Extended Forecast Discussion NWS Weather Prediction Center College Park MD 305 AM EDT Sat Apr 23 2022 Valid 12Z Tue Apr 26 2022 - 12Z Sat Apr 30 2022 ...Overview... An upper trough is forecast to track from the Midwest eastward and develop an embedded upper low spinning over the Northeast for the latter part of the week. This will initially push a cold front with rainfall along it across the Eastern Seaboard Tuesday, then strengthening surface low pressure could promote lingering precipitation in the Northeast as the week progresses. Meanwhile, mean troughing is likely in the West with multiple shortwaves tracking through causing rounds of precipitation, while ridging in between will promote warm temperatures across the Southwest to southern/central High Plains. ...Model Guidance Evaluation... Model guidance has remained consistent with better than average agreement for the eastern trough developing into an upper low. There is still some variability in the timing of the upper low forming (depending in part on what height contour interval is used), with the 12Z ECMWF on the slower side to develop it but the 00Z EC showing its development more in line with other guidance. Models also continue to show a strengthening surface low Wednesday into Thursday near New England, with good clustering in position for a medium range forecast. By Friday into Saturday, there is some uncertainty with the timing of the upper low lifting out northeastward, with GFS runs faster while the CMC runs appeared slow initially, but the 00Z ECMWF clusters with it. The latter cluster is slower than the ensemble means at this point though. Greater model differences are seen with the pattern over the West despite general agreement initially for troughing stemming from an upper low centered near British Columbia. One uncertain shortwave reaching the Northwest around Tuesday should continue onward into southern Canada, possibly forming a compact upper low, as it tracks around the northern periphery of the central U.S. mean ridge. The exact timing/track have varied a fair amount from run to run and model to model. There is also some suggestion of a weak shortwave reaching the Southwest around Wednesday but it should have at most modest effect and shear out as it heads downstream into the mean ridge. By Thursday, another shortwave is likely to drop into the West Coast while the bulk of guidance favors the trough axis shifting eastward by Friday-Saturday. There has been a general trend in recent guidance towards less of a persistent and strong upper low within the trough, though considering the uncertain Pacific origins of the associated shortwaves, confidence is low. At this point, the main outlier with the pattern appears to be the CMC runs, especially the 00Z, which shows stronger energy in the East Pacific around Friday that causes the trough to slow down considerably compared to other guidance that shows troughing moving into the north-central U.S. on Saturday. At least model agreement remains fairly good for upper ridging in between the West and East trough/low features. The WPC forecast utilized a multi-model 12/18Z deterministic blend early in the forecast period, phasing out the CMC and UKMET in favor of the pretty agreeable ensemble means as the period progressed. This approach yielded minimal changes in continuity. ...Sensible Weather/Threats... The cold front moving across the Eastern Seaboard Tuesday as the period begins will promote rain and possibly thunderstorms along it, but its reasonably fast progression should keep rainfall amounts in the light to moderate range. Then as surface low pressure consolidates and pivots, precipitation will remain in the forecast across the Northeast through the end of the workweek. There is some potential for higher elevations of New England to receive snow with this precipitation activity. How much precipitation falls will be sensitive to exact system details. Meanwhile in the West, unsettled weather is likely as shortwave energy moves through the mean trough south of a closed upper low positioned off British Columbia through Thursday. Guidance has persisted in showing a better defined episode of rain and high elevation snow in the early-mid part of the week with a leading shortwave followed by another later in the week with a separate shortwave and then inland progression of the overall trough by Friday. Heavier precipitation is likely with the latter shortwave, which should be in the form of snow over the northern Rockies, and there is once again some potential for the northern High Plains to see snow by the latter part of next week, though precipitation type in the lower elevations will be quite sensitive to system details. Meanwhile a frontal boundary draped over the central Plains may help to focus some rainfall as far southeast as the Mid Mississippi Valley late in the week. The core of post-frontal cold air is forecast in the Dakotas/Midwest on Tuesday and spreading into the Great Lakes region by Wednesday-Thursday, with temperatures 10-25F below average. As the cold front sweeps across the East, temperatures will transition from warmer than normal on Tuesday to below normal midweek, though with some temperature moderation. The Northwest should see near to slightly below average temperatures, but by late week temperatures in the northern Rockies/Plains may become much below normal especially in terms of highs with the possibility of more potent troughing aloft. Meanwhile farther south, ridging in place will spread warmer than normal temperatures from the Southwest/Four Corners states into northern and central parts of the Plains, with temperatures in the 90s in many areas. 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