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Composite Map Day 8-14

Experimental Probabilistic Outlooks Temperature Hazards Precipitation Hazards Wind Hazards Snow Hazards

Valid Friday June 09, 2023 to Thursday June 15, 2023

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT June 01 2023

Synopsis: Mid-level high pressure favored late next week is expected to sustain the risk of excessive heat for portions of the northwestern contiguous U.S. (CONUS) early in week-2. Potentially deepening surface low pressure supports increased chances for heavy precipitation over the south-central CONUS as well as over portions of Florida. Antecedent anomalous dryness combined with increased chances for below-normal precipitation continuing through mid-June increases the risk for rapid onset drought over portions of the Midwest and extending eastward towards the Central Appalachians. Localized flooding related to the spring thaw remains possible for portions of the western CONUS, and along portions of the James River in South Dakota.

Hazards

• Slight risk of excessive heat for parts of the Northwest, Fri, Jun 9.
• Slight risk of heavy precipitation for portions of the Southern and Central Plains and Lower and Middle Mississippi Valley, Fri-Sun, Jun 9-11.
• Slight risk of heavy precipitation for portions of Florida, Fri-Sun, Jun 9-11.
• Slight risk of high winds for parts of Southwest Alaska and Aleutian Islands, Fri, Jun 9.
• Possible flooding for portions of California, Nevada, Utah, Wyoming, and Idaho.
• Possible flooding along portions of the James River in South Dakota.
• Rapid onset drought risk across portions of the Middle and Upper Mississippi Valleys, the Ohio Valley, and Great Lakes.

Detailed Summary

For Sunday June 04 - Thursday June 08: WPC Days 3-7 U.S. Hazards

For Friday June 09 - Thursday June 15: By late next week, dynamical model 500-hPa height forecasts continue to advertise a split flow pattern prevailing over the western CONUS with more amplified troughing downstream forecast throughout much of the eastern CONUS. Much of the amplified mid-level ridging favored over western Canada is still expected to ease early in week-2, leading to a moderation of temperatures and a lowered risk of excessive heat persisting over the Pacific Northwest and Northern Intermountain regions. The GEFS and ECMWF ensembles differ somewhat on the timing of the deamplification of this feature . However, this cooling trend is reflected in the reforecast tools which generally depict lower chances for maximum temperatures exceeding the 85th percentile compared to previous guidance. A slight risk of excessive heat remains issued for day 8 (Jun 9) where there is still a 20% chance of daytime temperatures exceeding 90 degrees F, as this heat signal is expected to dissipate by next weekend tied to the weakening and northwestward shifting positive height departures.

As the negative mid-level height departures over the southwestern CONUS are favored to shift eastward over the Interior West later in the period, both the GEFS and ECMWF ensembles show more of a southerly flow component ahead of the trough axis to promote warming temperatures over portions of the Great Plains and Mississippi Valley. Since earlier this week, reforecast tools have been fairly consistent in regards to the onset of the increasing warm signal over southern Texas by next weekend, with increased chances of daytime temperatures exceeding the 85th percentile and 100 degrees F along and near the Rio Grande Valley. While the calibrated heat tools also support this growing warm signal, uncalibrated temperature guidance from the GEFS and ECMWF appear rather tepid, depicting a limited coverage of weak, positive temperature anomalies over southern and eastern Texas through the middle of the period. As a result, no temperature related hazards are issued at this time, but will continue to be monitored in subsequent outlooks.

In the Midwest, the ECMWF and Canadian reforecast tools indicate at least a 40% chance of temperatures exceeding the 85th climatological percentile and 90 deg F. However, due to these tools not showing enhanced risk of reaching 95 deg F and the lack of support from the GEFS, a corresponding heat hazard is not posted. This heat event would likely be driven by absolute temperatures rather than being moisture driven, thus reaching hazardous thresholds would be less likely.

For precipitation, there is fair agreement in the ensembles featuring surface low formation in the lee of the Rockies by the outset of week-2. Uncalibrated daily precip amounts are on the low side; however reforecast tools continue to indicate increased chances for 3-day amounts exceeding the 85th percentile and 1 inch to support a slight risk of heavy precipitation over the south-central CONUS during the first several days of the period. The moist, low-level return flow favored also increases the potential for thunderstorm activity in this highlighted region. Farther east, raw precipitation tools continue to point to a strong uptick in rainfall and tropical moisture over Florida and the Caribbean tied to a weakness in the subtropical ridge and a broad area of low pressure early in the period. A slight risk of heavy precipitation is added over the Florida Peninsula where there is better agreement in the reforecast tools indicating at least a 20% chance of 3-day amounts exceeding the 85th percentile, and the uncalibrated ECMWF depicting >40% chances for 3-day totals exceeding an inch early in the period.

Developing drought concerns persist across parts of the central and eastern Corn Belt and adjacent areas during week-2. Antecedent conditions are quite dry (2-4 inches below normal precipitation since early May) from the Mississippi Valley to the Central Appalachians. Although temperatures aren't expected to be very exceptionally warm to increase evapotranspiration rates, many areas in the Midwest and the Mid-Atlantic are now experiencing abnormal dryness based on the latest drought monitor, with little indications for improvement given the normal to below-normal precipitation favored through mid-June. Therefore, the rapid onset drought risk remains highlighted and is expanded to include additional areas in the Mississippi and Ohio Valleys, Great Lakes, and eastward to the Central Appalichians.

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. 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. With increased chances for above normal precipitation predicted for much of the western CONUS during week-2, possible flooding continues to be highlighted. Due to decreasing snow water equivalent levels across portions of south-central and northeastern Idaho, the corresponding flood hazard is removed in the updated outlook, as well as over portions of southern Utah, where flooding issues are expected to ease prior to the week-2 period. 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.

In the western Pacific, tropical cyclone Mawar is forecast to undergo extratropical transition and become absorbed in the westerlies, where an amplification of the mid-level pattern remains favored over the Bering Sea and the Aleutians late in week-1. While the strength of troughing aloft and surface low is expected to weaken by the onset of week-2, there is continued support of strengthening pressure gradients associated with this pattern and slight risk of high winds remains issued (Jun 9) across the Aleutian Islands and portions of southwest Mainland Alaska.

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