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Composite Map Day 8-14

Experimental Probabilistic Outlooks Temperature Hazards Precipitation Hazards Wind Hazards Snow Hazards

Valid Saturday June 03, 2023 to Friday June 09, 2023

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT May 26 2023

Synopsis: The synoptic pattern during the week-2 period is characterized by widespread mid-level high pressure over Canada and weak mid-level low pressure along both coasts of North America. This results in warmer and relatively quiet weather for most of the Contiguous U.S. (CONUS). Enhanced chances for excessive heat in parts of the Midwest are anticipated to wane early week-2, but as the anomalous mid-level ridge in Canada drifts west, there will be slightly enhanced chances for a period of nominally-excessive heat in the Northwest CONUS. A favorable environment for thunderstorms with heavy rainfall in the Plains and some adjacent portions of the Lower Mississippi Valley and Rockies should slowly relent by the middle of week-2, when moist low-level inflow eases. The potential for river flooding mostly related to the spring thaw continues for portions of the West CONUS, Alaska, and along portions of the James River in South Dakota and Souris River in North Dakota.

Hazards

• Slight risk for heavy precipitation for portions of the Plains and adjacent Rockies, Sat-Tue, Jun 3-6.
• Slight risk for excessive heat for portions of the Upper Midwest, and the Ohio, Middle and Upper Mississippi Valleys, Sat, Jun 3.
• Slight risk for excessive heat for parts of the Northwest Sat-Fri, Jun 3-9.
• Possible flooding for portions of California, Nevada, Utah, Wyoming, southwestern Montana, and Idaho.
• Possible flooding along portions of the James River in South Dakota.
• Possible flooding along portions of the Souris River in North Dakota.
• Possible flooding in parts of western Mainland Alaska and along the Yukon, Kuskokwim, Koyukuk, and Kobuk Rivers.
• Rapid onset drought risk across portions of the Upper Midwest, and the Middle Mississippi and Ohio Valleys.

Detailed Summary

For Monday May 29 - Friday June 02: WPC Days 3-7 U.S. Hazards

For Saturday June 03 - Friday June 09: The week-2 synoptic picture as depicted by ensemble model solutions for 500-hPa height anomalies resembles an omega block, with weak troughing along both coasts of North America, slightly below-normal mid-level 500-hPa heights along the southern tier of the CONUS, and expansive mid-level ridging over much of Canada. This general configuration continues through the period, although the anomalous mid-level ridge centered over Canada is expected to slowly retrogress toward western Canada and weaken slightly as the period progresses. As a result, warm and generally quiet weather is predicted to prevail across most of the CONUS. One exception to this is over part of the Plains and adjacent portions of the Rockies and Lower Mississippi Valley, where increased low-level jet (LLJ) activity creates enhanced probabilities of heavy precipitation with periodic thunderstorms for the region until the middle of week-2. Rainfall will be enhanced at times by upsloping orographic winds near the mountains. Reforecast tools from the GEFS, ECMWF, and CMCE all indicate at least a 20% probability of exceeding the 85th climatological percentile and 0.75 to over 1.0 inch for 3-day accumulated precipitation, supporting a slight risk of heavy precipitation across the region, Jun 3-6. This slight risk area is again extended farther north for the second successive day based on raw output from the CMCE, ECMWF, and GEFS ensemble means.

Expansive mid-level ridging over much of central North America is likely to result in warmer than normal temperatures across the central and northern portions of the CONUS, closest to the maximum 500-hPa height anomalies associated with the broad mid-level ridge centered in Canada. All reforecast tools indicate at least a 20% chance of maximum temperature exceeding the 85th percentile during Jun 3 from the northwestern Ohio Valley through the Middle and Upper Mississippi Valleys and the southwestern Great Lakes Region. The ECMWF reforecast tool and to a lesser extent the CMCE counterpart favor a warmer solution with 30% to 50% probabilities of exceeding the 85th percentile, but GEFS-based tools are cooler and bring in significant uncertainty, precluding designating any areas with a moderate risk of excessive heat. Dew points and associated heat indices are not expected to be extremely elevated, but nonetheless there is the potential for temperatures to exceed 90 deg F for portions of the Upper Midwest for the first time this season. The past one to two months have been drier than normal in this region, which increases the odds for higher temperatures but limits the potential for unusually high dew points. Taking all of this into consideration, a slight risk of excessive heat is posted from western Indiana westward through northern Missouri and northward across southern Minnesota, including part of the Chicago metro area, Jun 3.

Ensembles overwhelmingly favor a slow retrogression and de-amplification of the expansive ridge over Canada, which should bring much above-normal temperatures to the northwestern CONUS throughout the period. There is still a lot of uncertainty regarding the potential for hazardous heat, but with positive 500-hPa height anomalies over and north of the region throughout week-2, chances are sufficiently elevated to post a slight risk for excessive heat over the Northwest throughout the period (Jun 3-9), with the best chances near the middle of week-2. Probabilistic tools show lesser chances for impactful heat to the west of the Cascades, and the lower-level wind flow pattern is not particularly favorable to bring excessive heat west of the Cascades, so the hazard has been removed from this area. In contrast, probabilistic tools based on all three model ensembles show enhanced chances for temperatures topping 90 deg. F in parts of central and southwestern Idaho, so the slight risk hazard has been extended into this region.

Drought concerns are forecast to increase across parts of the central and eastern Corn Belt and adjacent areas during week-2 due to the aforementioned warmth, the expectation of below-normal precipitation, and precipitation totals one to three inches below normal for the past 30 to 60 days. Soil moisture is currently below the 30th climatological percentile over many of these areas. Due to these factors, a rapid onset drought risk is highlighted across parts of the central and eastern Corn Belt, specifically in much of southern Iowa, northern Missouri, central Illinois and Indiana, and southern Ohio.

Snowmelt continues across the western CONUS after accumulating a very large and in some cases record-breaking snowpack, inducing river flooding especially across higher elevations for some areas in the West, where a few sites still report over 40 inches of snow water content. As snowmelt continues in parts of the Sierra Nevada, Great Basin, and Northern Rockies, flooding may continue to occur along some rivers and in areas adjacent to the higher elevations. Flooding is also expected to continue along the James River in South Dakota as river levels have been very slow to recede due to saturated ground conditions and relatively flat topography. A similar situation has developed along the Souris River in North Dakota, which should cause ongoing minor to moderate flooding to continue into week-2.

Weak mid-level flow across Alaska favors only a slight tilt in probabilities for above and below normal temperatures across parts of northern and southern Alaska, respectively. However, regardless of temperatures, rivers are expected to continue to rise across western Mainland Alaska due to ice jams, while increasing snowmelt across eastern Alaska is anticipated to continue the potential for river flooding through a large part of the Alaskan Interior. Other rivers of concern for river ice break up leading to the potential for possible flooding and ice jams are the Kuskokwim, Koyukuk, and Kobuk Rivers. Another factor which could impact conditions over parts of Alaska is the evolution of Super Typhoon Mawar, currently west of Guam in the Pacific Ocean with top winds near 175 mph. Mawar is now the strongest typhoon since 2021, and the fifth-strongest May typhoon on record. The typhoon is expected to continue westward for the next few days before turning to the northeast just east of Taiwan. After recurvature, the storm is anticipated to slowly become extratropical while continuing northeastward, but still remain a potent storm system. Some tools bring the remnants of Mawar close to Alaska during week-2, which would significantly impact some parts of the state. However, model guidance has been inconsistent regarding how far north the storm tracks, and today’s runs are less indicative of a potential threat to Alaska than yesterday, so there are no highlighted hazard risks associated with the remnants of the typhoon at this time.

Forecaster: Rich Tinker

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

Resources

Week-2 Probabilistic Extremes Tool

GFS Ensemble Forecasts