Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sun Feb 18 2024 Valid 12Z Wed Feb 21 2024 - 12Z Sun Feb 25 2024 ...Overview... Upper troughing coming quickly into the West Coast as the period begins midweek will spread some lingering precipitation to California and snow into the Intermountain West. The weakening shortwave energy from this system will then quickly eject eastward and may combine with amplifying northern stream energy to develop a central-eastern U.S. surface low/frontal system by the latter half of the week. This feature should spread rain and northern tier snow to parts of the eastern third of the country as a larger scale upper trough settles over the East. Meanwhile, periods of upper ridging across the west-central U.S. will lead to much above normal temperatures on average through this week into next weekend. Upper and surface lows may approach the West Coast again by next weekend but with uncertainty in their placement and timing. ...Guidance/Predictability Assessment... Most models and ensembles agree fairly well on the large scale pattern evolution through the period, but with continued differences and/or trending for details within the deepening eastern U.S. upper trough and timing spread for the East Pacific system approaching California. By next weekend there are lower-predictability shortwave differences in northern tier U.S./southern Canada flow that affect the details of surface waves/fronts. Guidance maintains some degree of spread for timing of the upper trough moving into the Southwest on Thursday, with ECMWF runs tending to be slowest and GFS fastest and a compromise likely best for a single deterministic forecast. The new 12Z ECMWF has nudged a little closer to the intermediate 12Z UKMET/CMC. As this feature accelerates eastward and interacts with amplifying northern stream flow by Thursday-Friday, there is still a persistent theme of a defined surface wave tracking from the south-central Plains through Mid-Atlantic but with an emerging hint from recent GFS runs and the 12Z CMC that this wave could track farther north. Even with these differences, there is decent clustering toward the system tracking fairly close to the New England coast and then into the Canadian Maritimes. The 12Z UKMET/ECMWF have strayed a bit to the slow side relative to the majority model/ensemble cluster while some ECMWF runs (including the new 12Z version and other before the 00Z cycle) have been a little more offshore New England for the track. A consensus approach provides good continuity for this system. However the models/means have made a notable change with the core of the upper trough over Canada into the Great Lakes, now gravitating toward a deeper and better defined upper low tracking near James Bay. This changes the frontal depictions behind the system with a well-defined cold front dropping into the Upper Midwest/Great Lakes by Friday. The 00Z/06Z guidance maintained recent spread for the system offshore California, with the ECMWF/ECens on the eastern side of the envelope and the GFS farthest west. The GEFS has generally been a tad east of the operational run, while the CMC/CMCens is closer to the middle. ECMWF-initialized machine learning models also argue for a compromise approach, at times more emphatically tracking east of the GFS but then by the end of the period favoring slower timing than the ECMWF. The new 12Z ECMWF has adjusted a little slower to narrow the spread a bit. The updated forecast started with a 00Z/06Z operational model composite early in the period and then trended toward an even blend of the 06Z GFS/GEFS mean and 00Z ECMWF/ECens mean. The blend phased out the 00Z CMC after Friday as its northern tier features became out of sync with most other solutions. ...Weather/Hazards Highlights... An atmospheric river event mainly in the short range period will be winding down across California by Wednesday. There remains somewhat low confidence in how much precipitation may continue across southern California Wednesday behind the main cold front. Overall, totals should be reduced significantly from earlier days, but as the upper trough comes through, the lowering heights aloft can steepen lapse rates and allow for some instability to produce locally higher rain rates. There is a persistent signal in the guidance that the associated axis of deep moisture may still be progressing through the region very early in the period as well. Thus have maintained the Marginal Risk in the Day 4 Excessive Rainfall Outlook covering southern California for early Wednesday given the likely sensitivity to additional rain at the tail end of the event. Snow could also continue across the Sierra Nevada Wednesday. Farther east, moderate snow is likely in the higher elevations of the Intermountain West and Rockies, particularly in the Wasatch to central Rockies before the West dries out for late week. The next system dropping into the East Pacific late in the week may increase precipitation chances over California by next weekend, but details depend on how close the upper/surface lows get to the coast. The current array of guidance ranges from meaningful precipitation reaching the coast as early as Friday night (with moisture spreading farther inland thereafter) to even light amounts not reaching the coast until Sunday. Precipitation should increase in coverage/intensity over 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 an inch or two are likely in the Ohio Valley on Thursday, with locally higher amounts. The surface low and shortwave helping to force the precipitation should be moving rather quickly, but the west to east track means that rain has the potential to train west to east. Some instability combined with the good dynamical support (left exit region of the jet) may lead to high rainfall rates. However, streamflows and soil moisture are well below average along/north of the Ohio River but closer to average just to the south. Latest guidance continues to shift around a little relative to the best overlap of ensemble probabilities for meaningful QPF thresholds, favoring maintenance of the core of the existing Marginal Risk area depicted over the Ohio Valley in the Day 5/Thursday ERO. Some operational model/ensemble solutions supported a slight expansion on the northeast and west sides of the area for the daytime update. 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. The uncertain interaction of separate features ultimately leads to a wide range of possibilities for sensible weather over the Northeast, but most guidance suggests that that the region should see at least some precipitation/wind from this system. However confidence in exact magnitude remains fairly low. Continue to monitor forecasts over the coming days as important details become better resolved. Behind this system, parts of the Great Lakes may see mostly light snow from a combination of lake effect activity and one or more waves/frontal systems. Upper ridging pushing from the central U.S. Wednesday toward the eastern U.S. Thursday will promote above normal temperatures in those areas. Temperatures around 15-25F above normal will be in place across the Plains to Midwest on Wednesday, and 10-20F anomalies are likely to reach into the East by Thursday with will above normal lows extending into Friday morning along the East Coast. 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 (possibly declining to below normal for highs over New England during the weekend) but remain above normal in the central U.S., especially over northern/central parts of the region. Then expect renewed western U.S. into Plains ridging during the weekend to raise temperatures to 15-25F above normal once again over an expanding area of the central U.S. Meanwhile the West Coast should be near average for highs from Wednesday through next weekend but a few degrees above normal for lows at times. 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