Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Sat Apr 6 2024 Valid 12Z Tue Apr 09 2024 - 12Z Sat Apr 13 2024 ...Heavy rain and flash flooding concerns will increase in the southern U.S. next week... ...Overview... The overwhelming focus of the forecast will be on an upper low ejecting from the southern Rockies and eventually phasing with northern stream flow to yield an amplified and larger scale eastern U.S. upper trough by late next week. With this evolution aloft, expect surface low pressure to deepen as it tracks from Texas or near the western Gulf Coast northeastward through the Great Lakes and into eastern Canada. There will be a couple days or so of persistent Gulf inflow which will interact with the upper dynamics/surface fronts to produce a multi-day heavy/excessive rainfall event from parts of the southern Plains and Lower Mississippi Valley into the Southeast. A broad area of less extreme but still potentially significant rain will be possible over some areas farther north as well as along much of the East Coast. A western U.S. upper ridge that reaches the Plains by next Saturday should bring a pronounced warming trend to those areas. While there remains some uncertainty over the details of Pacific energy moving into western North America around the end of the week, guidance has trended towards increasing precipitation chances spreading inland from the Pacific Northwest into the Rockies. ...Guidance/Predictability Assessment... Guidance trends have remained similar overnight with respect to the handling of a split stream pattern over the central/eastern U.S. early in the forecast period for the middle of next week. The updated 00Z/06Z guidance shows the ECMWF/CMC/UKMET retaining a slower eastward progression of a southern stream upper trough/closed low moving eastward from the Southwest towards the Mississippi Valley Tuesday-Thursday compared to the more recent runs of the GFS, which have been trending faster. The ECMWF/CMC also have a similar handle on the eventual evolution/phasing of the southern stream system with the northern stream over the Midwest/Southeast mid- to late next week, continuing a slower progression of the southern stream energy with a more amplified trough approaching the East Coast compared to the GFS. The evolution in the 00Z ECens/CMC means is similar to their deterministic counterparts, with the GEFS mean similar to the GFS. The suite of 00Z EC AI models as well as the ICON follow a similar evolution early to the ECMWF/CMC/UKMET cluster. The EC AI forecasts do start to show increasing uncertainty as the streams phase, though in general due tend to be more amplified and slower. The upper-trough progression speeds up in the ICON, though it remains more amplified. There is more uncertainty upstream over the West and eastern Pacific into later next week. The guidance shows a similar overall picture, with a weak southern stream system approaching California and lead northern stream shortwave energy moving into the Northwest ahead of an upper trough/low over the northeastern Pacific. Run- to-run consistency and already increasing uncertainty in the details at this time frame leads to less confidence in the exact evolution, and the EC AI models also depict some variance in the solutions. The 00Z GEFS mean is also rather neutral compared to the other deterministic/ensemble guidance. However, decent clustering in regards to the overall pattern suggests leaning towards a potentially stronger system. This does bring a notable increase in precipitation chances/QPF from the Pacific Northwest into the Great Basin/Rockies later next week. The updated WPC forecast blend relied on a combination of the 00Z ECMWF/CMC/UKMET for the early to mid-forecast period. Increasing contributions from the 00Z ECens and CMC means were added for the later half of the period given increasing uncertainty in the details with respect to the evolution of the system entering the Northwest. The GFS was not included given the timing and evolution differences with respect to the first central/eastern U.S. system, with no contribution from the GEFS mean either given how it differed from the ECens and CMC means with respect to the western U.S. later in the period. The tendency of the EC AI models and the ICON to more align with the ECMWF/CMC/UKMET cluster, especially in the early to mid-forecast period, helped support this updated blend. This also generally tends to follow continuity with the prior forecast as a slower progression of the southern stream system has been favored. ...Weather/Hazards Highlights... The primary focus of next week's forecast from a hazard perspective will continue to be a multi-day heavy rainfall threat from parts of the southern Plains eastward as the upper low tracking out of the southern Rockies interacts with a persistent flow of moisture from the Gulf of Mexico and one or more leading surface fronts. This event should already be in progress over the southeastern Plains/Lower Mississippi Valley as of the start of the period early Tuesday, and will likely persist/expand thereafter. The Day 4 Excessive Rainfall Outlook valid Tuesday-Tuesday night maintains a Slight Risk area extending from northeast Texas to the southwest corner of Tennessee with a surrounding Marginal Risk area. The surrounding Marginal Risk area accounts for lower- consensus ideas from various guidance, including the GFS being on the northern side of the spread with a separate enhanced QPF axis, and the ECMWF/UKMET/CMC runs being consistently heavy with their rainfall axis over the southern High Plains just to the north of the upper low track. The synoptic pattern, influx of Gulf moisture, and deterministic QPF amounts/ensemble exceedance probabilities suggest that an upgrade to a Moderate Risk is likely at some point. However, the overlap in the guidance, particularly the current differences between the more northerly GFS/GEFS mean and southerly ECMWF/ECens mean do not provide enough confidence in the location of this higher threat at this time. The heavy rain threat should begin to push farther eastward by midweek, with the Day 5 ERO valid Wednesday-Wednesday night depicting a Slight Risk area from parts of the Lower Mississippi Valley into Georgia. While the system is beginning to accelerate in this time frame, guidance continues to show significant maximum totals/ensemble exceedance probabilities within what may be a line of south to north training convection. Similar to day 4/Tuesday, an eventual upgrade to a Moderate Risk looks increasingly likely if forecast trends remain the same, with the locally highest threat currently extending from southern Mississippi northeastward through central/southern Alabama and into northern Georgia. Again similar to Tuesday, 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, though confidence in location and amounts here is lower. Expect a broader area of meaningful rainfall to the north, with some pockets of heavy rainfall possible, with areas of moderate to heavy rain reaching the East Coast later in the week. At this time, the best focus for heavier rainfall looks to be across north Florida and along the Appalachians north through the Mid-Atlantic into New England. The deepening storm system may also produce brisk to strong winds over some areas. The Storm Prediction Center is also monitoring the potential for severe weather with this southern tier event, passing from the Southern Plains Tuesday east to the Lower Mississippi Valley Wednesday and the Southeast Thursday. Check their latest outlooks for more information. The upper low tracking through the southern Rockies may produce some areas of meaningful snow over higher elevations on Tuesday while the Northwest could see lingering light precipitation with a front reaching the area. An upper shortwave and surface system reaching the Northwest late in the week looks to bring some precipitation chances to portions of the Pacific Northwest, Great Basin, and Rockies, including lower elevation rain and higher elevation snow, particularly for the Cascades. The upper low crossing the southern Rockies/High Plains Tuesday into Wednesday will support high temperatures up to 10-20F below average over the region on Tuesday with some lingering below normal readings lingering over the southern Plains into Wednesday. To the east of the upper low, a majority of the eastern half of the country will see near to above normal temperatures, especially for morning lows. The Northeast may be one exception, with near to below normal temperatures due to cool high pressure behind a backdoor front that settles 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 building into the West and eventually reaching the Plains will steadily expand coverage of plus 10-20F anomalies during the Tuesday-Saturday period. Cooler temperatures look to settle into the Northwest by late week as a cold front passes through the region. Putnam/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