Extended Forecast Discussion NWS Weather Prediction Center College Park MD 305 AM EDT Fri Mar 18 2022 Valid 12Z Mon Mar 21 2022 - 12Z Fri Mar 25 2022 ...Strong low pressure system will likely produce heavy rain/severe storms across portions of the southern U.S. and heavy central Rockies snow by early next week... ...Overview... The main upper level feature of note during the medium range period will be an initially closed low/trough tracking from the Four Corners states Monday northeastward through the workweek with an associated surface low pressure system. This will spread a myriad of significant and hazardous weather to the U.S., including potentially heavy snow over the central Rockies and vicinity, heavy rain from the south-central U.S. to the Ohio/Tennessee Valleys and Southeast, and severe thunderstorms from Texas through the Deep South. Some precipitation in the northern fringe of the moisture shield could be in the form of snow for the central/eastern U.S. as well. Behind this system a strong upper ridge will build into the West, bringing mostly dry and warm weather to the region. ...Guidance Evaluation/Predictability Assessment... Model guidance agrees that a well-defined upper low should be closed with its centroid likely over New Mexico by Monday morning, lifting into the southern half of the Plains by Tuesday. The initial position of this upper low along with the associated surface low on Day 3/Mon did have a bit of west-east spread in the 12/18Z (yesterday) model cycle, with the ECMWF the farthest west and GFS runs a bit on the eastern side, but a compromise position of deterministic guidance worked well early in the medium range period and was consistent with ensemble mean guidance and continuity. Guidance also agrees that strong ridging coming into the West will serve to amplify the trough. By Day 5/Wed and beyond, there is more notable spread in the 12/18Z cycle (and run to run of guidance cycles) with the evolution of the upper low/trough, with questions about stream separation, width of the trough, and trough axis/how much energy is held back to the west. The 12Z ECMWF was particularly neutrally tilted and narrow with its trough pattern with more phasing in northern and southern streams for Day 6/Thu. GFS runs were positively tilted and with more stream separation, with some southern stream energy held back across the south-central U.S./Mexico for late week. The 12Z CMC appeared to be a broad, rounded outlier with its 500mb height pattern compared to other deterministic guidance and ensemble members by Wednesday through Friday, more separated from the polar jet and the slowest solution for its track eastward. However, the newer 00Z model cycle appears to be more agreeable, and actually trending overall toward the CMC with a more rounded upper low especially Wed with more separation from the polar jet, and a slower overall trough movement. At the time of forecast creation, the latter part of the medium range period went to an ensemble mean-heavy blend perhaps earlier than average given these differences, with manual adjustments to deepen surface lows. The 12Z EC ensemble mean was a good proxy for the preferred consensus at least in terms of mass fields, and the WPC forecast had good continuity with previous forecasts from this approach. Future changes to the forecast remain likely given run to run model differences, perhaps with a slower front track especially by Day 7/Fri in the East. ...Weather Highlights/Threats... Ample Gulf of Mexico moisture flowing into the surface/upper-level lows tracking across the country will lead to widespread rain and thunderstorms for the Southern Plains and Lower Mississippi Valley early in the week, spreading eastward into the Ohio and Tennessee Valleys and Southeast by around midweek. Per the Storm Prediction Center, storms could be severe in eastern Texas Monday and across the Gulf Coast states as the week progresses. Furthermore, heavy rain is likely to cause areas of flash flooding particularly due to heavy rain rates Monday into Wednesday, and Slight Risk areas are indicated near the Ark-La-Tex/eastern Oklahoma and for the Lower Mississippi Valley/Mid-South region for Days 4 and 5 in the experimental WPC medium range Excessive Rainfall Outlook. Rain should spread into the Eastern Seaboard around Thursday. Additionally, heavy snow is increasingly likely for the central Rockies Monday, with some potential for snow to spread into the High Plains as well, along with an area of enhanced rainfall totals likely near the Central Plains/Middle Mississippi Valley (though with lesser rainfall amounts than farther southeast). Precipitation coverage from the northern half of the Plains into the Midwest and Northeast has much lower confidence, but if moisture reaches that far north some precipitation could be in the form of snow. In the West, mainly dry conditions are likely after a period of precipitation in the Pacific Northwest on Monday. Somewhat below normal temperatures are expected underneath the upper level troughing, with particularly high temperatures 10-20F below average across the Intermountain West Monday and central/southern High Plains Tuesday and Wednesday. The cooler air should moderate closer to normal by the latter part of next week. Meanwhile, the Midwest will see warmer than average temperatures by around 20F on Monday, and generally warmer than normal conditions are forecast for the eastern third of the U.S. through much of the week ahead of the main system. The West should also see warmth build with the ridging, with temperatures 10-20F above normal expanding from the West Coast inland through the workweek. Daily record highs are possible particularly in California. 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 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml