Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 316 AM EDT Thu May 17 2018 Prelim Day 1 QPF Discussion Valid May 17/1200 UTC thru May 18/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Mid Atlantic, Southeast and FL... Widespread showers and thunderstorms will again be common today across much of the Southeast into the Mid Atlantic. Broad troughing over the area with embedded shortwaves and broad upper divergence will provide the synoptic support for convection. PWATs will remain well above normal, with much of the area above the climatological 90th percentile, and some near record values for this time of year. The broad synoptic support, well above normal moisture, and ample instability...will all set the stage for another day of locally heavy rains. The most pronounced wave is across south GA as of 06z...and will slowly track north northeast into the Mid Atlantic. In the lower levels a west to east boundary is forecast to drift slightly southward and set up across central VA into the Delmarva. Anticipate the heaviest rainfall totals during the day 1 period will end up focused near this boundary. As the aforementioned wave approaches from the south, should see gradually increasing 850 mb moisture transport into the front. The greatest instability should also focus along/south of this front. Thus the expectation is that the highest rates will generally be along/south of the front...and the front should act as a focus mechanism for some backbuilding and west to east training. Overall, the high res guidance was in good agreement on this evolution...although some latitudinal differences with the front, and thus max QPF axis, were noted. WPC generally went for a middle ground solution, accounting for all the 0z HREF members and the 0z Experimental HRRR. Ended up with a swath of 3"+ near the front across central VA into the Delmarva. The 0z HREF depicts moderate to high probabilities of 5"+ amounts...suggestive that localized amounts of this magnitude are certainly a possibility. Given the stationary front, PWATS, and forecast instability...localized amounts of this magnitude do seem reasonable. ...Lower/Mid MS Valley, OH/TN Valley into the Central Gulf coastal region... Main forcing here will be the presence of a mid/upper level shear axis and embedded shortwaves. The presence of these features, combined with moderate instability, should result in scattered to widespread convective development. Weak shear will limit the organization of storms, and most storms should thus be short lived. However, will likely see some cell mergers given the expected widespread nature of activity, along with some localized organization along outflows. Also, weak mean storm layer winds along the shear axis will also support slow moving cells. PWATs, while not as anomalously high as further east, will remain above average. All of this suggests, that while an organized widespread flash flood threat is unlikely, isolated flash flooding will again be probable today across this region. WPC did increase our QPF amounts quite a bit from our previous forecast over this area...and is generally in line with the 0z HREF guidance. ...Central and Northern Plains... Moderate to high instability will develop along/ahead of a boundary over the western Plains today. The approach of a mid/upper level trough from the west will result in increased upper level divergence over this boundary, and increasing 850 mb moisture transport. These factors will combine to produce a favorable environment for convective development by this afternoon. The increased southerly flow will also result in increasing PWATS, with values into the well above normal range by thus evening. The combination of high cape/moisture suggest the likelihood of intense storms capable of producing heavy rainfall rates. In general think storms will be moving quick enough off to the east to prevent any extreme rainfall totals. However does appear like there may be two areas to watch for the potential for locally heavier amounts. One across ND, where a stationary west to east boundary could act as a focus mechanism for training for a period this afternoon/evening. The other is across NE, closer to the core of 850 mb moisture transport. The increasing low level jet and moisture transport corridor this evening may allow for some backbuilding of convection for a period over NE. Although the corridor of moisture transport appears to progress enough to the east with time to suggest that long duration training/backbuilding is less likely. WPC QPF across this region relied heavily on a blend of the 0z HREF members, with a bit more weight given to the ARW members. ...Great Basin, Northern Rockies, MT... Troughing over the area will continue to support widespread shower activity into Thursday. Multiple shortwaves embedded within the trough will help focus convective activity. PWAT values near these shortwaves will be in the well above normal range. Thus locally heavy rains will be possible with each wave, with at least weak instability forecast as well. Thus anticipate locally heavy totals across northern NV into southern ID, eastern WA into northern ID, and much of MT. Anticipate mainly pulse convection given low shear...although slow storm motions and some localized organization along outflows will still allow for pockets of heavy rains and some flash flood threat. ...Southern Plains... Isolated to scattered convection is likely today ahead of the dry line over TX, possibly aided some by the approach of the subtropical upper jet. PWAT values above normal will support some locally heavy rains. Not thinking we will see much in the way of organization, but as usual, anticipate some brief repeat cell activity will be possible resulting in some localized heavier rains. Chenard