Composite Map Day 8-14

Experimental Probabilistic Outlooks Temperature Hazards Precipitation Hazards Wind Hazards Snow Hazards

Valid Thursday May 22, 2025 to Wednesday May 28, 2025

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT May 14 2025

Synopsis: Tied to forecast mid-level high pressure, there are elevated chances for excessive heat over the Southern Plains continuing from week-1 into week-2. There is stronger support compared to yesterday for a heat threat emerging over parts of the Central California Valley, Desert Southwest, and Interior West during week-2. East of the Rockies, mid-level low pressure in the model guidance supports cooler than normal springtime temperatures, with the potential for unsettled weather over parts of the Eastern Seaboard heading into Memorial Day weekend.

Hazards

• Moderate risk of excessive heat for portions of southern Texas, Thu, May 22.
• Slight risk of excessive heat for portions of the Southern Plains and the Lower Mississippi Valley, Thu-Sat, May 21-24.
• Moderate risk of excessive heat for parts of California and the Desert Southwest, Thu-Sun, May 22-25.
• Slight risk of excessive heat for parts of California, the Desert Southwest, and Interior West Thu-Wed, May 22-28.
• Slight risk of episodic high winds for parts of the Pacific Northwest and California, Thu-Sun, May 22-25.
• Slight risk of periodic heavy precipitation for portions of the Mid-Atlantic, Appalachians, and the Northeast, Thu-Sun, May 22-25.
• Slight risk of episodic high winds for portions of the Mid-Atlantic, Appalachians, and the Northeast, Thu-Sun, May 22-25.

Detailed Summary

For Saturday May 17 - Wednesday May 21: WPC Days 3-7 U.S. Hazards

For Thursday May 22 - Wednesday May 28: The week-2 hazards perspective remains mostly on track with a few notable developments in the updated guidance. Dynamical model 500-hPa height forecasts from the GEFS and ECMWF continue to advertise building heights over the West Coast and an amplified subtropical ridge centered over the Gulf of America towards the middle of next week. These two mid-level features remain the focus for excessive heat conditions potentially lingering from week-1 over the Southern Plains and Lower Mississippi Valley, with heat becoming more likely to emerge over the West Coast, Interior West, and Desert Southwest by early in week-2. Over the eastern CONUS, models have trended towards a more persistent and deeper mean 500-hPa trough during the period. This is expected to promote cooler and drier conditions over many parts of the Midwest, while potentially bringing unsettled weather to portions of the eastern Seaboard heading into Memorial Day weekend.

Analysis of normalized height anomalies over the southern tier of the CONUS shows that both the GEFS and ECMWF indicate reduced strength and coverage of these features since yesterday. Despite this weakening, raw temperature tools continue to show large positive temperature departures focused over southern Texas, translating to daytime temperatures in the upper 90s to triple digits early in week-2. Based on both the GEFS and ECMWF Probabilistic Extremes Tools (PETs) maintaining elevated chances for maximum temperatures exceeding the 90th percentile and the NWS Heat Risk tool favoring extreme levels (red and purple values) of heat late over southern Texas late in week-1, a moderate risk of excessive heat remains issued for day 8 (May 22). Within the highlighted area, the National Blend of Models (NBM) still shows a number of locations approaching or breaking maximum temperature records. By days 9 and 10 (May 23-24), PETs show a reduction in the heat signals tied to the aforementioned troughing favored and a northerly component of the mid-level flow over the Midwest. Despite indications of more northerly flow, tools still maintain at least 20% chances for daytime temperatures in the 85th percentile climatologically, supporting a corresponding slight risk of excessive heat area across the south-central CONUS, valid through May 24.

Across the West, ensembles have been consistent in bringing in more amplified ridging from the eastern Pacific, with the ridge center favored to shift into the Great Basin and Interior West by the middle of week-2. The associated surface temperature responses are well reflected in the temperature tools, with the PETs becoming more robust with the heat signals over the West Coast and expanding eastward in the Four Corners. While the GEFS remains comparatively weaker than the ECMWF, these tools support the inclusion of a moderate risk of excessive heat from the Central California Valley into the Desert Southwest, now valid for May 22-25 in the updated outlook. Within the moderate risk area, the PETs indicate 30-60% chances for daytime temperatures exceeding the 85th percentile and 95 degrees F as far north as Sacramento, California, and 105 degrees F in the lower elevations of the Desert Southwest through the middle of the period. While these temperatures would not meet hazardous criteria during the peak of the summer, such temperatures may reach advisory or warning levels based on NWS Heat Risk climatology for this of the year. A broader area is highlighted with a slight risk of excessive heat highlighted for the entirety of week-2, which now covers the higher elevations of the Great Basin and parts of the Rockies where there is at least a 20% chance of maximum temperatures exceeding 90 degrees F. Coupled with a more persistent mean trough downstream over the east, ensembles suggest a more stable ridge/trough pattern developing over the CONUS, which could bring a more prolonged risk of excessive heat conditions across the West through the end of May. This potential will continue to be assessed in upcoming outlooks this week.

In addition to the heat signals over the West Coast, enhanced offshore flow is favored associated with strong mean surface high in the eastern Pacific. While this is likely to bring a period of suppressed precipitation, a strong mean surface pressure gradient favored over northern California and the Pacific Northwest in the GEFS and ECMWF ensembles is expected to promote episodes of high winds. This is reflected in the PETs indicating increased chances for wind speeds exceeding the 85th percentile, thus a slight risk of high winds remains posted for May 22-25. It is worth noting that any elevated wind speeds across the Southern High Plains and the western CONUS may elevate the risk for wildfire conditions given high odds for above-normal temperatures and near- to below-normal precipitation favored during week-2. Such conditions would lead to drier fuels, where any episodes of dry thunderstorms/lightning could initiate wildfire activity.

East of the Rockies, the shortwave energy ejecting from the Rockies merging with negative height departures retreating westward from the northwestern Atlantic is favored to bring more cohesive and persistent mean troughing over the eastern CONUS, with the mean axis extending from the Great Lakes into the Southeast through the middle of week-2. As a result, more northwesterly flow is favored to promote unseasonably cooler temperatures for many parts of the northeastern CONUS, though guidance is pointing towards more above-normal precipitation and unsettled weather ahead of the trough axis over the eastern Seaboard, with the potential surface low development offshore of the Mid-Atlantic and Northeast. Ensembles still show ample spread with the position of the surface low, and PETs remain fairly modest in regards to heavy precipitation potential. However, there is good agreement between the GEFS and ECMWF depicting at least a 20% chance of 3-day amounts exceeding the 85th percentile supporting a corresponding slight risk area remaining posted, now valid through May 25. At the base of the amplifying trough over the eastern U.S., a slight risk of high winds is also continued for the same period. While confidence is low, any heavy precipitation and high winds realized may impact Memorial Day travel along the I-95 corridor from the Carolinas to Maine.

No hazards are issued over Alaska. Cooler than normal springtime temperatures are favored mainly north of the Brooks Range, with a slight tilt towards above-normal precipitation for much of the Mainland and Southeast, but nothing approaching hazard thresholds.

Forecaster: Nick Novella

$$ Please consult local NWS Forecast Offices for short range forecasts and region-specific information.

Resources

Week-2 Probabilistic Extremes Tool

GFS Ensemble Forecasts