Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 PM EDT Sun Apr 25 2021 Valid 12Z Wed Apr 28 2021 - 12Z Sun May 02 2021 ...Heavy rain threat from the Southern Plains into the Middle Mississippi Valley around midweek... ...Guidance/Predictability Assessment and Preferences... At least in the 00Z/06Z cycles there was momentary agreement on what was recently an emerging trend toward more progressive northern stream flow amplifying into the Northeast (with developing late week/weekend New England into Canadian Maritimes low pressure) and corresponding slower trend for an upper low that may drift from the U.S.-Mexico border into the far southern Plains/Gulf Coast region. Aside from typical spread and variability for forecasts several days out in time, guidance has been more stable with the upstream pattern that features a strong upper ridge building over the West after midweek followed by weakening/progression of the ridge as a Pacific shortwave arrives by next Sunday. A composite of the 06Z GFS and 00Z ECMWF/CMC/UKMET depicted the consensus for significant features in the updated forecast from day 3 Wednesday into day 5 Friday. Given the expected location of the southern upper low as of late week and recent trends for northern stream flow, preference remained with the GFS/ECMWF/CMC runs through next weekend in order to maintain reasonable definition--with the understanding of persistent uncertainty as seen in the ensemble spread. The operational model blend ended up close to the ensemble means for the upper trough reaching the West late in the period. Perhaps not surprising given recent history, new 12Z guidance is back to offering widely divergent solutions. The UKMET becomes slow and deep with upper troughing that amplifies into the Upper Midwest and Great Lakes as well as more progressive with the southern tier upper low, returning to a phased solution for which the GFS had been the last holdout before the 00Z run. The UKMET also becomes quite flat and progressive with eastern Pacific shortwave energy late in its run. The ECMWF has also jumped faster with the southern tier low and by the end of the period uses later northern stream energy to phase with the southern feature. On the other hand the CMC has held onto an evolution close to the 00Z CMC/ECMWF for most features while the GFS has nudged a little faster for the southern tier upper low but is still a member of the slower cluster. ...Weather/Hazards Highlights... Early in the period the forecast is fairly consistent in depicting a heavy rainfall event that should already be in progress as of early Wednesday from parts of Texas and Oklahoma into the Middle Mississippi Valley, as Gulf moisture interacts with a wavy surface front initially extending from the Great Lakes into western Texas. Slow movement of the wavy front around midweek could allow for training of convection and an increased flood risk. Continue to check Storm Prediction Center outlooks for the latest information on severe weather potential. The ultimate evolution and progression of this precipitation area after early Thursday is still very uncertain in light of new guidance diverging once again after models had reached a momentary consensus in the prior cycle. There is still a wide range of possible outcomes, from locally enhanced rainfall settling over Texas and the central Gulf Coast region to faster progression and a drier trend over the Plains. Either way some of the initial moisture should extend into parts of the Great Lakes/Ohio Valley and Northeast with some meaningful totals possible. Most of the Northwest will be dry Wednesday-Thursday as the best flow of moisture focuses on western British Columbia/Vancouver island, with light amounts of precipitation possibly extending into the Olympics and vicinity. The approach/arrival of the Pacific shortwave and leading front will spread mostly light to moderate precipitation from the Pacific Northwest into northern Rockies late this week into the weekend. The strong upper ridge building into the West after midweek will bring a brief period of much above normal temperatures to the region. Parts of northern California and Oregon may see highs reach 10-20F above normal already on Wednesday and a majority of the West should see such anomalies (with localized readings 20-25F above normal possible) from Thursday into Saturday. Some locations may reach daily records for highs/warm lows. The approaching Pacific trough aloft and leading cold front will bring a pronounced cooling trend from the Pacific Northwest eastward/southward by next weekend, possibly bringing highs down to slightly below normal over northern areas by Sunday. The western warmth will extend into the northern half of so of the Plains by Friday-Saturday with highs 10-20F or so above normal, and sagging into the Midwest/central Plains on Sunday as a front approaches. Farther south, the upper low forecast to drift near the U.S.-Mexico border will lead to cool daytime highs of 10-20F below normal for one or more days over parts of the southern Rockies/High Plains. Expect above to well above normal temperatures from Wednesday into early Friday ahead of a wavy front from the Northeast/Great Lakes into southern High Plains early Wednesday and pushing southeastward through late week. Warmest anomalies of plus 10-20F for highs, and perhaps locally a bit higher for morning lows, should extend from the southern Midwest through the Lower Great Lakes/Ohio Valley and Mid-Atlantic. Frontal passage will likely bring near to slightly below normal temperatures over the East into the weekend. 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