Extended Forecast Discussion NWS Weather Prediction Center College Park MD 307 AM EDT Thu Apr 28 2022 Valid 12Z Sun May 01 2022 - 12Z Thu May 05 2022 ...Overview... An active pattern is likely during the medium range period 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 Midwest to the Great Lakes and the northeastern U.S./southeastern Canada as it weakens Monday-Tuesday. This will spread rain across much of the central U.S. with some enhanced rain totals and potential for severe weather in the southern Plains to portions of the Mississippi Valley, with lighter rainfall extending 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 Monday across the Rockies, Plains, and potentially into the Midwest by Thursday. These latter two systems will bring rain and high elevation snow to the Northwest and then areas of showers and thunderstorms to parts of the central and eastern U.S., with specific areas of enhanced precipitation depending on system/frontal details. ...Model Guidance Evaluation... Recent model guidance has continued to show fair agreement with the overall pattern with typical differences in the details/specifics of individual systems. Model solutions have converged fairly well for the leading system tracking across the Midwest early next week, though with some divergence by Monday-Tuesday with the surface low track and speed as it heads into Canada while weakening. Meanwhile there are still meaningful detail and track issues with the trough over the Northwest Sunday--ECMWF runs have been persistent with an initial upper low, which the 00Z CMC has now joined, but guidance has a trend toward some weakening of the energy as it tracks east across the country during the first half of the week. However, the shortwave should still be strong enough to produce a surface wave over ~Illinois as shown by model guidance from the last day or so, with some variety in position from cycle to cycle. Newer 00Z guidance has shown a bit of a westward position with the low over the Mid-Mississippi Valley Tuesday morning compared to the 12/18Z cycle. The timing of the surface low Tuesday-Wednesday is affected by the potential for stream interaction over eastern North America and the timing of the absorption of the shortwave. Model guidance remains least agreeable with the last system of note during the period--significant energy spilling into the West Monday that likely forms a closed low as it tracks eastward eventually into the central U.S. by Wednesday-Thursday, accompanied by a surface low pressure/frontal system. The trend for eastern/faster GFS runs and western/slower ECMWF runs has stayed consistent through the 12/18Z and 00Z cycles, and the ensemble means for both have generally followed their operational model trends. The 12Z and now 00Z CMC as well as the 12Z CMC ensemble mean have provided a middle ground in between these slow and fast solutions with this feature. However, the CMC runs do show an upper low farther north in south-central Canada around Tuesday moving east through Thursday where other guidance handled these northern stream features differently--though the 00Z ECMWF is now a little closer to this--and has the northern feature phasing with the U.S. upper low unlike other guidance. Thus while the CMC runs seem to be a good middle ground over the U.S. its pattern farther north and the evolution of those features is somewhat questionable. The WPC forecast early in the period used a blend of the deterministic guidance led by the 12Z ECMWF and 18Z GFS. As the period progressed, lessened the influence of the fast GFS and slow ECMWF in favor of the ECMWF and the CMC ensemble means, while keeping the 12Z CMC in the mix. This attempt at an intermediate solution led to reasonably good consistency with the previous forecast at least at the 500mb level, though the continued uncertainty with the track of surface features could continue leading to some changes in the frontal, temperature, and precipitation forecasts going forward. ...Sensible Weather/Threats... The first surface low pressure system affecting the lower 48 early next week will spread precipitation across much of the eastern U.S. with generally light to moderate rain, but with potential for locally heavy to excessive amounts back across the central U.S. where fronts are expected to stall. Current forecasts show the heaviest rainfall amounts in the southern Plains Sunday-Sunday night and spreading into the Lower/Middle Mississippi Valley on Monday, but exact positioning of the heaviest rain could shift with future forecasts given lingering uncertainty. The same is true for severe weather, but the Storm Prediction Center currently indicates the best potential for severe thunderstorms across the southern Plains Sunday and Monday. 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. 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. Tate/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