Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1205 PM EDT Tue Apr 30 2019 Valid 12Z Fri May 03 2019 - 12Z Tue May 07 2019 ...Heavy Rain/Flooding Threat Fri/Sat for the Southern Plains/Lower Mississippi Valley... ...Overview... Upper ridging over the northeastern Pacific will remain stationary during the medium range period which will drive a split flow across the Lower 48. Features in the forecast will be the northern stream out of NW Canada and an incoming closed low in the southern stream into California around Sunday. Ridging off the Southeast coast will wobble in place allowing for wavy fronts along the east coast as systems exit into the Atlantic. ...Guidance/Predictability Assessment... In the southern stream (off the west coast), the GFS/Canadian and their ensemble means were quicker than the ECMWF/ECMWF ensemble mean to move the upper low into California either late Saturday or early Monday as the timing difference exceeded 36 hrs between the quicker/slower solutions. 00Z UKMET was about 2/3rds the way to the slower ECMWF from the GFS and this was preferred given: 1) the separation from either an upstream kicker or the northern stream in the Pac NW/Great Basin, 2) tendency of the GFS to be too quick to bring closed lows eastward in weak patterns and 3) continuity which favored the slower side of the ensembles. Over Texas, tail end of a front will linger on Friday and then slowly move southeastward on Saturday as a wave moves along the boundary into the Southeast then off the mid-Atlantic coast Sunday. This will provide a focus for the heavy rain potential (partly into the short range period) along and east of I-35. In the northern stream, consensus approach was maintained which resulted in only a few changes to frontal timing/placement. System moving out of the Dakotas on Saturday will lift northeastward toward Hudson Bay but the ECMWF/UKMET became much quicker than most of the ensembles whereas the GFS/Canadian were more in line with the ECMWF ensemble mean. Preferred to rely mostly on the ECMWF ensemble mean and continuity by Sun-Tue via a larger blend. Confidence decreased by next Mon/Tue in the latitude of the front in the east given a flatter flow aloft and spread in ensembles. Western upper low is forecast to push past the deserts around the middle of next week per current timing. ...Weather Highlights/Threats... The front over Texas will tap pooled moisture over its surface boundary aligned parallel to the upper flow Friday which supports a threat of heavy rain/thunderstorms and cell training/locally enhanced runoff issues as embedded hard-to-time weaker shortwaves move through. More defined shortwave should carry the front southeastward Saturday and move the rain threat eastward, focused mostly along the central Gulf coast Sunday. Rain threat remains into next week as the front lingers along the Gulf coast. This will generally keep temperatures closer to or just below typical values for early May. Pacific closed low will move into CA early next week which will spread scattered Southwest-to-Rockies showers/convection as height falls move eastward. Moisture anomalies may only be +1 to +1.5 sigma from normal per the guidance and other ingredients may be marginal at best, but expect at least isolate showers/storms especially over the terrain from the Sierra across the Great Basin next week. Temperatures will trend toward cooler than average as cloud cover and precipitation increase. In the east, modest rain will accompany/precede frontal passages with higher amounts south of about 40N in the warmer airmass. Northern system will bring more focused precipitation (some snow in the west at higher elevations) as the lead cold front dissipates Saturday to be overtaken by a trailing system moving into the Great Lakes Sunday. Baroclinic zone along the front may periodically focus convection contingent upon mesoscale enhancements. Temperatures will be above average ahead of and south of frontal boundaries by about 5-10 degrees F. Fracasso Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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