Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Fri Apr 21 2023 Valid 12Z Mon Apr 24 2023 - 12Z Fri Apr 28 2023 19Z Update: The 12Z model guidance suite is in good agreement on the synoptic scale pattern going into early next week, so a multi-deterministic model blend suffices as a starting point in the forecast process through Tuesday. The guidance has now settled on the more amplified side of solutions across the Intermountain West, with the GFS falling in line with the ECMWF and CMC. By the end of next week, there is a strong signal for an amplified northern stream trough with closed upper low dropping south from central Canada to the Northern Plains, with the CMC weaker than the GFS and ECMWF, and a strong ridge axis builds across the West Coast region. In terms of the excessive rainfall outlook, a small Slight Risk area was introduced for portions of the ArkLaTex and extending east to central Mississippi on Day 5 (Tuesday) to account for a more unified signal in the 12Z guidance for a concentrated QPF maxima centered over northern Louisiana, with strong return flow from the Gulf of Mexico near a developing wave of low pressure. Otherwise, no major changes from the previous WPC forecast were warranted, and the overnight forecast discussion is appended below for reference. /Hamrick ---------------------- ...Overview and Guidance/Predictability Assessment... Models and ensembles show a reasonably similar larger scale pattern evolution through medium range time scales, albeit highlighted by a tricky/transitional blocky flow focus from the northeastern CONUS through eastern Canada and the digging of a series of systems through the West that eject across the U.S. southern tier underneath the block. The WPC medium range product suite was primarily derived from best clustered guidance from the 18 UTC GFS/GEFS and 12 UTC ECMWF/Canadian/ECMWF ensemble mean along with the 01 UTC National Blend of Models. The 12 UTC UKMET was an outlier with deeper lead energies and much heavier QPF in the southern stream, but latest 00 guidance including the UKMET is in line with the aforementioned preferred blend. This certainly helps forecast confidence, but predictability may not be a good as normally would be the case in a pattern with reasonable forecast spread given differences with system timing/emphasis embedded within the southern stream flow and evolution aspects of a less defined than normal rex/omega style block at this point. ...Weather/Hazards Highlights... Expect lingering light to moderate wrap-around precipitation, mostly rain, from the Great Lakes through the Northeast through early-mid next week. WPC forecast progs show some system progressions and re-developments across the region into the Atlantic, but overall trends may prove slower given the complex/blocky nature. Accordingly, the experimental medium range Excessive Rainfall Outlook (ERO) plans to show a Day 4 "marginal" threat area into Monday from the White Mountains through southern Maine in case there is a protracted local focus emerging from short range time scales. A main upper trough and associated surface frontal system is slated to dig slowly southeastward across the West early-mid next week and spread cooled/unsettled conditions across a broad area as highlighted with a moderate precipitation/mountain snow focus from the Cascades through the north-central Great Basin/Rockies. Activity across the north-central Rockies/High Plains may be locally enhanced by lower atmospheric upslope fetch and added moisture. Mid-late next week system ejection across the South/Southeast may have less certain lead impulses focus and emerging pattern favorable for moderate to heavy convective rainfall. Activity may be greatly enhanced across the South with multiple wavy/reforming fronts, especially starting by Tuesday when a "marginal" ERO threat area is planned to be issued despite uncertainties. Activity may expand and spread out across the Southeast and southern Mid-Atlantic mid-late next week with steady system progressions within the southern stream to monitor, held underneath the block to the north. Meanwhile upstream, the next main upper trough from the Gulf of Alaska and western Canada are then slated to dig east-southeastward across the Northwest and north-central Rockies/Plains and perhaps eventually the Upper Midwest into mid-late next week. Run to run continuity is not stellar, but this general track would focus light-moderate precipitation across the region. Schichtel 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