Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 PM 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 latest model guidance continues to offer good agreement on the overall large scale pattern, but plenty of lingering uncertainty in the details. The most impactful part of the forecast surrounds details of the Southwest to Eastern U.S. upper low/trough. Guidance continues to show notable timing differences as the low shifts and opens up over the Southern Plains into the Tennessee Valley mid- week. The GFS remains on the faster side of the guidance envelope, but not out of the realm of possibilities, especially considering much of the incoming 12z guidance this afternoon also have trended slightly faster. A look at the 00z ECMWF-initialized machine learning models shows variability on the timing of this system too, which may affect eventual phasing with northern stream energy mid to late week. These seemingly subtle differences though, have a larger impact on the QPF forecast. While all the guidance shows increasing potential for an impactful heavy rainfall event across the South, there are plenty of lingering questions on where the heaviest rainfall axis may set up. Elsewhere, 00z/06z guidance struggled with shortwave timing late week into the Northwest (CMC was quite a bit faster/more south), but 12z guidance have come more in line with the ECMWF and GFS/ensemble means. The WPC blend for today was able to utilize the deterministic models for days 3-5, increasing contributions from the ensemble means later in the period. Did weight the blend more heavily towards the ECMWF which was most in the middle of the guidance spread for the Southern U.S. upper low. This maintained good continuity with the previous WPC shift as well. ...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. Gusty winds are possible on the backside of the system across parts of California too, mainly on Monday. 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. Santorelli/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