Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Fri Apr 07 2023 Valid 12Z Mon Apr 10 2023 - 12Z Fri Apr 14 2023 ...Gulf of Mexico low pressure could bring heavy rain concerns to the Gulf Coast/Southeast next week... ...Overview... Through the medium range period 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 Rockies. Farther east, model guidance is indicating that a Rex Block-type pattern could arise across the central to eastern U.S., as an upper ridge/high over the Four Corners region Monday-Tuesday lifts north into the Middle Mississippi/Ohio Valleys Wednesday-Thursday 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. Also of note, by the middle of next week differences in how shortwave energy drops down the northeast side of the eastern upper ridge lead to some uncertainty in the temperature forecast over parts of New England, within a pattern favoring well above normal temperatures from the north-central Plains into the Mid-Atlantic/Northeast. ...Guidance/Predictability Assessment... Minus the progressive and shallow UKMET, most model and ensemble guidance has generally displayed better than average agreement and continuity for the overall handling of the upper trough moving into the West through early Wednesday. More differences arise after a likely embedded upper low reaches the Pacific Northwest at that time, with most ECMWF/GFS/CMC runs either tracking or reforming the low farther south/east and then diverging some for timing. Latest CMC runs looked like a southern extreme with the upper low by day 7 Friday until the new 12Z ECMWF arrived with reforming the closed low even farther south. On the other hand, the 06Z GFS was on the more open and progressive side of the spread. The new 12Z GFS has come in fairly close to the 00Z ECMWF which was the operational model run closest to the GEFS/ECMWF mean trough axis. The means are still keeping the trough open due to the spread among individual members. Meanwhile, guidance has been stabilizing around the idea that a blocky pattern will develop as Plains energy dives south Monday-Tuesday and helps develop a southern stream upper low while ridging shifts north of it. Most model runs continue to indicate that a surface low is likely to develop in the Gulf of Mexico in conjunction with the upper low and a preexisting surface front. The low generally looks to track northeastward with time. However there is still lingering uncertainty in specifics. The GFS/ECMWF and their means provide the majority cluster for the system's overall evolution through the period, while the new 12Z CMC is sufficiently weak with its upper feature to support only an inverted surface trough over the Gulf and the 12Z UKMET (with questionable flow details well to the northwest) shifts the overall system well east/southeast. The new 12Z ECMWF has trended somewhat weaker aloft, leading to a weaker/southward Gulf surface low, but still better defined than the new CMC. There is still a lot of spread among the ensemble members so it remains possible for a seemingly low-probability scenario to occur. One other question mark in the forecast is with shortwave energy passing through New England around midweek. The 06Z and new 12Z GFS are on the deep and amplified side of the spread, with the 12Z run even closing off a compact upper low. The 12Z CMC has joined the GFS. The 12Z ECMWF has trended a lot more amplified than its 00Z run (that was flatter than average) but not to the degree of the GFS/CMC. A blend appears best for resolving this aspect of the forecast. The above considerations led to updating the WPC forecast with a 00Z/06Z model blend consisting of the GFS/ECMWF/CMC during the first half of the period, followed by partial inclusion of the 06Z GEFS/00Z ECMWF means. The blend still maintained enough model input to reflect a compact closed low over California into early day 6 Thursday, while emphasis on the GFS/ECMWF and their means for the Gulf Coast/Southeast system led to some detail/position adjustments but reasonable continuity for the overall evolution. ...Weather/Hazards Highlights... An atmospheric river aiming at the West Coast just ahead of a cold front on the leading edge of the approaching Pacific upper trough is likely to be ongoing at the start of the medium range period Monday, though decreasing in intensity compared to day 3/Sunday. As the precipitation forecast is heaviest across coastal areas of southern Oregon/far northern California that can generally handle considerably higher amounts of rain, and the area of enhanced rainfall appears fairly localized, left the Day 4 Excessive Rainfall Outlook with no areas expected to exceed 5 percent at this time. However, the chance may be nonzero for isolated flooding on Monday, and will continue to monitor timing of the peak of the AR in future forecasts. As the upper trough and leading front slowly progress eastward next week, expect Pacific Northwest precipitation to trend gradually lighter while the northern Rockies and vicinity should see multiple days of snow and low elevation rain. Eventual coverage and amounts across California and the Great Basin are more uncertain, depending on precise details of the upper trough/possible embedded low. This issue will also influence precipitation specifics over the northern Rockies by Wednesday-Thursday and spreading out into the Plains by Thursday-Friday. Central U.S. shortwave energy may produce some areas of rainfall lingering over the southern half of the Plains through Monday. As this energy forms an upper low along the Gulf Coast with a northern Gulf surface system, moderate to heavy rainfall will be possible over some areas from central and eastern parts of the Gulf Coast through the Southeast during Tuesday-Friday with a general east-northeastward progression during that time. Tracks of the surface low and the positioning of heavy rain amounts remain quite uncertain as model guidance continues to show considerable spread. There has been enough persistence in a cluster of guidance to maintain a Marginal Risk area in the Day 5 (Tuesday) Excessive Rainfall Outlook for parts of the central Gulf Coast, with the only adjustment from continuity being a modest west-southwestward shift per recent GFS/GEFS and ECMWF/ECens trends. Even amounts somewhere between the heaviest and dry extremes could still pose issues given wet conditions that may exist over some areas due rainfall in the short-range time frame. Ahead of this evolution, easterly flow behind a cold front may already support periods of rain over the Florida Peninsula starting earlier in the period. 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 early in the the workweek and shifting into the Midwest and Northeast later in the week. Uncertainty over the strength and southward extent of a New England cold front around midweek lowers confidence in the temperature forecast to some degree over that region. 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. Meanwhile the Southeast could see some cooler than average temperatures especially for highs for the first half of the week, 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