Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Wed Apr 27 2022 Valid 12Z Sat Apr 30 2022 - 12Z Wed May 04 2022 ...Overview... Latest guidance continues an active pattern with a series of Pacific systems entering and progressing across the lower 48. The first system will be a closed upper low and associated surface low pressure that should lift slowly northeastward from the north-central Plains, passing through the Northeast in weakened form by next Monday-Tuesday. This will spread rain across much of the central U.S. with some chance of wintry weather spreading into the northern High Plains on the backside of the low. Lighter rainfall will extend farther east. The next system should be somewhat weaker, progressing from the West Coast to near the East Coast during the period. Then a stronger system is likely to track from the West Coast into the Rockies/Plains during Monday-Wednesday. The latter two systems will bring rain and high elevation snow to the Northwest and then areas of enhanced precipitation to parts of the central and eastern U.S. depending on system/frontal details. ...Model Guidance Evaluation... Within what has been a fairly agreeable pattern from the multi-day mean perspective, some challenges are ongoing for specifics of individual systems. Solutions have converged fairly well for the leading system tracking out of the Plains during the weekend but still diverge a bit by day 5 Monday with the 06Z GFS and now the 12Z UKMET straying somewhat to the faster/northward side of the spread. Meanwhile there are still meaningful detail and track issues with the shortwave reaching the West during the weekend, with the new 12Z ECMWF at least adjusting somewhat north of its 00Z run whose upper low center was on the southern side of the envelope. Once this feature ejects from the West, model runs over recent days have differed greatly over specifics (including the surface reflection). However in a positive trend today's 00Z through 12Z runs seem to be clustering a little better toward maintaining enough definition of the shortwave to support a surface wave over or just east of the Mid Mississippi Valley by early Tuesday and near the East Coast Wednesday. There is still a significant question of stream interaction over eastern North America that complicates the forecast after early Tuesday. The final system in the series continues to provide a difficult forecast though. GFS runs remain on the faster side of the spread, bringing it well into the Plains by midweek. The 00Z ECMWF jumped back close to the persistently slow CMC (which itself changed to an intermediate timing in the new 12Z run) while the new 12Z ECMWF is close to its 00Z run. The 06Z GEFS mean hedged back to a compromise, close to the 00Z CMC mean and a bit ahead of the ECMWF mean, before gravitating closer to the operational run again in the 12Z cycle. The array and variability of latest runs favored a compromise approach between the GFS and 00Z ECMWF aloft, slower than continuity but offering some flexibility pending future trends. Based on the guidance available through the 06Z cycle, the updated forecast used a composite of 00Z/06Z models early to tone down the more extreme aspects of one or more solutions, followed by adding some input from the 06Z GEFS/00Z ECMWF means along with the models to yield the desired intermediate solution late. Admittedly the favored model/mean blend ended up producing a surface evolution a tad slower than ultimately desired by day 7 Wednesday though. ...Sensible Weather/Threats... The first surface low pressure system affecting the lower 48 over the weekend will spread precipitation across much of the central and eastern U.S., with some wintry weather possible in the Dakotas and the best chances in the higher elevations of the Black Hills. In portions of the Dakotas, precipitation may begin as rain and transition to mixed precipitation or snow, though details of these threats remain somewhat uncertain. Additionally, rain could become heavy to locally excessive from parts of the Plains into the Mississippi/Ohio Valleys on Saturday. Then as the low pressure system shifts eastward early next week, precipitation chances should spread eastward with heavier activity tending to become more localized. The trailing front will likely stall over the southern Plains as another system approaches from the West and current forecasts show potential for heavy rainfall on Sunday across this region. There is enough uncertainty with details of this incoming system to suggest potential for future shifts in the expected axis of heaviest rainfall. With another potential early-mid week wave plus a trailing one reaching the Plains by around Wednesday, rain/showers and thunderstorms will likely persist from the Plains eastward through much of next week. Some areas over the central U.S. will need to monitor forecasts of the midweek system given its potential to produce significant weather and sensitivity to additional rainfall. The latter two upper shortwaves/surface systems will bring rounds of mostly light to moderate precipitation (snow in the higher elevations and rain in the lower elevations) across the Northwest, mainly in the Saturday-Monday period and perhaps lingering over parts of the Rockies after then. Temperature-wise, the general trend over the lower 48 will be for cooler than average weather in the northern tier and warmer than average in the southern tier. The core of the coldest air should be across the northern High Plains to Midwest, where highs will be around 10-20F below average due to being in the cold sector of initial low pressure affecting the region followed by influence from the southern periphery of Canadian high pressure and moisture arriving with another system toward midweek. Lows should stay closer to normal though. Meanwhile, slightly above normal temperatures should stretch across much of the southern tier, with mainly warmer than average lows also spreading into the Ohio/Tennessee Valleys during the weekend and more of the East next week. Best potential for some plus 10-15F anomalies for highs will be over the southern High Plains early next week while similar anomalies for morning lows may be a little more common on some days. Rausch/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, 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