Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 553 AM EDT Tue Apr 17 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 17/1200 UTC thru Apr 20/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Northeast and Great Lakes... Snow showers will continue today off the Great lakes and into the terrain of northern New England underneath the closed mid/upper level low. Given the small scale features of lake effect and orographic enhancement...WPC QPF relied solely on a blend of the 0z HREF members across this region. ...Northwest into northern Rockies... Lingering onshore flow will result in showers into the the terrain of western WA/OR and the northern Rockies. Shortwave ridging building in later today into tonight should result in decreasing coverage of showers. WPC QPF blended the 0z HREF members. ...Central and northern Plains into the Mid and Upper MS Valley... Shortwave energy will push east out of the Rockies and into the Plains by Tuesday night, with a surface low ejecting across the Plains and into the MS Valley. Precipitation will move across portions of WY/MT early today, and into the western Plains this afternoon, ahead of the shortwave trough along a low level convergence axis. Through this period utilized a multi model blend for QPF, with the high res and global models showing some usefulness. Would appear like the 12z ECMWF may be too dry here, with the GFS/UKMET and HREF members generally wetter than it. At the same time, concerned the GFS and HREF members may be too wet over western ND, with the non NCEP models generally lighter here. Thus the multi model blend mentioned above seemed to result in a good middle ground solution. By later Tuesday night things get a little less certain as the low moves into the MS Valley. Given the strong dynamics in play (well defined shortwave and strongly divergent flow in the upper levels) would think we should see a compact but briefly intense area of precipitation northwest of the low. By this time the 0z HREF members generally seem too dry and quite a bit different than the global model consensus...resulting in lower confidence by this period. Often the high res models can struggle with non convective precipitation by the end of day 1 into the day 2 period. Thus given the more consistent global model signal, stayed away from the HREF by this period. WPC QPF is thus closer to a blend of the 0z GFS/UKMET and 12z ECMWF by this time...placing a max corridor from eastern NE into western IA. This is a bit south of our previous forecast. The GFS has shown a consistent trend south over the last several cycles, with the ECMWF and UKMET showing decent consistency further south. Thus the southward shift seems to be the way to go at this time. Days 2/3 ...West Coast/Intermountain West... An upper-level trough and closed low will move southeast through the Great Basin into the Four Corners/Central Rockies region by Friday morning. Available moisture will be fairly limited across much of the West as this feature approaches the Rockies... with PW of ~0.25 inch. Mid-level instability will support showery precipitation across portions of New Mexico and Colorado, with enhanced amounts in the highest terrain. Weak onshore flow with PW values around 0.25 - 0.50 inch across northern California and the Pacific Northwest will result in precipitation, mainly falling as snow, from southern California to Utah/Idaho. Model guidance continues to be fairly clustered and in agreement with the evolution and placement of the highest qpf. A general model blend would suffice across the West. ...Northern/Central Plains eastward to the Northeast/Northern Mid-Atlantic... A baroclinic zone over the Middle Mississippi Valley and the Central/Southern Plains will move northeastward off the East Coast by Thursday evening. A plume of moisture from the Western Gulf of Mexico will intersect the boundary with precipitable water values of 0.75 inches that will pool along the boundary until the front and the moisture plume move off the East Coast by Thursday morning. The axis of highest precipitation on day 2 is progged to set up west to east over northern Iowa, southern Minnesota and western Wisconsin where areal averages of 1 inch qpf is expected. On day 3 as the low continues to track east it will become reinforced as another lobe of energy surges south toward the Great Lakes region. The boundary will aid in producing a broad region of qpf from the Lower Great Lakes to New England Coast with maximum qpf amounts ranging from 0.25 to 0.50 inches from Wednesday evening to Thursday evening. WPC used a blend of the GFS, ECMWF and NAM as a starting point. ...Southern High Plains into the Rio Grande Valley... A mix of Pacific and Gulf moisture with transport northward into the Southern High Plains as the upper-level low progresses eastward into the Desert Southwest. A steady stream of PW values of 0.50 to 1.5 inches will wrap into the system, producing qpf all along the Southern/Central Rockies. The highest qpf was focused over the highest terrain of northeast New Mexico and southwest Colorado, were areal averages were nearing 0.50 to 0.75 inch on day 3. The models of choice for this region comprised of a mix of the ECWMF/GFS. Chenard/Campbell Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml