Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 621 PM EDT Sun Mar 11 2018 Final Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 12/0000 UTC thru Mar 15/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1... ...Lower Ohio Valley - Tennessee Valley - Southern-Central Appalachians - Mid Atlantic - Southeast... The well-defined shortwave over the mid MS Valley will pivot across the TN Valley and eventually off the southeast coast. Meanwhile, the northern stream wave - dropping south of Hudson/St. James Bay early this afternoon - will drop across the Great Lakes and Ohio Valley toward the end of the period (00Z Tue). These features will become more in phase by Monday, and in doing so, lend support to a southern "Miller-B" cyclone evolution as the surface low across the TN Valley weakens with time upon crossing the mountains and gives way to cyclogenesis off the mid Atlantic coast. The latest WPC reflected noteworthy changes with the 12Z guidance from the previous model cycle(s) - particularly a boost in amounts across central SC-NC where the model consensus depicts a widening area of coupled southern stream/northern stream dynamics. At the same time, we have curtailed amounts a little north of the VA/NC border during Day 1 in light of the upper shortwave ridge/lower level anticyclonic flow lingering a while longer. Did not go as dry as the ECMWF (see PMDHMD). ...Georgia / Florida... As of early evening / 22z, a linear MCS associated with the leading edge of large scale height falls was marching steadily across southern Georgia and northern Florida. Steering flow will generally keep the line moving eastward. Intensity may fall off over southeast GA and northeast FL where the MCS will outrun the supply of better instability, but it is then likely to flare up again offshore to the east when the outflow finds the warmer near-surface layer over the ocean. Some activity may survive on the tail end over the Gulf as well, but with time our attention will focus primarily on south Florida, where advection of moisture / axis of greater PW located south of the Keys / and some warm advection should lead to scattered showers - some of which were already ongoing at 22z. As the vigorous upper shortwave over the southeast U.S. digs in this direction, lower to middle level flow will become gradually more and more convergent, and coverage of rain/thunder should increase toward morning, particularly from south of Tampa Bay southward, with the potential over central Florida being a bit more limited by the presence of broad low level troughing stretched across the peninsula which will deflect the Keys PW axis eastward without as much northward progress. On the large scale the models form a decent consensus aside from the very dry GFS solution. There is a lot of variance in the smaller details, but when using the large scale forcing mechanisms as a guide, WPC QPF was close to resembling the WRF-ARW2 and NAM. Days 2/3... ...The West... Multiple shortwave spokes are expected to wrap around a large closed low which will edge slowly closer to the West Coast Monday night through Wednesday. While there is large scale agreement on the progression and depth of the large scale pattern, smaller scale details, such as those individual spokes, have yet to be ironed out. A blend of the 12Z NAM, 12Z GFS and 00/12Z ECMWF was used for QPF across the West with adjustments made based on hi-res data where available. One of the shortwaves is expected to near the northern California/southern Oregon coast beyond 00Z/13 with decent model agreement outside of the farther west 12Z UKMET. A surface low is forecast to deepen at the head of an occluded front and track north near latitude 130 W, while the front bows out to the east making landfall along the northern California/southern Oregon coast Monday night. Ahead of the front, roughly 50 to 65 kt 850-700 mb flow is forecast to be present (according to the 12Z GFS), parallel to the coast up north and a little more perpendicular to the coast farther south toward San Francisco bay. Precipitable water values are expected to peak in the 1.0 to 1.1 inch range along the coast but wind and PWAT values decrease as the progressive front continues down the California coast during the day on Tuesday. 12Z NAM_nest...which may be a worst case scenario....supports hourly rates peaking in the 0.25 to 0.50+ in/hr range Monday night into Tuesday, but the progressive nature of the anomalous moisture flux should limit any widespread flooding concerns with no risk on the excessive rainfall outlook at this time. For what it's worth, the 12Z NAM_nest 24 hour QPF ending 00Z/14 shows localized 3-5 inch totals from Coastal Ranges north of North Bay into the Santa Cruz and Santa Lucia ranges, but it tends to have a high bias in the terrain and limited to zero instability should preclude these higher amounts given the progressive nature of the boundary. WPC expects areal average values for these ranges to be in the 1 to 2 inch range, locally higher. Two-day areal average liquid equivalents for the Sierra Nevada ranges are expected to peak in the 3-6 inch range supporting a combination of lower elevation heavy rain and mountain snow. A second notable shortwave spoke is forecast by the 12Z NAM/GFS to near the northern California coast on Wednesday along with a burst of heavier precipitation, but given the ECMWF/UKMET/CMC show less support for this feature a blended approach was taken at WPC. Weakly anomalous, +1 to +2 standard deviations above the mean, are expected to be transported inland beyond the Sierra Nevada into the Great Basin with strongly diffluent upper level flow and left exit region divergence setting up atop the surface cold front. This general idea continues downstream, but moisture availability and dynamics weaken into the downstream ranges of the Rockies and Wasatch, still resulting in moderate to locally heavy rain/snow for a few inland locations of the Intermountain West. ...Carolinas to New England... Two main upper waves are expected to impact the Northeast U.S. early this week with a smaller scale shortwave swinging out across the Mid-Atlantic coast while acquiring a negative tilt Monday night, while a larger scale mid-level trough reaches down to its northwest over the northern Mid-Atlantic states. Models continue to struggle with the details of a strong coastal low, which is expected to continue strengthening northward through the western Atlantic Monday night, with the latest consensus taking the surface low just southeast of the 40N/70W benchmark Tuesday morning. The 12Z ECMWF/UKMET adjusted back to a low a bit farther offshore compared to the 12Z NAM/GFS which are closer to the coast. WPC leaned in the middle of these, but with more weight toward the 12Z NAM/GFS solutions given better ensemble support for this track. The 12Z CMC also agrees more with the 12Z NAM/GFS versus the 12Z ECMWF/UKMET. Future wobbles in the low track are expected which will have major impacts on where the heaviest liquid equivalents fall, with the max of 2-3 inches currently expected over southeastern Massachusetts. The 12Z NAM appeared too heavy here, given the expected steady movement to the surface low and duration/degree of moisture wrapping back around to the west. As the surface low nears offshore New England, there is a clear model signal for a deep layered frontogenesis arcing from south-southwest to north-northeast over southeastern New England between roughly 06Z/13 and 00Z/14. Strong banding with potential convective elements appear likely, but the exact location of this heavy precipitation axis is still uncertain. A strengthening deformation axis should impact eastern and northern New England while light to moderate snow falls farther west into the interior Northeast. 850-700 mb winds of 30-40 kt inland and peaking in the 60-70 kt range along the coast will support orographic influences as winds back from southeast to east and eventually northeast. The larger scale upper low referenced in the first paragraph of this discussion will ensure plenty of cold air to support lake effect/enhanced snow showers downwind of the Great Lakes with a focus southeast of Lakes Erie and Ontario and wrap around moisture into northwestern New England. Further info on snowfall can be found in the QPFHSD discussion. Hurley/Otto Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml