Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sun Apr 24 2022 Valid 12Z Wed Apr 27 2022 - 12Z Sun May 01 2022 ...Overview... For the latter half of this week, a surface low consolidating near New England in conjunction with a slow-moving upper low will cause precipitation chances to persist over the Northeast. Meanwhile, mean troughing with multiple shortwaves is likely in the West, and this trough axis should eventually track eastward by the end of the week, pushing a broad upper ridge initially over the central U.S. eastward. The trough will support formation of a central U.S. surface low and possibly another round of snow in the northern Rockies/High Plains on its backside, while rain is likely farther south across the central Plains/Mississippi Valley. The details of these threats remain quite uncertain at this point, however. ...Model Guidance Evaluation... Model guidance remains quite agreeable with the pattern in the eastern U.S. through the end of the week with the aforementioned upper low tracking slowly from the Great Lakes to Northeast and stalling/spinning, with an associated coastal surface low. There is also reasonably good agreement for general broad ridging behind in the central U.S., though with some uncertainty in embedded shortwaves. But models are considerably more varied with their handling of the western troughing, first with a shortwave/possibly a compact upper low moving through southwestern to south-central Canada Wednesday-Thursday and how and when the energy gets absorbed, and then with the trough's track eastward around Friday into the weekend. GFS runs had been on the faster side of consensus through the 12Z run, but fortunately the 18Z GFS slowed to be more in line with other guidance. However, even by Friday there are still some differences, with the CMC and ECMWF showing energy spilling into the Pacific Northwest to prompt the trough to move around Friday, while the GFS is oriented more inland with the energy and shows ridging across the Pacific Northwest. Differences in energy there and elsewhere lead to considerable model and ensemble member variability with timing of the trough movement eastward as well as its extent southward, affecting the surface low position as well. The newer 00Z guidance like the GFS and ECMWF show a more northern track of the upper and surface lows over the weekend compared to the 12/18Z cycle used for this forecast, further indicating the uncertainty. The WPC forecast was able to use a multi-model deterministic blend favoring the ECMWF and GFS early in the period, but gradually adding in GEFS and EC ensemble mean influence to about half by the latter part of the period as uncertainty in any particular solution increased. But ensemble mean guidance was certainly flatter/less amplified than preferred given the ample differences in the individual ensemble members in a pattern sensitive to smaller scale shortwaves that may not be handled too well. Thus future changes to the forecast especially by next weekend may be considerable as guidance evolves over the next few days. ...Sensible Weather/Threats... The northeastern U.S. could receive multiple rounds of precipitation through much of the week as the upper level and surface lows pivot and linger over the area. Individual precipitation events should generally be light to moderate; amounts will add up over time but still are not expected to be too hazardous. Higher elevations of New England may see snow with the precipitation activity. Some rain may begin in the central U.S. as early as Wednesday near a warm front and surface low, but rain is likely to increase in coverage and amounts as the week progresses given better upper-level support and strengthening low pressure that will eventually push eastward into the weekend. Unsettled weather should also occur in the western to north-central U.S., with amounts increasing from Thursday into Friday and Saturday. Conditions will likely be cold enough for snow in the northern Rockies on the backside of the surface low, and there is once again some threat of wintry precipitation spreading into the northern High Plains as well. But the details of these threats may not come into focus for a few more days given considerable model uncertainty. Cooler than average temperatures are forecast for the eastern third of the U.S. Wednesday into Thursday, with the core of the cold air in the Great Lakes/central Appalachians, with temperatures 10-20F below normal and the possibility of a few daily record lows on Wednesday. On the other hand warmer than normal temperatures should spread from the Southwest/Four Corners states into southern and central parts of the Plains underneath upper ridging. By Friday into the weekend, the pattern should generally shift to cooler than average temperatures in the North and warmer than average in the South. The northern Plains to Midwest are forecast to see highs nearing 20F below average, but lows may be closer to normal. Meanwhile the southern tier should be around 10F above average. 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