Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 432 AM EDT Thu May 17 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 17/1200 UTC thru May 20/1200 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. Days 2/3... ...Pacific Northwest/Northern Rockies/Great Basin... A long wave trough extending from the Northern Rockies to the Southwest states at the beginning of Day 2 weakens and takes on a negative tilt across the Pacific Northwest and Northern Rockies during Day 3. Moisture and instability associated with the mid level system feed convection that could produce heavy to locally excessive rainfall, especially during Day 2. There was generally good model agreement with the synoptic scale systems, so the WPC QPF was based on a blend of the 00z ECMWF/GFS. Day 2... As the long wave trough over the Rockies weakens early on Day 2, it begins to take on a negative tilts as it reorients itself over the Pacific Northwest by the end of the period. High pressure building south from west central Canada will focus the low level flow on the Northern Rockies, where precipitable water values of 0.75/1.00 inches (which is about two standard deviations above the mean) will feed convection developing in the marginal to locally moderate instability. The convection is expected to produce areas of 0.50 inches of QPF across the Blue Mountains in northeast OR and nearby central and northern ID, mainly between 18/18z and 19/03z. Three hour flash flood guidance values here are generally above these values, but short term training could produce local amounts over 1.00 inches (which is depicted by the 00z GFS) could result in runoff issues. Additionally, the National Water Model is showing high stream flows here, so a Marginal Risk was placed here for Day 2. Day 3... As the mid level system weakens over the Pacific Northwest, any synoptic scale forcing is expected to diminish. However, instability remains in place, though the moisture profiles are not expected to be as robust as Day 2. Convection developing over the terrain could produce local 0.50 inch QPF amounts over places like the Grand Tetons in WY, and the Uinta Mountains in UT, but elsewhere lesser amounts are expected. ...Northern and Central Plains/Mid and Upper MS Valley... Short wave energy ejected from a weakening long wave trough over the Rockies during Day 2 crosses a frontal boundary extending from eastern CO into the Upper MS valley. Moisture and instability along the front feed convection that produces heavy to excessive rainfall. The short wave energy then crosses the Central Plains into the Mid and Upper MS Valley during Day 3. There is generally good model agreement during Day 2, but there is growing spread in just how quickly the short wave energy crosses the region during Day 3. The 00z ECMWF/UKMET are slower than the 00z NCEP guidance, and based on trends, the WPC QPF was based more closely on the non-NCEP guidance during Day 3. Day 2... Short wave energy exiting ND early in the period interacts with moisture and instability along a frontal boundary extending from the Upper MS Valley into the Central Plains to produce an area of 1.00/1.50 inches of QPF over much of eastern ND. While the activity will be exiting, there could still be enough QPF for a Marginal Risk early in the period, as three hour flash guidance values are as low as 1.50 inches. The potential for flash flooding is expected to shift southwest along the front into central Plains and eastern plains of WY and CO. Model soundings showed a plume of instability extending from the southern High Plains into western NE/eastern CO and eastern WY, peaking between 19/00z and 19/06z. The strong convection may be outflow dominated at first, but as the column moistens (as a low level southeast flow transports 0.75/1.00 inch precipitable water air (which is about two standard deviations) into the region. There was a multi model signal for 2.50 inches of QPF over southeast WY and nearby western NE, with lesser amounts extending into northern NE and far south central SD. Locally higher amounts are possible where short term training exists. Three hour flash flood guidance values 1.00/1.50 inches over far southeast WY, so a Slight Risk was placed here for Day 2. A Marginal Risk was extended over portions of the Central Plains to account for activity along the front. Day 3... Short wave energy ejected from a weakening long wave trough over the Rockies during Day 2 crosses the Central Plains and Northern Plains, well as the Upper MS Valley, during Day 3. Lift with the energy spins up surface low pressure over southeast CO, which then tracks into KS and NE. There is increasing spread on the placement of the surface low and front on which it forms. The 00z ECMWF/UKMET indicated a slower solution, and may be onto a trend. A slower solution would allow deeper moisture and instability for form on the southeast side of the front extending from the surface low into northeast MN and western MI. The non-NCEP guidance placed a 2.00/2.25 inch QPF maximum over northeast NE/southeast SD and far northwest IA in the deepening moisture and instability, with local amounts over 4.00 inches. By contrast, the NCEP guidance was much more scattered with its QPF field, suggesting that the moisture and instability becomes wrapped around the mid level system. For now, the non-NCEP solutions were favored, and a Slight Risk area was placed in the aforementioned area. However, given the spread in the guidance, changes are possible (with the placement of the highest QPF, as well as the Slight Risk area) with subsequent forecasts. ...OH Valley/Mid Atlantic/Northeast... Deep moisture streaming northward from the Southeast states and FL rides up and over a frontal boundary extending from the OH Valley across the northern Mid Atlantic states during Day 2. There is still some spread concerning just how far north the front gets during Day 2, and instability parameters suggest that a more southerly solution is better than a more northerly solution. With this in mind, the Day 2 WPC QPF was based on the more southerly 00z GFS/ECMWF guidance. On Day 3, the front moves north across New England as short wave energy drives a cold front across NY states into the eastern OH Valley. There was better model agreement on Day 3, so the WPC QPF was based on a multi model blend. Day 2... Ongoing convection ahead of a short wave or MCV is expected to exit the northern Mid Atlantic coast early during Day 2. After that time, short wave energy in the deep layered south southwest flow provides synoptic scale lift over a slow moving frontal boundary extending from the OH Valley across central and northern VA. The best instability remains here for the first part of the day, as strong surface high pressure builds across Quebec and northern New England. After 19/00z, the front edges north as the surface high crosses the northern New England coast. A 25 to 30 knot low level south southeast flow transports 1.75 inch precipitable water air and marginal instability to the front, which becomes elevated over the front into PA and NJ. Because instability appears as though it will be the limiting factor, the highest WPC QPF was extended northern VA/northern MD/DE and far southern NJ, with amounts near 2.00 inches. The 00z GEFS has a few 3.00 inch amounts over central and northern VA, so a Slight Risk area was placed here. A Marginal Risk was extended from central NJ across eastern WV into western VA/western NC and eastern TN, where slow moving cells in the deep moisture could cause isolated flash flooding issues. Day 3... As the surface high move out over the Atlantic, and mid level ridging builds along the Mid Atlantic coast, the surface front is allowed to move into New England. While a 25/35 know low level south southwest flow transports 1.50/1.75 inch precipitable water into this area (which is almost three standard deviations above the mean), instability likely ends up being the limiting factor for a more widespread heavy rainfall/flash flood threat. Following a general model consensus, an axis of 1.00/1.50 inches of QPF was placed over the NYC Metro area and southern New England. Three hour flash flood guidance values where are high, but given the deep moisture in place, and a frontal boundary approaching NY state and northern New England, a Marginal Risk was placed here for Day 3. Finally, moisture streaming northward ahead of the Mid Atlantic mid level ridge will combine with instability to produce lines of northward moving of convection that impact eastern SC and portions of eastern NC. Three hour flash flood guidance values are fairly high here, but given the potential for training in the deep moisture channel, a Marginal Risk was placed here for Day 3. ...FL/Southeast... Deep moisture ahead of a weakening long wave trough over the OH Valley and Southeast states and mid level ridging off the Mid Atlantic coast is expected to feed convection riding north across the east coast of FL into the Southeast during Day 2/3. The combination of moisture and instability could result in heavy to locally excessive rainfall, especially across the east coast of FL. There was generally good model agreement with the overall synoptic setup, so the WPC QPF was based on a multi model blend. Day 2... The weakening long wave trough over the Southeast states and mid level ridging off the Mid Atlantic coast will funnel 1.75/2.00 inch precipitable water air along the east coast of FL, as well as across GA into SC. Spokes of lift riding around the trough could produce local support for clusters of thunderstorms in the moderate instability, and lines of storms are expected to cross the east coast of FL into coastal SC. Training along the northward moving lines could produce local 3.00+ rainfall amounts in these places. While three hour flash flood guidance values are generally above these values. short term training in the deep moisture could result in isolated flash flooding issues, so a Marginal Risk was placed over the east coast of FL for Day 2. Day 3... Similar conditions are expected on Day 3. The long wave trough weakens and becomes absorbed in the westerly flow over the northern Mid Atlantic states, but the mid level ridge along the Mid Atlantic coast moves further west. The flow ahead of the ridge will continue to supply deep moisture to the east coast of FL, as well as the SC coast. Like Day 2, training along the northward moving lines could produce local 3.00+ rainfall amounts in these places during Day 3. A Marginal Risk was placed over the east coast of FL to account for the isolated flash flood threat. Chenard/Hayes Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml