708 FXUS21 KWNC 121808 PMDTHR US Hazards Outlook NWS Climate Prediction Center College Park MD 300 PM EDT May 12 2026 SYNOPSIS: A pair of forecast mid-level high pressure centers over the eastern and western contiguous U.S. (CONUS) results in a more favorable pattern for unseasonably warm temperatures, with possible extreme heat conditions being felt over parts of the Mid-Atlantic, Southeast, and California early in week-2. Surface low development in the lee of the Rockies supports potentially heavy precipitation over the south-central CONUS, with the highest risk now timing off into week-1. Meanwhile, cyclonic mid-level flow over the southwestern CONUS may also bring episodes of high winds across coastal California and Oregon. HAZARDS Slight risk of extreme heat for portions of the Mid-Atlantic, and Southeast, Wed-Thu, May 20-21. Slight risk of extreme heat for the Central Valley of California, Wed-Thu, May 20-21. Slight risk of heavy precipitation for portions of the Southern Plains, Lower Mississippi, Tennessee and Ohio Valleys, Wed-Thu, May 20-21. Slight risk of high winds for much of coastal California and Oregon, Wed-Fri, May 20-22. DETAILED SUMMARY FOR FRIDAY MAY 15 - TUESDAY MAY 19: https://www.wpc.ncep.noaa.gov/threats/threats.php FOR WEDNESDAY MAY 20 - TUESDAY MAY 26: As previously advertised, a major pattern shift looks to be well underway during week-1. Dynamical models favor the development of amplified ridging over much of the eastern CONUS, troughing over portions of the Great Plains and Interior West, with another anomalous 500-hPa ridge center nudging eastward from the northeastern Pacific into the West Coast. By the start of week-2, both the GEFS and ECWMF ensembles are in fair agreement depicting a temporary bridging between the two 500-hPa ridge centers over the eastern and western CONUS, consisting of rising heights over the Interior West and Great Plains, followed by a breakdown of this pattern towards the end of next week. It is at this forecast lead where the ensemble mean solutions begin to diverge considerably in the latest guidance. The ECMWF favors more of a progressive evolution of the pattern, featuring more ridging shifting over the western CONUS from the eastern Pacific, with more downstream troughing east of the Rockies. In contrast, the GEFS is more steadfast with the amplified ridge over the eastern U.S. ridge, while favoring more longwave troughing upstream across the West. As a result, there is significant uncertainty in the hazards outlook beyond day 10 (May 22) due to these mid-level features being completely out of phase over the CONUS in the various models. Tied to the amplified ridge across the East at the start of week-2, a slight risk of extreme heat remains posted (May 20-21) over portions of the mid-Atlantic and the Southeast where Probabilistic Extreme Tools (PETs) maintain increased chances for maximum daytime temperatures exceeding the 85th percentile climatologically and 90 deg F. Both raw and calibrated temperature tools continue to show much of the anomalous warmth peaking late in week-1, before the ridging aloft deamplifies over the eastern U.S. in both the GEFS and ECMWF by the middle of next week. Due to the predicted larger and slightly stronger coverage of positive height departures overspreading the West Coast compared to yesterday, a slight risk of extreme heat is also posted (May 20-21) over the Central Valley of California which is supported by warm trends in the latest temperature tools. Although the GEFS is comparatively weaker with the heat potential than the ECWMF, both PETs show at least 20% chances of maximum temperatures exceeding the 85th percentile, with support in the raw tools indicating similar chances for actual temperatures exceeding 100 deg F. Based on historical heat risk data and classifications defined at https://www.wpc.ncep.noaa.gov/heatrisk/, such temperatures surpass orange and red level thresholds over this part of the country during May. If spring heat is realized over these highlighted areas, such temperatures may adversely impact unsuspecting populations without cooling or hydration systems in place, and also those who are not yet acclimated to this level of warmth. Across the Great Plains, surface low development and an accompanying frontal system continues to be favored, potentially bringing heavy precipitation to the south-central CONUS late in week-1. Much of the surface return flow to enhanced moisture advection into the Plains and Mississippi Valley is favored to ease heading into week-2, with the mid-level heights also favored to rise. Daily mean precipitation totals from the GEFS, ECMWF, and Canadian ensembles now show the highest amounts timing off, though raw and calibrated probabilistic guidance continues to indicate increased chances for 3-day amounts exceeding an inch focused over the Southern Plains and Lower Mississippi Valley, albeit lower chances compared to yesterday. Based on the drier trend in the tool and the less favorable synoptic pattern, the moderate risk of heavy precipitation is discontinued in the updated outlook, however a slight risk remains posted and valid through May 21 due to some of the frontal activity potentially lingering into early week-2. Flash flooding is possible in some areas with anticipated rainfall during week-1 and the beginning of week-2, especially in some areas in the Deep South that are currently experiencing river flooding. Given the transient nature of the pattern over the Great Plains, with anomalous troughing lifting out and mid-level heights beginning to rise, the risk of high winds is expected to diminish. Although there continues to be marginal signals in the PETs for winds speeds exceeding the 85th percentile, there is not enough forecast confidence that wind speeds will exceed hazard thresholds based on the favored surface pattern and uncalibrated wind guidance among the GEFS and ECMWF, thus the slight risk of high winds is removed over the central CONUS. However, the slight risk of high winds remains posted over portions of coastal California and Oregon where ensembles generally show cyclonic mid-level flow underneath the anomalous ridge center over the northeastern Pacific as well as periods of strengthening pressure gradients in the surface guidance. In Alaska, the typical peak of the river ice breakup season is approaching, and ice jam flooding can occur this time of year with little or no notice as conditions on frozen rivers can change quickly. Please check with the Alaska Pacific River Forecast Center for the latest conditions and advisories. FORECASTER: Nick Novella $$