Extended Forecast Discussion NWS Weather Prediction Center College Park MD 216 AM EST Sat Feb 17 2024 Valid 12Z Tue Feb 20 2024 - 12Z Sat Feb 24 2024 ...Atmospheric river will persist but start winding down in California Tuesday-Wednesday... ...Overview... An atmospheric river taking aim at California will be ongoing at the start of the medium range period Tuesday and could cause some heavy rainfall with flooding potential and heavy mountain snow to linger, but should be weakening compared to Sunday-Monday. Shortwave energy pushing eastward into the west looks to combine with amplifying northern stream energy and help form a low pressure/frontal system in the central U.S. that pushes into the eastern U.S. by the latter part of next week, spreading rain and northern tier snow to some areas as a larger scale upper trough settles over the East. Ahead of these features, temperatures are forecast to be above average in the central U.S. early-mid next week and shifting into the eastern U.S. mid-late next week. Meanwhile latest trends are suggesting that another upper low dropping into the eastern Pacific by late next week will stay offshore through the period, with at least a weak upper ridge building over parts of the West. ...Guidance/Predictability Assessment... Model guidance has come into better agreement over the past day with the surface low and associated upper trough atop the eastern Pacific to West Coast Tuesday-Wednesday. Models have trended toward a northern position with the surface low and indicate it weakening as it reaches the Pacific Northwest Wednesday. Most 12/18Z deterministic guidance was within reason and able to be used for these features, with the exception of the 12Z UKMET which held the upper and surface low farther southwest. The QPF of the ECMWF runs farther south across southern California with the AR/front have been questionable though. The timing of the ejecting West Coast trough and details of exactly how Canadian troughing amplifies into the eastern half of the lower 48 will affect the surface evolution over East by the latter half of the week. Fortunately most models show fairly good agreement with a surface low consolidating atop Oklahoma or so early Thursday, but unfortunately model spread increases as the low tracks quickly east by Friday. The 12/18Z suite showed some spread with the surface low's strength and position but all had a general theme of the low being atop the Mid-Atlantic, fitting well with the ensemble means. 00Z models have actually come in with increasing spread in the low position with the ECMWF showing a much slower trend and the GFS faster and to the north. A northward turn of the low is likely by Saturday but the differences in placement and strength continue and will certainly have sensible weather effects, including potential snow in the Northeast. After the western shortwave departs, some model runs have been signaling potential for lingering energy from the northern part of the feature and/or a shortwave dropping through western Canada to settle over or near the Northwest. The 12Z ECMWF brings its feature farther south than its previous runs or other guidance. The EC-based AI models did not show this evolution, which gave more confidence to lean away from the deterministic ECMWF solution. The new 00Z ECMWF seems in more reasonable alignment. Regarding another upper low over the eastern Pacific, guidance had been trending westward with its position and the 12Z ECMWF seemed exceptionally far west. Meanwhile the 12Z CMC was farther east than the best consensus. The most recent 00Z guidance has shown a bit of an eastward jog, so this waffling makes the positioning of the upper low uncertain. Thus the WPC forecast utilized the 18Z GFS and 12Z ECMWF/CMC/UKMET at the start of the period, but eliminated the UKMET by Day 5 and reduced the proportions of all deterministic models, more quickly with the CMC and ECMWF, to favor the EC/GEFS ensemble means. The ensemble mean proportion was about half by Day 5 but increased rapidly Days 6-7 given the issues with the deterministic runs in various locations. ...Weather/Hazards Highlights... An atmospheric river with ample subtropical Pacific moisture will cause rounds of precipitation that may continue to be heavy across southern California into Tuesday. A Slight Risk of excessive rainfall remains in place across the Los Angeles Metro and southward-facing terrain of the Transverse Ranges, with guidance agreeing on an axis of anomalous deep moisture and an advancing front extending from weakening eastern Pacific low pressure helping to enhance focus over this region. The expected rainfall should be decreasing by Tuesday compared to Monday, but considering that the area will likely be sensitive to additional rain after heavy amounts in the short range, felt it prudent to keep the Slight Risk going. A broader Marginal Risk covers much of the state farther north and east, until the higher elevations of the Sierra Nevada are reached where the precipitation will be in the form of heavy snow, possibly reaching multiple feet by midweek. By Wednesday precipitation totals look to decrease further, but the amount of remaining rain is uncertain. With most 00Z models coming in with near/over an inch of rain though, have a Marginal Risk for the Los Angeles metro to the Transverse Ranges given the likely sensitivity to additional rain. Farther east, moderate snow is likely in the higher elevations of the Intermountain West and Rockies, especially in central/north-central areas through midweek before the West dries out for late week. Precipitation chances are likely to increase in the Ohio Valley to Great Lakes Wednesday and especially Thursday under the influence of a couple of low pressure/frontal systems. Precipitation is most likely to be rain except in the far northern tier. Rainfall amounts of around an inch could be widespread in the Lower Ohio Valley, with locally higher amounts. The precipitation should reach the East Thursday-Friday with snow possible in parts of the Northeast. Northern New England seems most likely to get notable snow, but continue to monitor forecasts as the details are yet to be determined, given the uncertain interaction of separate features that will influence precipitation amounts/coverage/type. Upper ridging traversing the interior West to the central and then eastern U.S. through next week will promote above normal temperatures in those areas. Temperatures around 15-25F above normal will be in place across the Plains to Midwest on Tuesday and Wednesday. Highs are forecast to be into the 80s for much of Texas with 70s stretching into the Southeast. Temperatures should moderate closer to normal after the cold front comes through after midweek but may still remain above normal in the central U.S., especially for lows. In fact, temperatures look to warm once again in the central and northern Plains for late week. By Thursday above normal temperatures should push into the East with plus 10-20F anomalies, moderating there by Friday. Meanwhile the West Coast should be near average for highs through the week but somewhat above normal for lows Tuesday-Wednesday. Tate Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw