Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sun Apr 7 2024 Valid 12Z Wed Apr 10 2024 - 12Z Sun Apr 14 2024 ...Heavy rain and flash flooding threat across the southern U.S. next week with less extreme but potentially significant rain over other parts of the East... ...Overview... Guidance continues to show vigorous low pressure development from the Lower Mississippi Valley through the Great Lakes into Canada Wednesday onward as an upper low initially tracking through the southern Plains phases with amplifying northern stream flow to yield a large scale upper trough over the East by Friday-Saturday and finally starting to depart by next Sunday. Gulf inflow will interact with the upper dynamics/surface fronts to produce heavy/excessive rainfall across the South with a broad area of less extreme but still potentially significant rain over some areas farther north as well as along much of the East Coast. A trailing upper ridge crossing the lower 48 will bring a pronounced warming trend to the West and then the Plains/Mississippi Valley. Meanwhile guidance has been rapidly changing for how much upper troughing may move into the West toward the end of the week or next weekend, with trends toward a deeper trough leading to expanding precipitation coverage and higher totals. ...Guidance/Predictability Assessment... The updated forecast based on 12Z/18Z guidance started with an emphasis on the 12Z ECMWF with smaller input from other guidance (and less than usual GFS weight) early in the period, followed by a trend toward more balanced GFS/ECMWF input along with some incorporation of the 18Z GEFS/12Z ECens means. For the system affecting the eastern half of the country, the main consideration is that an overwhelming majority of guidance, including dynamical models/means and ECMWF-initialized machine learning models (MLs), suggests that the GFS continues to run about 6-12 hours fast with the developing surface low during Wednesday- Thursday. Deceleration of the GFS after Thursday allows other guidance to catch up by Friday, with better clustering observed from that time onward. The eastern Pacific/western U.S. forecast has been changing very rapidly in recent guidance runs. 24-36 hours ago the MLs were suggesting that the incoming upper trough could be deeper/sharper than most dynamical guidance, and this trend has continued into today. In fact in 18Z/00Z runs there is an increasing signal for this trough to contain an embedded upper low. The ML forecasts at least offered more support for including the 18Z GFS/12Z UKMET in the forecast blend than might have been the case otherwise. The new 00Z ECMWF has shifted to an embedded upper low as well, so it is likely the manual deterministic forecast to depict additional changes. ...Weather/Hazards Highlights... Expect heavy rainfall to be in progress across the South at the start of the forecast period on Wednesday, with developing low pressure beginning to track northeastward through the Lower Mississippi Valley on Wednesday-Wednesday night helping to pull Gulf moisture northward while dynamics aloft begin to phase into a larger scale upper trough. By the Wednesday-Wednesday night valid period for the Day 4 Excessive Rainfall Outlook, the heaviest convection should begin moving eastward into the Lower Mississippi Valley and Southeast where a Slight Risk area continues to be depicted. The main adjustment from the prior Day 5 outlook was to expand the Slight Risk area a little eastward over Georgia to account for a couple operational models that trended faster, and the occasional bias of guidance to be too slow with convective progression. There is still enough scatter with the location of maximum QPF to preclude introduction of an embedded Moderate Risk but the localized maxima do suggest such potential once better clustering develops. Also of note, guidance has been consistent in depicting a separate enhanced QPF axis to the north of the low track from the ArkLaTex northeastward through the Ohio Valley, but still with poor agreement for specifics. The surrounding Marginal Risk area accomodates this area of potentially enhanced rainfall. By Thursday-Thursday night, upper trough phasing and continued deepening/northward progression of low pressure will promote a broad southerly flow of moisture across the East. Within this large area of potentially enhanced rainfall (but likely with less extreme totals than forecast across the South), the Day 5 ERO proposes a Slight Risk area encompassing parts of the Mid- Atlantic and southern New England. This region shows the best guidance overlap and continuity for relatively higher rainfall totals as well as higher antecedent soil moisture/stream flows. Other areas within the large surrounding Marginal Risk have varying degrees of sensitivity and mixed messages from the guidance regarding location/timing of enhanced rainfall. Rainfall may continue into Friday over the Northeast. The deepening storm system may also produce brisk to strong winds over some areas. The Storm Prediction Center is monitoring the potential for severe weather with the convection forecast to track across the southern tier. Check their latest outlooks for more information. Recent trends for a stronger upper trough to move into the West suggest greater areal coverage and higher totals for precipitation from Thursday into the weekend. The best emphasis for rain and higher elevation snow should be from the Pacific Northwest into the northern-central Rockies. The more extreme side of the envelope, which is not out of the question given the significant model changes in the past day or so, could even bring meaningful moisture into California and the Southwest. The upper trough and leading surface system may reach far enough east to start developing some areas of rain over the central U.S. by next Sunday. The upper low crossing the southern Plains on Wednesday will result in some lingering moderately below normal highs before a moderating trend. To the east of the upper low and phasing northern stream flow with associated surface low and cold front, a majority of the eastern half of the country will see near to above normal temperatures, especially for morning lows, during Wednesday- Thursday. Coastal areas over the Northeast may be exception, with cooler highs due to onshore flow north of a backdoor front settling into the northern Mid-Atlantic around Wednesday-Thursday. Passage of a couple fronts will bring a cooler trend from west to east late in the week. Meanwhile an upper ridge progressing from the West through the Plains and beyond will support an area of well above normal temperatures (plus 10-20F anomalies) moving from the West into the Plains Wednesday Wednesday through Saturday, with more moderate warmth extending into the East by next Sunday. A trailing upper trough with leading cold front will then bring a cooler trend to the West and the northern High Plains by the weekend. 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 forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw