Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Mon Apr 10 2023 Valid 12Z Thu Apr 13 2023 - 12Z Mon Apr 17 2023 ...Gulf of Mexico low pressure could bring heavy rain concerns to the Gulf Coast/Southeast this week... ...Overview... The medium range period begins Thursday with a surface low/frontal system coming into the central Gulf Coast supported by an upper low aloft. This should lead to ample rain and thunderstorms possibly causing flash flooding across the Gulf Coast/Southeast including Florida on Thursday and pushing into the Carolinas on Friday. Meanwhile an upper trough is forecast to push across the western U.S. late week and through the central U.S. this weekend and into early next week, allowing for some snow in the northern/central Rockies and perhaps into the north-central U.S. while rain chances increase farther south in the Plains and Mississippi Valley and pushing into the East early next week. ...Guidance/Predictability Assessment... Model guidance continues to persist in showing a Rex Block-type pattern through Thursday as an upper ridge/high atop the Ohio Valley is present north of an evolving Gulf Coast upper low. There has been some improvement in models agreeing better on an upper low position, though the 12Z CMC remained on the south side of the guidance envelope. But differences in guidance continue especially with the positioning of the low at the surface, which affects QPF. The ECMWF and its ensemble have persistently had the tendency to have rain extending farther west than the bulk of other guidance. On the whole guidance suggests through late week that as the upper and surface lows lift northeast they should become increasingly ill-defined with time, with the surface low embedded within the gradient between Atlantic high pressure and the central U.S. front. Models show fair agreement with the trough initially across the West Thursday, though with typical spread in the details. The overall theme of the guidance shows a fairly phased/open upper trough that progresses eastward into the central U.S. through early next week. Most guidance from the past couple model cycles holds most energy in the northern part of the trough, but CMC runs have been the exception with rounding an upper low farther south within the trough. The trough should support defined surface low pressure reaching the Great Lakes by Sunday. This low has trended a little slower/westward compared to the previous forecast and the slower trend seemed to continue in the new 00Z runs. Then there is reasonable agreement for upper ridging to reach the West early next week behind this trough and ahead of a northeast Pacific upper low. The WPC forecast was based on a blend of the 12Z ECMWF/18Z GFS/12Z CMC/12Z UKMET (from most to least proportion in the blend) early in the period. Then the UKMET and CMC were phased out in favor of the agreeable GEFS and EC ensemble means to smooth out any individual model details. ...Weather/Hazards Highlights... A closed upper low is likely to be present near the central Gulf Coast on Thursday along with low pressure at the surface. Moderate to heavy rainfall will be possible over some areas from central and eastern parts of the Gulf Coast through the Southeast on Thursday, with potential for slow-moving storms with high rainfall rates that may be able to overcome high Flash Flood Guidance, especially if heavy totals overlap with areas that have seen notable rainfall recently. Thus there is a broad Marginal Risk of excessive rainfall in place for Day 4/Thursday; there is some potential an embedded Slight Risk upgrade might be necessary at some point if model guidance narrows in on the placement of heaviest totals and if those areas are sensitive to heavy rainfall. This feature's warm front extends across Florida along with ample moisture for convection there. By Friday the low pressure system should be pushing northeastward with some rain possible in the Carolinas, but with uncertainty in how far west (into more sensitive areas like the Columbia SC metro and back into the southern Appalachians) the convection will spread. Rainfall should trend lighter overall late week as the upper system opens up, though some showers could reach the Mid-Atlantic. The trough aloft and a couple of frontal boundaries in the West will lead to precipitation chances (lower elevation rain and higher elevation snow) across the northern Rockies continuing Thursday, with some light amounts possible into the central Great Basin. Snow chances then reach the higher elevations of the central Rockies by Friday behind surface low pressure in the central High Plains. Farther east, precipitation is likely in the northern half of the Plains/Mississippi Valley late week. There may be some enhanced rainfall in a band or two from the north-central Plains into Minnesota on Thursday, which combined with possible snowmelt could create isolated flooding issues. By Friday there may also be some enhanced convection in the central U.S. but with ample spread in its placement, with possibly higher rain amounts but on average farther south than any snow covered areas. No excessive rainfall areas are depicted in the outlook for now given ample uncertainty, but this will continue to be monitored. There is also some chance for springtime snow to extend into parts of the Plains and the Upper Midwest for Friday into the weekend. Meanwhile expect rainfall to expand in coverage to the south and east late week into the weekend, with moist flow ahead of the cold front likely helping to increase rainfall chances across the central Gulf Coast region once again. The most anomalous temperatures as the period begins Thursday should extend from the central High Plains through the Upper Midwest/Great Lakes to the Northeast with readings exceeding normal by at least 15-25F. These temperatures are forecast to persist in the northeastern quadrant of the U.S. into the weekend, with Friday seeing the largest coverage of potential record highs from the central Appalachians through the Mid-Atlantic to southern New England. Meanwhile the trough pushing through the western and central U.S. and its attendant cold front will help produce cooler than normal temperatures especially in terms of highs across the West initially and reaching the central U.S. by the weekend, and then lower temperatures in the East early next week. The West is expected to flip back above average early next week as upper ridging comes in behind the trough. Tate 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, 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