Extended Forecast Discussion NWS Weather Prediction Center College Park MD 137 AM EDT Sun Mar 21 2021 Valid 12Z Wed Mar 24 2021 - 12Z Sun Mar 28 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 the most significant features. ...Model Guidance/Predictability Assessment... One shortwave and associated deep surface low will be exiting the Upper Midwest Wednesday-Thursday, as a second follows on its heels (and a bit farther south) from the Southern Plains to the Northeast Thursday-Saturday. The deterministic models show fairly good agreement at this time with both systems so WPC leaned on a majority deterministic model blend for days 3-5. Bigger forecast questions and uncertainty begin to arise by late week as shortwave energy amplifies over the Western U.S. and shifts east into the Plains by Sunday/Day 7. The latest (18z/Mar 20) run of the GFS splits northern and southern stream energy more than the rest of the guidance, developing a deep closed low over Southern California by the end of the period. There is a little support for this from the latest run of the CMC as well. However, run to run continuity is poor for both pieces of guidance (as well as the ECMWF which is quicker to bring the trough into the Plains) so a blend towards the ensemble means was preferred for days 6 and 7. ...Weather/Threats Highlights... Some 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 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 continue to show 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 Great Lakes into the Northeast Wednesday-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 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. Santorelli/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, 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