Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 256 PM EDT Fri Mar 16 2018 Prelim Day 1 QPF Discussion Valid Mar 17/0000 UTC thru Mar 18/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 ...West of the Continental Divide... Deep low pressure will slowly nudge into the central Great Basin through the day 1 period, with spokes of vorticity pinwheeling cyclonically on all sides. One particular wave -- currently off the British Columbia coast -- will pivot south then southeast toward southern CA on Sat. This vort lobe and attendant compact upper jet streak will allow for modest (and short-lived) amplification on the southern periphery of the trough -- with more pronounced height falls toward late Sat and Sat night. As the trough buckles, while still largely suppressed, some subtropical moisture will be drawn northward as the warm conveyor belt (WCB) too becomes more meridional with time. PW values peaking around 0.50-0.60 inches is only marginally anomalous for the time of year (0.5-1.0 standard deviation above normal), however with the strengthening (albeit short-lived) 850-700 mb moist flow directed largely orthogonal to the southern Sierra and San Gabriel-San Bernardino Mtns into the Peninsular Ranges, some orographically-driven higher QPF totals above 1 inch are expected. Elsewhere throughout the West, cold air aloft and the broad scale, deep-layer cyclonic flow will support a showery regime over much of Oregon, and some semblance of a renewed frontal zone / secondary Pacific front behind the one which entered the Great Plains - will help to focus precipitation in Nevada and Idaho. WPC QPF here was derived from a blend of the 12Z HREF Mean, WRF-ARW2, our in-house ensemble summarizing the larger scale guidance, while also maintaining a degree of continuity from the previous forecast. ...Central Plains - mid-Mississippi Valley into the Ohio and Tennessee Valleys and Deep South... A compact cyclone over the central Plains will encounter a highly amplified shortwave ridge downstream, and in-turn open with time as it approaches the mid MS Valley and Ohio Valley. Thus the area of mod-heavy precipitation will shrink with time as per the model progs; however, as the wave opens, the uptick in forcing via the northern stream jet axis (right-entrance region) will favor an increase in low-level frontogenesis north of the surface boundary. As a result, expect to see a narrower (banded) structure with the higher totals -- averaging aoa 0.50 inch -- focused in a NW-SE fashion from eastern IA through central IL-IN. Farther east, warm advection and upglide on the 295-300 K surfaces will support rain arcing out through the lower Ohio Valley / Tennessee Valley and lower Mississippi Valley. A consistent signal has been for a more concentrated rainfall over parts of LA/MS, supported by a southern stream jet streak and proximity to deeper moisture seen on the evening soundings over south Texas. The SREF 6-hourly rainfall probabilities were useful, as was the previous WPC forecast which seemed to have a good handle. Hurley/Burke