Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 424 AM EDT Tue May 15 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 15/1200 UTC thru May 18/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Northeast/Mid-Atlantic/Central Appalachians/Ohio Valley... A mid-level shortwave associated with ongoing convection across the Great Lakes is expected to become embedded within the base of a broad upper trough centered over the Great Lakes/Northeast and move progressively across the Northeast. This is expected to spur a convective complex that is forecast to move across northern Pennsylvania and southern New York into New England during the late morning and afternoon hours. This may produce some locally heavy totals, but given the anticipated progressive nature of this system, widespread heavy amounts along the leading edge of this system are not expected. However, the tail end the convection is expected to hang up across southern Pennsylvania, northern Maryland into New Jersey as it merges with a quasi-stationary boundary extending across the region. Convection will continue to train across the region into the evening hours as a deepening moisture pool coinciding with the boundary interacts with energy emanating from the south and west. Meanwhile, additional convection development initiating near a weak wave over the Ohio valley Tue afternoon/evening is expected to move east -- resulting in an axis of moderate to heavy accumulations extending from the northern Mid Atlantic coast back into the Ohio valley. WPC QPF utilized the HREF mean as starting point across most areas. ...Southeast to the southern Appalachians... A closed low will continue to drift north over the northeast Gulf of Mexico this period. Southerly winds east of the center will continue to channel deep moisture along the Florida peninsula into southern Georgia, supporting the potential for moderate to heavy rains across the region. Upslope flow and pooling moisture along an inverted trough extending to the north is expected to extend the threat for moderate to heavy rains further to the north into the southern Appalachians. Here also, the HREF Mean was used as starting point for QPF. ...Southern Plains to the lower Mississippi valley... Several areas of thunderstorms are expected to develop as the atmosphere becomes moderately to strongly unstable later today. Models are showing the typical spread with respect to the finer details, but there is some consensus that a MCV may support some organized heavier amounts as it moves from eastern Oklahoma into Arkansas during the afternoon. Meanwhile, additional storms are expected to develop along the Texas dryline and within an upslope flow regime further to the north over high plains and into the foothills of northeast New Mexico and eastern Colorado. Slightly anomalous moisture, supported by weak southerly flow will support the potential for locally heavy amounts with these storms. ...Pacific Northwest and northern California to the northern Great Basin... Southerly inflow ahead of an approaching upper trough will support and increase in moisture, raising the potential for shower and thunderstorm development across the region later today. There may be enough available moisture (PW anomalies increase to around 2 standard deviations above normal) to support some locally heavy totals along the favored terrain of southwest Oregon into northern California. Days 2/3... ...Pacific Northwest/Great Basin/Rockies... As the closed mid level low over the the Great Basin and northern Rockies fills during Day 2, another closed mid level low crossing the central CA coast during Day 2 expands and elongates into a positively tilted long wave trough across the Rockies and Great Basin during Day 3. There should be sufficient instability and moisture for convection to produce to heavy to locally excessive rainfall each day, especially over the northern Rockies. There was generally good model agreement concerning the evolution of the synoptic scale features, so the WPC QPF was based mainly on a blend of the 00z ECMWF/GFS. Day 2... After the closed mid level low near the central CA coast early on Day 2 comes ashore, the system expands across the Pacific Northwest, Great Basin and northern Rockies. The mid level system brings with it 0.75/1.00 inch precipitable water air (which is greater than two standard deviations above the mean) on an upslope flow which becomes focused on the northern Rockies. There is a strong model signal for MUCAPE values greater than 1000 J/KG across portions of northern ID, western MT and nearby northeast OR, mainly between 16/18z and 17/03z. The resulting convection could produce local 1.00 inch QPF amounts, which are above the three hour flash flood guidance values. However, there is some question about coverage, and because of that, only a Marginal Risk area was assigned to these areas for Day 2. Day 3... A frontal boundary impinges on the northern Rockies during Day 3, which has the effect of increasing the upslope flow over northern ID and western MT. Moisture profiles are similar to those of Day 2, but the model signal for instability is not as robust as Day 2. As a consequence, the extent of instability is expected to be less on Day 3, but still focused over northern ID and nearby western MT. Local 0.50/1.00 inch QPF amounts are possible, which are close to three hour flash flood guidance. When combined with snow melt, this could result in runoff issues, so a Marginal Risk was assigned here for Day 3. ...Plains... Moisture and instability focused along and ahead of a surface trough or dry line is expected to fuel convection producing locally heavy rainfall across the central and southern Plains on Day 2. Moisture and instability on an increasing low level flow supports convection near a frontal boundary over the the northern Plains, and and the convection could produce heavy to excessive rainfall during Day 3. While there is still some latitudinal differences in the placement of the front (and the associated QPF maximum), there was enough agreement to base the WPC QPF on a blend of the 00z ECMWF/GFS/UKMET. Day 2... Short wave energy tracks from OK into the lower MS Valley during the first part of Day 2, as short wave ridging builds over the TX Panhandle. This should shift the main axis of convection along and ahead of a surface trough over dry line across north TX into central and eastern KS. A 25 knot low level southerly flow transports 1.25/1.50 inch precipitable water air and moderate to strong instability across these areas, peaking between 17/00z and 17/06z. Local 0.50/1.00 inch QPF amounts are possible along this axis. Day 3... The focus for convection with heavy to excessive rainfall shifts north during Day 3. A 30 to 40 knot low level southerly flow transports 1.25 inch precipitable water air across the central and northern Plains, mainly after 18/00z. Convection is expected to initiate in the axis of moderate to strong instability extending across eastern CO/western and central NE, where local 1.00 inch QPF amounts were placed. Both the 00z ECMWF/GFS showed local QPF amounts in excess of 2.00 inches along this axis, but in different locations. The storms are expected to be outflow dominated after 18/00z, but after that, local training could result in a flash flood risk. Thus, a Marginal Risk was placed here to account for the threat. Further north across the northern Plains, the moisture and instability becomes focused on a frontal boundary extending from central MT into ND and western MN after 18/00z. There is some model signal for a cluster of storms or an MCS to form on the front and track eastward. Depending on the placement of the front (as there is still some spread concerning this), some of the instability could be elevated. In any case, there is a multi model signal for 2.00/3.00 inches of QPF over east central ND and northwest MN. Three hour flash flood guidance values are as low as 1.00/1.50 inches along the ND/MN border, and the National Water Model showed high stream flows before 18/12z. Because of this, a Slight Risk was placed here for Day 3. ...Lower/Mid MS Valley across the TN Valley into the Mid Atlantic... Deep moisture drawn northward from the Southeast states will combine with instability to produce convection with heavy to excessive rainfall over portions of the the Mid Atlantic states during Day 2. Better forcing along the frontal boundary on Day 3 could result in a larger flash flood threat across portions of the Mid Atlantic during Day 3. There are still some model differences with respect to the placement of the front, and its associated QPF. However, the 00z ECMWF/GFS/UKMET were close with respect to the placement of the front (through the 00z ECMWF/GFS may yet be too far south, based on instability parameters). With this in mind, the WPC QPF was based on a blend of these three models. Day 2... As the best mid level support for a frontal boundary crossing the northern Mid Atlantic states shears out to the northeast during Day 2. Along and south of the front, the low and mid level southwest flow transports 1.75 inch precipitable water air (which is between two and three standard deviations above the mean) from the Southeast states over much of the central and northern Mid Atlantic states. Instability is expected to be marginal for the most part near the front, but the potential for training near the front could allow for local 1.50 inch QPF amounts. Since these areas have received heavy rainfall in the past few days, a Slight Risk area was placed over portions east central VA into the central Delmarva Peninsula. A Marginal Risk was extended back into KY/TN, as convection in the deep moisture could train, resulting in a lower end flash flood threat. Day 3... Short wave energy tracking from the Southeast states into the mid Atlantic states during Day 3 provides synoptic scale ascent over a slow moving frontal boundary. The position of the frontal boundary is still in question, with the 00z UKMET the furthest south. Given the placement of the best instability, a more southern solution is preferred. Ahead of the short wave, the deep layered southwest flow continues to pump 1.75 inch precipitable water air up and over the frontal boundary, with the strongest low level flow occurring after 18/06z over the northern Mid Atlantic. There was a decent model signal for an axis of 1.50/2.00 inches of QPF extending from western NC and far northern SC into central VA. North of this area, the instability becomes more elevated, and with surface high pressure building down from the south, there could be a sharp gradient of heavy rainfall near the Mason Dixon line by 18/12z. There were some 00z GEFS members that showed 3.00/4.00 inches of rainfall over north central VA, and given the moisture in place, these values could serve as local maxima. Based on heavy rainfall earlier this week, a Slight Risk was extended over the aforementioned areas. A Marginal Risk was extended to the Mason Dixon line to cover the model spread in the placement of the heaviest rainfall. ...Southeast/FL... As the closed mid level low over the eastern Gulf of Mexico moves slowly north across the Southeast states on Day 2, it evolves into a long wave trough that feeds deep moisture and instability over the region during Days 2/3. Slow storm motions in the deep moisture could result in heavy to locally excessive rainfall, especially during Day 2. There was enough model agreement with the synoptic scale features to base the WPC QPF on a multi model blend. Day 2... The mid level system coming ashore over the eastern Gulf states and Southeast during Day 2 aids in ushering deep moisture (with precipitable water values of 1.75 inches) over the region. Model soundings showed moderate to strong instability developing in advance of the convection, so scattered to broken convection mainly diurnal (favoring the terrain at first) is expected. Storm motions are expected to be be slow, so local 3.00+ inch QPF amounts are possible, especially where local training occurs. There could be some local rainfall enhancement with the surface wave associated with the mid level system, but the model signal is not nearly as strong in the 00z model guidance as it has been in previous model cycles. In any event, even though three hour flash flood guidance values are fairly high, the potential for local 3.00+ inch necessitates a Marginal Risk over much of the Southeast for Day 2. Day 3... The main focus for convection shifts to the north as short wave energy moves north into Mid Atlantic states during Day 3. Instability and deep moisture remains in place, but with less in the way of mid level focus, QPF amounts are expected to remain below 1.50 inches, with the highest amounts over central and southern FL. Pereira/Hayes Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml