Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Fri Apr 5 2024 Valid 12Z Mon Apr 08 2024 - 12Z Fri Apr 12 2024 ...Heavy rain and flash flooding concerns will increase in the southern U.S. next week... ...Overview... Behind a weakening system tracking slowly northeastward from the northern Plains/Upper Midwest, the dominant focus of the forecast will turn to digging western energy that may already form a closed upper low over the Southwest by Monday and then gradually track eastward into the Plains by midweek or so. Amplifying northern stream energy downstream from an upper ridge moving into the West should ultimately phase with the southern feature to yield a larger scale mean trough centered over the east-central U.S. by late in the week. However there are a lot of detail uncertainties with this phasing process and the resulting surface evolution after midweek. In the meantime there is greater confidence in the threat of a multi- day heavy/excessive rainfall event from parts of the southern Plains into Lower Mississippi Valley early-mid week, and extending eastward thereafter, as the southern upper low interacts with a leading front and persistent low level Gulf inflow. This system may also produce significant snowfall over portions of the Colorado/New Mexico Rockies. ...Guidance/Predictability Assessment... The dynamical models/ensembles and 12Z ECMWF-initialized machine learning models (MLs) offer good clustering in principle for significant features from Monday into early Wednesday. The primary stray solution is the new 00Z CMC which tracks the southwestern upper low farther southwest than other guidance from Monday onward, leading to delayed phasing to the east. The 12Z CMC was a bit on the southwest side as well but fit closer to the western side of the guidance envelope. By the latter half of the week, the dynamical models/ensembles and MLs significantly diverge with respect to exactly how the amplifying northern stream flow interacts with the southern system. This leads to widening spread for where the best-defined surface low pressure will track by late in the week, with solutions by Friday ranging anywhere from the Tennessee Valley into southeastern Canada. A relatively higher percentage of solutions would suggest a surface low in the vicinity of the eastern Great Lakes or southeastern Canada as of early Friday. Any meaningful delay in stream phasing would lead to a farther south surface low emphasis. Currently a model/ensemble mean blend provides a reasonable intermediate representation of the most likely evolution. Farther west, guidance becomes rather chaotic regarding details of Pacific energy that may try to push into the western North America ridge by next Friday. At the very least the deep trough reaching the Pacific Northwest coast in the new 00Z CMC is fairly extreme versus other guidance, with either a more subtle shortwave or a farther offshore trough being the more likely options. Guidance comparisons among the 12Z/18Z guidance led to starting with an operational model composite for the first half of the period, followed by incorporating some of the 18Z GEFS/12Z ECens means while also splitting GFS input between the 12Z/18Z runs as detail differences emerge. This led to an even weight between the ensemble means and GFS/ECMWF/CMC by next Friday. ...Weather/Hazards Highlights... The dominant focus of next week's forecast from a hazard perspective will be a multi-day heavy rainfall threat from parts of the southern Plains eastward as the upper low tracking out of the Southwest interacts with a persistent flow of moisture from the Gulf of Mexico and one or more leading surface fronts. The Days 4-5 Excessive Rainfall Outlooks covering the Monday through Tuesday night time frame depict Slight Risk areas centered over northern Louisiana and southern Arkansas, with the Day 5 area (plus the surrounding Marginal Risk) expanding relative to Day 4 as guidance shows increasing moisture anomalies aiding the potential for heavier rainfall. Depending on how consistent guidance is in showing Tuesday's activity overlapping with Monday's, the current Day 5 time frame has potential for a Moderate Risk upgrade at some point in the coming cycles. Beyond Tuesday night, expect the heavy rainfall to continue eastward across the southern tier while less extreme but still potentially significant rain expands across the East later in the week. Southern tier rainfall should gradually become a little more progressive after midweek assuming surface low pressure track northward with its trailing cold front becoming the primary rainfall focus. The Storm Prediction Center is also monitoring the potential for severe weather with this southern tier event. Check their latest outlooks for more information. Otherwise, the system tracking northeastward from the Upper Midwest may produce some light precipitation in its vicinity and farther east across the Great Lakes along its trailing front. To the west of the southern tier heavy rain area, the system tracking through the Southwest should bring a period of significant snowfall to portions of the Rockies in Colorado and New Mexico during the first half of the week. Greatest accumulations should be during Tuesday- Tuesday night. The northern Pacific Northwest/Rockies should see some light to moderate precipitation early in the week as a frontal system brushes the region. A little moisture could return to the Pacific Northwest by next Friday with another front, but confidence in details is fairly low at this time. Moderately below normal high temperatures will cover most of the West on Monday and then concentrate more strongly over the southern Rockies/High Plains Tuesday-Wednesday (up to 10-20F below average) as the Southwest upper low passes overhead. The West will see a warming trend from west to east next week as upper ridging builds in, with an expanding area of temperatures at least 10F above normal. This warmth will extend into the northern Plains as well. The eastern half of the country will generally be above normal due to being in the warm sector of an initial Upper Midwest system and then a trailing front and ejecting Southwest system. Frontal passage late in the week will bring a somewhat cooler trend to the East. 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