Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 PM EDT Sun Apr 23 2023 Valid 12Z Wed Apr 26 2023 - 12Z Sun Apr 30 2023 19Z Update: The 12Z model guidance suite has come into better agreement on the synoptic scale pattern going into the end of the week and into next weekend with a more amplified upper level flow pattern evolving. The models and ensembles now have a strong signal for a deep trough axis setting up from the Great Lakes all the way to the Gulf Coast region next weekend, with the 12Z ECMWF being most amplified at this time and becoming negatively tilted going into next Sunday night across the Southeast states. This will support a strengthening surface low along with a strong cold front reinforcing a colder airmass by the end of the forecast period, and perhaps a developing triple point low near the East Coast that could resemble Miller-B cyclogenesis. The forecast blend incorporated about 20 percent each of the GFS/ECMWF/previous WPC continuity for the weekend and the remainder being ensemble means. The 00Z CMC was dropped from the forecast blend by day 7 owing to differences across the Plains, but the 12Z run does align more with the consensus compared to previous runs. In terms of the excessive rainfall outlook, the existing Marginal Risk areas were slightly expanded to account for a little more model spread in heavy QPF signals, but still not enough confidence to delineate any Slight Risk or greater areas at this time. The previous forecast discussion is appended below for reference. /Hamrick -------------------- ...Overview and Guidance/Predictability Assessment... Guidance continues to show a reasonably similar larger scale upper pattern evolution for the coming week, but with less than stellar run to run continuity with flow embedded features. This is evident by early in the forecast period within blocky flow over the northeastern U.S. and eastern Canada as well as with a series of wavy rain focusing systems ejecting out from the West to across the U.S. southern tier states to underneath the block into the western Atlantic. This portends average to below average predictability. Subsequent late week and weekend upper ridge amplification and slow progression from the eastern Pacific across the West Coast actually has a decent guidance signal as additional blocking simmers, but forecast spread is larger with ample trough energies set to increasingly sandwich the ridge both over the east Pacific and with wavelength spacing/phasing variances downstream over the north-central U.S.. Overall given uncertainties, the WPC medium range product suite was primarily derived from a blend of the 18 UTC GFS/GEFS mean, the 12 UTC ECMWF/Canadian/UKMET and ECMWF ensemble mean along with the 01 UTC National Blend of Models. Applied more blend weighting to the ensemble means over time to maintain best possible WPC product continuity. The blend process tends to mitigate embedded system variances consistent with system predictability. ...Weather/Hazards Highlights... A main upper trough ejecting out through the West/Rockies through midweek will leave behind cooled temperatures and a locally enhanced precipitation focus over the central Rockies/High Plains. Mid-late next week main system ejection across the South/Southeast along with less certain lead impulses will act to re-focus lower atmospheric convergence/baroclinicity with a series of frontal waves. There will be several opportunities for ingredients to come together to fuel swaths of moderate to heavy convective rainfall given influx of Gulf of Mexico to western Atlantic moisture and instability. There is also some risk of severe weather. Downstream, deeper coastal low solutions bring heavier activity from the coastal Southeast and Florida up through the Mid-Atlantic. Meanwhile, northern stream upper trough energy remains slated to dig southeastward into the north-central U.S. by mid-late next week, in an amplifying and growing blocky pattern that settles in late week/weekend with strong upper ridge translation and building inland across the West Coast. Run to run continuity is not stellar, but this general track would increasingly focus light to moderate precipitation across the north-central states. This activity may gradually spreading downstream into the Midwest/Great Lakes, but latest 00 UTC guidance is trending slower that seems to make sense given flow. The pattern does favors development of much below normal temperatures with sharp southward frontal surges of cold Canadian air into the U.S. with system development and overall flow amplification. 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