Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 509 AM EDT Sun Apr 01 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 01/1200 UTC thru Apr 04/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 Central Plains...Mid MS Valley...OH Valley...lower TN Valley...central Appalachians into the Mid Atlantic Short wave energy rotating through the base of the very broad mean trof across the central portion of the nation will support an increasing area of overrunning precipitation from portions of the Central Plains...eastward into the Mid MS Valley...OH Valley...Lower TN Valley...central Appalachians into the Mid Atlantic day 1. These height falls will be moving very quickly eastward this period with a corresponding quick eastward movement to the broad area of moderate precipitation expected across these areas. Late season accumulating snows possible on the northern edge of this precipitation shield from southeast NE...far northeast KS...northern MO...southern IL...southern IN...southern OH...northern WV...northwest VA...western to northern MD...southern PA into central NJ. See the latest QPFHSD for additional winter weather information across these regions. Pacific Northwest into the Northern Rockies Models are in good agreement day 1 with the strong closed low dropping southeastward across southwest B.C....northern Washington State into northern ID. There is also good agreement with precipitation distributions and amounts from the combination of large scale lift along the associated inland moving front and then post frontal onshore cyclonic flow across these areas. This will support widespread moderate to locally heavy precip totals likely in the favored terrain regions of the northern Rockies from northwest WY into central to northern ID...northwest MT...northeast OR and through the Washington-Oregon Cascades and along the northern Oregon coast range into the Olympic range. Overall...confidence is high across these regions. Days 2 & 3 ...Pacific Northwest... As a potent shortwave shifts into Wyoming by Monday night, onshore flow along the Coastal Ranges of the Pacific Northwest will weaken. This, in turn, results in decreasing coverage of precipitation with additional amounts being light. After a brief respite, another plume of moisture is forecast to approach the PAC NW coast late Tuesday. The onset of rainfall with this plume remains somewhat uncertain, but precipitation should be underway by Tuesday evening. The 00Z runs of the GFS/UKMET remained more progressive than the 00Z ECMWF. ...Northern and Central Rockies Eastward Across the Plains to the Great Lakes... The system which moves into Wyoming on Monday night will turn east and reach the upper Midwest by Tuesday. As it does, a broad area of low pressure to the lee of the Northern and Central Rockies will become better organized overnight Monday night/Tuesday morning...with a deepening surface low pressure center making its way from the central Plains to the Great Lakes during the day on Tuesday. Precipitation will become increasingly widespread north of the surface low...with some models pointing to banding of precipitation. The problem is that the model agreement on many of the important details is poor which results in a wide range of solutions both in terms of the track and associated precipitation and did little to help refine the details of a significant late season snowfall event on day 3. The 00Z NAM looks to be too far north and weak with its precipitation field while the ECMWF remained weak and slow. The WPC precipitation forecast tried to take advantage of similarity in upper level fields such as H25 winds and low level moisture axis. This gave a solution which was closest to the 01/00Z run of the GFS from late day 2 into day 3. Confidence in the placement of precipitation or in the amount of precipitation was lowest from eastern Montana into the Dakotas and even into parts of Minnesota on day 2. By the time the system reaches the Great Lakes, the models begin to agree on the idea of deepening low pressure but arrive at the idea at different forecast hours. The uncertainty is greater in the Day 2 forecast than it is by the end of the Day 3 forecast. ...TN and OH Valleys to Gulf Coast Eastward to the Mid-Atlantic/Northeast... A cold front associated with the developing surface low pressure system over the Great Lakes region will be the focus for convection from the Gulf Coast northward into portions of the Tennessee and Ohio Valleys on Tuesday. Given the depth of the system and the magnitude of the forcing, we tended to put more weight on the stronger/faster solutions. Confluent southerly flow at H85 coming northward from the Gulf ahead of the front will result in precipitable water values to increase to roughly 1.5 inches along the Gulf and into the range from 1.25 inches to 1.50 inches into the Tennessee Valley by late Tuesday afternoon. Thinking is that this will be able to support some heavy rainfall rates in an area that has experienced multiple rounds of heavy rainfall over the past couple of months. Based on coordination with offices in the area, we opted to hoist a marginal risk of excessive rainfall in this area rather than a slight risk. An upgrade is possible in later forecasts if the signal for rainfall amounts, intensity or placement becomes better defined. Oravec/Bann Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml