Extended Forecast Discussion NWS Weather Prediction Center College Park MD 217 AM 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... An operational model blend provides a reasonable starting point for the system affecting the West midweek. Shortwave energies in the northern and southern stream then track quickly east into the central U.S. by early Thursday and cause some differences with the upper trough forming across the east-central U.S. that affect the associated surface low. GFS runs have been questionably sharp and slow with amplifying northern stream energy, causing the surface low track to be farther northwest into the Ohio Valley/Northeast rather than the flatter west-east track that the non-NCEP guidance shows. By Saturday the non-NCEP guidance diverges somewhat too, with the 12Z ECMWF tracking slower north (the newer 00Z run sped up) and the 12Z CMC farther offshore than the bulk of other guidance, though many CMC ensemble members agreed with an offshore track. Overall a model and GEFS/EC ensemble mean blend seemed the best intermediate solution. Models are pretty consistent showing ridging behind the eastern troughing, but could be disrupted by uncertain shortwaves coming through the northwest flow. On the larger scale, an upper low dropping southeastward in the eastern Pacific has been waffling in the models with its position on how near to the West Coast it will be. ECMWF runs have been the farthest east with GFS runs on the western side, particularly the new 00Z GFS. Many ECMWF ensemble members were farther east and pulled the EC mean east of the GEFS mean. This remains uncertain for now and a blended position of the means seemed okay. The WPC forecast was thus based on a blend of deterministic guidance early in the period but gradually reduced the proportion of the operational models in favor of the means. Reduced the GFS proportion most quickly due to the differences it had in the East. Ensemble means reached half the blend Day 6 and over half Day 7. ...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. Have maintained a Marginal Risk 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 start to bring returning precipitation chances to California by next weekend dependent on how close the upper/surface lows get to the coast. 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 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 quite dry across the Upper Ohio Valley but closer to average in the Lower Ohio Valley. There is also still model spread with the low track and heaviest QPF. This forecast shifted the axis of heaviest rain north from the previous issuance per model trends, which puts the heaviest QPF in an area with drier antecedent conditions, but the newer models coming in have shown a slight southward shift again. Considering all these competing factors, opted to issue a somewhat broad Marginal Risk in the Ohio Valley with the expectation that the area could narrow or change with time. 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 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 into the East by Thursday. 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, and renewed ridging over the weekend will likely raise temperatures again to 15-25F above normal there. Meanwhile the West Coast should be near average for highs through the week but a few degrees above normal for lows at times. 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