Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sat Mar 09 2024 Valid 12Z Tue Mar 12 2024 - 12Z Sat Mar 16 2024 ...Heavy rainfall potential returns to the South late next week... ...Overview... The medium range period starts Tuesday with one shortwave moving through the Central U.S. as the next one enters the West Coast. After this, the pattern looks to trend much more amplified with upper ridging building over the East Coast allowing for the second system over the West to dig southeastward into the Southwest. Guidance shows general agreement on a deep closed low developing over the Southwest/Four Corners region on Thursday, and lingering or only very slowly moving eastward into the weekend. This should promote a period of heavy rainfall potential across parts of the Southern U.S. late this week. Elsewhere, northern stream energy will slide across the Upper Midwest/Great Lakes late week, as a strong ridge shifts over the West Coast. ...Guidance/Predictability Assessment... Model guidance is generally agreeable on the larger scale pattern through much of the period, with some typical differences in the details that tend to increase with lead time. The 00Z CMC was on the slower side with the initial shortwave moving across the central U.S. Tuesday-Wednesday. Then by Thursday, 00Z and 06Z GFS runs were showing a notable shortwave in the Plains that produces stronger surface low pressure than the non-NCEP guidance, but the 12Z GFS seems to be in better alignment. Overall the 00Z ECMWF seemed to be the best proxy for a middle ground solution and favored it for the forecast blend. This continues into the latter part of next week when larger model differences arise, primarily with energy on the backside of the mean trough digging southward to form a southern stream closed low over the Southwest/Four Corners region while at the same time northern stream energy shifts across the northern Plains/Upper Great Lakes. Most operational and AI models show stream separation taking place, with the exception of the 00Z CMC that showed phasing. With the majority of models and continuity favoring the stream separation, this forecast did as well. The 12Z CMC has transitioned to more stream separation. However, there is still uncertainty among models with the placement of the southern closed low and with the depth and position of the northern stream trough. As the forecast progressed, transitioned toward a model blend composed of half EC/GEFS ensemble means by the Day 6-7 timeframe. ...Weather/Hazards Highlights... The Western U.S. remains unsettled for the early part of the period as a couple of shortwaves bring rounds of rain and higher elevation snow, affecting the Pacific Northwest/northern California eastward into much of the Intermountain West on Tuesday, and shifting more inland into the Rockies with time. The central Rockies can expect the highest snow amounts on Wednesday and Thursday, with some uncertainty with how much snow spreads into the Front Range urban corridor. Snow should continue into higher elevations of New Mexico for the latter part of the week. Farther east, rainfall will begin to increase in coverage and intensity over north-central parts of the Plains and Mississippi Valley by Wednesday as troughing amplifies over the West. Precipitable water values may be too low for any flooding concerns with this activity, and antecedent conditions are quite dry, so any excessive rainfall threat there looks to be at sub-Marginal levels. On Tuesday and Wednesday, considerable convection is likely over the Gulf of Mexico, which may sneak into the west coast of Florida by Wednesday, but model differences in amounts and placement preclude any area in the Excessive Rainfall Outlook at this point. But by the latter part of the week, moisture streaming ahead of the closed low developing over the Southwest should increase the threat for heavy rain and potentially flash flooding across the Lower Mississippi Valley/Gulf Coast/Southeast. Areas across the South may be more susceptible to flooding with this system considering recent heavy rains. Severe weather is also a threat with this convective activity--the Storm Prediction Center has 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 expanding area of above normal temperatures to the east of the Rockies. The core of warmest anomalies should cover parts of the northern Plains and Upper Midwest Tuesday through Wednesday, with highs potentially reaching 20-30 degrees above normal. Above normal temperatures will gradually expand into the South and East mid to late next week. Over the West, the wet/snowy pattern over the northern-central West Coast and eventually into the Rockies will maintain moderately below average highs and near/above average lows through early next week. Upper troughing that amplifies inland by Wednesday-Thursday should spread a broader area of cool daytime highs across the West, though temperatures should begin to rebound again along the immediate coast next Friday-Saturday as strong ridging builds in aloft. Tate/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