Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Fri Feb 16 2024 Valid 12Z Mon Feb 19 2024 - 12Z Fri Feb 23 2024 ...Atmospheric river to bring heavy rain and mountain snow to California early next week... ...Overview... Upper-level and surface lows over the eastern Pacific early next week will steer a stream of Pacific moisture into California and produce heavy rainfall with flooding potential and heavy mountain snow. These features should weaken by midweek while shortwave energy pushes eastward. This shortwave 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 farther offshore for a time, with at least a weak upper ridge building over parts of the West. ...Guidance/Predictability Assessment... Most guidance reflects similar ideas for the large scale pattern evolution through the period but there are some significant embedded differences that will take time to be resolved. For the system initially over the East Pacific and the associated upper trough, a guidance average provides a reasonable starting point in light of some offsetting considerations. Over recent days the dynamical models and means appear to have trended northward for the offshore surface low during the first half of the week, as initially advertised by the ECMWF-initialized machine learning models. There is still a fair amount of north-south spread for the surface low off the Pacific Northwest coast by midweek, with the 12Z GFS trending north of prior runs but the previously north CMC nudging back a little southward. The 00Z ML models suggest the northern side of the spread could be in play though. On the other hand, the ML models have insisted that the opening upper trough will become a little more progressive than the models/means by Wednesday. Thus far there has been no evidence of a faster trend in the consensus of dynamical models/means. 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. Latest GFS/ECMWF/CMC runs show what could be fairly vigorous development of a south-central Plains through Mid-Atlantic surface wave in response to the western shortwave, while the amplifying northern stream flow supports a leading cold front with one or more embedded waves. The 00Z ECMWF was on the southern edge of the spread for its Plains/Mid-Atlantic wave though, leading to a farther south than consensus rain-snow line by late in the week. The new 12Z GFS is slower than most other guidance in amplifying the eastern North America trough late in the week. By early next Friday the ensemble means show a more even balance for strength among the Mid-Atlantic wave and the separate northern stream frontal wave. On the other hand, slightly faster progression of the western shortwave in the ML models ultimately leads to more northern stream dominance for developing low pressure reaching the Northeast. An operational model/mean blend maintains reasonable continuity and tilts slightly to the Mid-Atlantic wave for strength. 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 GFS has trended weaker than its 00Z/06Z runs but the new 12Z ECMWF brings its feature farther south than its previous runs or other guidance. The ensemble means and large scale pattern do not appear too favorable for a defined shortwave/low, so by day 7 Friday the favored blend weakens what reflection could linger into Thursday. Multi-run continuity from the GFS/ECMWF and their ensemble means shows a noticeable westward trend for the upper low forecast to drop into the eastern Pacific late next week. New 12Z guidance is similar to or even westward of the prior cycle (in the case of the 12Z ECMWF). Interestingly, the ML model average is somewhat east of the latest dynamical guidance consensus. Based on 00Z/06Z guidance comparisons, the updated forecast used a 00Z/06Z model composite for about the first half of the period and then transitioned toward half models and half 06Z GEFS/00Z ECens means by the end of the period next Friday. ...Weather/Hazards Highlights... An atmospheric river with ample Pacific moisture will cause rounds of precipitation that will be heavy across California into Monday and Tuesday. For coastal/lower elevation areas, the Day 4 and 5 Excessive Rainfall Outlooks indicate Slight Risks across central to southern parts of California for the flooding potential. Rainfall rates in this region have the potential to reach 0.5-1" per hour, with over 1"/hour rates possible, and some areas are likely still sensitive from AR events early in the month/cool season. The Slight Risk Monday is likely more on the higher end of the probability range for a Slight 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 initially strong eastern Pacific low pressure helping to enhance focus over this region. Tuesday's Slight is lower-end as the expected rainfall decreases, with some shift south as well. Broader Marginal Risks cover 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. Locations over northern California and along the Sierra Nevada may also see high winds on Monday. Farther east, moderate snow is likely in the higher elevations of the Intermountain West and Rockies, especially in central/north-central areas. Precipitation chances are likely to increase in the Ohio Valley to Great Lakes Wednesday-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. The precipitation should reach the East Thursday-Friday with some snow possible in parts of the Northeast. 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 10-20F above normal will rise to 15-25F above normal across the Plains to Midwest on Tuesday and Wednesday. Highs are forecast to be into the 80s for parts 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. By Thursday the above normal temperatures should push into the East with plus 10-20F anomalies, also moderating there by Friday. Meanwhile the West Coast should be near average for highs but somewhat above normal for lows. Rausch/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