Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 400 AM EST Tue Mar 06 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 06/1200 UTC through Mar 09/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Northern Plains/Great Lakes/East/Gulf coast... An occluded cyclone and its attendant frontal boundaries are expected to spread precipitation, some of it wintry, across the region. The system should increasing amounts of precipitation across the Mid-Atlantic States due to strengthening 650-850 hPa frontogenesis across the region. In the wake of the system near the Great Lakes, areas of the Upper Peninsula of Michigan should see lake effect/enhanced precipitation as cyclonic flow strengthens and slight cooling occurs at 850 hPa. In general, the guidance has trended slower across the Southeast when compared to continuity, where the heaviest precipitation is expected due to 1.50"+ precipitable water values, instability near and offshore the coast, and 30+ kt inflow at 850 hPa. To account for the above, used a 30/15/15/20/20 blend of WPC continuity/the 02z National Blend of Models/00z NAM/12z ECMWF/00z ARW with some 12z NSSL WRF thrown in early. A little bit of the 00z UKMET/00z ECMWF was thrown in manually to further slow the evacuation of rainfall from the Southeast. Per coordination with our Winter Weather Desk, went much more towards the National Blend of Model output east of the Keeweenaw Peninsula of Michigan where the best lake effect is anticipated. The contributions of the ARW and NSSL WRF were to better account for the Gulf coast convection since they had a better handle on it than the other guidance, particularly the 00z GFS, whose mass fields indicate that the heavy rainfall in the FL panhandle should fall closer to the Gulf Coast than its precipitation pattern suggests. See the WPC Winter Weather suite of products for more details on winter weather impacts. Days 2/3... ...Pacific Northwest/Northern Rockies... A closed mid level low approaches the Pacific Northwest coast early on day 2 and gradually eases from the Pacific Northwest into the Northern Rockies during day 3. A Pacific moisture plume ahead of the mid level system, focused on a warm front, feeds the development of locally heavy rainfall, especially on day 3. There was generally good model agreement with the placement of the synoptic scale features, so the WPC QPF was based on a blend of the 00z ECMWF/GFS. QPF amounts are expected to be modest, ranging between 0.10 and 0.25 inches on day 2. On day 3, 40 to 50 knot low level flow ahead of the system will transport 0.50 inch precipitable water air across the Pacific Northwest from WA into northern CA and parts of the inter-mountain region. Given marginal instability still being shown by the model forecast soundings, saw little reason to make too many changes to the previous WPC forecast which accounted for a period of briefly enhanced precipitation rates and QPF amounts generally under an inch. Further east, as the mid level system crosses the Northern Rockies late on day 3, the combination of synoptic scale lift and upslope flow make the moisture of the paltry moisture in the column to produce local 0.35 inch qpf amounts over the higher peaks in southern and central ID. ...Great Lakes/Ohio Valley... There should be a closed mid level system over the Great Lakes...with two pieces of energy rotating around the periphery of the low. One piece of energy will head eastward while a second piece of shortwave energy drops south an reinforces the closed low. Lift and steepening mid level lapse rates associated with the mid level system will make the most of the moisture in the column to produce a broad area of 0.10 to 0.25 inches of qpf extending from eastern Michigan and into West Virginia with locally higher amounts on day 2. Another 0.10 to 0.25 inches are possible on day 3 close to the track of the mid level system and across the higher terrain of WV (where upslope flow becomes a factor). ...Mid Atlantic/Northeast... As a negatively tilted long wave trough tracks from the Great Lakes/Ohio Valley, a closed mid level low forms over NJ. Lift associated with the mid level systems induces surface cyclogenesis off the mid Atlantic coast, and the cyclone tracks north along the coast with the mid level system during day 3. The models continued to show signs of converging on a common solution, although the UKMET jumped from the eastern side of the model solutions on the 05/00Z run to being one of the western most models on the 06/00Z run tonight. We continued to favor a solution on the western side of the guidance, but given little run to run consistency we gave the UKMET little consideration. The ECMWF/GFS was used as a starting point and nudged a bit in deference to ensemble means. Day 2... Ahead of the developing mid level and surface systems, a 40/50 knot low level easterly flow transports 0.50/0.75 inch precipitable water across the northern Mid Atlantic states into southern New England, with the best mid level lifted centered on 07/18z. By the end of the period at 08/12Z, the surface low should be making its way northeast along the coast of ME, which will result in a fetch of moisture converging along the coast. The models tended to jump the placement of their 6 hourly maximum precipitation at this point. Given that the models tended to be fairly consistent with the placement of their boundary layer surface moisture flux convergence maxima, WPC tended to key on that to aid in the placement of the maximum. Day 3... The system will continue to depart on day 3 with enough upper level support and moisture transport into the region to support another half inch of precipitation or so in northern ME before the system pulls away. Heavy snowfall is possible on days 2 and 3, especially over interior portions of New England. Please refer to the most recent suite of WPC Winter Weather products for more information. ...South Florida... A cold front making its way south extending from the Southeast U.S. across the Florida peninsula will be the focus for scattered showers and thunderstorms. The deepest moisture and strongest dynamics should have moved offshore by the beginning of the day 2 period. Even so, some locally heavy rainfall is possible before the front moves far enough south and east for rain to end. Roth/Bann Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml