Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 446 AM EDT Mon May 14 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 14/1200 UTC thru May 17/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Mid Mississippi valley/Great Lakes/Ohio Valley/Mid-Atlantic... Low amplitude shortwaves embedded within progressive mid-upper level flow will continue to produce rounds of convection, producing locally heavy amounts, as they interact with a relatively deep moisture and instability pool coinciding with a quasi-stationary boundary stretching from the Mid-Atlantic back into the central Plains. Ongoing convection across the upper Midwest has continued to develop and become better organized over the past few hours and while the overnight guidance backed off some on the amounts shown by previous runs, moderate to heavy rains, with locally heavy accumulations are still expected as training convection moves southeast from the upper Great Lakes region into the upper Ohio valley this morning. The HRW-ARW, which appeared to have one of the better handles on the ongoing development, was more closely followed early in the period. Hi-res guidance shows a stronger signal for heavy amounts further downstream as the associated shortwave moves from the upper Ohio valley into the Mid-Atlantic region this afternoon-evening. Deepening moisture afforded by increasing southerly flow, along with daytime heating, may support heavy amounts from the eastern foothills of central Virginia to the coastal plains as it interacts with the approaching shortwave. Overall, the 00Z hi-res guidance is in generally good agreement across this region, with the HREF mean showing high probabilities of 6-hr amounts of 2-inches for the period ending 00Z Tue. Meanwhile, additional storms are likely to develop during the afternoon and evening hours from the mid Mississippi valley to the southern Great Lakes region as the low level jet intensifies and moisture deepens. While there is a fair amount of spread with regards to the finer details, there is fairly good signal for heavy amounts developing in the vicinity of the lower Missouri into the mid Mississippi valley Mon afternoon-evening as weak surface wave, fostered by an approaching shortwave from the west, may generate an area of enhanced low level convergence and ascent across the region. To account to the typical uncertainty, WPC QPF relied on the HREF Mean as starting point across this region through much of the period. Further to the north, an amplifying shortwave trough across the upper Midwest into the Great Lakes may help develop storms further to the north, generating another round of storms from Iowa into the southern Great Lakes Mon night. Here also, there was a good deal of spread in the guidance, so once again significant weighting was given the to HREF Mean. ...Southern and Central Plains... Frontal convergence and dryline mixing along with a string of shortwaves embedded within southwesterly flow aloft should support developing convection, with heavy amounts possible, as they interact with a relatively deep moisture pool associated with the low level jet. There is a fair amount of spread with respect to the details here as well, although the hi-res guidance does cluster fairly well showing two areas of focus for heavier amounts during the afternoon and evening hours-- northwest Texas and central Oklahoma. ...Central Rockies and High Plains... Moist upslope flow along with increasing instability is expected to support another round of developing convection later today. These storms could produce some locally heavy rainfall amounts as they develop and spread east from the southeast Wyoming and Colorado foothills into the plains. ...Florida... A closed low is expected to continue to develop and drift north through the northeast Gulf this period. Deep southerly flow east of the center will continue to support anomalous moisture values across much of the state. There remains a fairly good model signal for heavy amounts centering mainly along the Atlantic coast this period -- supported by convergent surface to low level onshore flow. By late in the period, may also start to see organized heavy amounts further north along the Gulf coast as well, as the low circulation approaches the Panhandle Tue morning. Days 2/3... ...Pacific Northwest/Great Basin/Rockies/Northern Plains... A closed mid level low the Great Basin on Day 2 weakens and drifts northward, as a second closed mid level low reaches the central CA coast. The second closed low expands to cover much of the Great Basin and northern and central Rockies on Day 3. Finally, a frontal boundary extending from eastern MT across ND/SD later on Day 3 could become the focus for convection that produces heavy rainfall. There was generally good model agreement with respect to the overall synoptic scale evolution, so the WPC QPF was based on a blend of the 00z ECMWF/GFS. Day 2... Ahead of the closed mid level low spinning down over the Great Basin on Day 2, a low level east southeast flow transports moderate instability and 0.75 inch precipitable water air (which is almost two standard deviations above the mean) across the plains and Front Range in CO. The resultant convection is expected to produce local 1.00 inch amounts over the CO plains, with locally higher amounts in places where short term training occurs. Three hour flash flood guidance values are as low as 1.50 inches in spots, so a Marginal Risk was placed here for Day 2. Further west, as the first closed mid level low weakens and drifts northeast, a second closed mid level low approaches the central CA by the end of the period. Ahead of the second closed mid level low, 0.75/1.00 inch precipitable water air (which is about two standard deviations above the mean) crosses northern CA into the OR Cascades. The resulting convection is expected to produce an axis of 0.50/0.75 inches of QPF in the Shasta/Trinity Mountains in northern CA, with slightly lower amounts in the central OR Cascades. Day 3... The central CA closed mid level low comes ashore and expands across the Pacific Northwest, northern Rockies and Great Basin during the period. Lift associated with the mid level system provides sufficient instability to fuel convection, which is initially tied to the terrain. There was a multi model signal for local 0.50 inch qpf amounts over the higher terrain of northern CA/Sierra Nevada Range into the Blue Mountains of OR and Bitterroot Mountains in ID. There was a strong model signal for the best instability to reside across northeast OR into northern and central ID, where three hour flash flood guidance values are as low as an inch. Since there could still be some snowmelt issues here, a Marginal Risk was placed in these areas for Day 3. Finally, moisture and instability funneled along a frontal boundary extending from eastern MT across ND/SD during the second half of Day 3 could fuel developing convection between 17/06z and 17/12z, mainly over ND. A 20 to 30 knot low level flow introducing 1.00 inch precipitable water air and marginal instability could support local 0.75 inch QPF amounts before the end of the period. ...Central and Southern Plains... Deep moisture and instability along and east of a weakening frontal boundary and dry line will fuel convection that produces heavy to locally excessive rainfall during Day 2. The low level flow weakens a bit on Day 3 across the region, so the threat for excessive rainfall does not appear as high. For the most part, there was good model agreement with the placement of the maximum QPF with the convection, so the WPC QPF was based on a blend of the 00z ECMWF/GFS. Day 2... Short wave energy tracking out of NM across north TX into OK will provide synoptic scale ascent for convection that forms ahead of a weakening front or dry line extending across the western Red River Valley during Day 2. Ahead of the surface focus, a 20/25 knot low level southwest flow transports 1.50 inch precipitable water air and moderate to strong instability across the region, which peaks between 16/00 and 16/06z. Convection that develops may be initially mostly outflow driven. but a moistening column, as well as short term training concerns, could result in a low end flash flood threat during this time. Flash flood guidance values are fairly high, so a Marginal Risk was placed here to cover the flash flood threat during Day 2. Day 3... Similar conditions are expected during Day 3, as the short wave energy exits, and short wave ridging builds over the TX Panhandle. This should shunt the convection threat further east into OK/and central and eastern KS. The best lift exits before the convection peaks, so the organization may not be as robust as Day 2. Still, local 2.00+ inch amounts are possible, especially over eastern KS. However, three hour flash flood guidance values are still fairly high, so for now no excessive area was assigned. ...Mid and Upper MS Valley/OH Valley/Mid Atlantic/Northeast... Short wave energy in the quasi zonal flow on the southern edge of the westerlies will provide pockets of lift over a frontal boundary that drops slowly south across the region during Days 2 and 3. The short wave energy interacts with deep moisture and instability to produce convective clusters capable of producing heavy to excessive rainfall, especially during Day 2 across the northern Mid Atlantic states. There are still some model differences concerning the placement of the front, especially during Day 2. In an attempt to mitigate some of the remaining differences, the WPC QPF was based on a multi model blend, with more weight given to a blend of the 00z ECMWF/GFS. Day 2... Convective clusters should be ongoing early on Day 2 across southern MI into northern NY state ahead of a frontal boundary dropping south across central and southern New England. Short wave energy in the mid level flow crosses the front as it drops slowly south, reaching a position extending from eastern New England across the NYC Metro area into lower OH Valley into the Mid MS Valley before 16/00z. Ahead of the front, a low level south southwest flow continues feeding 1.50/1.75 inch precipitable water air (which is between two and three standard deviations above the mean) across the northern Mid Atlantic (some of which is siphoned off from a system over the Southeast states). The best instability ahead of the front is expected here as well, centered between 15/18z and 16/00z. Strong convection in the deep moisture is expected to produce an axis of 1.00/1.50 inches of QPF extending from southern OH into Southeast NY state, with the highest amounts over southeast PA and nearby NJ. Three hour flash flood guidance values are as low as 1.00/1.50 inches over portions of northern WV/southwest PA and eastern PA and northern NJ. There is some potential for training ahead of the front here, so a Slight Risk area was extended over these areas. A Marginal Risk was extended from southern New England into portions of the OH Valley and Mid MS Valley as well, as both the 00z ECMWF/GFS showed the potential for higher QPF amounts in these locations. Day 3... As the best mid level lift exits into eastern Canada during Day 3, the frontal boundary slows across the OH/TN Valleys into the Mid Atlantic states. A low level southwest flow continues to pump 1.50/1.75 inch precipitable water air into the Mid Atlantic and OH Valley, with the source located over the Southeast states. With the slowing front comes slower storm motions, and the combination of high moisture content air and marginal to moderate instability (locally higher over the OH Valley close to the front) could result in cells producing local rainfall amounts in excess of 2.50 inches (as shown on the 00z GFS). The highest rainfall amounts are expected over the Delmarva Peninsula and southeast VA. A Slight Risk was placed here, as recent heavy rainfall could make this area more susceptible to flash flooding. A Marginal Risk was placed over much of the lower OH Valley and Mid Atlantic, where slow storm motions and high moisture content air could overcome relatively high flash flood guidance. ...FL/Southeast... A closed mid level low over the eastern Gulf of Mexico at the start of Day 2 moves slowly north, tracking into the Southeast states later on Day 2 into Day 3. A surface low associated with the mid level system approaches the central and eastern Gulf Coast states during this period. Deep moisture and increasing low level inflow will pose at least a low level flash flood threat each day. There is better agreement among the 00z model suite members with the placement of the mid level system, but there is some still some spread on the forward speed of the surface system (with the 00z NAM faster than the consensus). The WPC QPF was based more closely on the 00z ECMWF/GFS, which are close on the placement of the surface low. Day 2... As the closed mid level low crosses the Gulf Coast and reaches the Southeast states during Day 2, a low level southwest flow continues to transports 1.75/2.00 inch precipitable water air (which is between two and three standard deviations above the mean) across much of FL, as well as GA/SC into NC. Model soundings showed moderate instability over these areas, and the moisture and instability is expected to feed mainly diurnal convection. Slow cells motions associated with the mid level system could result in local 3.00+ inch rainfall amounts over GA/SC/NC, and despite the fairly high three hour flash flood guidance, may pose a flash flood threat during Day 2. Thus, a large Marginal Risk was placed here. Further south over the FL Panhandle and portions of southern AL, a surface system associated with the mid level low meanders just offshore. There is some spread on how fast the system moves northward, but there is some signal that the low level southeast flow ahead of it could begin to transports higher moisture to the Gulf Coast. While the highest QPF totals were kept offshore during Day 2, there could be enough convection across the Gulf Coast for localized flash flooding issues, so a Marginal Risk was placed here for Day 2. Day 3... The closed mid level low opens up into a broad long wave trough over the TN Valley and Southeast states during Day 3. Moisture and instability profiles are similar to those of Day 2, and slow cell motions are expected (though they may be a bit faster than Day 2). Once again, mainly diurnal convection is capable of local 3.00+ inch QPF amounts over the Southeast, so a Marginal Risk was placed here for Day 3. Perhaps the greatest flash flood threat on Day 3 will be associated with the surface low as it comes ashore over either MS or the FL Panhandle. As mentioned earlier, there are still some differences with respect to timing/placement, so it is not clear just how significant the flash flood threat will be. However, there should be enough low level southeast flow (generally between 25/30 knots) to usher rainfall amounts between 2.00 and 3.00 inches here. The three hour flash flood guidance values are fairly high here (mostly above 4.00 inches), so a Marginal Risk was placed here on Day 3. If better timing can be achieved with the surface low, this area could be upgraded to a Slight Risk, since the potential for several inches of rain exists). Pereira/Hayes Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml