Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM 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 is considerable uncertainty over details of Pacific energy moving into western North America around the end of next week. ...Guidance/Predictability Assessment... For the upper low ejecting from the southern Rockies and subsequent phasing with northern stream flow, 12Z/18Z guidance still showed quite a bit of spread but the new 00Z runs seem to be showing some consolidation. Over recent runs the GFS/GEFS have tended to be on the faster side with the overall evolution while the CMC has been a slow extreme. The 00Z GFS/GEFS are still somewhat fast but a tad slower than the 18Z version, while CMC/UKMET details are gravitating closer to the ECMWF (including the new 00Z run) that has been fairly consistent thus far with an intermediate timing. The 12Z ECMWF-initialized machine learning models (MLs) favored slower timing than the GFS as well, but with a variety of ideas for specifics beyond that. Overall preference based on the 12Z/18Z guidance was to stay fairly close to continuity given the ongoing spread, which led to greater emphasis on the 12Z ECMWF after Tuesday with some input of other models/ensemble means to account for detail uncertainty. The other significant issue in the forecast involves Pacific flow that reaches western North America toward the end of next week. Ensemble spaghetti plots have been quite messy over this region the past couple of days, with operational runs also varying for the depth and timing of a potential shortwave. Issues include the ultimate path/evolution of a Gulf of Alaska upper low, with poor agreement for that particular feature. Adding to the uncertainty, most of the 12Z MLs actually favored a more amplified and sharper upper trough reaching the West by next Saturday versus most of the dynamical models and ensemble means. This would lead to a slower northern tier surface system and farther south progression of the trailing cold front over the West. For now the manual forecast depicts the fairly conservative representation of the incoming shortwave per the majority of dynamical guidance but it is worth watching for any trend toward the ML ideas. ...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 new 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. There continues to be potential for an upgrade to an embedded Moderate Risk area in the shorter term if confidence increases in overlap of Tuesday's heavy rainfall with that of the prior day. 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 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 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 within what may be a line of south to north training convection. 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 that time, the best focus for heaviest rainfall may be northwestern Florida and vicinity plus portions of the northern Mid-Atlantic/Northeast. 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. 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 may bring some precipitation to the region but still with low confidence in the details at this time. 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 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. Confidence in the temperature forecast over parts of the West declines by the end of next week due to uncertainty in amplitude/timing of Pacific shortwave energy and southeastward extent of the leading cold front. 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