Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM 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. The weakening shortwave energy from this system will quickly eject eastward after early Wednesday and may combine with amplifying northern stream energy to develop a central-eastern U.S. surface low/frontal system by the latter half of next week. This feature should spread rain and northern tier snow to some areas as a larger scale upper trough settles over the East. Ahead of the developing system, temperatures are forecast to be above average in the central U.S. early-mid next week and shift into the eastern U.S. mid-late next week. Meanwhile guidance continues to show another upper low dropping into the eastern Pacific by late next week, likely staying offshore through next Saturday, while some upper ridging builds over parts of the West. ...Guidance/Predictability Assessment... An operational model composite provides a reasonable starting point for the system affecting the West into midweek. Among the 00Z/06Z runs, the 06Z GFS/GEFS were a bit on the faster side with the southern part of the upper trough by early Wednesday. Recently the ECMWF-initialized machine learning (ML) models were consistently faster with the trough than most dynamical guidance but today's ML models have adjusted a little slower, at least to the GFS timing. The 12Z GFS is consistent with the 06Z version while the new 12Z ECMWF/UKMET have adjusted a little faster. Guidance spread/variability over the past few days would recommend maintaining an intermediate solution. Also of note, ECMWF runs through the 00Z cycle have been questionably light for southern California QPF with the AR/surface front. The new 12Z run has at least trended heavier in the direction of other guidance. Precise timing of the ejecting West Coast upper trough and details of exactly how Canadian troughing amplifies into the eastern half of the lower 48 continue to be a forecast challenge with respect to surface evolution over the eastern half of the country by the latter half of the week. On the good side, today's 00Z ML models have trended more in the direction of recent dynamical guidance Thursday into early Friday--so now there is better agreement on a south-central Plains into Mid-Atlantic wave, with typical spread for depth and track. By early Friday the 00Z ECMWF and 12Z UKMET are on the southern side of the spread with the new 12Z ECMWF adjusting north by a state to align with the 00Z ECens mean. GEFS/CMCens means are a little north of that. The 00Z ML models favor an intermediate track near the northern Mid-Atlantic with a depth around the mid 990s mb. Already by Friday GFS runs over the past day have been questionably sharp and slow with amplifying northern stream energy. Going into the weekend this results in the 00Z/06Z runs holding low pressure near the New England coast as of late Saturday while remaining consensus (among both dynamical and ML models) is already over northern New England or the Canadian Maritimes by early Saturday. 12Z GFS details remain a question mark, and while the early Saturday surface depiction is a lot closer to the majority cluster, it still lingers near New England longer than consensus thereafter. On the other hand, 12Z CMC/00Z ECMWF runs track the system farther offshore New England than most other solutions. Specifics within general northwesterly flow behind the eastern U.S. upper trough late in the period become quite uncertain, favoring a mostly mean approach. The ECMWF system reaching the Great Lakes by day 7 Saturday may be on the strong side but there are hints in other models/means for at least a weak wave. For the upper low dropping into the eastern Pacific later in the week, some guidance has been reversing what had been a westward trend. This shift is most notable in the 00Z/12Z ECMWF plus ECens runs and the 12Z UKMET. This leaves the GFS/GEFS and 00Z CMCens mean on the western side of the spread. The updated forecast resolved this divergence with a compromise position that nudged the deterministic forecast a little eastward from continuity but kept it will within the current spread to maximize flexibility pending future trends. Forecast considerations led to starting with an operational model composite early in the period, followed by a rapid transition to mostly the 06Z GEFS/00Z ECens means late in the period. Day 7 Saturday reached 80 percent weight of the means with only lingering input from the 00Z ECMWF/CMC at that time. ...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 as the upper trough axis reaches the California coast (most likely early in the day), with exact amounts uncertain due to timing differences with the trough. Most 12Z guidance thus far has been nudging lower with totals over southern California with slightly faster upper trough progression. However there is a residual signal that the band of enhanced moisture could still be over the area very early in the day before departing, and guidance has persistently varied with exact timing of the trough over recent days. Thus the updated Day 5 Excessive Rainfall Outlook maintains the Marginal Risk area over and south of the Los Angeles Metro/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. The next system dropping into the East Pacific late in the week should remain far enough offshore to keep most of California dry into Saturday, though there is modest potential for a farther east track that would bring somewhat more moisture into the state at that time. 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 has a relatively higher probability of seeing notable snow along with the potential for a period of brisk winds. However the uncertain interaction of separate features ultimately leads to a wide range of possibilities for sensible weather, including a lower-probability storm track far enough offshore to bring minimal precipitation/wind to New England. Continue to monitor forecasts over the coming days as important details become better resolved. 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 over northern/central parts of the region where readings may rebound once again by next Saturday (up to plus 15-20F anomalies). 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. 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