Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sun Mar 19 2023 Valid 12Z Wed Mar 22 2023 - 12Z Sun Mar 26 2023 ...Heavy rainfall threat over parts of the east-central U.S. after midweek... ...Overview and Guidance/Predictability Assessment... Latest guidance is consistent with the large scale flow aloft, showing an energetic pattern with a series of features moving into and through a mean trough over the West Coast/western U.S. while an upper ridge meanders over parts of the Gulf of Mexico/northwestern Caribbean into the Bahamas. A vigorous system moving into the West as of Wednesday will initially bring rain and significant higher elevation snowfall to some areas and then the potential for heavy rainfall over the east-central U.S. as moist leading flow interacts with a front trailing from a northern stream system. The next system reaching the West Coast later in the week should amplify farther east and thus yield a farther north precipitation emphasis than the first one, followed by low pressure emerging over the Plains next weekend. Yet another system may approach the Northwest by next weekend as well. The forecast period begins with an amplifying pattern as a closed low over the Gulf of Alaska digs southward over the West and a ridge begins to build over the southeastern CONUS. The model guidance has also been consistent in showing a northern stream shortwave dropping southward from Canada over the northern tier and moving eastward from the Plains to the Northeast along the Canadian border. Guidance has also trended towards showing broad mean troughing over the CONUS with embedded shortwave energy rather than more individually phased troughs. While some of the broad details are similar across the guidance, the timing/location/strength of the upper-level energy shows increasingly significant differences, even beyond the expected increase in the spread of the solutions with increasing lead time, with implications on the evolution of the frontal pattern over the CONUS. As has been the case the past few days, guidance differences arise quickly with respect to both the northern stream shortwave and the progression of an embedded shortwave energy with a trough of over the West/Plains. The CMC continues to show a weaker shortwave compared to the the other guidance, while the ECMWF is the most aggressive with this feature. More subtle details in the timing of height falls reaching the Plains and the degree of downstream ridging are hard to resolve at that scale/forecast timeframe, though the overall pattern is at least similar. All of the guidance has also trended towards this trough becoming more embedded in the mean flow with a less individually phased downstream evolution in the mid- to late forecast period, with the 00Z ECMWF retaining the strongest embedded wave. The last few runs of the GFS have shifted quite significantly towards a much weaker wave translating downstream over the East, increasing concerns over predictability. Both the 00Z GEFS and ECens means retain at least subtle indications of a shortwave, especially considering the lack of any amplitude to the wave in the 00Z GFS. The 06Z run of the GFS agrees more with the the 00Z GEFS/ECens means. Similarly, the GFS and CMC are more aggressive with a strong wave digging southward over the southwestern CONUS following the departure of the initial trough downstream over the East, while the ECMWF has trended away from this feature significantly as well. In fact, the latest couple of runs of the ECMWF, including the 00Z ECMWF, show a much less amplified pattern across the CONUS in comparison to the other deterministic and ensemble mean guidance. Both the 00Z GEFS and ECens means retain a stronger wave. A combination of the deterministic guidance (00Z ECMWF/CMC/UKMET, 06Z GFS) is used for the WPC forecast blend initially, with a relatively lower contribution from the 00Z CMC due to more significant differences in the details of the northern stream shortwave. Reliance on the 00Z GEFS/ECens means is significantly increased for the mid- to late period of the forecast given the differences in the deterministic guidance regarding numerous small-scale and larger-scale features in the pattern and the notable increase in uncertainty given changes in recent trends. The means help to capture the general consensus across the guidance in handling the departing wave downstream over the eastern CONUS as the pattern looks to amplify over the West. Heavy reliance on the ensemble means also follows the previous WPC forecast for smoother continuity until the confidence level in more specific details of the pattern increases. ...Weather/Hazards Highlights... A leading system tracking out of the Upper Midwest should spread some light to moderate precipitation across parts of the northeastern quadrant of the lower 48. Some snow will be possible over far northern areas. Meanwhile, an initially strong system over the West will spread a broad area of rain and higher elevation snow across areas from California through the central two-thirds of the Rockies. Areas of heavy snow will be possible over favored mountain terrain on Wednesday, particularly in the Central Rockies. Anomalously high moisture will begin to spread into the central CONUS as low-level Gulf flow due to a downstream ridge and the incoming trough becomes established. The combination of this moisture and the front trailing from the departing Upper Midwest system may promote the development of storms overnight Wednesday producing an area of locally moderate to heavy rainfall. However, guidance signals for this continue to be mixed, and soil conditions over this area are neutral to dry, so the day 4 (12Z Wednesday-12Z Thursday) Excessive Rainfall Outlook continues not to depict any risk areas. Even though there is still a lot of model spread for surface specifics, there has been a persistent guidance signal for heavy rainfall over parts of the east-central U.S. in the day 5 (12Z Thursday-12Z Friday) period with a Slight Risk of Excessive Rainfall outlined from eastern Oklahoma to southwestern Ohio. This axis provides the best overlap of greatest moisture anomalies, instability, and guidance QPF. The Storm Prediction Center has also highlighted areas of the Southern Plains and Lower Mississippi/Tennessee Valleys for the risk of severe weather Thursday-Friday. Meanwhile, rainfall should continue spreading over the East late in the week and into the start of the weekend, with some snow possible over the Interior Northeast. Precipitation chances increase again for the West as another system looks to enter the Pacific Northwest Thursday/Friday and track southeast. Best rain/mountain snow emphasis over the West from late week into the weekend will be from the Pacific Northwest/northern California into the Rockies. There is potential for some moisture to extend a little farther south if the upper trough over the region leans toward the amplified side of the envelope. Currently the daily amounts do not look exceptionally heavy but areas of localized enhancement are possible. Cyclo/frontogenesis looks possible over the Plains this weekend with an increasing chance for rainfall over the Plains/Lower Mississippi Valley and the potential for some wrap-around snow over the Central High Plains. However, given the noted uncertainty in the overall pattern late in the period, specifics on locations and amounts may likely change. The overall pattern will keep much of the West and parts of the northern Plains quite chilly through the period, with broad coverage of highs 10-20F below normal each day. Temperatures over the Northwest should be less extreme but may still be mostly below normal. In contrast, an area of well above normal temperatures will spread from the central-southern Plains through much of the East mid-late week. Thursday-Friday will be the warmest days for highs with plus 10-20F anomalies and some morning lows that could be as much as 20-25F above normal Thursday into early Saturday. Frontal passage across these areas will then bring temperatures closer to normal. Putnam/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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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