Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM 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... The start of the extended period continues to portray a Rex Block-type pattern with the upper ridge/high over the Ohio Valley north of a Gulf Coast low. The past few days the models have shown a lot of variability on where and how the Gulf low will evolve. The Gulf of Mexico has had warmer then usual water temperatures for much of the year so far which is promoting a stronger Loop Current for this time of the year. The trajectory of this current would support a low track further east over the Gulf of Mexico. The 06Z/00Z runs of the GFS, CMC and UKMET are fairly clustered closer to the stronger current, whereas, the ECWMF is showing its westward/slower bias at least the with Gulf feature. However, the EC ensemble means were better clustered with the trio of global guidance. The general consensus through late week suggests 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. Meanwhile upstream, a fairly phased/open upper trough will track from the West into the central U.S. by early next week. Most guidance have been persistent in favoring the most energy in the northern part of the trough. The CMC continues to depict a stronger upper low tracking much further south with the trough while the latest UKMET has trended with a southward shift but not as much as the CMC. For this feature a combination of the 00Z ECWMF/EC ensemble means and 06Z/00Z GFS and GEFS means was preferred as it supported a defined surface low pressure reaching the Great Lakes by Sunday. Additionally, 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 06Z GFS, 00Z GFS/CMC/UKMET and 00Z EC ensemble means early in the period. Then the UKMET and CMC were phased out in favor of the agreeable GEFS means and the ECWMF for the middle and latter periods of the extended forecast. ...Weather/Hazards Highlights... Moderate to heavy rain will likely overspread the central and eastern Gulf states into the Southeast, 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. There will be an elevated threat for isolated areas of flash flooding Thursday, as such, WPC has a Day 4 Marginal Risk area in effect spanning from southeast Louisiana/southern Mississippi eastward to Florida and southern Georgia. 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. By Friday the low pressure system should be lifting to the northeast 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. 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. Campbell/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