Extended Forecast Discussion NWS Weather Prediction Center College Park MD 404 PM EDT Tue Apr 18 2023 Valid 12Z Fri Apr 21 2023 - 12Z Tue Apr 25 2023 18Z Update: The latest model guidance continues to indicate an amplifying upper trough over the central U.S. that becomes negatively tilted going into the weekend across the Ohio Valley and Great Lakes region. There is also an increase in QPF across portions of the Ohio Valley Friday and also the Northeast U.S. on Saturday compared to earlier forecasts. There is still noteworthy latitudinal spread with the location of the main surface low by Sunday morning, with the UKMET well south of the ensemble consensus, and the past two runs of the GFS have also been south of the ensemble means and the ECMWF and CMC models. The forecast for this time period was hedged more towards the ECMWF/CMC given the better ensemble support. Going into early next week, the guidance is in good agreement on a low developing over the southern Plains and a surface high over the eastern U.S., but greater differences exist with shortwave timing across the northeast Pacific and also southern Canada. Inclusion of the ensemble means increased to about 40% by next Tuesday, while initially starting with a deterministic blend for Friday. The previous forecast discussion is appended below. /Hamrick ------------------------- ...Overview and Guidance/Predictability Assessment... The upper trough progressing eastward from the central U.S., as well as one or more embedded upper lows, have been providing various forecast challenges for surface evolution and precipitation in recent days. Latest consensus shows an initial upper low over or near Minnesota on Friday tracking northeastward into Canada, followed by formation of another low over the central/upper Great Lakes by Sunday. The combination of southern Greenland and Canadian Maritime ridging that may push into far eastern Canada could stretch the upper low early next week while holding mean troughing farther south over the East. This evolution aloft will most likely correspond to a leading system initially tracking out of the Great Lakes, followed by a Texas wave that deepens as it reaches the Ohio Valley/Great Lakes (with some cold sector snow potential over the Great Lakes), and then another wave evolving near the New England coast as ridging to the north begins to prevent northward progression. Meanwhile, most guidance shows a mean ridge building into the western U.S. by the weekend. Then models and ensembles have been showing considerable spread and variability for how strongly incoming Pacific shortwave energy pushes into the ridge, with uncertainty in the offshore pattern as well. Based on the 12Z/18Z guidance used to produce the updated forecast, the GFS/ECMWF and to some degree CMC provided decent clustering for the central/eastern U.S. details through the period. The 12Z UKMET was a notable slow solution with the Texas into Great Lakes wave. Exactly where the Great Lakes upper low closes off during the weekend will be an important consideration for the forecast, and will likely require more time to resolve given the finer-scale shortwave details involved. For the time being, the ensemble means support the operational model cluster which represents a slight southward adjustment from previous cycle. Likewise, the means and models are similar in principle for how the upper low stretches northwest-southeast underneath far eastern Canada ridging. Overall, the favored operational model composite worked well for the first half of the period, followed by a trend to half total 18Z GEFS/12Z ECMWF means by day 7 Tuesday to downplay lower confidence details from a particular guidance source. Over the West, the aforementioned blend reflected what appeared to be continuation of improved clustering from the prior 00Z cycle with a ridge holding on over the West while an eastern Pacific mean trough approaches. CMC/CMC mean runs have still held onto the prior idea of more troughing moving into the West early next week, and now the 00Z GFS brings more of a shortwave into the West. ...Weather/Hazards Highlights... Ahead of the advancing central U.S. upper trough, the heaviest rainfall during the period should extend from the Lower Mississippi Valley northeastward. The Day 4 (12Z Friday-12Z Saturday) Excessive Rainfall Outlook proposes the addition of a Slight Risk area over parts of the Lower Mississippi Valley, within the existing Marginal Risk area from yesterday's Day 5 outlook. Guidance is starting to show a more coherent signal for where heaviest rain is most likely ahead of a frontal wave tracking from Texas into the Ohio Valley. Some heavy rain may continue into the Day 5 period (12Z Saturday-12Z Sunday) over the Mid-Atlantic/central Appalachians, with the Excessive Rainfall Outlook initially depicting a Marginal Risk from North Carolina into southeastern West Virginia, where initial ground conditions are not as dry as areas to the north. Increased clustering for heavier totals elsewhere would merit expansion of the area. Thereafter, the Northeast should become the primary focus for highest totals as the surface front/embedded wave support a period of Atlantic inflow and the overall system decelerates/stretches out due to a blocking ridge well to the north. As for snow potential, light amounts will be possible over the northern tier on Friday behind the leading Great Lakes system. The next system tracking into the Great Lakes could produce meaningful accumulations depending on exact evolution/track. Across the West, the majority of precipitation should be with rain and higher elevation snow over the Pacific Northwest and northern Rockies. Persistent detail differences early in the period, and larger scale issues by late weekend/early next week continue to temper confidence in determining precipitation coverage and amounts. Areas near the East Coast should see above normal temperatures late this week into the weekend, with Friday being the warmest day (some highs 20F or more above normal and possibility to challenge a few record highs). In contrast, expect below normal temperatures over much of the central U.S. Friday-Saturday, with fairly broad coverage of minus 10F or greater anomalies. Parts of the northern Plains could see highs 20-25F below normal on Friday. The colder air will push into the East later in the weekend into early next week, with gradual moderation. Meanwhile, moderately above normal temperatures should drift across West and reach the Four Corners states and High Plains by next Tuesday. 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 Southeast, the Lower Mississippi Valley, the Southern Plains, the Ohio Valley, and the Tennessee Valley, Fri, Apr 21. - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Tue, Apr 25. - Heavy rain across portions of the Mid-Atlantic, the Northeast, and the Great Lakes, Sun, Apr 23. - Severe weather across portions of the Lower Mississippi Valley and the Southern Plains, Fri, Apr 21. - Flooding possible across portions of the Middle Mississippi Valley, the Northeast, the Great Lakes, the Upper Mississippi Valley, and the Northern Plains. - Flooding occurring or imminent across portions of the Lower Mississippi Valley, the Great Lakes, the Upper Mississippi Valley, and the Northern Plains. - Flooding likely across portions of the Great Lakes, the Upper Mississippi Valley, and the Northern Plains. - Much below normal temperatures across portions of the Central Plains, the Middle Mississippi Valley, and the Southern Plains, Sat-Sun, Apr 22-Apr 23. - Much below normal temperatures across portions of the Central Plains, the Lower Mississippi Valley, the Central Appalachians, the Tennessee Valley, the Great Lakes, the Middle Mississippi Valley, the Southern Appalachians, the Upper Mississippi Valley, the Southern Plains, and the Ohio Valley, Sun-Mon, Apr 23-Apr 24. 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