Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Sat Apr 08 2023 Valid 12Z Tue Apr 11 2023 - 12Z Sat Apr 15 2023 ...Gulf of Mexico low pressure could bring heavy rain concerns to the Gulf Coast/Southeast next week... ...Overview... Through the medium range period beginning Tuesday and lasting through next week, expect an upper trough axis to track through the eastern Pacific and across the West, which combined with a front or two will lead to multiple days of rain and high elevation snow over some areas from the West Coast into the northern/central Rockies. This trough may reach far enough east to support an expanding area of precipitation over the Plains by the end of the 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 southern Rockies Tuesday lifts northeast into the Middle Mississippi/Ohio Valleys above an evolving Gulf Coast upper low. The upper low and associated surface reflection may produce areas of heavy rainfall along the Gulf Coast into the Southeast next 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 continues to be fairly agreeable and consistent up to the point when the initial embedded upper low reaches the Pacific Northwest by early Wednesday but then diverges thereafter. At least through the 00Z/06Z cycles, the GFS/ECMWF were both showing some trends toward a slower/farther south upper low closing off within the southern part of the overall trough by late Thursday/Friday--first suggested by some CMC runs. Enough 00Z ECens/CMCens members gravitated toward some variation of this idea for their means to hint at a southern upper low as well. The GEFS mean through the 12Z cycle continues to depict a trough with no discernible separation. UKMET runs have continued to be somewhat fast/broad with the northern part of the trough but the 12Z version finally pulls off enough southern energy to close off a low like the other operational models. Additional uncertainty comes into play regarding influences on this trough/low by days 6-7 Friday-Saturday. There is good agreement on rapid trough amplification south of Alaska during that time but with significant differences on the broadness of the trough and its exact longitude. Latest GFS runs have been on the sharp/slow side of the spread, with the downstream ridge in turn allowing some northern stream energy to dig into the West (especially pronounced in the 06Z/12Z runs) and influence the Southwest upper low. On the other hand, the 00Z ECMWF/CMC and their ensemble means are all broader/eastward with the developing trough and push the downstream ridge farther east. Those operational runs even offer the potential for the Southwest upper low to decelerate significantly, though their means are somewhat more progressive. Blending an operational model composite (more ECMWF and GFS influence than CMC/UKMET) and then eventually adding in partial 06Z GEFS/00Z ECens mean input helped to nudge the forecast a bit slower late in the period per recent trends, while not committing to either extreme of the widening solution spread. Northern stream attributes of the 00Z ECMWF become more questionable late, so ECMWF input was eventually split between the 00Z and 12Z/07 runs. The 12Z CMC/ECMWF have both dramatically changed their eastern Pacific/western U.S. solutions after Wednesday, ultimately yielding a more progressive and phased western trough. Guidance continues to develop the expected blocky pattern farther east as Plains energy dives south Monday-Tuesday and helps develop a southern stream upper low while ridging shifts north of it. However the models also continue to show significant detail differences that affect the rainfall forecast over the Gulf Coast and Southeast. Latest consensus is a tad weaker with the upper low early in the period, which results in a slight delay for development of surface low pressure over the Gulf of Mexico. Then by early Wednesday the latest GFS runs become noticeably stronger/northwestward with the surface low versus other guidance. Through the rest of the period the guidance average has trended a bit slower with the northeastward progression of the system versus 12 hours ago. The new 12Z CMC has come in closer to the majority cluster for the upper low, versus the 00Z run that was a southern extreme and other runs that were weak. On the other hand the UKMET remains a southeastern extreme. The new 12Z ECMWF is deeper with the upper low and deeper/northward at the surface, reversing prior trends. Needless to say, confidence in specifics of this system remain below average given the persistent spread and variability among models and ensemble members. The blend described above for the western trough provided a reasonable intermediate starting point for this system based on 00Z/06Z guidance. Guidance is steadily coming closer together for the shortwave/possible embedded low crossing New England and far eastern Canada around Wednesday. Consensus suggests a fairly strong shortwave but northward GFS trends for the embedded upper low, and 12Z ECMWF continuity, are helping to add confidence in the upper low track staying north of New England. ...Weather/Hazards Highlights... Shortwave energy across the northern Gulf of Mexico Tuesday will likely form an upper low at least by around midweek, 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 Tuesday and Wednesday across the central Gulf Coast given the 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. However, tracks of the surface low and the positioning of heavy rain amounts remain quite uncertain as model guidance continues to show considerable spread. There is still some chance most rain falls offshore over the Gulf, but the Marginal Risk areas reflect the relative possibility for the heavier side of the guidance spread to occur. Ahead of this evolution, easterly flow behind a cold front may already support periods of rain over the Florida Peninsula early in the period, with continued chances for rain and storms later in the week as well with the main low/frontal system. There may be a nonzero threat for excessive rainfall over particularly the east coast of Florida, but again with some uncertainty in amounts onshore. 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 Pacific Northwest and northern Rockies on Tuesday. Expect Pacific Northwest precipitation to trend gradually lighter for midweek, but the northern Rockies and vicinity should see multiple days of snow and low elevation rain, while some precipitation spreads into northern parts of the Plains as well during the latter half of the week. Farther south, only light amounts of precipitation are forecast for California and the Great Basin at this point, but amounts will depend on precise details of the upper trough/possible embedded low. Then as the low/frontal system ejects into the Plains late week, snow amounts may be enhanced for the central Rockies with rainfall coverage and totals increasing over the Plains. There is also some question as to whether snow could extend into parts of the High 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 next 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 Tuesday-Wednesday and shifting into the Midwest and 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 next Saturday. Warmer than normal conditions in the southwestern U.S. for the first half of the week could lead to temperatures nearing 100F in the Desert Southwest and a few daily record highs. Then the West should cool 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 next Saturday as upper ridging builds in. Meanwhile the Southeast could see some slightly cooler than average temperatures especially for highs for Tuesday-Wednesday, but may moderate closer to normal as the week progresses. Rausch/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