Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 PM EDT Sun May 12 2024 Valid 12Z Wed May 15 2024 - 12Z Sun May 19 2024 ...Another southern tier heavy rainfall threat mid-late week... ...Overview... A system reaching the Mid-Atlantic by early Wednesday and a cold front just to its north/west will support a wet midweek over portions of the East before the surface low continues eastward into the Atlantic. Farther west, shortwave energy emerging from the Rockies should support a wavy frontal system pushing eastward from the Plains, spreading another broad area of rainfall over the central/eastern U.S. with highest totals still expected to be across the southern tier. Guidance is still having difficulty in resolving some important details though. Potential stalling of the system's trailing front near the Gulf Coast could support at least a localized heavy rainfall threat into the weekend. Models/ensembles still differ for specifics of flow rounding eastern Pacific ridging from Wednesday onward with effects translating eastward with time, but at least the prior extremely broad solution envelope has narrowed somewhat. ...Guidance/Predictability Assessment... There was good enough agreement the first half of the period (Wednesday-Friday) to warrant a forecast blend between the latest GFS, ECMWF, CMC models. The UKMET was a little quick/strong with western Canada energy into the northern tier as early as Thursday, and was not used in the blend today. After Friday, there is a lot of uncertainty with timing and amplitude of shortwave energy over the Upper Midwest-Great Lakes and possible phasing with the southern stream shortwave lifting towards the east-northeast. Another shortwave should drop into the Northwest next weekend as an upper low well off the California coast gets drawn eastward. Models struggle with the details of how these two systems may interact and also with more weak energy into the Southern Plains downstream. Given this, opted to trend more towards the ensemble means to mitigate the differences. This maintains good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... The primary focus for heavy rainfall potential still looks to be in the southern half of the Plains by the Wednesday-Wednesday night time frame covered by the Day 4 Excessive Rainfall Outlook. Reflecting some developing spread for approaching dynamics aloft (from the Rockies, as well as a separate Southwest shortwave that may have some interaction with northern stream flow), model/ensemble guidance continues to differ for convective specifics over this region. Although guidance doesn't show increasing instability and moisture in this region, models are showing increasing uncertainty in where heaviest rainfall sets up, and backing away from the previous general theme of a central Texas focus. Given this, and after collaboration with the affected WFOs, have opted to remove the previous slight risk area from the Wednesday ERO given the downtrend in QPF. There is still somewhat of a developing signal from some dynamical and machine learning output toward better convective emphasis a little farther north-- over an area where short term forecast rainfall could lead to wetter soils heading into Day 4 than currently observed. Will hold off on any slight risk upgrades to gain better confidence in any potential adjustments. Meanwhile continuity looks good for the lingering Marginal Risk over northern Florida as the front reaching the area stalls, accompanied by instability and above normal moisture. For the Day 5 ERO (Thursday-Thursday night), aforementioned spread for details aloft continue to influence convective details from the Plains into the Gulf Coast states. The outlook maintains a Slight Risk area extending from southeastern Texas into southwest Mississippi, where guidance shows a pronounced surge of moisture into this region with adequate instability as well. An eventual need for an upgrade to a moderate risk, especially across very sensitive regions of southeast Texas into central Louisiana, is possible but more dependent on additional rainfall in the short term period. A front farther north may help to focus less extreme but still locally heavy rainfall, which is accounted for by a Marginal Risk area that extends into parts of the south- central Plains and Midwest. Expect the moisture shield to continue across the eastern U.S. late in the week, with heaviest rainfall most likely extending across the Southeast. Depending on specifics of upper flow, the front trailing from the surface system associated with this episode could stall near the Gulf Coast and provide a focus for additional rainfall during the weekend. An eastern Pacific upper ridge extending into the West should support above normal temperatures over most of the West Coast states and parts of the Great Basin during Wednesday-Thursday, with some areas seeing plus 10-15F anomalies. Thereafter, expect the warm readings to persist over California/Nevada with eventual expansion into the southern Rockies while the Northwest trends closer to normal (possibly below normal over Montana). Most of Florida may see several days with highs up to 5-10F above normal, in a warm pattern to the south of a couple fronts expected to stall upon reaching the far northern part of the state. Combined with high dew points, these temperatures may bring heat indices as high as 105-110 across southeast/southern Florida, signaling the first hazardous heat days of the summer. Otherwise, areas to the east of the Rockies will tend to be within a few degrees of normal for highs, aside from below normal over the southern Plains on Thursday and possibly an axis of modestly above normal readings dropping southeast from the northern Plains Friday onward. Most of the eastern U.S. should see somewhat above normal lows through the period. Santorelli/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