Extended Forecast Discussion NWS Weather Prediction Center College Park MD 303 AM 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, an upper trough axis is forecast 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. ...Guidance/Predictability Assessment... Thus far the guidance has displayed better than average agreement and continuity for the overall handling of the upper trough moving into the West by mid-late next week. There are some differences with the timing of an embedded upper low existing within the trough and how much separation may develop with this upper low by Thursday-Friday (the new 00Z CMC may be a bit aggressive with the separation). The exact timing of the trough/upper low track also varies a bit, but well within typical spread for the medium range period. Meanwhile, it has been the case for a few model cycles now that models indicate a blocky pattern developing as Plains energy dives south Monday-Tuesday and helps develop a southern stream upper low while ridging shifts north of it. The 12/18Z model cycle and the newer 00Z model cycle 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, but there is still considerable spread on position of this low and the timing of its track though. It is hard to consider any model an outlier with this feature as ensemble members show a lot of spread as well. Given the considerations above, the WPC forecast was based on a blend of the 12/18Z operational models for the first half of the period, with some ensemble mean inclusion to almost half by day 7. This provided middle ground positions for both the western trough and the eastern low. ...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, 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 is looking likely over some areas from central and eastern parts of the Gulf Coast through the Southeast during Tuesday-Friday with a general trend east-northeastward 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. But did add a Marginal Risk to the new Day 5 (Tuesday) Excessive Rainfall Outlook for parts of the central Gulf Coast, as most guidance indicates at least some heavy totals making it onshore during that time. 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. 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. But 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. 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