Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 547 AM EDT Thu Apr 12 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 12/1200 UTC thru Apr 15/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Northern Rockies into the Plains and Upper MS Valley... The trough over the west coast will move across the Great Basin and Rockies through the day Thursday...with a strong area of low pressure ejecting into the Central Plains Thursday night. Likely to see a large area of snow/rain with this trough. Good model agreement is seen across most of the terrain where orographic effects will dominate...and thus here WPC QPF follows a multi model blend weighted towards the higher res guidance. Likely to see significant QPF north of the low across the MT plains into the western Dakotas through the day 1 period. A corridor of increased 700 mb convergence, an inverted surface trough, and easterly upslope flow in between the low to the south and high to the north will all provide a favorable setup for this precipitation. Likely to be a pretty tight northern gradient given the strength of the high to the north pushing low level dry air south. Thus the biggest model differences here seem to be with the northern extent of the QPF axis across MT. Have noted a slight northward shift in most of the new 0z guidance...including the high res models. WPC thus trended a bit in this direction as well...with our axis generally in between the furthest north GFS and south ECMWF. On Thursday night the better mid/upper level forcing ejects into the plains...with pieces of shortwave energy and dual left exit and right entrance region upper jet dynamics. Thus expect to see precipitation increase in coverage across the Northern Plains into the Upper MS Valley. In fact, looks like we could see some elevated convection form north of the surface warm front Thursday night where elevated instability is forecast to develop. This convective activity should impact portions of western NE north into portions of central and eastern SD. Some differences with exactly where this activity focuses Thursday night...and for row WPC favored a middle ground consensus solution incorporating both the global and high res guidance. This focuses some higher amounts from north central NE into southeast SD east towards the MN/IA border. ...Great Lakes into New England... Isentropic lift ahead of a quick moving wave will continue to produce an area of rain/snow from the Great Lakes into New England through Thursday night. Generally favored a multi model blend to handle QPF here...although think the GFS is probably too high along the NY/PA border...being the only piece of guidance suggesting higher amounts there. ...Pacific Northwest... Onshore flow will continue showery conditions into Thursday...although mid level ridging building into the region should result in decreasing coverage and intensity. This lull is short lived however, as another front will move into the Pacific Northwest Thursday night. Models were in close enough agreement that staying close to a blend should suffice here. Days 2/3 ...Pacific Northwest... Shortwave energy will round a low over the Gulf of Alaska Friday through Saturday before the low ejects south Saturday night. A persistent southwesterly flow south of this low will direct a narrow plume of 0.75 inch PW into western WA Friday morning through Friday night before it drifts south to OR through Saturday night. The 24hr long fetch over the same area from 12Z Friday to 12Z Saturday allowed the Marginal Risk to be maintained for the Day 2 Excessive Rainfall Outlook (12Z Fri-12Z Sat) for lower western portions of both the Olympics and the northern WA Cascades. ...Northern and Central Rockies to the Great Lakes to the Northeast... A potent mid-level trough will continue advancing eastward from the central Rockies on Friday, closing at 500 mb near the NE/CO/KS border Friday afternoon. Moisture originating from the eastern tropical Pacific and western Caribbean then across the western Gulf will advect up southern Plains on a 50kt southerly jet ahead of an amplifying trough south of the mid-level low center. This moisture will wrap around a 700 mb low center across the central Great Plains with heavy precipitation as this moist air rides over the cold conveyor belt from eastern Montana east to the northern Great Lakes Friday. As the surface cyclone takes shape over the central Plains, a broad area of warm advection/frontogenetically enhanced precipitation will stretch across the Plains into the upper Midwest while increasing upper divergence/diffluence enhances precipitation to the west within a developing deformation axis. The northward extent of the precip will be determined by the presence of a strengthening surface high over northern Manitoba/Ontario. Dry air from this high will make for a sharp cutoff on the northern side which has wavered in recent runs and will continue to do so with hopefully lesser magnitude going forward. The potential for mesoscale banding with low level convergent flow is supportive of locally heavier totals from Nebraska/South Dakota and eastward until the axis weakens through Saturday with lesser totals toward the Northeastern CONUS. Continued to prefer the UKMET/ECMWF/GEFS solution for the 00Z suite with the addition of the 00Z 3km NAM which is in agreement with the previous. ...Lower Mississippi Valley/ArkLaTex... A potent mid-level trough will continue advancing eastward from the central Rockies on Friday, closing at 500 mb near the NE/CO/KS border Friday afternoon. Moisture originating from the eastern tropical Pacific and western Caribbean then across the western Gulf will advect up southern Plains on a 50kt southerly jet ahead of an amplifying trough south of the mid-level low center. The 00Z ECMWF continues to produce slightly higher PW ahead of this trough than the 00Z GFS, both of which are over two standard deviations above normal (closer to three in the ECMWF). The 00Z operational GFS continues to be the most progressive by far, but the axis of greatest QPF was quite similar between the 00Z GEFS mean, 00Z ECMWF, 00Z 3km NAM, and 00Z UKMET for both Days 2 and 3 (12km NAM for Day 3) which was the basis for WPC QPF. That said, mesoscale processes will dictate the movement of the prefrontal linear thunderstorm activity. Furthermore, as the trough amplifies over the southern plains, a backing of low level flow is possible which could slow progression of the line and enhance already heavy precip. As of now northern LA/southern AR is the most likely location for such delay, which is highlighted in global and high-res guidance for Friday night with 5-6" QPF. Furthermore, this area is highlighted for having some remnant soil saturation from antecedent activity in the HRRR percent of soil saturation product. Therefore, a Moderate Risk for excessive rainfall was embedded in the existing slight risk for Day 2 (12Z Fri-12Z Sat) over this area. The upper trough swings to a negative tilt over the lower MS Valley through Saturday and Saturday night. This should help the front continue to progress and limit max QPF. However, the anomalous moisture and expected presence of mesoscale processes could enhance rainfall further. For now a slight risk for excessive rainfall was issued for Day 3 (12Z Sat-12Z Sun) from the central Gulf Coast up to central KY where areal averaged QPF is 3-5". A moderate risk may be needed if a particular focus area is identified. Chenard/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml