042 FXUS02 KWBC 031915 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 315 PM EDT Sat May 3 2025 Valid 12Z Tue May 06 2025 - 12Z Sat May 10 2025 ...Blocky pattern with closed lows over the West and the East with areas of heavy rainfall potential early to mid next week... ...Overview... An omega block will remain in place over the lower 48 into Tuesday, with an upper low atop the Four Corners region, one over the Ohio Valley, and upper ridging in between. Both lows should slowly drift east by Wednesday/Thursday. Moist inflow over the south-central and southeastern U.S. ahead of the Four Corners upper low will produce areas of significant rainfall and potential flooding over portions of the Central and Southern Plains into the Lower Mississippi Valley on Tuesday and near the central Gulf Coast Wednesday-Thursday as a front stalls there. Meanwhile some locally heavy rain is possible in the Northeast ahead of the eastern low. This eastern low is forecast to be swept up toward the Canadian Maritimes later next week as a northern stream trough reestablishes over the Northeast. The western low will likely remain isolated as it slowly tracks over the Plains toward the Lower Mississippi Valley next weekend. ...Guidance/Predictability Assessment... Model guidance seems to be in reasonable agreement early in the forecast period considering the blocky pattern. The eastern upper low is forecast to track from the Ohio Valley/Great Lakes Tuesday and atop the Northeast Wednesday, before pivoting northeast as northern stream energy drops into the Northeast. This northern stream energy potentially closing off an upper low shows some timing differences in the 00/06Z and newer 12Z guidance, with the 12Z GFS the fastest/farthest east into late week. The eastern trough will also impact surface low pressure systems in the Northeast and western Atlantic that vary. Tended to favor the ensemble means with the trough's timing despite the means not being as deep as the deterministic runs. The western upper low is forecast to remain cut off at least through midweek, and recent operational and AI/ML models show it staying cut off from the main flow even into late week as it drifts into the Lower Mississippi Valley or Southeast, possibly interacting with the eastern trough but with some energy held back. Thus the forecast trended in the direction of an upper low stalling over the Lower Mississippi Valley even into early next Saturday, with less phasing with the northeastern trough. The newer 12Z guidance kept this trend but will continue to monitor as the position of that upper low would considerably affect sensible weather. Smaller scale features like shortwaves also show spread in the West, even by Tuesday-Wednesday with the potential for a compact low in the northern Rockies or High Plains as shown by many 00/06Z models. The WPC forecast used a blend of the 06Z GFS and 00Z ECMWF and CMC early in the period, and introduced and gradually increased the proportion of ensemble means to just over half by the end of the period to reduce too many individual model differences. ...Weather/Hazards Highlights... Anomalous moisture is forecast to persist over the southwestern to south-central U.S. near and ahead of the Four Corners upper low. Precipitation is likely to remain over the central Great Basin/Southwest into Tuesday and the southern/central Rockies on Wednesday, including moderate snowfall in the highest elevations. But farther east, ample instability pooling ahead of a low pressure/frontal system in the south-central U.S. will allow for widespread rain and thunderstorms across the south-central Plains into the Lower Mississippi Valley on Tuesday. Portions of the Plains into the Lower Mississippi Valley states will be especially vulnerable to heavy rain/flooding due to recent heavy rains and saturated soils. Widespread rainfall amounts of 2-5+" remain likely across portions of the ArkLaTex and Lower Mississippi Valley, and a Moderate Risk remains in place for the Day 4/Tuesday ERO for this activity. Back westward, this ERO issuance expanded the Slight Risk across the south-central High Plains where rain totals of 1-3" will be well above average and likely exceed lower-end average recurrence intervals. Then by Wednesday, the heaviest rain is likely to shift toward the central Gulf Coast pooling along a frontal boundary starting to lay even more west to east. Maintained a Slight Risk similar to the previous issuance, as heavy rain rates and training could cause scattered flash floods, but the potential may be limited slightly by drier antecedent conditions and higher flash flood guidance. Heavy rain could persist into Thursday along the central Gulf Coast with some rain and storms likely across the Southeast in general into late week. Under the eastern upper low, a low pressure system will still be in the vicinity of the Great Lakes with its frontal system stretching into the Northeast on Tuesday. After a few days of moderate to heavy rain in the northeastern U.S. during the short range period, showers and storms on Tuesday may cause isolated flooding risks especially in urban and poor drainage areas. By Wednesday the surface low should be tracking northeastward a little more progressively, but some wrapback moisture and instability near the low track could lead to locally heavy rain in the sensitive terrain of the Interior Northeast. Marginal Risks remain in place across portions of the Northeast Tuesday and Wednesday, though shrinking in coverage in this forecast cycle as models come into slightly better agreement with where rainfall may focus, and as rain amounts generally decrease. The upper low over the Southwest early in the week will result in below to well below normal temperatures for the Southwest and south-central Plains where daytime highs could be 15-20 degrees below normal. This should moderate with time but may still remain modestly below normal into late week for the southern Plains. Upper ridging building over the West as the upper low shifts east will allow for expanding coverage of above normal temperatures there as next week progresses, including 100 degree temperatures in the Desert Southwest. Above average temperatures will stretch into the northern tier as well, with highs reaching the 70s and 80s in the northern Plains to Upper Midwest. Tate/Santorelli 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 forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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 https://www.wpc.ncep.noaa.gov/#page=ovw $$