Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Thu Mar 01 2018 Prelim Day 1 QPF Discussion Valid Mar 01/1200 UTC thru Mar 02/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...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. We maintained the previously issued slight risk of excessive precipitation area, with amounts maximized where the onshore flow intersects the axis of mountain ranges from central to southern coast/coastal ranges. The concern about run off, mudslides and debris flows remains in areas with recent burn scars. Otherwise, most of the moisture will flourish in the Sierra with ideal for orographics and extremely heavy mountain snow right through early Friday. Refer to the WPC winter weather desk for details about snowfall amounts here. Bann