Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 425 PM EST Wed Mar 07 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 08/0000 UTC thru Mar 11/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... For the 00z QPF issuance: the only changes made were to increase amounts across portions of southeast MA, based on radar trends, as well as the trends in the HRRR. ...Great Lakes - Northeast... "Miller B" type cyclogenesis is well underway early this afternoon, as the decaying Great Lakes surface low continues to give way to the rapidly intensifying low off the mid Atlantic coast (just east of the DELMARVA at 1800 UTC). The deep upper low across the Great Lakes and associated negatively tilted trough axis pivoting southeast-east will provide robust deep-layer Qs vector forcing -- i.e. strengthening DPVA and low-mid layer waa/theta-e advection leading to steepening easterly moist isentropic ascent (strong 850 mb negative U-wind or easterly flow anomalies). The new day 1 WPCQPF comprised a multi-model blend of the 12Z guidance -- including the GFS and ECMWF as well as the high-res WRF-ARW, ARW2, HREF mean, and the NBM. The variance between all aforementioned solutions is as low as it has been thus far for this event -- not surprising given the timing (now within 24-30 hours). Highest additional totals after 00Z Thursday (1-1.5") will be across coastal New England from eastern MA though eastern ME. Please refer to the latest QPFHSD for further details regarding the heavy snowfall potential with this system. ...Northern CA/Northwest... Onshore flow with precipitable water values of 0.5-0.75" near and ahead of a frontal boundary is expected to lead to moderate-heavy totals across northern CA into the coastal ranges in OR and the coastal-Cascade ranges in WA (max areal-average totals of 1-1.5+ inches). Used a blend of model guidance and WPC continuity to come up with the QPF in this region, which ended up best resembling the 12Z GFS, ECMWF, HREF mean, and NBM. Days 2/3... ...West Coast into the Upper Mississippi Valley... A short wave over parts of the Pacific Northwest into British Columbia will move southeastward to the Northern/Central Plains by Saturday evening. As the short wave moves out of the Northwest late Friday, upper-level ridging develops over the Northwest Friday evening into Saturday evening. Onshore flow streams moisture into the Northwest with precipitable water values of 0.50 inches that will come to an end as the low-level flow becomes parallel to the coast overnight Friday as the upper-level ridging builds. Onshore flow will continue into California through Saturday morning as the flow, likewise, becomes parallel to the coast. The moisture flowing into California will have precipitable water values ranging from 0.75 to 1.00 inches through Friday morning dropping below 1.00 inches on Friday afternoon into Saturday afternoon before beginning to climb over 1.00 inches by Saturday evening ahead of the next short wave. The system will produce maximum qpf amounts ranging from 1.25 to 1.50 inches over the Cascades on Thursday evening into Friday evening. A secondary maximum will develop over parts of the Northern Rockies with maximum qpf amounts ranging from 1.00 to 1.25 inches. As the moisture decreases and the energy moves inland much lighter qpf will develop inland over the Northern Rockies into the Upper Mississippi Valley with the maximum qpf amounts ranging from 0.25 to 0.30 inches over parts of the Northern Rockies on Friday evening into Saturday evening. In addition, the qpf amounts over California will be very modest with maximum qpf amounts less than 0.25 inches over Northern California on Friday evening into Saturday evening, too. WPC used a blend of the GFS and the ECMWF as a starting point for the manual graphics. ...Great Lakes into the Central Appalachians into the Northeast... A deep closed upper-level low over parts of the Northeast will move northeastward into the Canadian Maritimes by Saturday evening. Easterly low-level flow over Northern New England will become northerly and offshore by early Saturday morning. As the storm wind down the maximum qpf amounts will range from 0.50 to 0.60 inches over Northern New England with a secondary maximum over parts of the Lower Great Lakes and Central Appalachians with maximum qpf amounts ranging from 0.25 to 0.40 inches on Thursday evening into Friday evening. On Friday evening into Saturday evening, the qpf amounts continue to lessen with the maximum qpf amount generally less than 0.25 inches over parts of the Northeast/Lower Great Lakes. WPC used a blend of the GFS and the ECMWF as a starting point for the manual guidance. ...Central Plains/Western Gulf Coast into parts of the Middle/Lower Mississippi Valley and the Tennessee Valley... A baroclinic zone will move into the Southern Plains early Saturday morning moving to the Lower Mississippi Valley by Saturday evening. Moisture will begin to move northward from the Western Gulf of Mexico on Thursday evening that will move northeastward into the Lower/Middle Mississippi Valley by Saturday evening. The precipitable water values will increase from 1.00 inches over the Western Gulf Coast on Thursday evening to 1.25 inches over the Lower Mississippi Valley by Saturday. Maximum qpf amounts will range from 0.75 to 1.00 inches over Lower Mississippi Valley on Friday evening into Saturday evening. Modest amounts of qpf will develop over parts of the Western Gulf Coast on Thursday evening into Friday evening with amounts less than 0.25 inches. WPC used a net blend of the GFS and the ECMWF for the starting point for the manual qpf. The GFS was a lot heavier with the qpf and in a location farther west than the ECMWF. There was no consensus on the location and amount of the qpf with this system with the global models. The 12 Z ECMWF did come in closer to the GFS with the placement of the qpf, therefor the net blend of the GFS and ECMWF. Higher qpf amounts are possible with this system. Hurley/Ziegenfelder Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml