Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 PM EDT Sun May 8 2022 Valid 12Z Wed May 11 2022 - 12Z Sun May 15 2022 ...Blocky pattern next week to support record heat over the Southern Plains/Lower Mississippi Valley into the Midwest along with episodes of central U.S. convection and an emerging onshore rainfall focus to the Carolinas... 19Z Update: The 12Z model guidance suite is in excellent synoptic scale agreement through the end of the week, with the main exception being the UKMET that is more amplified with the trough exiting the Desert Southwest and emerging over the western High Plains by Friday, and also slightly faster with the inland progression of the hybrid/closed low system approaching the southeast U.S. coast. Although some timing and magnitude differences are more apparent with the north-central U.S. trough and the storm system approaching the Pacific Northwest for next weekend, there is still above average agreement on the overall pattern with the deterministic GFS and ECMWF close to their respective ensemble means. In terms of QPF, there was enough of a signal in the UKMET/ECMWF/CMC to add a small Slight Risk area for excessive rainfall on Day 4 across portions of north-central Minnesota. Taking this into account, the WPC forecast was primarily derived from a CMC/GFS/ECMWF blend with a little bit of the UKMET, and then about 60% of the deterministic guidance and 40% of the means by next Sunday. The previous forecast discussion follows below. /Hamrick --------------- ...Overview... Latest models and ensembles maintain the idea of a blocky pattern through late week. An upper ridge will support a prolonged period of well above normal temperatures (challenging daily records over some areas) over the Mississippi Valley/central-southern Plains extending into New England. At the same time, mean troughing containing a couple separate features will produce chilly and unsettled weather over the West while waves/fronts and a dry line will promote episodes of rain and thunderstorms between these two regimes. A retrograding upper ridge over the Atlantic will likely push an initial western Atlantic upper low back toward the southern part of the East Coast, increasing rainfall over central and southern portions of the East. Most guidance suggests that the pattern will open up during next weekend, leading to a ridge building over the West and a mean trough axis reaching the Plains/Mississippi Valley. This would finally bring a drying trend over the northern Plains and vicinity and increasing rainfall coverage over the eastern half of the country. ...Guidance Evaluation/Preferences... Guidance continues to display some meaningful differences with important forecast details. The interaction among an ejecting western U.S. trough/possible embedded upper low and incoming Pacific flow has been a persistent uncertainty over recent days, with the character of the Pacific energy as it continues across the lower 48 also in question. Meanwhile the models appear to have become a little worse over the past day in terms of spread and run-to-run variability for the upper low expected to retrograde into the Southeast. On the positive side for the western evolution through late week, previously fast GFS/slow ECMWF runs have now trended closer to each other (GFS starting its slower trend in its 18Z run) in bringing the initial California upper low into the northern Plains by early Friday--leading to similarly deep surface low pressure in 18Z/00Z GFS and 12Z ECMWF runs. CMC runs have tended to be an eastern extreme with this system. The new 00Z run is still well east of those GFS/ECMWF runs but at least not to such a pronounced degree as the prior 12Z version. On the other hand the latest UKMET runs have been tracking the upper low extremely far south from late Thursday onward due to being flatter with the arriving Pacific flow. Based on guidance available through the 18Z cycle, a blend emphasizing the 18Z GFS/12Z ECMWF (tempered a bit due to uncertainty) offered the most reasonable starting point for the forecast. The new 00Z ECMWF is just a bit faster/weaker aloft, resulting in weaker/eastward adjustment for the northern Plains surface low. By next weekend the operational models have been varying with the details of the overall trough reaching the central U.S. as an upper ridge builds into the West. While not perfectly consistent over the past couple days, the ensemble means have been more stable than individual model runs so the preference was to add steadily more ensemble mean weight later in the forecast. As for the Atlantic upper low tracking into the Southeast, the track and timing appear to have become more sensitive to embedded shortwave details. As of the 12Z/18Z cycles, the GFS/ECMWF and their respective means provided the best clustering for most of the period while the UKMET strayed farther west and the CMC was to the east. In the new 00Z runs the UKMET/CMC look more reasonable into Friday while GFS is farther northeast, but then the CMC strays increasingly northward. The new 00Z ECMWF has also made a pronounced northward adjustment from Friday onward. Progression/opening of the low will depend on unresolved details of the trough reaching the Plains/Mississippi Valley. Above guidance comparisons/preferences led to starting the updated forecast with a blend of the 18Z GFS/12Z ECMWF early in the period followed by gradually more input from their ensemble means, reaching a total of 40-60 percent by next weekend. ...Weather/Threat Highlights... Strong upper ridging will lead to persistent much above normal temperatures over the central-southern Plains and most of the Mississippi Valley through the week, with some of this warmth also extending through the Great Lakes and Northeast. Expect a broad area of highs/morning lows 10-20F above normal with some pockets of anomalies reaching higher than that. Numerous daily records are likely through the week over the southern Plains and Lower Mississippi Valley, and may also extend into the central Plains/Midwest especially around midweek. Temperatures could also challenge record highs over a few places in the Northeast late in the week. Highest heat index values will tend to be from the Lower Mississippi Valley into southern Texas with maximum values approaching or reaching 100F at some locations. The excessive heat, gusty winds, and dry conditions leading to an elevated fire weather threat across parts of the southern High Plains into midweek should moderate thereafter. Weakening of the upper ridge and an approaching cold front should help to start a cooler trend over the Plains/Mississippi Valley by the weekend. Over the West, the initial system beginning to push inland should bring highs of 10-15F below normal to parts of Nevada and California on Wednesday and then incoming Pacific energy will bring a brief episode of minus 5-15F anomalies for highs over the Northwest/northern Rockies Thursday-Friday. Otherwise the region will see a warming trend as an upper ridge builds in by next weekend, bringing highs up to 10-15F or so above normal over and just east of California at that time. Highs over the Southwest could even approach daily records at a few places on Sunday. Periods of rain and higher elevation snow will tend to be light to moderate, but a system arriving around Thursday may enhance totals somewhat over favored terrain in the Northwest. Between the two contrasting regimes through late week, the central U.S. will see periods of rain and thunderstorms. Initial focus on Wednesday will be along a warm front lifting through the northern Plains/Upper Mississippi Valley, followed by a system emerging from the West and tracking over the northern Plains Thursday-Friday. This system will then push a wavy north-south front across the central U.S. with timing and wave details (and thus specifics of highest rainfall totals) still dependent on specifics of trailing dynamics flowing through the West. These fronts and the ejecting western system may produce some areas of significant rainfall that would be a runoff threat over northern tier areas that already have very wet soil. Some areas over the central U.S. may see a threat for strong to severe convection, with details likely to become more clear over coming days. The western Atlantic upper low forecast to retrograde into the Southeast should start to spread moisture farther inland around Friday. Expect moisture to expand in coverage during the weekend as the upper low lifts northward and opens up in response to the upper trough reaching the central U.S. Currently expect the highest rainfall totals to be over portions of the Carolinas into Virginia. 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