FXUS02 KWBC 101857 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 PM EDT Fri May 10 2024 Valid 12Z Mon May 13 2024 - 12Z Fri May 17 2024 ...Excessive Rainfall/Flooding Threat for the Gulf Coast and Southeast... ...Overview... A system progressing across the southern half of the central- eastern U.S. during the first half of the week will bring the threat of heavy rainfall to areas near the Gulf Coast. This system and a northern stream front will combine to spread varying intensity of rain across areas farther north as well. Another system currently expected to reach the Plains/east-central U.S. after midweek may produce another broad area of rainfall with highest totals again possible over parts of the southern tier. Meanwhile guidance diverges significantly for flow from the eastern Pacific into western North America by mid-late week, lowering confidence in forecast details over that region and to some extent downstream at that time frame. ...Guidance/Predictability Assessment... The latest operational model guidance provided a reasonable starting point for the first half of the forecast period including the initial system through the Mid-South and whatever influence from a northern stream shortwave through the Northeast early-mid next week. After this, the guidance shows some variability in timing and details of the next shortwave into the Central U.S. later next week, but the greatest uncertainty at this time continues to be out West. There are significant differences in how much ridging will build over the West/Northwest, seeming to stem from very uncertain shortwave energy across western Canada. The ECMWF and CMC seem to suggest some weak troughing will suppress ridging south and west (moreso from the ECMWF), but the last few runs of the GFS shows a very strong and potentially blocky ridge building over the region by next Thursday-Friday. The ensemble means generally align with their deterministic counterparts, but the ECMWF-initialized ML models do suggest more troughing than the current suite of guidance. Given this, the WPC forecast trended more towards the ECMWF with the ECENS and NAEFS (the GEFS seemed too strong with ridging). This maintained fairly good continuity with the previous WPC shift as well. ...Weather/Hazards Highlights... The system progressing eastward from the southern Plains during the first half of the week will be one generator of potentially heavy rainfall. The best signal currently exists along and near the Gulf Coast Monday-Tuesday, in association with a warm front lifting north through the region and followed by the system's trailing cold front. The new Day 4 Excessive Rainfall Outlook maintains continuity with a Slight Risk area extending from southeastern Texas through southern Alabama and a portion of the Florida Panhandle while the Day 5 ERO continues a Slight Risk area over southern Georgia and northern Florida. Over these two days, this region shows the best combination of instability and anomalous moisture along with synoptic features to enhance focus. A Marginal Risk area extends farther north into parts of the Middle Mississippi Valley/Ohio Valley into southern Mid-Atlantic over the two-day period, corresponding to a favorable combination of instability, guidance signals for scattered heavy QPF maxima, and currently wet ground conditions. There are two other areas worth monitoring for Day 5, one over the central Plains/Midwest where a front may stall, and another over the Northeast where a northern stream front may enhanced rainfall. At this time there is too much spread among the guidance to merit any risk area but that could change if solutions gravitated toward the heavier side of the envelope. From early Wednesday onward, first expect some lingering rainfall near the East Coast as the leading system departs. Then areas from the southern Plains eastward may see yet another episode of heavy rain as a front pushes into and through the Plains/Mississippi Valley and the first system's trailing front lifts back north as a warm front. Rain of varying intensity will also be possible farther north across parts of the central/eastern U.S. Uncertainty with the flow aloft over the northern half of the West by mid-late week leads to a potential range between dry conditions with above normal temperatures or some precipitation spreading through the Pacific Northwest/northern Rockies with temperatures closer to normal. Farther south, California and Nevada should be more consistently above normal next week with a scattering of plus 10-15F anomalies. The northern Rockies/High Plains should also be on the warm side early in the week. The clouds and heavy rainfall potential over the Gulf Coast/Southeast early in the week should initially keep highs below normal over that region, but particularly Florida may warm up to a few degrees above normal by mid to late week as another warm front lifts through. Combined with high dew points, this 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 while the rest of the eastern U.S. should see somewhat above normal lows Tuesday-Friday. 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 $$