Composite Map Day 8-14

Experimental Probabilistic Outlooks Temperature Hazards Precipitation Hazards Wind Hazards Snow Hazards

Valid Saturday July 27, 2024 to Friday August 02, 2024

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT July 19 2024

Synopsis: An eastward shifting area of strong mid-level high pressure during week-2 leads to an increased risk for excessive heat over parts of the north-central and northeastern contiguous U.S. (CONUS) later in July. With hotter and drier than normal conditions predominately favored over the Interior West and Great Plains, an expanded Rapid Onset Drought (ROD) risk area is issued for parts of the Northern and Central Plains where poor soil moistures and below-normal rainfall amounts have been registered.

Hazards

• Moderate risk of excessive heat for parts of the Northern Plains and Upper Mississippi Valley, Sat-Sun, Jul 27-28.
• Moderate risk of excessive heat for parts of the northern Great Lakes and the Northeast, Sun-Tue, Jul 28-30.
• Slight risk of excessive heat for parts of the Northern Rockies, Northern and Central Plains, Upper Mississippi Valley, Great Lakes, Ohio Valley, Mid-Atlantic, and the Northeast, Sat-Fri, Jul 27-Aug 2.
• Slight risk of episodic high winds for much of the western CONUS, Sat-Mon, Jul 27-29.
• Rapid Onset Drought risk for portions of the Northern and Central Plains.

Detailed Summary

For Monday July 22 - Friday July 26: WPC Days 3-7 U.S. Hazards

For Saturday July 27 - Friday August 02: By late next week and into the following weekend, 500-hPa height forecasts from the GEFS and ECMWF ensembles continue to feature an amplifying trough over western Canada and the West Coast, and an eastward progression of a strong ridge center with positive height departures overspreading much of southern Canada and the northeastern CONUS. Later in week-2, dynamical models generally agree on the mid-level pattern evolution over eastern North America, favoring the continuation of the anomalous ridging (albeit some differences in strength) over the Canadian Maritimes with more subtropical ridging developing in the Southeast, but diverge in regards to the evolution of troughing upstream over the West. The GEFS maintains a more persistent solution of the trough, whereas the ECMWF is quick to build more ridging back in over the Four Corners. Given this, outlook confidence is higher east of the Rockies, favoring warming temperatures and the potential for excessive heat across the northern tier of the CONUS, with the hazards picture becoming less clear out West through the end of July and heading into August.

Tied to the amplifying trough and the associated northwesterly flow to bring cooler temperatures over parts of the western CONUS late next week, the slight risk of excessive heat is discontinued in today's outlook as the predicted heat event over the West now appears to be largely within the week-1 timeframe. But in response to the enhanced southwesterly flow upstream of the amplified ridge center to the south of Hudson Bay, there continues to be increased odds for above-normal temperatures mainly over the north-central CONUS early in the period. Good agreement exists between the GEFS and ECMWF Probabilistic Extremes Tools (PETs) showing elevated chances (30-40%) for maximum temperatures exceeding the 85th percentile to support the continuation of the moderate risk of excessive heat over the Northern High Plains and Upper Mississippi Valley through day 9 (Jul 28).

As the troughing over the northwestern CONUS weakens, and the anomalous ridge center shifts eastward into southeastern Canada towards the middle of week-2, PETs show an expansion of the heat signals into portions of the Great Lakes, Ohio Valley, Mid-Atlantic and the Northeast. Given increased support in the raw temperature tools and the better agreement in the PETs depicting 30-50% chances for maximum temperatures exceeding the 85th percentile, a moderate risk of excessive heat is issued for the region from Jul 28-30. A broad slight risk of excessive heat remains issued along the northern tier of the CONUS, east of the Rockies, in proximity to the prevailing positive height anomalies over southern Canada and remains valid for all of week-2. Of note, the GEFS PET continues to favor increased chances for excessive heat further south into the Middle Mississippi, Ohio and Tennessee Valleys towards the middle of week-2, however the ECMWF PET, as well as raw temperature tools are less supportive of this realization. Excessive heat conditions may be felt across parts of the Corn Belt and lower Great lakes as the heat signals begin to shift south, but there remains uncertainty as to how long the colder air established over the central U.S. early in week-2 takes to moderate. There are also questions as to reliability and bias of the GEFS PET in this part of the country during the warm season.

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

Lee side surface troughing remains favored over the Northern and Central Plains where ensembles continue to show periods of strengthening pressure gradients in the region, as well as further south in the Desert Southwest. Based on continued support from the ECMWF PET favoring increased chances for wind speeds exceeding the 85th percentile, a slight risk of high winds remains issued for Jul 27-29 before the pattern deamplifies over the western CONUS. Hot, dry and windy conditions are likely to lead to enhanced risk of wildfires in the highlighted region, and inhibit efforts to contain any active wildfires.

For precipitation, the threat of heavy rainfall looks to have timed off over the lower Plains and Mississippi Valley, with much of the troughing favored to lift out late next week, and more subtropical ridging overspreading the Southeast from the western Atlantic. While there continues to be modest support in the PETs for 3-day amounts exceeding the 85th percentile early in week-2, raw precipitation amounts from the ensembles have become tenuous, and the slight risk of heavy precipitation is removed in the updated outlook. It should be noted however, that above-normal precipitation predominately favored in the next two weeks over portions of southern Texas and the eastern Carolinas may lead to localized flooding in these regions during the period.

Over Alaska, a highly amplified trough/ridge couplet is favored over the Bering Sea and northern Pacific during week-2. The resultant enhanced onshore flow is expected to lead to periods of elevated wind speeds and enhanced precipitation over parts of the Mainland, however, PETs show neither wind nor precipitation amounts exceeding hazard thresholds, and no corresponding hazards are issued. 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