Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Tue May 22 2018 Prelim Day 1 QPF Discussion Valid May 22/1200 UTC thru May 23/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...East... The wet pattern will continue today across much of the east. A moderately unstable airmass, moderately anomalous PWATs, and broad troughing will support scattered to widespread convective activity. A few forcing mechanisms within the broad troughing will help focus convection. Will have stronger synoptic forcing and wind shear across the central and northern Mid Atlantic, being closer to the mid level shortwave and upper jet to the north. Thus would anticipate convection will be a bit more organized here, probably generally in small linear segments. Will also have whats left of the backdoor front that is lifting north as a warm front, which should locally enhance convergence. Instability will drop off north of the front, which should cap rates there, however along and south of the front will have ample instability to produce locally heavy rates across portions of northern VA into MD. Storms should form near the terrain of WV/VA/MD this afternoon and progress eastward. Generally expect storms to be progressive in nature, limiting the overall flash flood threat. Although a localized threat probably still exists given the saturated conditions over portions of the central Mid Atlantic. Also likely to have storms form along the actual cold front further west over the OH Valley, and move eastward into portions of western PA. These should weaken after sunset as they head into the terrain. Across the southeast a lingering mid level shortwave will once again act as a focusing mechanism for storms this afternoon. This feature is currently over southern AL, and is responsible for the small scale training shower activity over southeast AL. With diurnal heating, anticipate a broader area of scattered convection to form around this wave across portions of AL and GA. As with past days, not expecting this activity to be organized, and cells should generally be pulse in nature and thus short lived. However briefly intense rates are likely, and where cells are able to merge extending rainfall duration, a localized flash flood risk exists. Activity should diminish after sunset, with a more localized band of showers likely near the center of the wave (similar to this morning). In between these two areas, the mid/upper level focus for convection is less evident. However seems likely that we will again see cells form over the terrain of the western Carolinas, eastern TN and southwest VA with diurnal differential heating. These cells will tend to drift eastward off the terrain with time. Normally this activity would not really pose a flash flood risk, but given the saturated conditions over portions of this area, a localized threat likely exists once again. May need to watch northeast GA into western SC and southwest NC a bit closer this afternoon. Some chance that we see a bit more enhanced cell merger threat here, with cells moving northward on the periphery of the wave over AL/GA possibly merging with cells moving east off the higher terrain. If this were to occur then may see a bit more focused flash flood risk develop. Confidence is not high on this though, and will just need to monitor through the day. A separate weak shortwave over the Lower MS Valley may also act to focus some convection over portions of AR/LA/MS today. The QPF signal amongst the 0z HREF member suggest this activity will be even more isolated and short lived than the activity further east. Thus while very localized heavy totals are possible, areal averaged rainfall should not be all that high. ...Central and Northern Rockies into the Plains... The approach of the mid/upper level trough will increase convective chances across the Rockies into the Plains. Upper level flow will become very divergent over WY by this afternoon, which combined with increasing instability,and the presence of a low level boundary, will result in scattered convective development. Instability should be high enough to fully take advantage of the anomalous PWATs and produce some locally intense rates. Storms will generally be progressive in nature off to the north, however could see multiple rounds of storms given the favorable synoptic environment. Thus at least a localized flash flood risk probably exists. Eventually these cells should take a turn to the right and move into the northern plains, with one or more convective clusters likely. Low confidence on how this evolves, but locally heavy rains are possible into SD/ND as convection moves across. Storms should generally be progressive, although one area to watch may be southwest SD. Some signs that storms that initiate off the Black Hills could merge with convection moving northeast out of WY/NE. Should this happen a more focused flash flood risk could develop. As we head into the overnight hours appears to be a pretty good setup for an axis of elevated convection across portions eastern SD into southwest MN and northwest IA. 850 mb moisture transport increases and persists through the night, with this elevated activity likely focusing on the nose of this feature. Storm motion vectors appear to have a large component parallel to this moisture convergence axis...suggesting that storms may tend to regenerate and train for a period from northwest to southeast. Would appear that both instability and PWATs would be high enough to produce some pretty heavy amounts should this materialize. Still a good amount of uncertainty though with both the organization of this activity and the latitude it sets up at. Generally favored an axis close to the 0z nam nest and ARW2 for the WPC QPF, although likely will need to make some adjustments to this as things become clearer by later today. Either way, something to watch, as certainly could see some flash flood potential with this setup. Chenard