Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 534 AM EDT Sun May 20 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid May 20/1200 UTC thru May 23/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Texas... Two separate convective complexes are expected across Texas today. One is ongoing this morning across north central TX. Anticipate this complex will slowly shift southeastward with time this morning. As we go through the morning, 850 mb moisture transport does weaken some ahead of the line, and think the thunderstorm complex will generally become less organized. With that said, still think some convection will persist through the day, along with a well defined outflow boundary. The airmass ahead of the convection/outflow should become increasingly unstable through the day. And while not overly strong, as mentioned above, 850 mb southeasterly moisture transport into the line should be strong enough to support some uptick in convective activity as instability increases today. All of the normally better performing 0z high res models do indeed maintain the complex to some degree, adding confidence to this idea. In general expect the complex as a whole will become more progressive through the day. However, a good consensus that cells should form ahead of the complex in the unstable airmass, then move off to the north with the mean flow, and merge with the southeastward moving MCS. This evolution, combined with PWATs forecast 1.5"+, would suggest that some flash flood risk will likely persist with the MCS through the day. Given this threat, will go ahead and issue a Slight risk of excessive rainfall for portions of east central TX. Confidence is only low to moderate that we will see organized exceedance of the relatively high FFG across this area. However the conditional threat appears to be high enough, given the remnant outflow, moderate/high instability and cell merging potential, to warrant the Slight risk area. WPC QPF for this MCS followed closest to a blend of the 0z ARW/ARW2 and Experimental HRRR (weighted towards the HRRR and radar trends to start the period). The other area of focus will be near the Rio Grande this afternoon into tonight. Convection should initiate early to mid afternoon off the higher terrain of northern Mexico and near the dryline. These storms should organize as they move towards the TX border, where they will encounter an axis of stronger southeasterly 850 mb moisture transport. This interaction may result in storms backbuilding for a period into the flow, before progressing more off to the southeast along the Rio Grande. This evolution would suggest a flash flood threat where the cells backbuild and make the turn more southeasterly. Will go with a small Slight risk across this area to account for this risk. WPC QPF for this area again stayed closest to the ARW, ARW2 and Experimental HRRR. ...Mid MS Valley into OH Valley... Low confidence on convective evolution today across this region. Will, however have opportunities for multiple rounds of convection through the day. Already seeing some activity this morning over MO forming in a broadly convergent low level regime in the right entrance region of an upper jet streak passing by to the north. Depending on cloud cover and recovery, additional storms will be possible this afternoon into tonight ahead of the shortwave energy moving east across the Plains. Also may see some interaction this afternoon/tonight with the front draped across the area, possibly helping focus convection north with time into portions of IA/IL and east into portions of IN/OH. No strong signal that an organized flash flood risk will develop across this region. However, with PWATs above average, the potential for multiple rounds of storms, and a stationary front that could act as a focus for some brief training, at least an isolated flash flood threat probably exists. Will thus carry a broad Marginal risk area. Again confidence is low on the details and thus for QPF generally took a consensus of the 0z high res guidance. ...Lower MS Valley into the TN Valley and Southeast... Anticipate we will see widespread diurnally driven convection across this region today. Moderate to high instability will develop, and will generally be uncapped given the broad troughing remaining over the area. PWATs, while not extreme, will continue to run above normal. Generally not anticipating much organization to convection, however weak mean flow supports some slow moving cells, and likely to see some cell merging along outflows. Thus, while any individual cells may be short lived, some chaotic cell motions and cold pool generations should allow for localized heavy rainfall through the period. Tough to really pin down areas of focus at this point, although one area of more focused activity should be across portions of AL/GA along a weak cold front as it drifts southward. Also possibly some remnant outflows from the Plains activity act as a focus across portions of AR/KY/TN/MS...although nearly impossible to pin down these details at this point. Overall, high confidence in scattered to widespread pulse convection across this area, but low confidence on the details and areas of greatest focus. Will carry a broad Marginal risk here. Given the uncertainty WPC stayed pretty close to the 0z HREF for QPF across this region. ...Mid Atlantic into Northeast... A weak cold front will push across the Mid Atlantic and Northeast today. Showers and thunderstorms are expected to develop along this front. The greatest coverage should be across the Northeast closer to the better forcing. Further south activity should be more isolated. The exception may be across the western Carolinas and southwest VA, where differential heating in the terrain may act as an additional trigger for storms. Most cells with the front will be progressive in nature. However areas of the Mid Atlantic that have saturated soils could see a localized flash flood risk. The best chance of this is probably across the terrain of the western Carolinas, where activity may be more intense and could briefly hang near the terrain increasing the duration. PWAT values will not be as high previous days, but still high enough for briefly heavy rates. Will carry a Marginal risk where the threat of briefly intense rates overlaps with saturated soil conditions. The lack of convective organization and the isolated to scattered nature of storms should generally keep the flash flood risk localized in nature. ...Florida... Will be another wet day across Florida, with moisture remaining high. As usual, low confidence with the details today. The potential will certainly be there for heavy rates and some flooding concerns, just a question as to if/where we see enough instability and localized convergence to generate more intense convection. Some signs a weak mid level wave may be approaching the state this morning, and southeasterly 850mb flow will remains strong enough to support the persistence of organized convection shall it develop. Thus will just need to continue to monitor trends through the day and will carry a Marginal risk. ...Northwest... Troughing will move into the northwest today, with multiple embedded shortwaves present. Steepening lapse rates underneath the trough will result in an uptick in shower and thunderstorm activity today. PWAT values are forecast around the 90th percentile, suggesting locally heavy rates are possible. Main uncertainties regarding a flash flood threat will be whether we get unstable enough to generate strong enough convection to get the more intense rates, and cells should generally be moving at a decent clip off to the north. With that said, the widespread nature of showers, anomalous PWATs, and at least marginal instability, suggests that at least a localized flash flood risk may develop across more susceptible areas. Will carry a Marginal risk across portions of eastern OR into western ID and northern UT, where the overlap of instability, anomalous PWATs and higher HREF probabilities of 2" exists. Days 2/3... ...Southeast/eastern Gulf Coast/FL... Deep moisture within confluent flow with corresponding lift provided by the passage on an inverted 850 mb trough will support bands of showers over the eastern Gulf coast....including the FL panhandle inland to AL/GA on day 2 and 3. The NAM/UKMET/GFS showed a closed low developing which led to strong low level wind fields and resultant convergence and lift. The inflow off the gulf advects the high moisture into the Florida panhandle across into southern Alabama and Georgia, so combined with diurnal instability and sea breeze initiated lift combined with convergence near the low, the higher amounts are shown in these areas for Monday and Tuesday. Manual progs blended continuity with the GFS/ECMWF/UKMET. Marginal risks for excessive rainfall were shown covering parts of Alabama and Georgia in the proximity to the low-mid level circulation, continuing down into the Florida panhandle. ...Great Basin/Northern Rockies/Northern Plains... A long wave trough will slowly move east out of OR and WA towards ID and MT on Tue. The focus of rainfall develops with upper divergence maxima just downstream from the trough/near the low level front in the ranges of ID and western/central MT and northwest WY on Mon/Mon night. Further south, showers continue in the proximity of the upper trough and embedded closed low as it drifts out of CA towards NV...with QPF maxima in the CA Sierra Nevada and favored terrain of northern NV. On Tue as the upper low ejects north across Utah to the ID border, southerly deep layer flow advection moisture north across Wyoming and the northern Plains. The warm/moisture advection impinges on the ranges of northeast Wyoming, including the Bighorn Mountains. As upper divergence maxima eject northeast across the northern Plains Tue night, clusters of showers/storms develops with the NAM/UKMET/Canadian showing higher amounts over a larger area. The NAM intensifies the 700 mb low over North Dakota in response to convective heating, usually a sign of convective/grid scale feedback from convection, in this case in southeast ND. Consequently the NAM was under weighted with amounts shown more in line with other solutions. Manual QPF showed a maxima near the SD/MN/IA border in line with the 12-00z UKMET/Canadian Global/GFS/00z ECMWF. ...West Texas/eastern NM... East of the developing western U.S. trough, southeasterly confluent low level flow over Eastern NM and across much of Texas will advect moisture off the Gulf of Mexico inland. The airmass will be characterized by a precipitable water value around 1.25-1.5 inches on Monday...which is 2 standard deviations above normal. This, combined with high CAPE and low shear, which favor slow moving storms. On day 2 with a couple of the models, including the 00z GFS, show locally 2-3 inches of rain. The excessive rainfall outlook was maintained at a slight risk category on day 2, but not forecast on day 3 following the lower amounts forecast by the models on Day 3/Tue. Nonetheless, bands of enhanced relative humidity and ascent centered on 700 mb keeps showers in the area Tue afternoon and evening. Manual QPF used a multi-model blend with continuity. ...Great Lakes to the Ohio Valley/Central Appalachians/mid Atlantic... A low-mid level wave over eastern Iowa on Sunday evening moves towards the Great Lakes, with the deformation zone focus over eastern Iowa/northern Illinois/northern Indiana. The threat for showers continues in northern IL across southern Lake MI to southwest lower MI with slow moving cells possible near the elongated cyclonic circulation. More progressive showers/storms develop in an axis of the low level jet with well defined 700 mb convergence near the jet maxima crossing Ohio into PA Monday, supported by upper divergence maxima. This continues downstream into NJ/southern NY in the zone of warm/moist advection at 700 mb on Monday evening. ON Tuesday bands of low level moisture convergence stream across much of Pennsylvania and MD, with diurnal instability contributing to showers/storms. The mid level westerly flow provides an upslope component of flow so moisture is lifted by the mountains with showery conditions expected in the terrain of West Virginia and Virginia. Manual progs used a blend of the 12-00z ECMWF/00Z NAM/00z GFS. Chenard/Petersen Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml