Extended Forecast Discussion NWS Weather Prediction Center College Park MD 204 AM 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 southward 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 continues to show good agreement on the very large scale pattern evolution during the period, but some significant details still in the details and timing of individual systems. Right out of the gate, yesterdays 12z and 18z GFS runs were faster than consensus with the shortwave through the Central U.S., but tonight's 00z run (available after forecast generation time) trended slower. The GFS was also much stronger with initial energy into the West Coast mid week which moves across the north-central U.S., with fairly decent surface low pressure tracking through the Midwest later this week. The rest of the guidance is more subdued with this energy showing only a wave of low pressure along the associated frontal boundary. Energy on the backside of the mean trough should then dig southward eventually with a closed low over the Southwest/Four Corners region while at the same time northern stream energy shifts across the northern Plains/Upper Great Lakes. There are still some very notable differences in the details/placement of this system and also how much phasing there is (or is not) with the northern stream energy. Also uncertainty on how quickly the upper low ejects eastward next weekend. The WPC blend for tonight used a general model blend for days 3 and 4 (using the 12z GFS over the 18z GFS as it looked more reasonable than the 18z/00z runs with shortwave energy through the Midwest). For Days 5-7, incorporated more and more of the ensemble means to account for the late period uncertainties. Maintained good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... Western U.S. remains unsettled early week as a couple of shortwaves bring rounds of rain and higher elevation snow initially shifting more inland into the central Rockies with time. Rainfall should also begin to increase in coverage and intensity over the Plains and Mississippi Valley by Wednesday as troughing amplifies over the West. Currently, no areas are depicted on the Days 4 and 5 Excessive Rainfall Outlooks. Then, 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 next Thursday and Friday. Areas across the South may be more susceptible to flooding with this system considering recent heavy rains. Some threat for locally heavy rain/mountain snows will exist across the Southwest as well in association with this system. 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 week. Over the West, the wet/snowy pattern over 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 with temperatures beginning to rebound again along the immediate coast next Friday-Saturday as strong ridging builds in aloft. 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