Extended Forecast Discussion NWS Weather Prediction Center College Park MD 359 PM EDT Tue Mar 21 2023 Valid 12Z Fri Mar 24 2023 - 12Z Tue Mar 28 2023 ...Heavy rainfall threat over parts of the east-central U.S. continuing into Friday... ...Overview and Guidance/Predictability Assessment... Upper troughing over the Great Plains will promote diffluence aloft over the Midwest/Mid-South beginning on Friday. The pattern begins to become more uncertain as early as this Saturday when a broader cyclonic flow develops over the northern tier states with bits of shortwave energy propagating through the rest of the weekend. Meanwhile, shortwave energy glides down the West Coast of North America this week and intensifies into a potent upper-level low over the eastern Pacific. Details surrounding this feature are quite uncertain but as of right now it appears that this upper low will stall out off the U.S. West Coast early next week. Despite guidance being all over the place at the end of the period, the synoptic pattern at the beginning of the medium range period was captured reasonably well by this morning's suite of 00/06z guidance. Thus, a general model blend consisting of the 00z EC/CMC/UKMET and 06z GFS were utilized through day 5. The ensembles were introduced on day 5 and continued through the end of the period. ...Weather/Hazards Highlights... Expect heavy rainfall over portions of the east-central U.S. late this week as guidance continues to suggest that developing low pressure emerging from the Plains will focus abundant moisture along its trailing cold front and a leading wavy front. The new day 4 (12Z Friday-12Z Saturday) Excessive Rainfall Outlook will be quite similar to the previous day 5 issuance, reflecting only minor adjustments for latest guidance with respect to a Slight Risk area from parts of Arkansas/northern Louisiana northeastward into southern Ohio and parts of West Virginia. Within this area, guidance is starting to cluster toward heaviest rain totals being along an axis from northeastern Arkansas into Kentucky or the Ohio River. As low pressure continues northeastward Saturday into early Sunday, some localized enhancement of rainfall is still possible but system progression will help to reduce totals compared to the prior couple days. Meaningful snow will be possible over parts of interior/northern New England as a triple point low develops and tracks off the New England coast. Continue to monitor Storm Prediction Center outlooks for the latest information regarding severe weather threats across the southern tier. Shortwave energy and possible upper low moving into the West by the weekend will support periods of rain and mountain snow from the northern/central West Coast into the Rockies. Most amounts should be in the light to moderate range but localized heavier enhancement will be possible, most likely in the Cascades on Friday. Once this trailing energy departs from the West, it may support a system bringing some precipitation of varying intensity to portions of the central/eastern U.S. Sunday onward. Locally moderate to heavy rainfall will be possible over the South while some snow could be possible in the northern fringe of the precipitation shield. Also in that time frame, the upper trough likely amplifying just off the West Coast may start to increase precipitation totals along the central coastal areas and over northern California based on the consensus trough position. However there has been enough spread and variability for the upper trough and possible embedded closed low to reduce confidence in specifics thus far. The forecast pattern will keep much of the West and portions of the northern Plains well below normal for temperatures during the period. Expect broad coverage of highs at least 10F below normal each day and the Great Basin/central Rockies could see highs 20-25F below normal during the weekend. The central Plains may see chilly highs around Sunday-Monday as well. Some locations over the West could see one or more days that challenge record cold highs. Temperatures over the Northwest should be less extreme but still be mostly below normal. Meanwhile the East will tend to see near to above normal temperatures and some variability with system progression. Warmest readings will likely be over central-southern parts of the East late this week ahead of the leading storm system, with highs 10-20F above normal and morning lows 15-25F above normal. Some record warm lows will be possible if morning readings hold for the calendar day, while a few isolated record highs could also be possible over the South. Frontal passage across these areas will bring a cooling trend to near or moderately above normal levels by Saturday, followed by a modest rebound of temperatures ahead of early next week's system. Kebede/Rausch 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 Hazards: - Heavy rain across portions of the Lower/Middle Mississippi Valley, the Tennessee Valley, the Great Lakes, the Ohio Valley, and the Southern/Central Appalachians, Fri, Mar 24. - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, the Southern Appalachians, and the Tennessee Valley, Sun-Mon, Mar 26-Mar 27. - Heavy rain across portions of California and the Pacific Northwest, Mon-Tue, Mar 27-Mar 28. - Heavy snow across portions of the Pacific Northwest and the Northern Great Basin, Fri, Mar 24. - Heavy snow across portions of the Northeast and the Great Lakes, Sat, Mar 25. - Heavy snow across the Sierra Nevada, Mon, Mar 27. - Severe weather across portions of the Southern Plains, the Lower/Middle Mississippi Valley, the Tennessee Valley, the Southeast, and the Ohio Valley, Fri, Mar 24. - Flooding possible across portions of the Middle Mississippi Valley, the Great Lakes, the Ohio Valley, and the Tennessee Valley. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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