along the spine of the Appalachians and into the mid-Atlantic. Low-level moisture is quite impressive as shown by morning dew point temperatures with numbers into the upper 60s. Little change in this overall synoptic pattern is expected over the course of the next 24 hours with right entrance region upper jet dynamics focused over the mid-Atlantic and the adjacent Delmarva Peninsula region. Additionally, the 12Z GFS shows only minor adjustments in the position of the west-east oriented frontal zone so it should remain an anchor for continued heavy rainfall, particularly as embedded mid-level shortwaves lift northward along the Eastern Seaboard. Generally speaking, left much of what was inherited from the overnight crew which favored the heaviest amounts from southwestern Virginia northeastward to just south of the DC metro area into the Delmarva Peninsula. 2 to 3 inches of rainfall is in the forecast but pockets of higher amounts are surely possible which would be on top of already saturated soils from previous days of rainfall. Thus, the Excessive Rainfall Outlook continues to advertise a threat for hydrologic issues for the Day 1 as well as into the further periods. ...Northwestern U.S./Central Great Basin/Sierra Nevada... A disorganized/chaotic band of shortwaves continue to rotate cyclonically about the Central Great Basin. Such mid/upper-level impulses will remain foci for patches of semi-organized shower and thunderstorm activity. There is limited predictability of the placement of these embedded features so confidence is below average in placing the convective maxima across the region. However, there should be the usual focus across the higher terrain and regions of enhanced upslope. It appears Montana could see a bit more storm organization as the upper-level flow is much more diffluent in nature as shown by current 12Z upper air analysis and forecast streamlines. Overall, attempted to maintain continuity as much as possible given the low predictability of such convective elements. Rubin-Oster Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml