Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 556 PM EDT Wed Apr 11 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 12/0000 UTC thru Apr 15/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Northern California---Pacific Northwest---Great Basin into the Northern Rockies... The strong area of northeast Pacific height falls that will be the driver of a high impact weather event over the next several days over large portions of the nation will be pressing inland tonight across the Pacific Northwest-northern California---eastward into the Great Basin and Northern Rockies during Thursday. An axis of above average pw values along the associated frontal boundary will support a concentrated region of frontal precip through the Pacific Northwest-northern California region tonight---across the northern Great Basin-northern Rockies and northern high Plains region during Thursdays. This frontal precipitation will be followed by post frontal showers from northern California into the Pacific Northwest to the west of the Cascades as westerly onshore flow persists across these regions. Model mass fields are in good agreement this period with the evolution of this trof---with subsequent good agreement also with the precipitation distribution and amounts across these regions. A broad region of moderate precipitation potential likely across these areas---with locally heavier totals in the favored terrain regions from the Sierra---northwest California coast range---northward through the Oregon-Washington Cascades and along the Oregon-Washington coast range into the Olympic range. Locally heavy totals also likely from the Wasatch of UT---northward into the Northern Rockies from northwest WY---far eastern ID---central to northern ID---northeast Oregon and western MT. See the latest QPFHSD for heavy snow threat across portions of these regions. ...Upper MS Valley---Great Lakes---northern Mid-Atlantic into western to central New England... Ahead of the strong height falls moving into the western U.S. this period...two areas of height falls will be pushing eastward across the northern tier of the country from the Northern Plains into the Upper MS Valley---Great Lakes and into New England. The lead area of height fall moving from eastern NY state into New England early day 1 will be the weaker of the two and have mostly light precip amounts associated with it from the northern Mid Atlantic---into NY state and New England. The second area of height falls pushing east northeastward from the Northern Plains into the Upper MS Valley tonight and across southeast Canada Thursday will be stronger. This will result in stronger isentropic lift ahead of the associated surface low moving east northeastward across these areas. Model consensus is for an expanding region of overrunning precipitation to spread across the Upper MS Valley into the Great Lakes tonight and across NY state into western to central New England Thursday---with a broad region of moderate precip amounts in the .10-25"+ range. Accumulating snow potential will be confined to the northern edge of this forecast precip region from northern MN---far northern WI---the U.P. of MI into southeast Canada. Days 2/3 ...Pacific Northwest... An persistent onshore flow in the wake of a departing upper trough Thursday, and south of a Gulf of Alaska closed low will allow PWATs of 0.50 to ~0.75 inches over land from the Pacific Northwest Coastal Ranges into the Cascades of OR/WA. 850-700 mb wind speeds of generally 30-40 kt can be expected through Friday evening, well in advance of a cold front related to the Gulf of Alaska system. Wind speeds increase roughly 10 kt as the cold front approaches but with a lack of instability and PWATs only weakly marginal, rainfall rates should stay under control for most locations to prevent much in the way of a flood threat. However, the longer duration of the 48 hour period ending 12Z/14 may allow for 3-5 inches from the Olympics into the northern Washington Cascades. A Marginal Risk was maintained on the Day 3 Excessive Rainfall Outlook (12Z Fri-12Z Sat) for lower western portions of both the Olympics and the northern WA Cascades. ...Intermountain West to the Upper Midwest/Great Lakes... A potent mid-level trough will continue advancing eastward on Friday, through the Intermountain West into the central Plains while closing off a low center. Moisture originating from the southern Plains will wrap around a 700 mb low center early in the Day 2 period (00Z/13) with heavy precipitation impacts into eastern Montana and the northern High Plains. 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 strength and latitude of this feature is still uncertain but 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. Ensemble support was greatest for a surface low evolution nearest the 12Z NAM/00Z UKMET with the 00Z/12Z ECMWF appearing to quickly transfer the energy over to a triple point low compared to the better remaining consensus. ...Northeast... Zonal flow will push a shortwave trough east from the Great lakes Thursday morning to across Maine by Friday morning. 0.75 inch PW in a southwesterly jet with Gulf origins allows moderate intensity QPF in the quick moving system. The models showed reasonable agreement here with a general blend used to make minor adjustments from continuity. ...Lower Mississippi Valley/ArkLaTex... An amplifying trough around a closed low over the central Rockies Friday will enhance a 50kt southerly jet off the western Gulf and across east TX/OK and the lower MS Valley before shifting farther north along the east side of the developing surface low over the KS/NE border. PW of 1.75 to 2 inches is 2.5 to 3 standard deviations above normal with a heavy rain threat. The timing of the upper trough remains a little uncertain but greater convergence was seen in today's 12Z models compared to yesterday's runs. Strong capping Thursday evening across the southern Plains gives way to convection by Friday morning as 700 mb temperatures cool with the advancing upper trough. Timing of the cold front toward the east was weighted more toward the ECMWF/UKMET versus the faster 12Z NAM/GFS, however, the handling of the precipitation movement faster toward the east could be right from the NAM/GFS for the wrong reasons given an often seen bias for these larger-scale models to be too slow with convective propagation. Despite this uncertainty, the strongly dynamic system is likely to have sufficient instability and moisture to support heavy rainfall, given initial rainfall with the front near the ArkLaTex advancing eastward Friday night with the advancing cold front. A second round of heavy rain is expected to develop farther south as mid-level energy swings around the closed low early Saturday in the vicinity of the upper Texas coast propagating eastward/northeastward, resulting in a rather elongated axis of heavy rain stretching from eastern Texas into and across the lower Mississippi valley into western Tennessee. Rainfall of 3-6 inches appears likely to generate possible runoff concerns with rainfall rates of 1.5 to 2.5 in/hr and periods of training given unidirectional flow...extending from the Sabine River through the Delta of northwestern Mississippi and perhaps into western Tennessee. Northern extent of this heavy rainfall axis will be limited by reduced CAPE as seen in the latest ECMWF/GFS. The Marginal and Slight Risks on the Day 3 Excessive Rainfall Outlook were expanded to cover the range of possibilities seen in the latest 12Z model guidance regarding timing. Oravec/Otto Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml