Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Sun Mar 21 2021 Valid 12Z Wed Mar 24 2021 - 12Z Sun Mar 28 2021 ...Wednesday-Thursday heavy rain threat most likely centered over parts of the South... ...Overview... Through late in the week two vigorous shortwaves within a large scale mean trough with an axis over the Plains/west-central U.S. will support fairly strong surface systems and a very warm/wet pattern over much of the eastern half of the country. Meanwhile incoming energy will produce areas of rain and mountain snow extending from the Pacific Northwest into much of the Rockies along with below normal temperatures. By next weekend guidance is suggesting that upper ridging over the eastern Pacific will start to build into the West and bring a warmer/drier trend for the region, but consensus is lacking for the shape/strength of the ridge as well as for evolution of the downstream trough. The central-eastern U.S. should see additional precipitation and moderately warm temperatures ahead of a third system next weekend but a slightly flatter trend to the pattern will likely lead to lower coverage of significant rainfall and moderate the warm anomalies. ...Model Guidance/Predictability Assessment... With guidance finally coming together for the system tracking out of the Upper Midwest from early Wednesday onward, the primary forecast issues that remain are for the timing and evolution of strong energy expected to eject northeastward from Arizona/New Mexico along with associated low pressure, followed by the combined details of upstream shortwave energy reaching the western/central U.S. and Pacific ridging that should build into the West. Guidance is continuing to have difficulty in resolving how quickly the upper low just south of the Four Corners on early Wednesday ejects northeastward, and how strong/closed the feature will be as it continues through/beyond the Plains. Over the past 24 hours most guidance has generally held onto the slower timing introduced by yesterday's 12Z cycle (after consensus had trended somewhat faster for the prior two or three runs). Within the latest envelope the 00Z ECMWF is slowest and most closed with latest CMC runs just a little faster and also closed aloft/deep at the surface. GFS runs already become faster and/or more open with the upper feature versus most guidance by Thursday. Recent parallel GFS (GFSP) runs have generally been a tad slower than operational runs. Recent persistence of the slower progression closer to valid time provides support to lean somewhat in the slower half of the spread and a tad slower than continuity. However the slow 00Z ECMWF may not be ideal either as the system is carried along by southwesterly mean flow aloft. The old 12Z ECMWF would be closer to representing current preference. Upstream the general theme of recent trends has been toward some combination of a stronger ridge moving into the West along with a somewhat weaker/less amplified version of the western-central U.S. trough. Individual models/ensembles also display varied ways in which energy on the western fringe of the late week western trough may pull off an upper low near or offshore the West Coast. Parallel GFS runs are more reflective of recent consensus though could be a bit deep relative to ongoing trends. Based on forecast preferences, the operational model blend early in the period used the 06Z GFSP for the GFS input. Then ECMWF input split the past two runs and trended toward half 06Z GEFS/00Z ECMWF mean weight to yield conservative adjustments to account for recent trends over the lower 48 from late week through the weekend. ...Weather/Threats Highlights... Lingering precipitation (rain and/or snow) is likely on the north side of the surface low exiting the Upper Midwest on Wednesday. South of this however, the interaction of moist Gulf inflow with the trailing front--which should stall for a time while awaiting ejection of upstream dynamics from the West and southern Plains/Gulf Coast wave development--is likely to produce heavy rainfall over parts of the South through Wednesday-Thursday. Recent slower trends by the average of guidance have adjusted the northern part of the heaviest rainfall axis a bit westward, now from the central Gulf Coast northward into the Tennessee Valley. Other surrounding areas could see significant rainfall as well and the surface/upper system could be strong enough to produce another band of enhanced precipitation on the northwest side of the surface low track. System strength will also determine how much if any snow may occur in the northwest fringe of the moisture shield. Greater progression of rainfall should lead to lower totals over areas closer to the East Coast. Some warm sector convection may be strong/severe and future Storm Prediction Center outlooks will provide more information on this threat as guidance becomes more agreeable for important details. Before helping to generate heavy rainfall downstream, the upper system initially south of the Four Corners will produce a period of enhanced snowfall over the southern Rockies around Wednesday. The next upper trough moving into the West will bring a mid-late week episode of mostly light-moderate rain/mountain snow to the Pacific Northwest and areas to the east and southeast through the Rockies. Some precipitation (rain and/or snow) should then spread across the northern tier as low pressure continues toward the Great Lakes. Rainfall will likely expand over the East again ahead of this low pressure and trailing front. A warm front may enhance amounts over some southern areas while lighter activity should prevail farther north. Above normal temperatures over the eastern half of the country will be the most extreme mid-late week, in the warm sector ahead of the leading system tracking out of the Upper Midwest on Wednesday and trailing system that may reach into the Great Lakes/Northeast late in the week. Plus 10-25F anomalies for highs/morning lows will be most common from the Great Lakes/Ohio Valley into the Northeast/Mid-Atlantic. Moist flow from the Gulf will also promote a broad area of very warm lows farther southward. Some locations in the East may see lows close to daily record warm values. Most of the East will see a cooler trend next weekend but should stay moderately above normal. The upper ridge strengthening over Cuba mid-late week and persisting into the weekend will promote very warm temperatures over the Florida Peninsula from Thursday onward, possibly bringing high temperatures close to daily records at sites with a relatively shorter period of record. Areas from the western U.S. into Rockies/High Plains will see below normal temperatures mid-late week followed by a warming trend. The most extreme anomalies of 10-20F below normal for highs are likely over the central-southern Rockies and High Plains on Wednesday. Pockets of highs 10F or so below normal are possible over parts of the Great Basin/central Rockies into Friday. The arrival of the Pacific ridge aloft should bring highs up to 5-15F above normal from the West Coast states into northern Rockies by Sunday. Rausch/Santorelli 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, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml