Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 552 AM EDT Wed Apr 18 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Apr 18/1200 UTC thru Apr 21/1200 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...Northern to Central Plains across the MS Valley and Great Lakes and into the Northeast... A compact and well defined shortwave is crossing the Plains early this morning. Overall a pretty dynamic system, with a strong upper level divergence signature noted along with some elevated instability feeding into the east side of the storm. Limited moisture and quick storm motion will end up limiting total QPF amounts, however the strong dynamics will overcome this to some degree through this morning, with a swath of moderate to locally heavy rain/snow from portions of eastern NE into far southeast SD, and portions of IA. The system begins to lose some of its dynamics by this afternoon as the wave opens and divergence decreases in the upper levels. Thus generally expect to see decreasing intensity of precipitation as things push into the Great Lakes. Still will however see a swath of mainly light QPF spread east with time as the wave begins to interact with a northern stream shortwave by the end of the day 1 period. Still some model differences noted with the exact axis of higher QPF totals today into tonight across this region. Overall there continues to be a slight southward shift in the expected axis, and thus WPC trended our QPF in this direction as well. Thought going for a model consensus approach would probably be the best option to account for the various north/south differences in the guidance and differences with the northern gradient. Going for a consensus approach resulted in WPC QPF looking pretty close to the 0z HREF mean. Trends and the overall consensus suggests the 0z GFS is probably a bit too far north over the MS Valley. ...West... A trough will move into the West today with a strong closed mid level low moving into CA by tonight. Overall a pretty dynamic system, but moisture will be limited. Thus in general only looking at light QPF amounts across portions of OR/CA/NV. WPC QPF blended the GFS/ECMWF and HREF mean. Days 2/3 ...California to Desert Southwest... An upper-level trough with a closing low off the northern CA coast Wednesday evening will deepen and shift southeast through the Great Basin the turn east over the Four Corners by Friday evening. Inflow from northern Mexico into this system will limit moisture with PW around 0.25 inch. Light shower activity is expected over CA/NV/UT/AZ Wednesday night through Thursday night with max QPF around a quarter inch for preferred higher elevations in the cyclonic flow. The slower 12Z ECMWF was preferred in WPC QPF with some 12Z NAM and a little 12Z GFS (which seems too fast given the closed low) included. ...South-Central Rockies and South-Central High Plains... Low-level moisture, tapping from both the Pacific and the Gulf of Mexico, will transport northward and wrap into the system moving across the Southwest. Model consensus shows PW values of 0.50 to 1.00 inch advecting northward along the Southern Rockies and into the adjacent Plains, where weak CAPE will be present. Higher elevation snow will be limited for the region. The model blend of choice was that of the 00Z ECWMF/GFS with hints of the NAM. ...Lower Rio Grande Valley... Showers and thunderstorms will develop of over the Lower Rio Grande Valley where moisture convergence sets up along the Mexican Plateau. Flow from the Gulf will pool along the terrain, aiding in persistent convection through Friday. Little precipitation is expected in south TX from this system due to lack of lift. ...Great Lakes to the Northeast/Northern Mid-Atlantic... A quick moving low pushes through the mid-CONUS ridge Wednesday which weakens the system over the upper Midwest until it merges with a lobe of the exiting low over the eastern Great Lakes Thursday and redevelops over the northeast through Friday. Expect diminishing QPF rates Thursday evening as the low moves from WI to MI then increasing to moderate in places over NY state and New England Thursday into Thursday evening as 0.75 inch PW air flows into the system. Some lake effect enhancement is expected for the favored areas downwind of Lakes Erie and Ontario with the wrap around flow. A general model blend will work for this sector, as timing and placement was fairly clustered. Chenard/Campbell/Jackson Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml