Composite Map Day 8-14

Experimental Probabilistic Outlooks Temperature Hazards Precipitation Hazards Wind Hazards Snow Hazards

Valid Friday July 26, 2024 to Thursday August 01, 2024

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT July 18 2024

Synopsis: An eastward shifting area of mid-level high pressure during week-2 is expected to sustain an increased risk for excessive heat over many parts of the western contiguous U.S. (CONUS) early in the period, with increasing chances for heat across the Northern Great Plains and expanding over the northern tier of the CONUS later in July. Mid-level low pressure focused over the lower Plains and Mississippi Valley is likely to promote cooler and wetter conditions early in week-2, with increased chances for heavy precipitation in the Southern Plains and Lower Mississippi Valley. With hotter and drier than normal conditions predominately favored, an expanded Rapid Onset Drought (ROD) risk area is issued for parts of the Northern and Central Plains.

Hazards

• Moderate risk of excessive heat for parts of the Northern Plains and Upper Mississippi Valley, Fri-Sun, Jul 26-28.
• Slight risk of excessive heat for parts of California, Desert Southwest, Great Basin, Rockies and Northern Intermountain, Fri, Jul 26.
• Slight risk of excessive heat for parts of the Northern Rockies, Northern and Central Plains, Upper Mississippi Valley, Great Lakes, and the Northeast, Fri-Thu, Jul 26-Aug 1.
• Slight risk of episodic high winds for much of the western CONUS, Fri-Sun, Jul 26-28.
• Slight risk of heavy precipitation for parts of the Southern Plains, Lower Mississippi and Tennessee Valleys, and the Southeast, Fri-Sat, Jul 26-27.
• Rapid Onset Drought risk for portions of the Northern and Central Plains.

Detailed Summary

For Sunday July 21 - Thursday July 25: WPC Days 3-7 U.S. Hazards

For Friday July 26 - Thursday August 01: Dynamical model guidance appears to remain on track over the past few days. Both the GEFS and ECMWF continue to favor a weakening 500-hPa ridge over the western CONUS with cooling temperatures to alleviate the excessive heat risk over many parts of West for late next week. Anomalous mid-level troughing developing over western Canada and into the West Coast is expected to lead to eastward progression of the amplified ridge center to the south of Hudson Bay where there is better agreement between the GEFS and ECMWF ensembles in the timing, location, and strength of this mid-level feature. As a result, there is added confidence for above-normal temperatures to overspread the northern tier, and into parts of the eastern CONUS later in July, with a renewed risk of excessive heat for parts of the Great Plains and Upper Midwest. Across the southern tier of the U.S., much of the troughing that is favored to linger over the lower Plains and Mississippi Valley during week-1 looks to be finally lifting out late next week, but not before another round of potentially heavy precipitation could be felt over parts of the Plains, Mississippi and Tennessee Valleys early in the period.

A slight risk of excessive heat remains posted for parts of California and the Interior West for day 8 (Jul 26), where there are residual heat signals in the Probabilistic Extremes Tools (PETs) tied to the weakening ridge aloft. Much of the slight risk coverage has been withdrawn over the Northern Intermountain region in the updated outlook due to the encroaching mid-level trough favored over the northwestern CONUS. Across the northern tier of the CONUS, there is better agreement between the GEFS and ECMWF PETS depicting an expansive area with at least 20% chances of maximum temperatures exceeding the 85th percentile. Higher probabilities (30-40%) are indicated across northern parts of eastern Montana, North Dakota, and Minnesota and while chances for actual temperatures exceeding 95 deg F appear rather weak in the PETs, raw temperature tools are more robust in exceeding this threshold, with the potential for enhanced Heat Index values. Therefore, a moderate risk of excessive heat is added for the region (Jul 26-28) and a broader slight risk area for excessive heat is continued for Jul 27-Aug 1. The slight risk area is expanded eastward to include northern portions of the Great Lakes, and the Northeast, where PETs show building warm signals later in the period in proximity to the amplified ridging over southern Canada. 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, though the ECMWF PET as well as raw temperature tools are much less supportive of this realization, as much of the colder air established over the central U.S. early in week-2 may take some time to mix out.

The combination of continued above-normal temperatures and below-normal precipitation supports the continuation of a Rapid Onset Drought (ROD) risk over parts of eastern Montana, Wyoming and the Dakotas. Based on soil moisture trends, rainfall registered during the past month and in the next two weeks, the ROD risk has been expanded to include parts of the western Dakotas, Nebraska, northeastern Colorado and Kansas in the updated outlook. 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.

At the base of the troughing aloft over the northwestern CONUS, lee side surface troughing is 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 26-28 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.

With much of the troughing favored to lift out over the lower Plains and Mississippi Valley, raw tools have begun to back off on the continued enhancement of daily precipitation amounts. Notwithstanding, return flow from the Gulf of Mexico and a stalled frontal boundary that is expected to linger late next week sustains a heavy precipitation risk early in week-2. There remains good agreement in the PETS showing increased chances for 3-day amounts exceeding the 85th percentile, and a corresponding slight risk remains posted for Jul 26-27. A key change in the PETs is the reduction of the wet signals over the eastern U.S., and the slight risk coverage is removed over this part of the country. As a result of the reduced potential for heavy precipitation further east, as well as a lowered risk in OWPs latest experimental Flood Hazard Outlook, the possible flooding is likewise removed over the eastern Carolinas and southeastern Virginia in the updated outlook. It should be noted however, that above-normal precipitation predominately favored during week-2 may still lead to localized flooding in the region.

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