Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 PM EDT Mon Apr 25 2022 Valid 12Z Thu Apr 28 2022 - 12Z Mon May 2 2022 19Z Update: The 12Z model guidance suite is in above average agreement across the continental U.S. through Friday, with the best agreement noted with the closed low off the Northeast U.S. coast, and the shortwave exiting the Rockies and crossing the Plains for the end of the week. By early Saturday, the 12Z CMC is much less amplified with the shortwave reaching the Pacific Northwest, and the prior 00Z run is preferred over the 12Z run. The UKMET has now trended closer to the consensus and not as far north with the surface low across the north-central U.S. for this weekend. There has been a slight westward trend and increase in the axis of heaviest QPF across the Dakotas compared to the overnight issuance, with some potential for a change to snow across western portions of the Dakotas before the event ends, so this is something that will continue to be monitored closely. The previous forecast discussion is appended below for reference. /Hamrick ------------------------ ...Overview... A mean trough axis initially near the West Coast as the medium range period begins Thursday will push eastward through the rest of this week, potentially forming a closed upper low over the north-central U.S. over the weekend. A surface low is likely to consolidate in conjunction with the trough/low, spreading rain to much of the central U.S. while snow is forecast on the backside of the low across the northern Rockies with some chance of wintry weather spreading into the northern High Plains as well, although the details of precipitation amounts and placement remain uncertain at this point. Meanwhile, surface and upper lows will move slowly away from the Northeast over the weekend after spreading some light to moderate rain and snow across the area for the latter part of the week. ...Model Guidance Evaluation... Model guidance remains quite agreeable in keeping a slow-moving low pressure system (both in the upper levels and at the surface) over the Canadian Maritimes through the weekend. There is also good agreement on the broad ridge over the central U.S. Thursday narrowing and becoming suppressed Friday into the weekend as western troughing comes in, though with various details on embedded shortwaves tracking through the overall ridge pattern. The more uncertain part of the forecast lies in the upper trough over the western U.S. Thursday pushing eastward with time. Recent model guidance is in somewhat better agreement than a day or so ago though, with a general consensus for shortwave energy to consolidate within the trough and form an upper low by the weekend in the north-central CONUS. There are still some differences particularly with the latitudinal position of the upper low. With the 12/18Z cycle, it appeared the the 12Z GFS was a bit south of consensus by around Saturday so preferred the 18Z GFS instead that was better clustered, but some newer 00Z guidance (particularly the CMC) came in farther south. Similar north-south spread was seen with the surface low positions through the weekend, with even more divergence in position by Monday depending on how the upper low may lift and/or weaken. Upstream of these features, models are even less agreeable with the pattern--the ECMWF and CMC runs have shown a strong shortwave somewhere in the Northwest by early Sunday whereas GFS runs have been weaker with that energy, and the ECMWF/CMC are also quite strong with possibly a larger closed low in the eastern Pacific by Monday with the GFS runs showing a different pattern. The WPC forecast was based on a multi-model blend of deterministic 12/18Z guidance favoring the ECMWF early in the period, gradually transitioning to a blend including the ensemble means (particularly the EC mean) as spread and uncertainty increased considerably. The results were pretty similar to the previous forecast through Sunday. ...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. Periods of precipitation 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 during the night but mixing with rain during the day. Some rain is likely in the central U.S. on Thursday, which should increase in coverage and amounts Friday over northern/central parts of the Plains as the main upper trough exits the Rockies and consolidates a low pressure system in the central U.S. 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. In portions of the Dakotas, precipitation may begin as rain and transition to mixed precipitation or snow. However, the exact details of these threats may not be determined for a few more days given continued model uncertainty. By the weekend, the current expectation is that rain will spread into the East and could linger over the southern Plains as the front there may stall. The northeastern quadrant of the U.S. can expect cooler than normal temperatures by around 10-15 degrees on Thursday with a few daily record lows possible, but temperatures there should gradually moderate closer to normal by the weekend and early next week. But below average temperatures especially in terms of highs will be reinforced with the incoming trough/low across the north-central U.S., with highs potentially around 15-25F below normal through much of the medium range period. Meanwhile the Four Corners states to the central/southern High Plains will see warmer than normal temperatures especially Thursday and Friday before the upper ridge promoting the warmth is suppressed over the weekend. Slightly above normal temperatures should then stretch across much of the southern tier. 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