Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 337 AM EST Mon Mar 05 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 05/1200 UTC through Mar 08/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Northern Plains... An eastward moving upper trough has led to cyclogenesis with the surface system deepening Monday with the development of a closed low aloft. Moisture increases with time as precipitable water values rise in excess to 2.5+ sigmas above the mean for early March near and ahead of the low. Precipitation should increasingly expand/organize within areas of low- to mid-level frontogenesis, including the cyclone's comma head. See WPC winter weather desk products for more information on the winter weather impact from this system. A slight westward trend continues in the model guidance this system, which led to some westward shift in the QPF pattern when compared to WPC continuity. Slight increases in liquid equivalent were seen in the latest guidance so made modest increases in amounts as well. Overall, the preferred QPF pattern best resembles a compromise of the 04z National Blend of Models and the 00z UKMET. ...Gulf Coast/Deep South... A slow-moving frontal zone erodes the cap near the western Gulf coast, with 700 hPa temperatures falling below the 6C threshold with time. The combination of high moisture (precipitable water values of 1.5-1.75"), 0-6 km bulk shear of 30-45 kts, sufficient 850 hPa inflow, MU CAPE values up to 2000 J/kg, and propagation vectors roughly parallel to the boundary across the upper TX coast and southwest LA led to an increase in amounts in this area. The deep layer flow becomes increasingly west-southwesterly with time. Hourly rain totals could reach 1.75" were cells train/merge. The 1000-500 hPa thickness pattern suggests convection will try to drift east with time, which should keep totals from becoming too extreme. The QPF pattern resembles a compromise of the 00z GFS/12z ECMWF or a southwesterly shifted version of the 00z HREF mean. This was based on 00z GFS mass fields and its expected instability field. Did not go towards the high end of the guidance across MS, AL, and GA as instability/MU CAPE values Monday night looks low north of the Gulf coast, and 850-650 hPa frontogenesis appears weak across the Southeast. Elsewhere, no hydrologically significant precipitation is expected. Days 2/3... ...North Carolina to the Mid-Atlantic and Northeast... The upper low over the Great Lakes persists with upper level jet energy crossing the central Appalachians and then the mid Atlantic coast, spawning low level cyclogenesis. The quickly developing low results in a growing shield of precipitation across the mid Atlantic states Wed and into eastern New York/New England Wed night-Thu. The models are in good agreement on the cyclone development with a gradual track spread as the UKMET/ECMWF/Canadian global were a bit east of the 18-00z Nam/GFS and respective SREF mean and GEFS mean. The coastal low is expected to develop a well defined deformation zone underneath the upper divergence maxima as it lifts up the Mid-Atlantic coast, with moderate to heavy precipitation spreading north from NJ into New York and New England. Given the excellent clustering of the 18z GEFS mean/21z SREF mean/18z nam/18-00z GFS cyclone track, the most weighting was given to these solutions. Less weighting was given to those solutions with a further east track, like the 00z ECMWF and UKMET (would prefer to see a 50N 50W low persist to force a more offshore solution). Please see the QPFHSD and snow graphics online for potential snow amounts and axis/orientation of the areas of heavy snow. ...Great Lakes... The central US cyclone drifting from the MS Valley into the northern Oh Valley and Great Lakes undergoes cyclolysis. Thus causes decreasing precip rates Tuesday, with the models coming into better agreement on light snow precip across the Great Lakes with local bands of lake enhancement in MI and on the other side of Lake MI in WI. Please refer to WPC's Heavy Snow Discussion (QPFHSD) and graphics for more details on the resultant snow amounts. ...South Carolina to the eastern Gulf Coast and Florida... The cold front south of the developing mid Atlantic low moves east across the southeast Tuesday and then Florida Wednesday. Precipitable Water (PW) around 1.50 inches on the south side of this front is two standard deviations above normal which should allow a narrow swath of heavier showers in the FL panhandle Tuesday, and then down into west central Fl along the inflow/PW maxima. Manual QPF used a multi-model blend with continuity, with slightly more weighting to the 18-00z GFS as a result. ...Northern Ca/Pacific northwest... The large cyclone in the eastern Pacific drifts east and approaches close enough that a band of enhanced moisture and lift nudges onshore into northwest CA and western OR Wed night to 12z Thu. General tenth to quarter inch amounts are forecast each 6 hour period by the NAM/GFS/12z UKMET/ECMWF/ECMWF Ensemble Mean, so a blend of these models was used. The 00z UKMET was a few hours slower, but not far from the aforementioned well defined cluster of models. Roth/Petersen Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml