Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Thu Mar 28 2024 Valid 12Z Sun Mar 31 2024 - 12Z Thu Apr 04 2024 ...A low pressure/frontal system will spread potentially heavy rain and mountain/northern tier snow to much of the country next week... ...Overview... Upper troughing with an embedded upper low will move across California and the Four Corners states early next week. This feature and its surface low pressure system will direct moisture into California and the Southwest/Great Basin into the Rockies on Sunday and produce heavy rain and higher elevation snow. These upper/surface features will interact with northern stream troughing in the north-central to northeastern U.S. to consolidate a surface low that should move from the central Plains to the Northeast during the workweek. Expansive precipitation coverage is likely with this system, with snow and freezing rain possible on its north side, and heavy rain in the warm sector as moisture pools along the surface low's fronts. ...Guidance/Predictability Assessment... Recent model guidance continues to be in fairly good agreement through the early part of next week with the overall pattern consisting of an upper low moving into the southwestern U.S. and a broad ridge ahead of it across much of the central and eastern U.S. while northern stream shortwaves track through the far northern tier. A multi-model blend of the deterministic models worked well for the first part of the medium range period. Through the workweek, more model differences arise as the West low moves eastward and energy combines/interacts with a northern system dropping into the Great Lakes. The details of possible phasing of these systems are very uncertain, with considerable variations in the operational and AI/machine learning models. Generally the model lean seems to be toward phasing by Tuesday or Wednesday rather than the southern stream low and the northern stream trough remaining separate. Even among the more phased solutions, some energy may be left behind in northern Mexico or so though. The 12Z GFS and ECMWF were reasonably agreeable in showing development of a closed upper low atop the Northeast late Wednesday-early Thursday while a tongue of energy/troughing stretches southwest. However, the 18Z GFS seems to dig more vorticity from the northern stream southward, combining with southern stream energy to produce a closed low earlier and farther southwest of the 12Z guidance, which naturally also affects the surface low position. This 18Z GFS run seemed like an outlier. The 00Z model suite fortunately shows somewhat less spread in the developing closed low position. Upstream, some troughing coming into the Pacific Northwest by midweek has some model spread as well. The 00Z ECMWF came in quite a bit faster than its previous run. By the mid to late medium range period, used increasing amounts of the GEFS and EC ensemble means in the blend to 60 percent by Day 7 given the increasing model spread. For QPF, made a notable change to the NBM to bring down precipitation amounts across the Midwest to Great Lakes around Monday. The NBM/bias-corrected model blend seemed to be heavily influenced by the 18Z GEFS mean that had heavier precipitation farther north there, while even individual GFS runs as well as other model suites have QPF focused farther south. ...Weather/Hazards Highlights... A potent storm system will move west to east next week and lead to widespread precipitation across much of the country. On Sunday, a weak atmospheric river should be pushing into Arizona. A Marginal Risk is in place in the Excessive Rainfall Outlook for much of Arizona on Sunday where heavy rainfall may occur, but from the Mogollon Rim northward across the Intermountain West to Rockies, snow is the likely precipitation type in higher elevations. Moderate to heavy snow totals are possible. Additionally, though the atmospheric river will have passed beyond southern California by Sunday, convective rainfall could occur there as steeper lapse rates come in with the upper low overhead. These showers may have some higher rain rates that could cause some isolated flooding concerns especially after a wet couple of days, so a Marginal Risk is delineated in southern California too. A drying trend is expected in the West Monday-Tuesday, except for the Pacific Northwest that will see increasing support for precipitation by Tuesday and beyond. The upper low will continue shifting inland through early next week, and with consolidating surface low pressure in the central Plains, a period of gusty winds is possible across much of New Mexico and southwest Texas. Farther east, moisture likely from the Pacific and Gulf will combine and produce rain and thunderstorms especially near frontal boundaries. The Ohio Valley will be one area of focus for potentially heavy rainfall. Some rain is likely there on Sunday but amounts are forecast to increase by Monday. A Marginal Risk is in place from the Middle Mississippi Valley into the Ohio Valley on Monday as a starting point for the Day 5 ERO, with certainly some potential for future upgrades. Moisture and instability are anomalously high for this time of year, and showers and storms could train along the west to east oriented warm front nearby. By Tuesday the cold front and low should continue to push east and the heaviest rainfall at that point is forecast to focus in the Tennessee Valley into the Appalachians. On the northern side of the storm system, snow and freezing rain are potential threats from the north-central Plains on Sunday, stretching into the Upper Midwest and Great Lakes region Monday and higher elevations of the Northeast by Tuesday-Wednesday. However, there is still considerable uncertainty in the placement and amounts of snow and ice, so continue to monitor forecasts. The Interior Northeast may see the heaviest snow amounts overall because of the duration of the potential snow event there compared to the north-central U.S./Midwest/Great Lakes. Chilly temperatures linger across the north-central U.S. through Monday, but across the southern half of the Plains and Mississippi Valley into the Tennessee Valley and Southeast/southern Mid- Atlantic, notable warmth is expected into Sunday as upper ridging builds. Temperatures of 10-20 degrees above average will become widespread, with highs well into the 70s reaching Kansas to Virginia while highs climb into the 90s to even 100F in southern Texas. A cold front is forecast to drop temperatures in the south- central U.S. by Tuesday, focusing this warmth in the Southeast before the front pushes through there too by midweek. Meanwhile in the Southwest, cooler than average temperatures particularly in terms of highs are likely underneath the upper low. The Pacific Northwest could see above average temperatures for a period early next week though. Tate 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