Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 428 AM EDT Sun May 13 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 13/1200 UTC thru May 16/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Midwest to Mid-Atlantic... A series of shortwaves embedded within the base of a broad mid to upper level trough will continue to interact with a pool of relatively deep moisture and instability coinciding with a quasi-stationary boundary -- supporting additional rounds of convection, with the potential for moderate to heavy rainfall. Overnight guidance suggests two areas with a greater potential for organized heavier amounts -- the upper Ohio valley to the northern Mid-Atlantic region and the mid Mississippi valley to the southern Great Lakes region. The shortwave associated with ongoing convection across the southern Great Lakes region is forecast to continue east, spurring convection north of the boundary from the upper Ohio valley to the the northern Mid-Atlantic coast. Guidance shows PWs increasing to over 1.5 inches during the afternoon/evening hours, which along with daytime heating and the favorable forcing aloft may produce some moderate to heavy amounts across the region. Across the upper Ohio valley to the Mid-Atlantic, WPC QPF gave strong weighting to the HREF Mean throughout the period. Back to the west, another round of moderate to heavy amounts is possible from the mid Mississippi valley into the southern Great Lakes region Sun night to Mon morning as a shortwave rounds the crest of an upstream ridge and moisture deepens along the nose of a strengthening low level jet (30-40 kts). Guidance begins to show significant latitudinal spread with respect to the axis of heavier amounts late in the period. The general trend of the overnight guidance has been toward a more northerly solution across eastern Iowa into southern Wisconsin and northern Illinois. This is reflected in the WPC QPF. However, did not go as far north as some of the solutions, including the NAM, which appear to be developing heavier amounts too far north into more stable air. ...Florida... Low level southeasterly inflow east of developing closed low over the northeastern Gulf will support deepening moisture across much of southern into central Florida. By early Monday, PWs are expected to increase to near or above 2 inches (2 to 3 standard deviations above normal) across much of the region. This moisture, coupled with low level frictional convergence and divergent flow aloft may result in the onset of some very heavy rainfall totals, especially from the Keys northward along the Atlantic coast. WPC QPF continued to follow a multi-model blend across this area. ...Southern Plains... Overnight hi-res guidance, as well as the GFS, show a good signal for moderate to locally heavy amounts from the eastern Texas Panhandle into western Oklahoma where convection is expected to develop later today. GFS shows PWs increasing to around 1.5 inches as the low level jet amplifies late this afternoon and evening. WPC QPF gave significant weighting the HREF Mean and GFS across this area. ...Great Basin to the central High Plains... Overnight guidance shows a deep closed low over the western U.S. beginning to gradually fill as it remains centered over the Great Basin this period. Close to the low, favorable mid-level lapse rates, in addition to diurnal heating, is expected to support additional showers and storms, with the potential for some moderate to locally heavy amounts across the region. Meanwhile further to the east, moist low level upslope flow coupled with ejecting vort energy from the base of the Great Basin closed low will foster clusters of showers and thunderstorms from the southeastern Wyoming and northeastern Colorado foothills into the High Plains. WPC QPF largely followed a blend of the HREF Mean and GFS across this region. Days 2/3... ...Pacific Northwest/Great Basin/Rockies... A closed mid level low over the Great Basin on Day 2 continues to spin down and weaken during Day 3. Another closed mid level low approaches the Pacific Northwest late during Day 3. There was generally good model agreement with the evolution of the synoptic scale systems, so the WPC QPF was based mainly on a blend of the 00z ECMWF/GFS. Day 2... Spokes rotating around the slowly weakening closed mid level low over northern NV/southern ID/northwest UT during Day 2 interact with 0.50 inch precipitable water air and marginal instability to produce convective clusters over portions of WY and CO. There was a fairly strong model signal for an axis of 0.50/1.00 inches of QPF stretching from southeast WY to the Front Range in CO, with lesser amounts (closer to 0.50 inches) over the Grand Tetons. It should be noted that the 00z GFS continues to be much higher than any other 00z guidance with its QPF amounts across these areas, and these amounts seem out of reach, except for local maxima. Short term training is possible as the convection develops over southeast WY/northern CO, but after the convection leaves the terrain, there should be some movement. Three hour flash flood guidance values are generally between 1.00/1.50 inches, with the lowest amounts over eastern WY. Snow levels drop low enough to suggest some of the QPF falls as snow, which should mitigate the flash flood threat. While heavy rainfall is expected, flash flooding is not expected during Day 3. Further west, convection associated with the mid level low itself is expected over much of eastern NV, as well as the Sierra Nevada range in CA. While there is sufficient moisture for convection, most models show 0.50 inches or less of QPF in these locations. Day 3... The mid level low over NV is expected to weaken as it crosses southern ID during Day 3. East of the mid level circulation, the low level upslope flow continues to supply 0.50 inch precipitable water air and marginal instability for convection over the Front Range and plains of CO. An axis of 0.50 inches of QPF was placed over the plains of CO, and once again the 00z GFS is higher with its QPF amounts. Three hour flash flood guidance values are higher here, so while there may be locally heavy rainfall, flash flooding is not expected during Day 3. Further west, as the next closed mid level low approaches the Pacific Northwest later during Day 3, lift ahead of it interacts with marginal instability across the higher terrain of Northern CA and the OR Cascades. There should be enough moisture to support convection in these locations, which is expected to peak between 16/00z and 16/06z. An axis of 0.50 inches of QPF was placed over the Shasta/Trinity Mountains in northern CA, as well as the southern and central OR Cascades, in line with the model consensus. ...Central and Southern Plains... Short wave energy tracking from the Southwest states into the Central Plains interacts with moisture and instability streaming northward along and east of a weakening frontal boundary extending from KS into central and West TX during Days 2 and 3. The developing convection produces heavy to locally excessive rainfall over portions of north TX each day. There was generally good model agreement with the timing of the short wave energy and placement of the front, so the WPC QPF was based mainly on a blend of the 00z ECMWF/GFS. Day 2... Short wave energy tracking out of NM early during Day 2 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 1.50 inch QPF amounts are possible with the convection, with the 00z GFS showing the potential for local 3.00+ inch QPF amounts. Given the signal in the 00z GFS (and to a lesser extent the 00z NAM), a Marginal risk was placed over portions of north TX for Day 2. Day 3... The next short of short wave energy crossing from NM to OK during Day 3 interacts with deep moisture and strong instability along and ahead of the weakening frontal boundary extending from central and eastern KS into OK and north TX. Convection is expected to develop in the mid to late afternoon hours, peaking between 16/00z and 16/06z. There is once again a strong model signal for 1.50 inches of QPF over the western edge of the Red River Valley, with local 3.00 inch amounts possible where short term training occurs. As was the case during Day 2, a Marginal Risk was placed in this area for Day 3 to cover the convective threat. ...Mid and Upper MS Valley/OH Valley/Northeast/Mid Atlantic... Short wave impulses riding in the mid level flow over a frontal boundary vacillating from the MS Valley across the OH Valley into the Mid Atlantic and Northeast states taps deep moisture and instability to support convective clusters that produce heavy to locally excessive rainfall each day. There are still some differences in the placement of the frontal boundary, especially over the OH Valley and Northeast, particularly on Day 3. The 00z ECMWF has come further south with the front during day 3, reaching the Mid Atlantic states by the end of the period. The 00z GFS/NAM are further north with the front, though instability gradients from each model (as well as the most recent SREF) suggest that the front could be further south than these models. The WPC QPF was based on a blend of these models, then dropped a bit further south to better fit the expected instability pattern, especially during Day 3. Day 2... Short wave energy in the relatively fast mid level flow (at the southern edge of the westerlies) is expected to interact with deep moisture (with precipitable water values near 1.50 inches, which is between two and three standard deviations above the mean) and moderate to strong instability stretching from southern IA/northern MO across north central IL/northern IN into central OH. The moisture and instability will feed convective clusters associated with the short wave energy along and south of the front. As has been the case the past couple of days, there is still some model spread on the placement of the frontal position, which will be the key to the placement of the QPF maxima associated with the convective clusters. Using the latest SREF instability as a guide to the frontal placement, the WPC QPF depicted an axis of 1.00/1.50 inches of QPF stretching from southwest IA/northernmost MO into central OH. The highest amounts were placed over southwest IA/northern MO, where there has been a more consistent signal for between 1.50/2.00 inches of QPF. While rainfall from previous storms has not yet affected this region, the convective clusters in a high moisture content environment necessitated the placement of a Slight Risk area over this area for Day 2. Further east into the remainder of central and southern OH/northern WV and southwest PA, the moisture profiles are similar to those further west, instability is not expected to be as robust, which would suggest lesser QPF amounts over these areas. However, three hour flash flood guidance values are as low as 1.50 inches over portions of WV/western PA, and short term training could allow storms to excess these values. Since there is not as strong a signal for flash flooding here, a Marginal risk was placed over the area for Day 2. Day 3... The placement of the frontal boundary becomes more problematic on Day 3, as there are some important differences, especially over the OH Valley and Northeast/northern Mid Atlantic state. The 00z NAM/GFS are further north, waiting for the passage of a surface wave over New England before dropping south. The 00z UKMET/00z ECMWF are further south, closer to the positioning suggested by the SREF instability probabilities. It should be noted that the 00z ECMWF sends a shallow front into northern MD, as high pressure from Quebec drops south, but the convection is not expected to be tied to this shallow front. Thus, the WPC QPF was tied more closely to the position of the 00z ECMWF/UKMET frontal placement. Along and ahead of the front, a low level west southwest flow continues to spread 1.25/1.50 inch precipitable water air over southern OH/northern WV/central and southern PA into portions of southeast NY and NJ. In fact, some solutions suggest that that moisture from a system in the Southeast becomes infused into the frontal zone, but it is not clear yet whether than occurs. In any event, the moisture and instability should be sufficient to support convection along the line, which could be organized in the right rear quadrant of a 110 knot jet streak crossing Quebec. A long axis of 0.75/1.25 inches of QPF was spread over the aforementioned areas, with some variability in the placement of 00z model QPF maxima. This is to be expected, given the timing of the mid level short waves, as well as the placement of the front itself. Given the deep moisture in place, and the potential for training as the 850-300 mb mean flow becomes better aligned with the propagation vectors, local 2.00+ inch amounts are possible. Lowering flash flood guidance values (due to earlier heavier rain) will make these areas more vulnerable to flash flooding, so a long Marginal Risk was extended from central IL into NJ. If there is better agreement with the placement of the front in later model runs, a Slight Risk could be warranted over portions of PA/southeast NY/northern NJ in subsequent forecasts. ...FL/Southeast... As a closed mid level low over the eastern Gulf of Mexico slowly weakens and moves north toward the Gulf Coast during Days 2/3, a low level south to southeast flow transports deep moisture and instability across FL and the Southeast states each day. There is better agreement with respect to the synoptic scale features in this area, but there are still some important QPF placement differences, especially during Day 3. In an attempt to mitigate some of the differences, the WPC QPF was based primarily on a blend of the these three models, with less weight given to the 00z ECMWF, as it appears to be once again suffering from gird scale feedback. Day 2... Ahead of a closing mid level low over the eastern Gulf of Mexico during Day 2, a low level southeast flow transports 2.00+ inch precipitable water air (which is better than two standard deviations above the mean) and marginal instability along an inverted surface trough over the east coast of FL. The funneled moisture and instability should fuel arcs of convection over the east coast of Fl, mainly north of the Miami metro area. There has been a consistent model signal for 2.00/3.00 inches of rainfall along the east coast for a couple of days, and that is represented in the WPC QPF. Even though three hour flash flood guidance values are high, the potential for training in the high moisture content air necessitated a Marginal Risk for Day 2. Day 3... The closed mid level low weakens as it moves ashore over the central and eastern Gulf Coast states during Day 3. The low level flow becomes more southerly, and begins to lose its deep tropical connection. Instability becomes better upstream of the Southeast states, so mainly diurnally driven convection is expected to cover much of GA/SC and portions of NC. Given the deep moisture in place, a large area of 1.00/1.50 inches of QPF was placed in these areas. However, there was a fair amount of spread associated with the placement of the QPF maxima. The 00z ECMWF, while its QPF placement might be OK, its amounts seem high, and that could be due to that model dealing with grid scale feedback. That having been said, slow storm motions in the high moisture content airmass could lead to high rainfall rates in places where the three hour flash flood guidance values are fairly high. Without a clear mid level trigger, it becomes very difficult to pinpoint where the highest QPF totals will be, so a large Marginal Risk was placed here for Day 3. Pereira/Hayes Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml