Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 406 PM EST Thu Mar 01 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 02/0000 UTC thru Mar 05/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Eastern U.S... Northern and southern streams were in the process of becoming phased Thursday afternoon. Ascent was shifting toward the developing cyclone in the Ohio Valley / lower Lakes, which will hand off to vigorous coastal cyclone development tucked into the climatologically favored area off Long Island from tonight through early Saturday. As such, the connection to the CAPE axis / convective weather over the southern states will dissipate, with the exception of some smaller scale convection and a few showers in the surging frontal zone. The main precipitation story will shift up north. Already moderate rainfall was developing over the OH Valley and Great Lakes. Warm advection will contribute to the initial expansion of rainfall into the Mid-Atlantic and southern New England states overnight. This event will become more intensely focused as the coastal low develops, leading to deep frontogenesis and anomalous easterly onshore flow from the Atlantic. All these ideas fit well with the previously issued WPC forecast, which we retained in part, and blended in the 12z GFS and 12z WRF-ARW. The GEFS re-forecast data, based on analog events, suggests a swath of 3 to 4 inch areal average QPF through Friday from eastern northern New Jersey to Long Island to eastern Massachusetts. This gave us one more reason to push toward the WRF-ARW magnitudes and raise QPF somewhat. The predicted rates of perhaps 1.5 inches in 3 hours are of increasing concern, especially given expected river levels and onshore flow backing up drainages. Also concerning to see these kinds of rates in the models a good 24 hours in advance of the event. We therefore are considering an upgrade to Moderate Risk of excessive rainfall for a sliver of coast from Long Island to eastern Mass, focusing on the early Friday period. QPF across the eastern states was based on roughly a 40/30/30/ blend of the previous WPC / 12z GFS / 12z WRF-ARW. ...Western U.S... Another phased cyclone will move south along the west coast of North America through the period. Heavy rain focuses along the California coastal mountain ranges early in the period before 850 mb onshore flow relaxes...and a frontal zone pushes farther inland. Watch for mesoscale discussions detailing an flash flood risk. Days 2/3... ...West... The active pattern across the Western U.S. on Friday will continue into the Day 2/3 period as an expansive and anomalous closed low off the Northwest coast edges inland this weekend. The highest precipitation totals should be along the Sierra Nevada, where a strong surge of Pacific moisture ahead of the leading edge of height falls will enhance amounts along the windward terrain. Also, moisture focusing along a frontal boundary should result in an axis of higher totals extending from Nevada and northern Utah into Wyoming. Deterministic models remained in very good agreement with 24-hr precipitation totals, so minimal adjustments were made to WPC's QPF. The Day 2 Marginal Risk of excessive rainfall was also maintained over Southern California. Please refer to WPC's Heavy Snow Discussion (QPFHSD) for more details on winter weather. ...Northern Plains... A swath of moderate precipitation should begin to develop over the Northern Plains on Sunday as an anomalous upper low/trough over the Western U.S. begins to pivot out into the Great Plains. There is still some uncertainty with the evolution of the upper trough and an associated surface low deepening in the lee of the Rockies, so the WPC QPF forecast was based off a non-NAM consensus since it appeared to offer a good middle ground solution. The bulk of this precipitation will fall as snow, so please refer to WPC's Heavy Snow Discussion (QPFHSD) for more details. ...Northeast... Heavy precipitation from a surface cyclone rapidly deepening off the Northeast coast on Friday should begin to edge offshore Friday night as the powerful storm exits out into the Atlantic. There are still some subtle detail differences with the depth and track of the low, which are leading to some uncertainty with how quickly precipitation clears the coast and resulting precipitation totals across southern New England during the Day 2 Period (Valid 00Z Sat - 00Z Sun). The WPC QPF forecast attempted to stay in the middle of model spread and used a blend of the 12Z GFS/00Z EMCWF, which resulted in slightly higher amounts over western Massachusetts and Connecticut. For the Day 2 Excessive Rainfall Outlook (Valid 12Z Fri - 12Z Sat), the slight risk area was upgraded to a moderate risk from eastern Long Island to just south of Boston, where models are coming into better agreement that rainfall totals will exceed flash flood guidance values. Please refer to WPC's Model Diagnostic Discussion (PMDHMD) for more details on model spread and WPC's Heavy Snow Discussion (QPFHSD) for more details on the winter weather threat with this system. Burke/Gerhardt Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml