Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Thu May 16 2024 Valid 12Z Sun May 19 2024 - 12Z Thu May 23 2024 ...Overview... At least for the moment, most guidance seems to be starting to gain somewhat better agreement and consistency for the larger scale evolution during the period. An initial western trough plus an incoming shearing-out southern stream Pacific low should eject eastward Tuesday onward while a Bering Sea into Gulf of Alaska upper low may drop into the Northwest U.S. by next Thursday to reinforce the mean trough over the West. The leading ejecting western energy should support Plains into eastern Canada surface low pressure with a trailing front expected to push through most of the eastern half of the lower 48 Tuesday-Thursday. This will support areas of locally heavy convection across parts of the Plains/Midwest/Great Lakes with activity continuing east/southeast with time. The Northwest should trend colder with gradually increasing precipitation, including high elevation snow in the northern Rockies. A system near the East Coast on Sunday may linger for a while just offshore the Southeast/southern Mid-Atlantic coast. ...Guidance/Predictability Assessment... While there are ongoing detail uncertainties and/or a stray solution or two for certain aspects of the forecast, continuity of general ideas from the 12Z/18Z guidance majority plus 00Z model trends of prior less agreeable solutions have incrementally improved confidence for the general ideas of most forecast features. That said, the forecast of the system along/offshore the southern half of the East Coast continues to be a challenge for the guidance. Once again the average of dynamical guidance is a little slower than most 12Z ECMWF-initialized machine learning (ML) models, while the dynamical solutions are still shuffling around a little for timing. The 12Z/18Z dynamical cluster was a little slower than 12 hours prior, but the 00Z UKMET has strayed to the fast side. Behind this system, there is a more coherent theme today that the combination of ejecting initial western U.S. troughing, an incoming southern stream Pacific low that shears out underneath or just behind the ejecting trough, and a lingering southern Canada upper low will support Plains through Great Lakes into eastern Canada surface low development next week. By day 6 Wednesday there has been noticeable convergence among a majority of dynamical models/means plus ML models toward low pressure reaching eastern Ontario/western Quebec. Note that there is still significant potential for detail adjustments given the influence of three separate features. The ECMWF/GFS and the ensemble means have developed a reasonable cluster for the upper low expected to reach the Northwest by mid- late week, with typical scatter for timing and longitude. The 12Z UKMET/CMC disagreed with this scenario, instead having more sheared flow reinforcing the mean trough, but the new 00Z runs look more like the GFS/ECMWF cluster. The ML models offer general support for this evolution as well. Guidance comparisons among the 12Z/18Z guidance led to starting with an operational model composite early in the period followed by transitioning out the less favored CMC/UKMET after early Tuesday and then trending to an even mix of GFS/ECMWF and their ensemble means by the end of the forecast Thursday. ...Weather/Hazards Highlights... During the Sunday-Monday night time frame covered by the Days 4-5 Excessive Rainfall Outlooks, the primary region of interest will extend from the central Plains into the Midwest. A Plains surface wave should lift a stalling front northward as a warm front late in the weekend, while a wavy boundary should hang up for a time over the Plains/Upper Midwest early next week while awaiting the approach of western U.S. dynamics. Compared to 24 hours ago, models at least show better agreement in the development of some convection during the current Day 4 period, even with continued scatter of details. The Day 5 period looks to be more favorable for potential repeat activity/training in light of slower frontal motion and persistence of favorable instability/moisture anomalies. For now both days depict a Marginal Risk area based on the best combination of guidance parameters, with threats on Day 5 depending somewhat on how Day 4 transpires. The Day 4 risk area extends from eastern Kansas/Nebraska into southern Minnesota while the Day 5 area broadens eastward somewhat. Meanwhile the system lingering near the East Coast could still produce some isolated moderate to heavy rainfall into Sunday but without enough of a signal to warrant a risk area. From early Tuesday onward expect northeastward progression of developing low pressure to push the trailing cold front and accompanying rain and thunderstorms of varying intensity into the eastern and southern U.S. It will take additional time to resolve most favored areas of heaviest rainfall in the Tuesday-Thursday time frame. Initial upper troughing over the West will support some precipitation over northern areas with scattered activity possibly reaching as far south as the Great Basin. Some snow will be possible in the high elevations of the Rockies. Approach/arrival of an upper low as currently advertised by midweek or so would produce somewhat more organized precipitation at that time. Changes in snow levels will depend on the depth of the upper low. Expect South Texas to see multiple days of hazardous heat during the period with highs persistently running 10-15F above normal with max heat index values possibly reaching at least 110F. Highs near or over 100 degrees could stretch farther north across the southern High Plains at times. Some daily records for highs/warm lows will be possible. Southern Florida may also experience hazardous heat into Sunday with highs locally up to 5-10F above normal and max heat index values reaching 105-110F, followed by continued above normal but slightly less extreme readings. The forecast pattern will favor below average highs over the Northwest for most of next week, and including the northern Plains by early next week. The warm sector from the High Plains into the Great Lakes/New England early in the week should see highs up to 10-15F or so above normal. Cold frontal progression should bring temperatures closer to normal over most of the eastern half of the country by mid-late week. Clouds and possibly lingering pockets of rainfall should keep highs on the cool side over the Mid-Atlantic during Sunday-Monday. 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 WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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