Extended Forecast Discussion...UPDATED NWS Weather Prediction Center College Park MD 315 PM EDT Sun Apr 7 2024 Valid 12Z Wed Apr 10 2024 - 12Z Sun Apr 14 2024 ***Heavy rain and strong storms for the Gulf Coast and Southeast states on Wednesday, then the Mid-Atlantic and Northeast on Thursday*** 19Z Update: Model guidance has improved on the more amplified solution across the Western U.S. for later in the forecast period, with a well defined upper low becoming more likely. The CMC is slightly slower with the passage of that trough, whereas the ECMWF is more progressive and a little weaker, with the GFS a middle- ground solution. For the storm system across the eastern U.S. Wednesday through Friday, there is decent overall model agreement, even though the GFS/GEFS mean are faster bringing the low towards the Great Lakes. The UKMET differed with the surface pressure pattern across the Plains by bringing the surface high farther north into the Midwest by Friday compared to the model consensus, which indicates this region will still be more under the influence of the departing surface low. Taking these factors into account, the updated forecast was derived mainly from a CMC/ECMWF/GFS blend through Friday, along with some UKMET on Wednesday. Since model agreement has improved overall through next weekend, the ensemble means increased to just 30 percent of the forecast along with some previous WPC continuity. The previous discussion, along with the updated ERO discussion, are appended below. /Hamrick ---------------- ...General 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 the 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... Heavy rainfall will likely be ongoing across the central Gulf Coast and Deep South on Wednesday, with developing low pressure beginning advecting Gulf moisture northward while dynamics aloft begin to phase into a larger scale upper trough. For the Wednesday- Wednesday night time period for the Day 4 Excessive Rainfall Outlook, the broad Slight Risk area remains valid with a slight expansion to the north to include southern Tennessee and farther northwest in Mississippi. The models are showing the potential for two main axes of enhanced QPF, with one across northern Mississippi and Alabama, and a second and potentially heavier band closer to the Florida Panhandle. There is still enough model variance in the 12Z guidance with the location of maximum QPF to preclude introduction of an embedded Moderate Risk for now, but the localized maxima do suggest such potential once better clustering develops in future model runs. There is also a separate enhanced QPF axis north of the low track from the ArkLaTex northeastward through the Ohio Valley, but still with limited agreement for specifics. The surrounding Marginal Risk area accommodates this area of potentially enhanced rainfall. With the storm system lifting to the northeast by Thursday, the Day 5 ERO maintains the Slight Risk area, but expanded to include northwestern North Carolina. This region shows the best guidance overlap and continuity for relatively higher rainfall totals as well as higher antecedent soil moisture and current 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 the 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 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/Hamrick 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