Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 428 AM EDT Sat May 12 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 12/1200 UTC thru May 15/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Mid Mississippi Valley to the Northern Mid-Atlantic and Southern New England... As a deep mid-upper level low remains centered over the Great Basin, broad cyclonic flow is expected to persist from the upper Midwest to the Northeast this period. At the surface, a wavy slow-moving boundary will extend from the northern Mid-Atlantic coast back into the central Plains. Showers and storms will continue to develop along and north of the boundary as a series of mid-upper level shortwaves moving along the base of the trough interact with a relatively deep moisture pool, fed by broad southwesterly flow into the boundary. A mid level shortwave associated with ongoing convection across Iowa is expected to reach the upper Ohio valley by the afternoon hours. This energy along with daytime heating and deepening moisture along the boundary, is expected to support shower and storm development from the upper Ohio valley to the northern Mid-Atlantic coast Sat afternoon-evening. While locally heavy amounts are possible, the anticipated progressive nature of these storms is expected to limit the threat for widespread heavy accumulations. Further west, another round of showers and storms are expected to develop and move east from the mid Mississippi valley into the southern Great Lakes region during the evening into the overnight hours Saturday into Sunday. Upstream energy interacting with a deepening moisture pool on the nose of a strengthening southwesterly low level jet (PWs increasing to over 1.5 inches) is expected to support moderate to heavy amounts from east-central Iowa through northern Illinois and Indiana into northwest Ohio late in the period. Guidance suggests a period of training storms may bolster the potential for heavy amounts across this region. Overall, WPC QPF axis of heavier amounts across the Midwest remains similar to previous forecasts, but with an increase in areal average amounts across northern Illinois to northwest Ohio. This was supported by a majority of the hi-res guidance and reflected in the overnight HREF mean. Further east, model trends supported a more southerly shift in heavier amounts across the northern Mid-Atlantic region. ...Great Basin/Northern Rockies and High Plains... The previously noted upper low is expected to continue to support widespread precipitation from the northern Great Basin into the northern Rockies and High Plains. Enhanced upper divergence along the left-exit region of an upper jet streak is forecast to support some moderate to locally heavy accumulations (including some significant mountain snows) across portions of northern Utah and Nevada, western Wyoming and southern Idaho. Days 2/3... ...Great Basin/Rockies... A closed mid level low over the Great Basin and the northern and central Rockies on Day 2 will slowly wind down into Day 3. Moisture and instability on a low level flow becomes upslope over the northern and central Rockies, and resultant convection is expected to produce heavy to locally excessive rainfall, especially during Day 2. There was generally good model agreement with the overall synoptic setup, so the WPC QPF was based primarily on a blend of the 00z ECMWF/GFS. Day 2... As the closed mid level low remains nearly stationary over northeast NV/southern ID/northwest UT during Day 2, a low level easterly flow transports 0.50/0.75 inch precipitable water air (which is between two and three standard deviations above the mean) and marginal instability over northern CO and much of eastern WY. The resulting convection is expected to produce an axis of 0.50/1.00 inches of QPF over southeast WY and the Front Range in CO, where the upslope is most efficient. It should be noted that the 00z GFS is much higher with its QPF here than any other piece of 00z guidance. Given the inflow, locally higher amounts are possible, especially over the higher terrain. Three hour flash flood guidance values are as low as 1.00/1.50 in this corridor, so runoff issues are possible, especially in the lower elevations. A Marginal Risk was placed over portions of eastern WY and northern CO to account for the low end flash flood threat. With the closed mid level low remaining nearly stationary over NV, developing convection is expected to be enhanced by lift associated with spokes in the mid level flow. A large area of 0.25/0.50 inches of QPF was placed over much of central and northeast NV to account for the convection. Day 3... The closed mid level low begins to spin down during Day 3, eventually stretching from northern CA across NV into southern ID by the end of the period. The low level inflow across southeast WY and northern CO is not quite as robust as Day 2, though instability profiles are roughly the same. Convection over this axis is expected to peak near 0.50 inches. Since there is expected to be a notable downturn in moisture, no Marginal Risk was placed here for Day 3. Elsewhere across the Great Basin and northern and central Rockies, local 0.25 inch QPF amounts are expected over the higher terrain, with moisture being the limiting factor for higher amounts. ...Central and Southern Plains... Short wave energy tracking out of the Southwest during Days 2 and 3 will interact with deepening moisture and strong instability along and ahead of a slowly moving front extending from the central Plains into west TX. The resulting convection is expected to produce locally heavy rainfall each day. There was good model agreement with the placement of the highest QPF amounts and the front, so the WPC QPF was based primarily on a blend of the 00z ECMWF/GFS. Day 2... A developing 30/40 knot low level south southwest flow transports 1.25/1.50 inch precipitable water air and moderate to strong instability along and east of a front extending from central KS into west TX. The instability is expected to peak between 14/00 and 14/06z, when most regional/global models showed local 0.50/1.00 inch QPF amounts expected from central KS into west central OK. Locally higher amounts are possible where short term training occurs, but three hour flash flood guidance values are generally fairly high. While locally heavy rainfall is expected, organized flash flooding is not expected. Day 3... Short wave energy tracking out of NM early during Day 3 crosses the TX Panhandle/western OK around 15/00z, before moving northeast. The low level southwest flow continues to pump moisture and moderate to strong instability east of the front, mainly across the TX Panhandle, with convection peaking between 15/00z and 15/06z. Local 1.00 inch QPF amounts are possible with the convection, with the 00z GFS showing the potential for local 3.00+ inch QPF amounts. At this point, it is not clear how widespread the convection will be, so no risk area was assigned for this region on Day 3. ...Mid and Upper MS Valley/OH Valley/Northeast and Mid Atlantic... Short waves in a fast flow on the southern edge of the westerlies in the mid levels will interact with deep moisture and instability to produce convective clusters along and south of a frontal boundary extending from the mid and upper MS Valley across the OH Valley into the Mid Atlantic and Northeast during Days 2 and 3. As has been the case the past couple of days, the placement of the front and timing of the short waves will ultimately determine the positioning of the highest QPF amounts, especially on Day 3. The 00z GFS remains further south than the consensus with the frontal zone, and seems to better depict the axis of best instability. With this in mind, the WPC QPF was based mainly on the 00z GFS for placement, and was augmented by high resolution guidance early on Day 2, especially over the Northeast and northern Mid Atlantic. Day 2... Ongoing convection is expected along and south of a frontal boundary extending from IA/IL across the OH Valley into the northern Mid Atlantic states on Day 2. The convection is expected to weaken as it moves into a more stable airmass, but additional convective clusters are expected along the frontal boundary during Day 2. The convective clusters are expected to be tied to impulses riding through the fast mid level flow near to just north of the front. A low level westerly flow will continue to supply 1.25/1.50 inch precipitable water air (which is between two and three standard deviations above the mean) extending from northern IL into eastern PA. Model soundings showed moderate to strong instability, centered over IL into the OH Valley, peaking between 14/00z and 14/06z. As the short wave energy interacts with the moisture and instability, convective clusters are expected to track from west to east along and south of the front. As mentioned earlier, the 00z GFS placement of the maximum QPF looks more realistic, as it seems to place the axis of maximum instability south of the front. The convection, is expected to produce pockets of 0.75/1.25 inches of QPF extending from northern IL into southwest PA and far northern WV. It should be noted that most of the 00z guidance has maximum values higher than the WPC QPF, and these amounts could serve as maximum values, especially where short term training occurs. Since there is no consensus concerning the location of the best convective clusters, a large Marginal Risk was stretched from northern IL into central PA for Day 2. Day 3... The frontal boundary sags a bit further south across the OH Valley and the northern Mid Atlantic during Day 3, and the front remains the focus for deep moisture and marginal instability. Convective clusters over this area could produce local 1.00+ inch over OH Valley, and the additional rainfall could result in local runoff issues. Flash flood guidance values should be lower here on Day 3, so there could be an enhanced for a time, especially over southern OH and northern WV. Due to uncertainty, a Marginal Risk area was placed here on Day 3. Further west, additional short wave energy crosses the frontal boundary over southern IA and northern MO, mainly after 15/00z. The low level flow continues to supply 1.25/1.50 inch precipitable water air and moderate to strong instability along the front, and as the short wave energy interacts with the moisture and instability, a convective cluster is expected over the aforementioned areas. An axis of 1.50 inches of QPF was placed here, and the placement of the highest QPF was closer to the more southern 00z GFS placement. If there is better consensus with the placement of the front and QPF here, a Slight Risk could be placed here in subsequent forecasts. ...FL/Southeast... An inverted surface trough over the east coast of FL on Day 2 will focus instability and deep moisture to produce convection with heavy to excessive rainfall during Day 2. A developing closed low off the FL west coast during Day 3 will provide a low level southeast flow of deep moisture and increasing instability that feeds convection producing heavy to locally excessive rainfall over portions of the east coast of FL and coastal sections of the Southeast. There is better agreement with the placement of the synoptic scale features, but there is still some spread on the placement of the highest QPF. In an attempt to mitigate some of the differences, the WPC QPF was based mainly on a multi model blend. Day 2... The inverted trough is well established over the east coast of FL during Day 3. The 00z ECMWF/GFS appear to be suffering from some grid scale feedback, which in turn results in a stronger surface system (and more low level convergence). The QPF from each of these models appears to be too high on Day 3, as does the 00z NAM. The placement of the QPF was based more closely on the 00z UKMET, which appeared to have the best handle on the placement of the inverted trough. A 20 knot low level southeast inflow transports 2.00 inch precipitable water air (which is between two and three standard deviations) and marginal instability across South FL. The moisture and instability are expected to feed more organized convection that produces 1.50/2.50 inches of QPF along the FL east coast. Higher amounts are possible where short term training occurs (with amounts of 4.00 inches possible), so a Marginal Risk was placed here for Day 2. Day 3... A closed mid level flow forms over the eastern Gulf of Mexico during Day 3, and ahead of the forming mid level flow, a 20 knot low level southeast flow continues to transports 2.00 inch precipitable water air over the east coast of FL, as well as coastal sections of southeast GA and far southern SC. Model soundings showed more in the way of instability in the flow from the Bahamas, and the deep moisture and increasing instability is expected to feed arcs of convection coming ashore over far northeast FL, southeast GA and a small portion of SC. There was a model consensus for 2.00/2.50 inches of QPF over southeast GA, with some spread on the placement. Even though three hour flash flood are generally above 3.00 inches, the training deep convection could result in local flash flooding, so a Marginal Risk was placed in these areas for Day 3. Pereira/Hayes Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml