Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 613 AM EST Thu Mar 01 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 01/1200 UTC thru Mar 04/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Morning update... Made some adjustments to the northern periphery of the rainfall shield across parts of AR into the TN valley based on overnight radar trends. Satellite imagery showed warming cloud tops upstream...but kept the precipitation going based on moisture flux convergence from the RAP and HRRR. Beyond that...made few changes. ...TN Valley area eastward and southward... An ongoing heavy to excessive rainfall event should be slowly tapering off during the overnight hours as developing area of low pressure forms/deepens over the Ohio valley and begins to push a cold front south and east of the area. In doing so, the best moisture flux convergence will be focused away from the area that has been saturated multiple times over the past couple of weeks. As a result, the better precipitation amounts and rainfall rates will be gradually be directed over areas where flash flood guidance is higher. Even so, rain should still be falling there early in the day today with the potential for over an inch of rain over far southeast TN or the northeast corner of AL before drying out later in the day. Thinking was that the NAM continued to be a bit too wet...but overall the guidance was in pretty good agreement on the placement and amounts including the WRF-ARW and WRF-ARW2. This approach largely maintained continuity with the previous WPC guidance. ...OH Valley to the Northeast States... Low pressure taking shape over the OH early today will be tracking eastward...with precipitation becoming more widespread as shortwave in the northern stream begins to interact with shortwave energy moving into the TN valley from the southwest. The result will be a rather broad and complex area of low pressure by Friday morning over the eastern part of the country. Given its location, onshore flow of deep moisture off the Atlantic Ocean will be directed towards the far southern portion of New England into New York and New Jersey. The previous WPC forecast already had a precipitation max in this area and we saw little reason to make many changes in this forecast. The placement was supported by the GFS/NAM and WRF-ARW and WRF-ARW2. The amount in the WPC QPF was close to a GFS/WRF consensus value...which was similar to a toned-down version of the NAM QPF. ...Western U.S... Another phased cyclone will move south through along the west coast of North America through the period. As it does so, the system will deliver widespread heavy mountain snow and possible heavy to excessive rainfall along coastal sections of California. As the mid level circulation forms and becomes better defined, an associated frontal zone will sweep inland and a slight anomalous plume of Pacific moisture will drop down almost the entire CA coastline. Days 2/3... ...Northeast... Confidence has increased for an evolving late season winter storm that will impact much of the Northeast Friday into the upcoming weekend. Following the heavy to excessive rain event over the southern tier of the country, the remaining southern stream upper dynamics and front will sweep off the southeast coast on Thurs. Meanwhile along the northern stream, a rather active short wave with a slight neg tilt will move into the Great Lakes and OH Valley, with a surface low transitioning from the western OH Valley to Lake Erie and nwrn PA/western NY. Thus when the forecast period begins on Fri, a full phasing or interaction between the two streams will result in a sub 985mb low near the bench mark. This low will deepen rapidly, go through a full cycle of cyclogenesis and drop about 15-20 mb over the 24hr period, while spreading a wealth of Atlantic inflow into the Northeast. Perhaps a true rare Nor'easter for this winter with the combination of heavy to excessive rains along with wet snow. Compared to 24 hrs ago, the two leading global models are night and day closer on this forecast scenario with only very slight detail differences. WPC followed a blend of the two on Fri, which features 1 to 3 or 4 inch areal avg qpf amounts, especially from eastern MA into RI/CT and Long Island/NYC. 1 to 1.5 inch amounts will be possible along the extension of an inverted surface trough up through northern NJ/nern PA and the lower Hudson Valley/Catskills of NY state. This is where the likelihood of heavy snow will be possible and for more on the frozen precip, please refer to the latest qpfhsd. Finally by Sat, the complete cyclone should be far enough offshore with only residual moisture over the nern section of the country. ...West... Another phased cyclone and large closed 500mb low is beginning to impact the west with a round of heavy coastal rains and perhaps the heaviest snowfall amounts of the wet season. This large scale upper trough will linger through the weekend and on Fri, the closed low will fill some while moving just inland across OR. The associated baroclinic zone will press east through the northern Inter-Mountain West, while extending back toward central to southern CA. The gfs and ecmwf show additional upper dynamics rounding the base of the trough this day, which may allow another burst of pac moisture transport to impact the Santa Barbara to Los Angeles metro area. WPC felt the issuance for a marginal was needed, as runoff issues will be possible with perhaps even a mudslide threat. Otherwise, most of the focus will be with moisture flourishing again in the Sierra for extremely heavy mountain snow. Then on Sat, the closed low and entire upper trough will move into the interior West. Expect lighter but continued onshore flow into CA, while moisture focuses along the frontal zone from NV/UT toward WY. Overall WPC stayed very close to the reliable gfs and ecmwf for qpf amounts each day. Bann/Musher Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml