Composite Map Day 8-14

Experimental Probabilistic Outlooks Temperature Hazards Precipitation Hazards Wind Hazards Snow Hazards

Valid Saturday August 03, 2024 to Friday August 09, 2024

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT July 26 2024

Synopsis: A strengthening and westward shifting area of mid-level high pressure supports an elevated risk of excessive heat conditions along with increased chances for high winds for the western half of the contiguous U.S. (CONUS). A secondary area of strong mid-level high pressure centered over New England is expected to maintain elevated chances for excessive heat for much of the Mid-Atlantic, eastern Great Lakes and the Northeast through the middle of week-2. A healthier monsoon circulation, as well as possible tropical cyclone activity in the eastern Pacific leads to increased chances for heavy precipitation over parts of the Desert Southwest. Possible tropical cyclone development in the Caribbean leads to increased risk for heavy precipitation and high winds over portions of the Gulf Coast and Southeast.

Hazards

• High risk of excessive heat for parts of the Northern intermountain, northern Great Basin, Rockies, and Northern and Central High Plains, Sat-Mon, Aug 3-5.
• Moderate risk of excessive heat for parts of California, Pacific Northwest, Great Basin, Rockies and the Great Plains, Sat-Fri, Aug 3-9.
• Moderate risk of excessive heat for the Northeast, eastern Great Lakes, Central Appalachians, and Mid-Atlantic region, Sat-Wed, Aug 3-7.
• Slight risk of excessive heat for much of the CONUS, except for parts of the Southern Plains, Lower Mississippi and Tennessee Valleys, Southern Appalachians, and Southeast, Sat-Fri, Aug 3-9.
• Slight risk of heavy precipitation for parts of the Desert Southwest, Sat-Wed, Aug 3-7.
• Slight risk of heavy precipitation for parts of the Southeast and Lower Mississippi Valley, Sat-Tue, Aug 3-6.
• Slight risk of high winds for much of the western CONUS, and High Plains, Sat-Wed, Aug 3-7
• Slight risk of high winds for parts of the Southeast and Lower Mississippi Valley, Sat-Wed, Aug 3-7.
• Rapid Onset Drought risk for much of the Great Plains, and into western Missouri.

Detailed Summary

For Monday July 29 - Friday August 02: WPC Days 3-7 U.S. Hazards

For Saturday August 03 - Friday August 09: Analysis of the mean week-2 500-hPa forecast height anomaly patterns from the GEFS and ECMWF continues to feature strongly amplified ridging overspreading nearly the entire CONUS, with near to slightly below normal heights over the southern tier of the U.S. Since yesterday, both model means have converged on the development of two separate height anomaly maximas in the CONUS, one situated over the northern Intermountain and the other centered over New England where the latest temperature guidance favors the greatest odds for above-normal temperatures and risks for excessive heat. As a result of the amplifying ridge center upstream across the across the West, however, both the GEFS and ECMWF favor more northwesterly flow across the Midwest, which is anticipated to lower the risk of excessive heat conditions, particularly for parts of the Plains and Mississippi Valley that remain favored to experience the strongest positive temperature departures late in week-1.

Based on raw and calibrated temperature tools, a broad slight risk of excessive heat remains posted for all of week-2, and is expanded to include nearly all of the West Coast in today's outlook. Western Washington is excluded from the slight risk due to the lack of support for temperatures reaching hazard criteria throughout the period. Within the broad slight risk area, a moderate risk of excessive heat is continued for all of week-2, covering the western half of the CONUS, and is likewise expanded westward to include parts of the Great Basin and the Central California Valley given shifts in the prevailing height pattern, increased warm signals in the Probabilistic Extremes Tools (PETs), and support in the National Blend of Models (NBM) indicating several locations reaching near record breaking daytime temperatures within the highlighted region. Relative to yesterday's outlook, the high risk for excessive heat is removed across the Central and Southern Plains due to the aforementioned reduction of heat potential over this part of the country, but a high risk is introduced over parts of the northern Great Basin, Northern Intermountain, Rockies and the upper High Plains closest to the anomalous ridge center. Within the high risk area, PETs depict 50-60% chances of maximum daytime temperatures exceeding the 85th percentile and 95 (90) degrees F over the lower (higher elevations). Tied to the secondary height center over New England, a moderate risk of excessive heat remains issued for through the middle of week-2, where PETS show 30-50% chances of temperatures exceeding the 85th percentile. The addition of a high risk was considered based on the ECMWF PET favoring >60% chances for exceeding the 85th percentile for parts of Maine, however the GEFS PET maintains relatively lower chances and there are little to no signals in the raw tools for maximum temperatures exceeding 90 degrees to support a higher hazard designation.

With the amplifying ridging favored aloft over the northwestern CONUS, and possible surface low development over the Northern High Plains early in the period, strengthening pressure gradients to induce episodes of high winds are favored in the region, as well as further south in Desert Southwest. A slight risk of high winds is issued (Aug 3-7) for much of the western CONUS where there are increased chances for wind speeds exceeding the 85th percentile in the ECMWF PET. The hot, dry and windy conditions are likely to lead to an enhanced risk of wildfires in the highlighted region, and inhibit efforts to contain any ongoing incidents.

The combination of continued above-normal temperatures and below-normal precipitation also supports the continuation of a Rapid Onset Drought (ROD) risk from most of the Northern and Central Plains into western Missouri. Dry soils, stressed vegetation, and reduced water availability due to low ponds and streams may lead to increased impacts to agriculture and livestock, with some locations already experiencing impacts.

For precipitation, there is little support in the models for substantial mid-latitude frontal forcing as any heavy precipitation risks look to be more tropical in origin during week-2. Analysis of upper-level velocity potential anomalies from the GEFS and ECMWF continue to feature an envelope of enhanced divergence aloft likely tied to the passage of a convectively coupled Kelvin wave over the tropical Americas. Although the ECMWF reveals more of an Madden-Julian Oscillation (MJO) presence in these fields, regardless, the large-scale environment appears to be more favorable for Tropical Cyclone (TC) genesis in the Western Hemisphere, which is supported in the GEFS and ECMWF ensembles featuring one or more areas of deepening low pressure to the south of Mexico by next weekend. Given a more favorable monsoonal ridge axis forecast over the Four Corners, potential TC activity to trigger a Gulf of California surge event (as supported in deterministic solutions with increased dewpoints), as well as increasing signals in the PETs across the Desert Southwest, a slight risk of heavy precipitation is issued for Aug 3-7.

Across the tropical Atlantic, the hazards perspective has become more interesting. Over the past several runs, the ECMWF deterministic and ensemble solutions have latched onto a tropical wave tracking westward from the western Atlantic, with increased chances for development based on probabilistic TC genesis tools by next weekend. As this potential disturbance moves closer to the southeastern U.S., both raw and calibrated precipitation tools show an increasing risk of heavy precipitation and high winds over many parts of the Southeast and into the Lower Mississippi Valley through the middle of week-2. However, both the GFS and GEFS remain largely unsupportive of this tropical potential, with little to no precipitation signals in the raw and calibrated PET guidance. Despite this, there is some support in the Canadian for this potential, and the latest Tropical Weather Outlook from National Hurricane Center shows at least a 20% chance of formation over the western Atlantic and Caribbean during the next seven days tied to the tropical wave. Given this, and good continuity in the models in regards to large scale conditions aloft becoming more favorable for TC development, a slight risk of heavy precipitation is highlighted for much of the Southeast and parts of the Lower Mississippi Valley for Aug 3-6. In addition, a slight risk of high winds is also posted (Aug 3-7) where the ECMWF PET shows increased chances for wind speeds exceeding the 85th percentile.

No hazards are posted for Alaska during week-2. However, there is a significant chance of accumulating snow for portions of the central and eastern Brooks Range, which could have an impact on people within that area. Near Juneau, localized flooding may be a concern associated with the glacial outburst flood for the Mendenhall River.

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