Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 PM EDT Sat Mar 20 2021 Valid 12Z Tue Mar 23 2021 - 12Z Sat Mar 27 2021 ...Midweek heavy rain threat over parts of the South, most likely from the central Gulf Coast into southern Appalachians... ...Overview... Expect a large scale mean trough aloft to persist over the Plains/west-central U.S., between a ridge over the eastern Pacific and a strengthening Cuba/Bahamas ridge whose influence will extend into parts of the Atlantic and Gulf of Mexico. Shortwaves within the trough will spread areas of rain and mountain snow from the Pacific Northwest through much of the Rockies, and with the help of Gulf moisture generate a broad area of rainfall over the eastern half of the country. The greatest potential for heavy rain currently exists from the central Gulf Coast into the southern Appalachians. Models and ensembles still agree on the overall pattern while gradually attempting to come close together or stabilize for important timing and detail aspects of significant features. ...Model Guidance/Predictability Assessment... The primary systems of interest will be a fairly vigorous low tracking northeast from the central Plains Tuesday onward, upstream energy digging into the West/Rockies into early Wednesday then ejecting eastward/northeastward (with east-central U.S. wave tracking into eastern Canada), and finally another shortwave settling over the Interior West/Rockies late in the week (associated low pressure reaching the Plains by Friday-Saturday). For the midweek Plains/Midwest storm, a slower trend by the GFS/GEFS mean over the past day has narrowed the timing spread somewhat. However new 12Z guidance does not improve confidence as the GFS has actually nudged a bit faster again while the CMC leans slower and the UKMET continues to hold its position on the slow side of the spread. The 12Z ECMWF maintains its intermediate continuity. Consensus continues to show the core of the upstream shortwave reaching a little south of the Four Corners by early Wednesday. Through the 00Z-06Z cycles the timing spread was becoming a little less dramatic compared to prior days. A compromise 70 percent toward the 00Z ECMWF/06Z GFS and their means relative to the somewhat slower 00Z CMC/06Z parallel GFS (GFSP) maintained continuity well and reflected the relative balance of 00Z-06Z guidance. The average of new 12Z solutions has adjusted a little slower. Meanwhile the late week shortwave reaching the West still shows better than average clustering and continuity in the guidance though the 00Z ECMWF could start to get a little fast by day 7 Saturday (slower in the new 12Z run). Models/means are making gradual adjustments with specifics of associated low pressure reaching the Plains late in the period. The operational model blend employed for the first half of the forecast excluded the 00Z UKMET as it was slow with upstream features in addition to the Midwest storm. Favorable comparison of an average among GFS/GFSP/ECMWF/CMC runs with the 06Z GEFS/00Z ECMWF means allowed for a somewhat higher model-to-mean weight than would typically be the case by late in the period. ...Weather/Threats Highlights... Locally moderate to heavy precipitation may accompany the system tracking northeast from the central Plains Tuesday onward. Much of this activity should be in the form of rain but the northwest fringe of the precip shield may contain some snow. Best potential for any snow that occurs would extend from near central Nebraska into northern Minnesota. The interaction of Gulf inflow with the trailing front--which should decelerate and possibly stall for a time while awaiting ejection of upstream dynamics from the West--is likely to produce heavy rainfall over parts of the South around midweek. Locations from the central Gulf Coast region into the southern Appalachians currently have the best potential to see the highest rainfall totals with this event. There is still enough uncertainty that nearby areas will need to monitor forecasts as well. Expect the aforementioned western energy to generate a wave tracking west of the Appalachians Thursday into Friday. This system will spread areas of moderate to heavy rain through more of the East late in the week, with somewhat lower totals than over the South due to faster progression/shorter duration. Another area of rain may develop over the South next weekend as a warm front returns northward ahead of the next upper trough reaching the West. The two systems affecting the West will bring rain and mountain snow from the Pacific Northwest eastward-southeastward into much of the Rockies, with highest totals over favored terrain in the Olympics, northern Cascades, and northern-central Rockies. The large scale pattern from Tuesday into the weekend will tend to favor above normal temperatures over the eastern half of the lower 48 and below normal readings from the West into the Rockies and parts of the High Plains. Expect the highest warm anomalies of plus 10-25F to extend from the Upper Midwest/western Great Lakes into the Northeast Tuesday-Friday, ahead of the system tracking northeast from the Plains and trailing wave lifting through the east-central U.S. Morning lows could be close to daily record warm values over parts of the East Coast. Proximity of the strengthening Cuba/Bahamas upper ridge may push Florida Peninsula high temperatures close to daily records at sites with a relatively shorter period of record. Farther west there will be two episodes of coolest temperatures. One around Tuesday-Wednesday should feature highs 10-20F below normal over the Great Basin and central/southern Rockies. The second will likely progress from west to east Thursday-Saturday with most highs 5-10F or so below normal, possibly 10-15F below normal over the north-central Rockies/High Plains by next Saturday. 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