Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sun Apr 09 2023 Valid 12Z Wed Apr 12 2023 - 12Z Sun Apr 16 2023 ...Gulf of Mexico low pressure could bring heavy rain concerns to the Gulf Coast/Southeast this week... ...Overview... The medium range period begins Wednesday as an upper trough arrives for the West Coast, which is forecast to push through interior parts of the West late week and into the Plains through the weekend. Some precipitation will likely be enhanced over the northern Rockies in particular for Wednesday-Thursday and then into the central Rockies and Plains/Mississippi Valley late week. Meanwhile the model guidance still shows a Rex Block-type pattern developing across the central to eastern U.S. during mid-late week, as an upper ridge/high atop the Middle Mississippi Valley/Ohio Valleys is present north of an evolving Gulf Coast upper low Wednesday-Thursday. The upper low and associated surface reflection may produce areas of heavy rainfall along the Gulf Coast into the Southeast this week but guidance has had a difficult time resolving important details so far. The pattern will favor well above normal temperatures from the north-central Plains into the Mid-Atlantic/Northeast, though with some uncertainty in temperatures over New England on any given day dependent on less certain shortwave and surface frontal positions. ...Guidance/Predictability Assessment... For the upper trough forecast to move into the West, guidance has been persistent with timing of its arrival around day 3/Wednesday. However there has been more spread regarding the closing off of an upper low within the trough, also affecting the speed of its movement eastward. A day or so ago particularly the non-NCEP models trended toward more upper low separation from the northern stream and thus a slower trek eastward through late week. But the 12/18Z model cycle and the newer 00Z model cycle went away from these solutions toward a more phased and faster progression of the trough, with the 00Z even faster on the whole compared to the previous cycle. Thus the WPC forecast trended faster with the upper trough and the associated surface fronts as well, but with somewhat low confidence as flow across Alaska and the Pacific remains uncertain and could impact the trough downstream. Models do remain persistent in developing the expected blocky pattern over the central/eastern U.S. beginning midweek, but with continued spread in significant details that affect the rainfall forecast over the Gulf Coast and Southeast. Most 12/18Z models happened to converge on a position for the upper low for Wednesday-Friday, with the exception of the 12Z UKMET that was farther southeast even of any ensemble members by Friday, but surface lows remained scattered in deterministic models and ensemble members. GFS runs have been developing the surface low more quickly than the non-NCEP guidance overall. The 12Z CMC was on the southern side of the guidance envelope with its low position in the Gulf for Thursday, but jumped closer to other models in its 12Z run but becomes slower than consensus Friday. Meanwhile the ECMWF has been showing more QPF farther west in Louisiana compared to other guidance. Needless to say, confidence in specifics of this system remain below average given the persistent spread and variability among models and ensemble members. There was perhaps some trend for more QPF to come onshore by midweek compared to the previous cycle. Given these considerations, the WPC forecast favored a blend of the 12/18Z GFS and 12Z ECMWF with some 12Z CMC and EC ensemble mean through much of the period, with the blending process providing middle ground solutions for these and other features like a strong shortwave over New England Wednesday. The proportions of the GEFS and EC ensemble means were increased by the latter part of the period to smooth out uncertain model details. ...Weather/Hazards Highlights... An upper low is likely to close off sometime around Wednesday over the northern Gulf of Mexico, while low pressure develops along a front 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 during the week, likely with a general east-northeastward progression during that time. Marginal Risks of excessive rainfall are in place for Wednesday across the central Gulf Coast and for the central/eastern Gulf Coast into parts of the Southeast Thursday, given most model guidance has rain/storms reaching land areas by that time frame. There is 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 and/or in the short-range time frame. On Day 5/Thursday there is some possibility for a Slight Risk upgrade at some point over perhaps the western Florida Peninsula, but uncertainty in placement and that area being not particularly sensitive to all but the heaviest rain/rain rates precluded anything higher than a Marginal Risk at this time. Additionally, easterly flow could bring enhanced rain totals to the Atlantic coastline of Florida and Georgia/South Carolina for a nonzero excessive rainfall risk. 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 Wednesday-Thursday. Some light amounts may spread into parts of California and the Great Basin depending on precise details of the upper trough/possible embedded low. Some precipitation could spread into northern parts of the Plains as well during the latter half of the week. Then as the low/frontal system ejects into the Plains late week, snow amounts may be enhanced for the central Rockies with precipitation coverage and totals increasing over the Plains and Mississippi Valley. There is some possibility that springtime snow could extend into parts of the Plains. At this point there has been too much model spread and variability for the details of the emerging western trough/low to provide much confidence one way or the other, but it is a possibility worth monitoring. The most anomalous temperatures for the latter half of this week look to be across the northern tier of the U.S., where warmer than normal temperatures by 15-25F are forecast in the northern half of the Plains and Midwest Wednesday and shifting/extending into the Northeast later in the week. Some scattered daily records for highs/warm lows will be possible within this broad area of very warm temperatures. A leading cold front may briefly bring temperatures closer to normal over New England around Wednesday, with another front possibly moderating temperatures again around Saturday. Warmer than normal conditions by a few degrees in the southwestern U.S. Wednesday should flip to below average in terms of highs and near to slightly below normal for lows as the upper trough comes in. The Northwest could rebound to slightly above normal by Saturday as upper ridging temporarily builds in. Meanwhile the Southeast could see some slightly cooler than average temperatures especially for highs on Wednesday, but may moderate closer to normal to above normal as the week progresses. Tate/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, 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