Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM 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 dominant focus of the medium range forecast will be an upper low expected to track northeast from the Four Corners states Monday onward, possibly opening up as northern stream energy phases with it after midweek. This feature will support strong low pressure that should track from the southern High Plains through the Great Lakes and New England/southeastern Canada. Expect the system to spread a myriad of significant and hazardous weather across the eastern two-thirds of the lower 48, 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 over 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. The anomalous warmth over the West could bring record highs to parts of California around Tuesday-Thursday. Pacific shortwave energy may erode the western ridge somewhat late in the week. ...Guidance Evaluation/Predictability Assessment... During the first half of the period, the latest 00Z/06Z operational model consensus provides a good depiction of significant features. Guidance maintains the recent tendency to keep the central U.S. upper low intact into Wednesday as it reaches the Midwest. There are still noticeable differences with specifics at the surface but these have lower predictability due to sensitivity to exact shortwave details aloft. Meanwhile guidance has become more agreeable with the cyclonic flow over the Northeast during the first half of the week, with solutions generally gravitating to an evolution between the extremes seen in earlier GFS and ECMWF clusters. Meanwhile the upper ridge building into the West should be strongest during Tuesday-Wednesday. The latter half of the forecast still contains various uncertainties, in particular exactly how Pacific shortwave energy will dampen the northern part of the western North America ridge. These details affect how southern tier Canada flow may interact with upper low reaching the Midwest by midweek, and then how much if any flow separation may occur in the vicinity of the Northeast U.S. and southeastern Canada plus timing of the rest of the trough over the lower 48. Thus far the ensembles suggest that the 00Z GFS/CMC may be overdone with how much separation occurs over eastern North America and the 06Z GFS has partially nudged back toward consensus. Farther south/southwest, the majority says the upper trough should extend back into northern Mexico into late week. The 00Z ECMWF could be a bit fast, though much improved over its prior run and not extreme versus its ensemble members. The 00Z CMC leans to the weaker side of the full spread with the western ridge late in the period. Guidance comparisons led to trending the operational model blend toward 40-50 percent 06Z GEFS/00Z ECMWF means by days 6-7 Thursday-Friday with lingering model input from the 06Z GFS/00Z ECMWF. ...Weather Highlights/Threats... Ample Gulf of Mexico moisture flowing into the surface/upper-level lows tracking northeastward over the central U.S. 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. An area of enhanced rainfall also appears likely from the Central Plains into Middle Mississippi Valley/Midwest, though with lesser totals than those expected farther southeast. There is a gradual improvement in agreement that at least some precipitation should reach areas from the northern parts of the Plains through the Northeast, with some of this moisture possibly producing snow. Probabilities for at least a quarter inch liquid in the form of snow are still relatively low though. In the West, the Pacific Northwest should see an episode of rain/high elevation snow on Monday and possibly a little light/scattered activity later in the week. Otherwise the West will see dry weather through the week. The strong upper ridge building into the West will lead to a broad area of highs 10-20F above normal Tuesday-Friday with slightly higher localized anomalies possible. Best potential to reach daily record highs will be over California during Tuesday-Thursday. Dampening of the northern part of the upper ridge by Friday may start a gradual cooling trend over some areas but also spread the warmth across the eastern Great Basin and Southwest/Four Corners region. Leading upper level troughing will support high temperatures 10-20F below average across the Intermountain West Monday and the central/southern High Plains Tuesday and Wednesday. The cooler air should moderate closer to normal by the latter part of next week and could trend moderately above normal Friday. Ahead of the Plains storm, the Midwest will see warmer than average temperatures by around 20F or so on Monday and generally warmer than normal conditions are likely across much of the eastern third of the U.S. for most of the week. Isolated record highs are possible over parts of Florida Tuesday-Wednesday. Rausch/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