Quantitative Precipitation Forecast Discussion NWS Weather Prediction Center College Park MD 500 PM EDT Sat Mar 17 2018 Prelim Day 1, Day 2 and Day 3 QPF Discussion Valid Mar 18/0000 UTC thru Mar 21/0000 UTC Reference AWIPS Graphics under...Precip Accum - 24hr Day 1 Southern Appalachians into the southern Mid-Atlantic The relatively small closed low initially over the upper MS valley region expected to continue to weaken day 1 as it presses east southeastward from the upper OH valley into the central Appalachians and southern Mid Atlantic tonight into early Sunday. The model consensus is for precipitation ahead of this closed low to refocus across the southern Appalachians this evening and move fairly quickly east southeast across the southern Mid Atlantic tonight. The quick movement likely to be a detriment to any widespread heavy totals with model consensus for areal average moderate to possibly isolated heavy amounts across these areas. Southern Plains---lower MS Valley---central Gulf coast Height falls in the southern stream flow expected to push east northeastward out of northern Mexico into the southern Plains this evening---eastward toward the lower MS valley by early Sunday. There is a fairly consistent signal in the hi res guidance for convection to develop late this afternoon over portions of north central TX and move fairly quickly downstream. Despite the expected fast movement---intense rainfall rates over a short period will support locally heavy to isolated excessive rainfall amounts---especially in the first 6 to 9 hours of the upcoming day 1 time period---with rainfall intensities and amounts expected to decrease after this as the initial activity moves toward the lower MS valley/central Gulf coast region. The marginal risk area on the latest excessive rainfall potential outlook over portions of northeast TX was not changed with runoff issues possible in the most intense heavy rain cores---especially if they occur over any urban areas in the vicinity of Dallas to Waco. Late Sunday morning into Sunday afternoon---another round of precip expected to break out over portions of the Southern Plains in an increasingly diffluent pattern ahead of additional southern stream height falls moving northeastward from northern Mexico and stronger upstream height pushing east from the southwest into the southern high plains. This next area of precip looks like it will form farther to the north than the lead area---from north TX into eastern OK Sunday afternoon---pushing into the lower AR river valley toward the end of the day 1 period---with model consensus for .25-50" amounts. Western U.S. The broad mean upper trof initially over the large portions of the western U.S. will become increasingly separated into distinct circulations day 1. The northern portion of this trof will be comprised of three fairly slowly moving circulations. One moving very slowly southward off the Pacific Northwest coast---while one over the northern Rockies and another over southern Alberta/southern Sask combine near the MT/Alberta/Sask border region. Meanwhile---stronger and faster moving height falls will eject out of the base of the trof tonight---pushing from southern California---through the Southwest and emerging into the southern high plains Sunday afternoon. Overall---there is good model agreement on what will be a large region of precip associated with each circulation center. Model consensus is for areal average moderate totals---with locally heavy amounts possible in the favored terrain regions from the Mogollon Rim of AZ---the central Rockies of CO---through the Wasatch of UT and into the northern Rockies of north central ID into western MT. Days 2/3... ...Northern/Central Rockies into the Northern/Central Great Plains... An upper low sweeping out of the central Rockies will remain a focus for active precipitation over the center of the country extending up into the northern Plains. The primary area of interest will likely be northeastern Colorado given a favorable low-level moist, upslope flow on Sunday evening into the overnight hours. Eventually by Monday morning the low-level flow will turn more parallel to the Rocky Mountain chain which should considerably decrease the coverage of precipitation over the region. During the 19/0000Z to 19/0600Z window, some solutions show anywhere from 0.50 to 0.75 inches of liquid which should mostly be in the form of snow. Additionally, a north/south convergence zone extending across the Northern Plains will also be a focus for active precipitation. This area may persist slightly longer given weak upstream height falls aiding in further lift on Monday. Farther to the south, a quick-moving, locally intense band of convection may spawn in close proximity to the deep surface low pushing along the Kansas/Oklahoma border late Sunday into Monday. The progressive nature of this wave should limit a flash flood threat but cannot rule out brief local issues given the extent of deep convection near the cyclone. As a whole, favored a combination of the 12Z NAM/GFS with the 00Z ECMWF back toward the terrain while utilizing a blend of the 12Z/06Z GFS with the 00Z ECMWF for the warm sector activity. ...Lower/Middle MS Valleys and TN/OH Valleys into southern states and much of the Eastern Seaboard... As the mentioned upper low slides eastward toward the Ozarks, the system should evolve into more an open wave and accelerate toward the lower Ohio Valley on Monday evening. Models vary with the intensity of this feature with the 12Z NAM being more robust which supported a heavier swath of precipitation streaking through the mid-Atlantic on Tuesday. Ensemble low plots support a wave emerging off the southeastern Virginia coast on Tuesday morning with quick progression toward the northeast thereafter. While progressive in nature, solutions favor rather potent omega fields supporting a quick burst of precipitation tracking from the lower Ohio/upper Tennessee valleys into the central Appalachians and mid-Atlantic region. Please check out the heavy snow discussion (QPFHSD) for more information on the wintry aspects of this system. At this point, felt the 12Z NAM was displaced too far to the north while favoring a consensus formed by the 12Z/06Z GFS and 00Z ECMWF given building model agreement. However, confidence remains only average given how much the model runs have fluctuated in recent cycles. Within the warm sector, active convection should continue from the Ozarks eastward into the mid-South and southeastern states. Much of the activity will focus along the advancing cold front with additional thunderstorms likely within the downstream warm sector. Further, an amplifying trough sweeping through in the wake should provide a further impetus for convection over the region. While the guidance is scattered a bit with QPF solutions, some of the high-resolution guidance shows heavier swaths, particularly the 12Z HRW-ARW with higher amounts from central Mississippi eastward into southern Georgia. ...West Coast... A powerful mid/upper-level low out of the central Pacific will be quite anomalous in nature with 500-mb height departures on the order of 3 to 4.5 sigma below climatology. Steady mid-level southwesterly flow will push through the state of California accompanied by an impressive subtropical moisture connection. An atmospheric river should unfold with 1.50 inch and above precipitable water values streaming toward the southern/central California coast on Tuesday evening. While the best vertical motions lurk just offshore by 21/0000Z, some quicker solutions would bring this heavier slug of precipitation inland late on Day 3. Although the 12Z NAM solution was not recommended, it did show 1 to 1.50 inch 24-hour amounts reaching the San Francisco Bay Area by Tuesday afternoon. However, per the Model Diagnostics discussion, stuck closer to a non-12Z NAM consensus while leaning more closely on the 12Z GFS/00Z ECMWF output. Based on the QPF from the medium range, some profilic amounts appear primed to reach much of California by mid-week. Oravec/Rubin-Oster Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml