Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 436 PM EST Wed Feb 28 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 01/0000 UTC thru Mar 04/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 Update for the 00z issuance: little in the way of substantive change was made from the 18z qpf. Some tweaks were made to the lower MS Valley, where radar trends and the most recent trends in the HRRR/experimental HRRR guided the tweaks. some tweaks were also made to the southern Appalachians, to account for orographic effects suggested by the latest HRRR and 12z HREF mean. ...Eastern U.S... ...High Risk of Excessive Rainfall parts of OK/TX/AR/LA/TN/MS... Southern and northern stream troughs will phase during this period along the length of the Mississippi River. The southern stream component will pull height falls across the central corridor of Gulf return moisture, driving a pronounced heavy rain and flash flooding risk from east Texas toward Tennessee. Low levels were nearly saturated and topped by dry mid levels / steep lapse rates to produce sizable CAPE which will contribute to strong convection and enhanced short term rain rates. Owing to extraordinarily wet antecedent conditions, expect a high percentage of surface runoff and associated hazards. See Excessive Rainfall Discussion and Mesoscale Precipitation Discussion for details. The phasing troughs will also lead to cyclogenesis across the Ohio Valley, wrapping in some of the Gulf return moisture, with PW values rising to around 0.75 inches as far north as the lower Great Lakes. This will yield a secondary axis of heavier QPF. Whereas areas in the southern states will see a swath of areal average 2 to 5 inches (locally greater), the northern Ohio Valley and lower Great Lakes should see average amounts 1 to 2 inches. The system may pivot around southeast lower Michigan and northeast Ohio as it assumes a negative tilt, meaning somewhat heavier precipitation may be found there. WPC QPF was influenced by our in-house ensemble, given an increasingly strong consensus, along with some hi-res influence, the WRF-ARW and WRF-ARW2. This approach kept good continuity while increasing QPF over southern Arkansas where the greatest overlap of CAPE and low level moisture transport is expected, as well as increasing it over the lower Lakes area where models had come into better agreement as to the strength and placement of the developing cyclone. ...Maine... A clipper type system seen over Quebec at midday will spread snow over primarily the state of Maine tonight and early Thursday, affecting especially northern Maine. This well defined wave should be well handled in the models, and the coverage of half inch liquid amounts has increased via a QPF consensus approach. ...Western U.S... Days 2/3... ...East to Northeast... Confidence has increased for an evolving winter storm that will impact much of the Northeast and coastal areas Friday and Saturday. A negatively tilted shortwave will pass from the Great Lakes region/Ohio Valley into the Northeast and will garner strong mid- and upper-level support along with low-level moisture wrapping into low during this period. Perhaps a rare Nor'easter for this winter with the combination of heavy rain and a marginal/slight threat for excessive amounts along with wet snow. The latest model guidance still had a fair amount of spread with the placement and timing of the surface low. The 12Z GFS was further North/Northwest of the cluster with the 00Z ECWMF appearing as an outlier to the south. The best representation for a compromise was the 12Z Nam with a position roughly between the GFS and ECWMF. The 12Z ECWMF trended a little north of the previous run, situated near the position of the NAM. Therefore for a weighting of 50% 12Z NAM and 25% 12Z GFS/ECWMF was incorpated for the QPF. This new blend result in a slight shift in the axis of higher QPF, spanning from western New York to Long Island, and a significant increase in the totals across southeast New York, New Jersery and Connecticut. With current conditions already close to exceeding flash flood guidance and another 2 to 4+ inches forecast, a slight risk for excessive rainfall was introduced for both days 2 and 3 for Long Island, coastal Connecticut, Rhode Island and eastern Massachuetts. Prior to the dynamic cooling expected with this system over the Northeast, pockets of heavy rain will be possible over the beginning of day 2 for the Great Lakes and northern OH Valley. Areal averages of 1 to 1.75+ inches over lower Michigan, northern Ohio and western Pennsylvania warrented the issuance of a marginal risk. ...West... ...SLIGHT RISK for excessive rainfall on Thurs (12Z Thu-12Z Fri) for the CA coast from the Southern Coast Ranges to the Transverse Ranges... Another phased cyclone will move south through the wesetern states and deliver widespread heavy mountain snow and possible heavy to excessive rainfall along coastal sections of CA. Multiple shortwaves over the northern Pacific and near the British Columbia coast will phase on Wed. A large closed 500mb circulation will form and dig south on Thursday, the associated frontal zone will sweep inland and a slight anomalous plume of Pacific moisture will drop down almost the entire CA coastline. Thus a slight threat for excessive rain will be possible and kept in the forecast from central to southern coast/coastal ranges and in some burn scar areas. Thus runoff issues will be possible with associated mudslide threat. Otherwise, most of the moisture will flourish in the Sierra with ideal for orographics and extremely heavy mountain snow. Then on Friday, the closed low will fill while moving inland over OR and the baroclinic zone will press east into the Inter-Mountain West into southern CA. Overall WPC stayed very close to the reliable gfs and ecmwf for qpf amounts each day. Burke/Campbell/Musher Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml