Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Fri Mar 01 2024 Valid 12Z Mon Mar 04 2024 - 12Z Fri Mar 08 2024 ...Wet pattern for parts of the West Coast and the South... ...Overview... Latest guidance for next week generally agrees upon large scale mean troughing over the western half of the lower 48 and upper ridging off the East Coast. This pattern will favor cool conditions over the West with periods of enhanced precipitation over some areas, while above normal temperatures prevail farther east with the Lower Mississippi Valley into Southeast seeing the potential for some heavy rainfall. Rain may extend farther north across the eastern half of the country as well. ...Guidance/Predictability Assessment... While models and ensembles are similar for the very large scale pattern, there are significant embedded differences that affect the forecast over some areas. Already early in the week the guidance essentially sorts itself into two clusters, with the GFS/GEFS runs showing broader and more amplified upper troughing over the western half of the country versus other solutions that show flatter Interior West/Rockies flow between a shortwave reaching Vancouver Island and the Canadian system just north of the Plains/Upper Midwest. 12Z ECMWF-initialized machine learning models favored the majority ECMWF cluster. Primary results of leaning away from the GFS scenario for this part of the forecast were somewhat greater precipitation potential along the north-central West Coast versus GFS forecasts, along with weaker surface waviness over the Midwest/Great Lakes. At least the new 00Z GFS has toned down its Midwest wave versus the 12Z/18Z runs. Through the rest of the week there are a lot of shortwave differences across the northern tier within a general theme of enough dynamics existing to support some degree of surface frontal evolution, while one or more Pacific shortwaves should amplify/move into the West after midweek, likely supporting a well-defined surface system that may reach the Plains by next Friday. Individual ensemble members and ML models vary considerably for the timing of upstream Pacific flow by late next week while generally suggesting that the trough moving through the West (and associated leading surface system) could end up being somewhat faster than the current model/ensemble mean majority. Farther east, the 12Z ECMWF has minimal support from other guidance for its slow upper low that develops. Instead a more progressive upper trough/surface reflection appears to be the more likely scenario. The early part of the forecast started with primary emphasis on the 12Z ECMWF/UKMET/CMC with minority input of the 18Z GEFS which was a bit less extreme than the GFS for the pattern over the western half of the country. Later on the favored blend rapidly increased 18Z GEFS/12Z ECens input to 50-60 percent total weight while reincorporating a little 18Z/12Z GFS and reducing 12Z ECMWF influence. This approach emphasized what similarities existed from the large scale perspective and maintained a reasonable degree of continuity where feasible. ...Weather/Hazards Highlights... During the first half of the week expect a front lingering over/near the Gulf Coast to provide a focus for rain and thunderstorms that may extend along and north of the Gulf Coast, with a front dropping southeastward from the central U.S. also possibly enhancing Gulf inflow somewhat toward midweek. The Day 4 Excessive Rainfall Outlook valid Monday reflects a Marginal Risk area centered over the Lower Mississippi Valley while the Day 5 Marginal Risk area reflects the consensus eastward progression of heaviest rainfall potential toward the Southeast (while still possibly lingering over the central Gulf Coast region). Expect some rainfall to extend farther north along the advancing central U.S. front as well. From midweek onward the Mid-Atlantic and Northeast may become the most favored region for meaningful rainfall as the wavy front progresses eastward. Meanwhile, upper level and surface systems will promote episodes of rain and higher elevation snow over the West with guidance differences keeping confidence lower than desired for the details. The non-GFS majority cluster would have an early week system focusing a narrow axis of enhanced moisture aimed at the West Coast near the California/Oregon border. The ECMWF/ECens mean still depict fairly modest moisture anomalies with this event whose heavy rainfall may be fairly localized, so have opted not to depict any risk area in the Day 4 ERO for now. A mid-late week system may bring additional rain/snow to the West. How quickly this system progresses through the West will determine whether a rebound in rainfall could occur along/north of the Gulf Coast late in the week. Under the influence of mean troughing aloft, most areas from the West Coast into the Rockies will see highs 5-15F below normal while morning lows will tend to be near or slightly below normal. In contrast, expect well above normal temperatures to prevail over the eastern half or so of the country. Plus 20-30F anomalies will be possible from the central-southern Plains into the Northeast early-mid week with locations from the Midwest into Northeast seeing the best potential for some daily records. Rausch 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