Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 701 AM EDT Fri May 11 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 11/1200 UTC thru May 14/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Upper Mississippi Valley/Great Lakes/Northeast... Broad warm advection ahead of an amplifying trough in the West will continue to support moderate to locally heavy rains to the north of a wavy slow-moving boundary extending across the mid Mississippi and Ohio valleys. Heaviest amounts are expected to occur in association with a shortwave trough moving from the upper Mississippi valley across the southern Great Lakes region late Fri into early Sat. Limited instability north of the boundary is expected to hamper rainfall rates. However, given the wet antecedent soil conditions, rainfall totals may pose runoff concerns across portions of the region from southern Wisconsin and lower Michigan to western New York and northern Pennsylvania. Overall, WPC QPF is similar to the previous forecast -- leaning closer to the more southerly camp of solutions, similar to the HREF Mean. ...Great Basin/Northern Rockies/High Plains... The previously noted trough in the West will continue to amplify this period, with a closed low dropping south from the Northwest into the Great Basin. Some of the heaviest amounts of the period are expected to center across Wyoming -- where anomalous moisture and sufficient instability, supported by low level easterly inflow, is expected to interact with larger scale forcing along the left-exit region of the upper jet. Heavy snows are expected for portions of the Big Horns and western Wyoming ranges, with moderate to locally heavy rainfall totals across the lower elevations. Further south, some moderate to locally heavy accumulations (including heavy snows in the Uintas) are possible along a low-mid level frontal boundary dropping south into the region. Overall, guidance was in good agreement, with WPC QPF reflecting a blend of the HREF Mean with the GFS and 12Z ECMWF. Days 2/3... ...Great Basin/Rockies... A closed mid level low over NV on Day 2 tracks slowly northeast into Day 3, then weakens. Ahead of the mid level system, a low level flow of increasing moisture is expected to result in locally heavy QPF across the higher terrain, especially during Day 2. For the most part, there was good model agreement concerning the synoptic scale features, so the WPC QPF was based on a blend of the 00z ECMWF/GFS. Day 2... A strong closed mid level low over NV early during Day 2 moves slowly northeast, reaching a position over northeast NV/southern ID/northwest UT by the end of the period. Ahead of the closed mid level low, surface low pressure tracking across UT will aid in focusing moisture and marginal instability to produce local 0.50/0.75 inch QPF amounts in the higher terrain of northeast NV across southern ID. Further east, a 15 to 25 knot low level easterly flow transports 0.05/0.75 inch precipitable water air (which is about two standard deviations above the mean) to the higher terrain of WY and north central CO, mainly after 13/00z. Because of the upslope flow, there was a multi model signal for 1.00+ inch QPF amounts over the Grand Tetons and Wind River Ranges, as well as the Big Horn Mountains in WY. In the lower levels, that could fall as locally heavy rainfall, but the bulk of the higher QPF amounts in the higher terrain are expected to fall in the form of snow, which should mitigate the threat for flash flooding. Day 3... The mid level system moves little during Day 3, as it slowly weakens and begins to spin down. The moisture source across southeast WY and nearby north central CO remains, at least during the first part of the period. The combination of moisture and instability could produce local 0.75/1.00 inch QPF amounts here, some of which could fall in the form of snow in the higher terrain. This could mitigate any flash flood concerns, though runoff issues are possible in the lower terrain. Further west, closer to the weakening mid level system, there should be sufficient instability to feed developing convection, especially over the higher terrain of northern and eastern NV. Moisture will be the limiting factor for higher QPF amounts, so for now local 0.50 inch QPF amounts were placed in these locations. ...Central and Southern Plains... Moisture and instability surging along a quasi stationary front extending from the western OK Panhandle into west TX during Day 3 will feed convection that produces locally heavy rainfall during the second half of the period. The 00z GFS was much higher with its QPF amounts associated with convection that other 00z model solutions, so the WPC QPF was based more closely on a multi model blend. Short wave energy tracking from NM to the western OK Panhandle during the second half of Day 3 will provide synoptic scale ascent over a frontal boundary stretching from western OK into west TX. Ahead of the front, a 30 to 40 knot low level south southwest flow is expected to transport 1.25 inch precipitable water air, mainly after 14/00z. Model soundings showed moderate to locally strong instability ahead of the front as well, and the combination of moisture and instability is expected to produce convection that becomes organized in the synoptic scale lift, peaking around 14/06z. Local 1.50 inch QPF amounts are possible over the northern TX Panhandle, as well as central KS. Three hour flash flood guidance values here are generally above three inches, so while heavy rainfall is possible, organized flash flooding is not expected. ...Upper MS Valley/Oh Valley/Northeast/Mid Atlantic... Impulses riding along in the quasi zonal mid level flow extending from the Upper MS across the OH Valley into the Northeast states during Days 2/3 will tap moisture and instability focused on a frontal zone dropping slowly south. Convective clusters are expected to produce heavy to locally excessive rainfall amounts along the front each day. The 00z GFS was a bit further south with its maximum QPF amounts, which seemed to better fit the positioning of the best instability. Because of this, the WPC QPF was based on a blend of the 00z ECMWF/GFS, then dropped a bit south to better fit the expected instability placement. Day 2... The first impulse in the mid level flow crosses IL/IN into OH during Day 2. The impulse provides synoptic scale lift over a front dropping slowly southward over the region. Along and south of the front, a 20/30 knot low level west southwest flow transports 1.25/1.50 inch precipitable water air (which is between two and three standard deviations above the mean) along the axis of best instability. The 00z GFS appeared to have the best handle on the placement of the instability axis, which is further south than much of the 00z guidance. Based on this, an axis of 1.00/1.50 inches of qpf was extended across northern IL and northern IN into northern OH and southwest PA. Since there has been a fairly consistent signal for 1.50+ inches of QPF over northern IN, a Slight Risk area was placed here. Further east, along and south of the front, moisture and instability should support convection that produces an axis of 0.75/1.00 inches of QPF extending across central PA into northern NJ and the NYC Metro area. Outside of the metro areas, these amounts are lower than the three hour flash flood guidance values. However, the deep moisture and the potential for short term training necessitated the inclusion of a Marginal Risk for Day 2 over the aforementioned areas. Day 3... The front drops a bit further south during Day 3, extending from northern IL across southern OH into southwest PA and central NJ. Impulses in the mid level flow over the front will once again provide synoptic scale ascent for convective cluster along and south of the front. As might be expected in a relatively fast mid level flow, there are some timing differences with respect to the mid level impulses, and this will lead to placement differences concerning the expected convective clusters. A large area of 0.50/1.00 inches of QPF was extended along the front across the aforementioned areas, with local 1.00+ inch amounts possible over northern central IL and a portion of southwest and central PA. Both the 00z GFS and 00z ECMWF had local QPF maxima in excess of 2.00 inches, but in different places. In an attempt to take into account the uncertainty, a large Marginal Risk was extended from northern IL across central and southern IN/OH/northern WV into portions of central and eastern PA. If there is better model agreement concerning the placement of the convective clusters, Slight Risk areas could be assigned in subsequent forecasts. ...FL... Deep moisture and instability become focused on an inverted trough over the east coast of FL during Days 2/3. Convection along the trough is expected to produce heavy to locally excessive rainfall over portion of the east coast of Fl, especially during Day 3. Both the 00z ECMWF/GFS appeared to be suffering from convective feedback to some degree, allowing the surface feature to strengthen, and likely producing too much QPF. Because of this, the WPC QPF was based more closely on the 00z UKMET, especially with respect to QPF placement. Day 2... A broad long wave trough over the central Gulf of Mexico will help focus deep moisture (with precipitable water values in excess of 2.00 inches) over portions of south FL, as well as the east coast of FL. Model soundings showed marginal instability in the column for much of the region, so it is not clear just how widespread the convection will be over land during Day 2. However, with the deep moisture in place (and more available just offshore), an area of 2.00 inches of QPF was placed over southeast FL, as well as portions of the Keys. Given the deep moisture available, locally higher QPF amounts, especially as an inverted surface trough becomes established over the east coast of FL. Because of this, a Marginal Risk was placed over portions of the east coast of FL for Day 2. Day 3... 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. Moisture profiles are similar to those of Day 2, with better instability in place. 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 3. Pereira/Hayes Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml