####018007729#### FXUS02 KWBC 110705 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 305 AM EDT Sat May 11 2024 Valid 12Z Tue May 14 2024 - 12Z Sat May 18 2024 ...Excessive rainfall/flooding threat reaching the Southeast by Tuesday, followed by another potential episode of southern tier heavy rain mid-late week... ...Overview... The combination of Mississippi Valley through Mid-Atlantic low pressure and a front to the north will spread a broad area of rainfall across the East Tuesday-Wednesday, with some flash flooding concerns over parts of the Southeast. Then a frontal system and one or more supporting upper shortwaves will likely spread another large area of rainfall across the central/eastern U.S. with highest totals again possible over portions of the southern tier. Behind this system, guidance is still having difficulties in resolving exactly how flow separates within central Pacific troughing around Tuesday-Wednesday plus details of trailing northern stream energy. This leads to continued lower than average confidence in flow details across the West and eventually farther downstream from Wednesday onward. ...Guidance/Predictability Assessment... Regarding the system crossing the East early the period, the primary differences in guidance arise from about early Wednesday onward as the surface low tracks away from the Mid-Atlantic. The 18Z GFS/12Z CMC in particular showed more involvement from eastern Canada upper flow, pulling the surface low northward of other guidance. 12Z ECMWF-initialized machine learning (ML) models generally favored greater separation between Canadian flow and the shortwave supporting the system of interest (along with surface pressures not as deep as the deeper side of the dynamical model spread). This scenario points toward the 12Z ECMWF and 12Z/18Z ensemble means. Near the West Coast, the guidance continues to diverge significantly from Wednesday onward in response to differences in how flow separates within central Pacific troughing around Tuesday-Wednesday, with differences in trailing northern stream energy also adding to the spread. 18Z/00Z GFS runs are slower to lower heights over the West and then eastward across southern Canada and the northern tier U.S. but at least they have trended somewhat more toward the ECMWF/ECens/CMCens versus some earlier runs that had a strong ridge over the Northwest. The 18Z GEFS mean also trended flatter aloft but the new 00Z GEFS has reverted back to a stronger ridge. MLs continue to offer fairly strong support for the 12Z ECMWF and 18Z GEFS/12Z ECens-CMCens in principle, leading to favoring those solutions from midweek onward. Farther east, this cluster agrees fairly well in bringing a modest surface wave close to the Southeast or Mid-Atlantic coast by next Saturday. Confidence in surface details is fairly low by that time as solutions differ a fair degree for how shortwave energy will be distributed within the overall upper trough. ...Weather/Hazards Highlights... As the system forecast to be near the Middle Mississippi Valley/Lower Ohio Valley as of early Tuesday continues eastward, the best signal for highest short-term rainfall rates during the Day 4/Tuesday Excessive Rainfall Outlook continues to be over parts of the Southeast. This region will have a favorable combination of instability and anomalous moisture along with a leading warm front lifting through early in period followed by a cold front approaching from the west. Thus far there has been no compelling need for much adjustment of the Slight Risk area centered mostly over northern Florida and southern Georgia, or the Marginal Risk area extending north into parts of the Ohio Valley and Mid-Atlantic. Guidance signals are more diffuse within the Marginal Risk area but there will be a favorable combination of instability and moisture along with currently wet ground conditions. Over the Northeast, a cold front may focus some enhanced rainfall but guidance has been showing fairly minimal instability, so there is still uncertainty as to whether short- term rain rates will be high enough to pose a flash flood threat and thus no risk area is depicted for now. Meanwhile a leading stalling front and another front heading into the central Plains may generate some isolated moderate to heavy rainfall but at this time the magnitude and organization do not appear sufficient to merit a risk area. By Day 5/Wednesday, the primary focus for heavy rainfall potential will return back to the southern half of the Plains. Model/ensemble guidance is not yet in great agreement for some details, but a decent number of ingredients appear to favor convective development around midweek--instability and increasing moisture, along with multiple surface features plus supporting dynamics aloft (and a southern stream shortwave emerging from the Southwest). As a starting point, the Day 5 ERO depicts a Slight Risk area centered over portions of Texas, representing the best overlap of GFS/GEFS and ECMWF/ECens signals plus currently wet ground conditions, with a surrounding Marginal Risk extending into parts of the central Plains. There is a lingering Marginal Risk over northern Florida as the front reaching the area stalls. Expect central U.S. rainfall to continue eastward after midweek, reaching the East toward the end of the week. Heaviest rainfall will most likely be across the southern tier which will be sensitive to additional rainfall given prior events. Sensible weather over the Northwest continues to be a question mark in light of the divergence of guidance aloft. The most likely scenario is currently for upper heights to decrease enough for some light precipitation to reach northern parts of the Pacific Northwest/Rockies around mid-late week while temperatures stay modestly above normal. Farther south, California and Nevada should be more consistently above normal next week with some locations seeing multiple days of plus 10-15F anomalies. Florida may warm up to a few degrees above normal by mid to late week as a warm front lifts north of the state Tuesday and trailing cold fronts likely stall over the far northern part of the state. 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 eastern U.S. should see somewhat above normal lows for most of the period. 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 $$