Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 541 AM EDT Mon Apr 16 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 16/1200 UTC thru Apr 19/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Mid Atlantic into the Northeast and Great Lakes... As of 08z a line of heavy showers and embedded thunderstorms stretches from southern PA into eastern NC...with this line continuing to progress off to the northeast through the day Monday. This remains a very dynamic system with the negatively tilted mid/upper level trough supporting strong upper level divergence. While mid/upper level forcing will persist through the day, would appear the overall dynamics become a bit less impressive with time...as the shortwave responsible for the increasingly negative tilt to the trough weakens along with a weakening upper jet. Also, anticipate CAPE will be harder and to come by as this area of rain moves northeast, with it taking on an increasingly stratiform nature with time. Some limited MUCAPE should however allow for continued embedded heavier convective cores within the rain shield. In general, would anticipate these factors to result in decreasing rain rates after 12z, although localized rates peaking in the 0.5"-0.75" in an hour range...and up to around 1.5" in any three hour period...will remain possible given the impressive PWATs and moisture transport in place. It appears likely that as the system occludes we actually see a focused, and slightly more persistent, 850 mb moisture transport axis across portions of southeast NY into adjacent MA/CT. This should result in a bit longer duration of the moderate rainfall rates...resulting in storm total amounts at least locally getting into the 2-3" range. These 1-3" amounts should stretch from southeast NY across southern New England and into eastern NH/ME...with the potential for a few locally heavier amounts, especially where orographic impacts may be maximized in the strong southerly flow. Will continue to carry a Marginal risk for these areas. Think rainfall rates will generally not be high enough to exceed 1 or 3 hour FFG. However, rainfall of the magnitude forecast may still be enough to cause some urban and small stream flooding concerns. Model agreement was very good with the evolution of this more convective precipitation as it moves northeast. Thus WPC mainly just blended the 0z HREF members along with the HRRR to derive QPF totals through day 1. Further northwest under the closed mid level low will continue to see precipitation into the Great Lakes region. Did increase amounts some here over our previous forecast, with the closed low continuing to be slower to exit. Again followed close to a blend of the 0z HREF members here. ...West... A trough will continue moving east into the Great Basin by Monday night. This will continue to support precipitation over much of the west, with as much as 1-3" over western WA, around 1" in the northern Rockies and up to 1" in the central Sierras. WPC QPF blended the 12z ECMWF and the 0z HREF members. Days 2/3 ...West Coast/Intermountain West... Embedded energy within an approaching long wave through will spread showers to portions of the Pacific Northwest, Great Basin and Intermountain West. Strong onshore flow in the wake of the northern shortwave will support westerly flow of 30kts to 45ks at 85-7H at or near orthogonal to the Cascade range and though moisture is fairly limited WPC forecast QPF are still in the range of 1.0" through 12z Thursday in Washington and .5" in Oregon. Areas of higher terrain will have some orographic enhancement and will likely have periods of heavy snow. Please refer to the latest suite of WPC Winter Weather products for more information. Deterministic models and the ensemble means are fairly clustered for the evolution of the West Coast trough, therefore a general model blend was preferred. ...Northern and Central Plains/Upper MS Valley... Rapid surface cyclogenesis and rapid precipitation development is forecast across Nebraska to Iowa Tuesday night into Wednesday, eventually occluding late in the day and past 00z Thursday. Gulf moisture feeding into the central low pressure will wrap precipitation into the cold sector. Snow, possibly a wintry mix, will spread across the Northern High Plains and Upper Mississippi Valley. Northeast Nebraska will likely have the highest QPF/heavy snow as this is where model consensus shows the strongest low-level moisture convergence and upper level divergences aligns. The 00Z GFS persisted in being the faster solution with this feature; however it did trend slower a bit slower for day 3. Largely a general model blend worked well for this region with minimal weighting of the operational GFS. At this time, it appears a portion of the QPF will be wintry in nature (snow/ice/mix) and likely reduce any flooding threat so no excessive rainfall area is suggested. ...Great Lakes/Northeast... During these periods, a large closed low will continue to lift northeast up the Eastern Seaboard with widespread showers and thunderstorms accompanying the front as it moves through the Mid-Atlantic and Northeast. The southerly warm conveyor belt will shift out of the CONUS late Tuesday though the vast nature of the upper low will support internal vorticity centers returning across the Great Lakes into northern NY/New England Tues into Wed increasing forcing/cooling aloft to activate some NW to SE oriented Lake Effect Belts as well as some weak comma head return moisture in N NH/ME by later in the Day Wed. Still this QPF is not substantial but please refer to WPC Winter Weather products for more information. Chenard/Campbell/Gallina Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml