Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 424 AM EDT Fri May 18 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid May 18/1200 UTC thru May 21/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Mid Atlantic and Southeast... Another wet day with locally excessive rainfall expected across the area. The mid and upper levels remain broadly diffluent ahead of the large scale trough, with multiple weak embedded shortwaves continuing to progress northward in the southerly flow. PWATs remain in the well above normal to near climatological record range, supportive of efficient rainfall processes. A nearly stationary west to east boundary will remain draped across central VA through much of Friday. This boundary will once again act as a focus for backbuilding/training convection, with continued 850 mb moisture transport into the boundary. Modest instability should again develop along this front, allowing for an uptick in rainfall rate potential this afternoon/evening. North of the front, rainfall will be more stratiform in nature, resulting in generally lower rates. South of the front, convection will generally be less organized, but still capable of briefly intense rates. Thus the best focus for heaviest rains and an increasing flash flood threat will be along/near the front, and into the northwest NC and southwest VA terrain, where differential heating across the higher terrain and some upslope component to the flow may enhance convective coverage and rainfall magnitudes. Also will need to watch portions of eastern NC/SC through the day. Looks like a low level convergence axis will try to set up here, with a long fetch of moisture streaming north out of the Caribbean as well. Thus possible we see south to north streamers of convection through the day, capable of producing very heavy rainfall rates. By later tonight the whole synoptic system begins to shift northward, which should dissipate the stationary boundary over VA. Thus, think by this time we should see an end to the better backbuilding/training setup over this area. However should still see showers and embedded thunderstorms shifting north across the Mid Atlantic, which may focus into a narrower north/south axis. Instability should be decreased by this point, thus think rates will be tempered some. WPC QPF followed a blend of the 0z HREF members and the 0z Experimental HRRR. This results in a pretty significant uptick in amounts over VA near the aforementioned boundary. ...Central Plains... West to east training of convection will continue across NE into the morning hours on the nose of strong 850 mb moisture transport. Anticipate this convection will weaken through the morning as we see the diurnal weakening of the low level jet. By afternoon another round of storms will fire across CO/WY. Will be an even more favorable synoptic pattern today compared to yesterday, with the broad mid/upper level trough and embedded shortwaves moving closer. PWAT values will remain well above normal to near climatological record values as well. Anticipate a complex evolution to convection across this area with multiple factors at play. Will have a very strong gradient in place over the western Plains, with moderate/high instability forecast. An 850 mb low over southern CO will strengthen southerly and easterly flow ahead of the instability gradient feeding into convection. Will also heave a northern stream cold front progressing south with time across the area. In general anticipate we will see widespread convective development, with some chaotic storm motions resulting in areas of training and repeat convection. Will likely end up seeing all this convection result in a well defined MCV taking shape as well, adding even more complexity to the evolution. Thus for QPF, WPC generally followed a blend of the 0z HREF members. Tough to pin down exactly where the heaviest amounts will fall, but think at least some flash flood risk will likely evolve with time into tonight from northeast CO into southeast WY, western and central NE, and western KS. Storms further south into OK/TX should generally be less organized and more progressive being displaced further form the better deep layer forcing and better 850 mb moisture convergence. ...Mid MS Valley into the OH/TN Valley... Scattered to widespread convection is likely across this region today underneath the mid level shear axis and embedded shortwave features. This setup supports slow moving convection, and periodic cell mergers, and with PWATs running well above normal, locally excessive rainfall will be possible. WPC generally followed close to the 0z HREF. As was the case in previous days underneath the shear axis, amounts will likely be higher than the global models suggest, and thus not surprising that this blend results in a pretty significant increase in amounts from our previous forecast. ...Northern Plains... Convection over the northern Plains this morning is aiding in the development of an MCV near the SD/ND border. This feature will move northeast with time today, interacting with the right entrance region of the upper jet to the north. Thus will remain favorable for showers and embedded thunderstorms ahead of this feature. Instability will be lower with this system resulting in generally lower rainfall rates. May see some weak instability develop east of the MCV with time, which could allow for the development of some heavier convective cores from eastern ND into northern MN. Although generally think this activity will remain progressive, keeping rainfall totals from getting too high. ...Great Basin, Northern Rockies, MT... Embedded shortwaves within the broad troughing will continue to support showers and embedded thunderstorms over this area. PWATs will remain above normal, but instability will generally be weak, inhibiting rainfall rates. A relatively narrow axis of weak instability is however forecast across ID, although probably not enough to really get much intense activity going. Thus generally thinking we will see light to moderate shower activity, with any heavier rates generally brief in nature. Days 2/3... ...Northern and Central Plains/Mid and Upper MS Valley/OH Valley Day 3 (Sun)... A low-mid level wave develops in the lee of the WY/CO Rockies Saturday with surface low moving from KS towards IA 12z Sun. Gulf moisture wraps around the low and confluent flow north of the developing circulation creates a deformation zone which also encounters topographical lift in the high plains. PW of 1 inch in this elevated area is 2 standard deviations above normal and should result in moderate to heavy rainfall. The 00z UKMET and Canadian global have the rain persisting into Sunday over Nebraska while the GFS and ECMWF are more progressive in moving the rainfall downstream into IL and the OH Valley. More weighting was given to the latter cluster. Day 2 QPF was based on the 18z GFS/12z ECMWF/00z NAM and 00z GEFS ensemble mean. The low shifts east through Sunday with the heavy rain/deformation zone focus over eastern IA/northern IL on Day 3/Sun. The Day 3 QPF was based on a blend of the 18Z GFS/12z ECMWF/00z Canadian Global (In IA/IL but not NE)/00z GEFS reforecast, which shows the area of one to two inch rains in eastern IA to northern IL. Confidence is low on day 3 due to the phasing timing difference of the 700 mb wave and resultant wide spread in QPF solutions among the models. ...Southeast/FL... Deep moisture within confluent flow west of a Bermuda high will support bands of showers over eastern FL on day 2 and then shifting west on day 3 to the western half of the FL peninsula. Showers continue on day 3 in southern peninsula to the keys as the moisture stream originates from the Caribbean and crosses the FL straights into the area with 2-2.25 inch pw values. The combination of moisture and instability should result in moderate to heavy rainfall, especially across the southern peninsula of FL and into eastern NC. WPC QPF was based on the 18Z-00z GFS/12z ECMWF/00z NAM for both Day 2/Day 3. A slight risk was continued for southern portions of eastern NC Day 2 as agreement increases for a narrow persistent band of heavy rain Saturday...which follows a rainy stretch on Thu-Fri. ...Day 2 OH Valley/Mid Atlantic/Northeast... Deep moisture streaming northward from the Southeast states crosses the OH Valley into the northeast with a pair of 700 mb waves forecast to produce lift and resultant showers across the lower Great Lakes/New York/New England during Day 2 and early on Day 3. Once the wave moves off the coast on day 3/Sun night, the coverage/amounts of rainfall taper in NY/New England. On Sat, the surface warm front ejects north from the central Mid-Atlantic to New England. The front lifting the above normal moisture results in widespread areal average of one inch QPF in portions of interior New York/New England. Good agreement among the NAM/GFS/ECMWF QPF resulted In a consensus based approach being used. ...Pacific Northwest/Northern Rockies/Great Basin... A long wave trough will slowly amplify Saturday as it cross OR and CA. The focus of rainfall develops with upper divergence maxima at the base on the trough crossing northwest CA to northern NV on Sat and continues in northern NV to OR as additional upper divergence maxima support widespread showers. Manual QPF and the GEFS reforecast show pockets of half an inch of rain on Sunday. QPF is based on a blend of the 18Z GFS/12z ECMWF/00z NAM/00z GEFS reforecast. ...West Texas/eastern NM... As the western US trough develops. confluent flow over Eastern NM and Texas establishes a return low level southeast to southeast flow. Pockets of 700 mb relative humidity of 70-90 percent develop in southwest TX to the NM border, with instability and bndry layer moisture convergence along the dryline helping showers/storms to develop, with increased amounts Sun compared to Sat. Manual progs blended the 00z NAM/GFS/ECMWF/UKMET QPF, with higher amounts near the southeast NM/western TX border close tot he UKMET amounts. Chenard/Petersen Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml