Extended Forecast Discussion NWS Weather Prediction Center College Park MD 303 PM EDT Sun Mar 10 2024 Valid 12Z Wed Mar 13 2024 - 12Z Sun Mar 17 2024 ...Heavy rainfall potential returns to the South late next week... ...Overview... A weak shortwave will be moving through the mid-Mississippi Valley Wednesday while the next one amplifies over the western states. The general flow across the contiguous states will transition to being more amplified with upper ridging building over the East allowing for the trough over the West to dig southward into the Southwest. A deep closed low developing over the Southwest/Four Corners region on Thursday, and lingering into the weekend. This would promote a multi-day period of heavy rainfall potential across parts of the Southern U.S. late this week. Elsewhere, northern stream energy will amplify across the Upper Midwest/Great Lakes next weekend, as a strong ridge shifts over the West Coast. ...Guidance/Predictability Assessment... The latest suite of guidance depict a growing signal for a deep, closed low to develop across the southwest and exit into the plains as the week progresses. Some of the details regarding this feature vary from one model to the next however consensus suggests it should linger or very slowly drift southward across the Southwest/far northern Baja. There has been a trend towards less separation between the northern stream and the Southwest upper low, showing the northern stream system further deepening/amplifying over the Midwest/Ohio Valley next Sunday as the upper low meanders/is cut off over the Southwest/far northern Mexico and a blocky ridge over the West Coast. WPC maintained a similar forecast approach as the previous for continuity. The WPC blend utilized 06/00Z GFS and 00Z CMC/ECWMF/UKMET for the Days 3-4, with a majority weighting towards the GFS and the ECMWF. By Day 5, replaced the CMC and the UKMET (which phases off after Day 5 anyways) with the ensemble means and gradually increased their influence through Day 7. This helps to mitigate some of the low confidence smaller scale details late period. ...Weather/Hazards Highlights... Early week precipitation over the Northwest will shift into the Rockies by Wednesday, with potentially heavy snowfall across parts of the Colorado Rockies into Thursday, and more uncertain potential for snowfall across the Front Range urban corridor as well. Across the higher elevations of New Mexico snow is anticipated to persist later in the week. Rainfall is expected to begin across portions of the Northern/Central Plains and the Mississippi Valley by Wednesday as the western trough amplifies. Although there will be ample moisture and instability in proximity to the frontal for efficient rainfall to develop, the risk for excessive rainfall will more than likely be less than Marginal given recent drier conditions. the areal coverage of convection will greatly increase across the Gulf and surrounding locations, which will help increase soil moisture across the South. A surge in moisture ahead of the approaching low by Thursday will bolster QPF rates and amounts over a vast area and should increase the threat for heavy rain and potentially flash flooding across the Lower Mississippi Valley/Gulf Coast/Southeast. There is still plenty of uncertainty in exact placement of the heaviest rains, but there is support in the dynamics and areal QPF coverage to introduce a fairly broad Slight Risk area on the Day 5/Thursday Excessive Rainfall Outlook across Arkansas, Louisiana and Texas region into northwest Mississippi. The heavy rainfall threat may shift slightly eastward more into the Gulf Coast states/Southeast Friday and Saturday, with a likely flash flood threat given recent heavy rains in this region. Severe weather is also a threat with this convective activity per the Storm Prediction Center with a severe weather area delineated for parts of the southern Plains on Thursday. Mean ridging aloft over the east-central and eastern U.S. will support an expansive area of above normal temperatures to the east of the Rockies. The core of warmest anomalies should cover parts of the Upper Midwest and Upper Great Lakes on Wednesday, with highs potentially reaching 20-30 degrees above normal. Temperatures will trend warmer into the South and East mid to late next week as well with daytime highs well into the 70s to near 80 for some. Over the West, moderately below average highs and near/above average lows will shift from the Northwest on Wednesday into the Great Basin/Four Corners Thursday and beyond. Temperatures along the immediate West Coast should begin to rebound again by next Friday-Saturday as strong ridging builds in aloft. Campbell/Santorelli 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, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: 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