Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Sat Apr 20 2019 Valid 12Z Tue Apr 23 2019 - 12Z Sat Apr 27 2019 ...Overview and Guidance Preferences... Guidance spread and variability for details of flow across the Pacific and extending into/across the northern Lower 48 and southern Canada continue to keep confidence fairly low for many aspects of the forecast. The one part of the forecast that has better confidence is the slower moving southern stream trough that progresses from the southwestern U.S./northwestern Mexico into the southern Plains Tue-Thu. There are still some differences and run-to-run adjustments in this time frame but a strong signal persists for associated heavy rainfall over the southern Plains and likely extending at least into the Lower Mississippi Valley. After early Thu the character of northern stream flow will become increasingly important to the timing/progression of the southern trough. The latest forecast update reflects a blend of the past two 12-hourly ECMWF/6-hourly GFS runs along with minority input from the 06Z GEFS/00Z ECMWF means during the Tue into Thu time frame. The rest of the forecast eliminates the GFS runs, holds onto ECMWF/ECMWF mean/GEFS mean input, and introduces some WPC continuity. The preferred blend reflects gradually improving consensus toward fairly slow departure of the initial New England upper low, but not as slow/amplified as the 00Z CMC becomes. Behind this low the blend remains close to continuity (an intermediate solution) for southeastern Canada shortwave energy--depicted in the guidance as anything from a weak open wave to a concentrated closed upper low. Thus far models and means have held onto or even furthered a bit yesterday's faster trend for the front crossing southern Canada and the northern tier of the U.S. By late next week prefer to stay closer to ECMWF-based continuity which reflects greater stream separation over the East versus the aggressive northern stream amplification (and western Atlantic or New England storm development) depicted in the GFS/CMC runs. Evolution of recent systems would also favor more stream separation. Farther westward one favorable trend in the guidance is better clustering among GFS/ECMWF runs and their means toward northeastern Pacific cyclonic flow during the latter half of the period--as supported by teleconnections relative to the upper low over northern Canada. Based on this cluster/teleconnection relationship, ridging in the 00Z CMC mean looks rather suspect. The forecast looks as uncertain as ever farther south over the eastern Pacific. The 00Z ECMWF and some ECMWF ensembles quickly pull off energy from a shortwave around 140-145W by Thu while a greater proportion of models/ensembles bring more of the shortwave energy toward the West Coast. Thus a more even blend of GFS/GEFS and ECMWF-based guidance may be more suitable over this region at least into day 6 Fri. ...Weather Highlights/Threats... The system forecast to affect the southern Plains Tue-Wed should produce widespread showers and thunderstorms, with models/ensembles showing a strong signal for heavy rainfall potential over Texas and Oklahoma. Some convection may be strong to severe as well. The Storm Prediction Center currently indicates best chances for severe weather in the early Wed-early Thu time frame. Check latest SPC products for further information. Expect scattered to numerous showers and thunderstorms to accompany the system eastward into the Lower Mississippi Valley and Southeast later in the week but increasing uncertainty in system strength and timing reduce confidence in duration and intensity of rainfall to the east of the Plains. Current preference for a somewhat slower solution would maintain decent potential for locally heavy rainfall into portions of the Gulf Coast and Southeast. Waviness extending from the Midwest/Great Lakes into New England and associated front(s) early in the period will provide a focus for rainfall over parts of the East. Still expect snow to be confined to locations north of the Canadian border except perhaps for northern Maine. A front pushing into the Northwest and then continuing eastward may generate some rainfall but most activity should be in the light-moderate range. Precipitation may persist for a time over parts of the Interior West/Rockies as the western part of the front stalls. Some moisture may reach the Northwest late in the period but timing/extent are uncertain given the wide spread in guidance for details of flow aloft at that time. There will be a broad area of above average temperatures with plus 10-20F anomalies over the West, with some of this warmth extending into the northern and then central Plains. Daily records (especially for warm lows) are most likely on Wed-Thu and some are possible on Fri as well. The front pushing into the Northwest will produce a cooling trend over northern parts of the West. Below normal highs will prevail over the southern Plains especially on Tue-Wed. Parts of the northern tier should trend a little below normal for highs by Fri-Sat. Rausch 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