Extended Forecast Discussion NWS Weather Prediction Center College Park MD 237 AM EDT Wed Mar 17 2021 Valid 12Z Sat Mar 20 2021 - 12Z Wed Mar 24 2021 ...Overview... Semi-progressive pattern will feature upper ridging moving through southern and eastern Canada Saturday-Monday with successive troughs through the West. Over the Southeast and off the coast, a weakness aloft may spur an area of low pressure along a frontal boundary but should slowly move away from the nearby waters. Main precipitation threat will be out of the Rockies and into the Plains to the Mississippi Valley. ...Model Guidance/Predictability Assessment... Aside from the 12Z Canadian, which handled the exiting eastern trough quite differently than the rest of the guidance, a blended solution of the 12Z/18Z GFS and ECMWF/UKMET offered a reasonable starting point this weekend with both the system off the Southeast and the incoming lead western trough. By next Mon-Wed, ECMWF was slower to move the lead trough out of the Four Corners region than the GFS while the ensembles were mostly in the middle and were the preferred route as no clear trends have been evident the past few cycles. As the system emerges onto the Plains early next week, ensemble means continued to cluster well as the deterministic runs showed a bit more spread (especially the 18Z parallel GFS which was rather quick to the east). Increased ensemble weight couple with the 18Z GFS and 12Z ECMWF maintained good continuity, albeit perhaps a bit slower than 24 hrs ago, as the system lifts toward the Great Lakes next Wednesday. Also at that time, second western trough is forecast to dig down into the Southwest with good agreement overall and only slight timing differences for this lead time (GEFS a bit quicker but well within tolerance). Aforementioned blend for the central system worked well in the Southwest/Four Corners region, roughly 50/50 ensemble/deterministic. ...Weather/Threats Highlights... System offshore Florida may wrap back some showers along the coast but likely keep the heavier rainfall over the Gulf Stream this weekend before slowly pulling away. High pressure to the north will keep temperatures chilly to start but then warm up quite smartly as southerly flow brings in milder temperatures to the Great Lakes and then Northeast. Temperatures in the High Plains/Upper Midwest could approach record highs ahead of the cold front. As the lead trough exits the Rockies, return flow out of the Gulf will steadily increase available moisture to the central states ahead of the front. This should first occur near the best dynamics (central Plains) Sunday then expand on Monday as inflow and moisture increase from the Gulf (lower MS Valley northward). Second/reinforcing upper trough (and weak surface reflection) will dig through the West Mon-Tue bringing some light valley rain and mountain snow to northern California, parts of the Great Basin, and the central Rockies. This could act to slow the front over eastern Texas as the northern part of the system heads into the Great Lakes, perhaps enhancing rainfall into the lower MS Valley. Fracasso 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