Prognostic Meteorological Discussion
Issued by NWS
Issued by NWS
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856 FXUS21 KWNC 301948 PMDTHR US Hazards Outlook NWS Climate Prediction Center College Park MD 300 PM EDT May 30 2024 SYNOPSIS: Strong and persistent mid-level high pressure has the potential to bring an extended period of excessive heat to many areas from the High Plains to the Pacific Coast. A slight risk of excessive heat covers this large area for the entirety of week-2, with a moderate risk posted for much of the interior West, also for the entire period. A high risk of excessive heat is limited to the first day of the period in the central California Valley. Toward the end of week-2, the slight excessive heat risk expands across southern Texas. Meanwhile, episodes of high winds from the southern High Plains through portions of the Rockies and Desert Southwest, along with dry and unusually hot conditions, may promote the rapid development and spread of wildfires. To the east, an excessive heat risk extends through all of week-2 across most of Florida and adjacent areas, with the greatest potential in portions of the interior Florida Peninsula. HAZARDS High risk for excessive heat for the Central Valley of California, Fri, Jun 7. Moderate risk for excessive heat for the Central Valley of California and adjacent lower foothills, Fri-Sat, Jun 7-8. Moderate risk for excessive heat for much of the interior West, Fri-Thu, Jun 7-13. Moderate risk for excessive heat over part of the interior Florida Peninsula, Fri-Thu, Jun 7-13. Slight risk for excessive heat in most areas from the High Plains to the Pacific Coast, Fri-Thu, Jun 7-13. Slight risk for excessive heat in much of coastal California and part of southwestern Arizona, Fri-Mon, Jun 7-10. Slight risk for excessive heat in southern Texas, Tue-Thu, Jun 11-13. Slight risk for heavy precipitation in part of the Northeast, Fri, Jun 7. Slight risk for episodes of high winds across much of the Texas Big Bend, Four Corners states, Great Basin, and Desert Southwest, Fri-Thu, Jun 7-13. Rapid onset drought risk across parts of southern Texas. Rapid onset drought risk in part of the central Florida Peninsula. DETAILED SUMMARY FOR SUNDAY JUNE 02 - THURSDAY JUNE 06: https://www.wpc.ncep.noaa.gov/threats/threats.php FOR FRIDAY JUNE 07 - THURSDAY JUNE 13: The GEFS, ECMWF ensemble mean (ECENS) and the Canadian ensemble mean (CMCE) all show a moderate to strong mid-level ridge over the western contiguous states (CONUS) as week-2 begins, with the greatest 500-hPa heights (>590 dm) over the Four Corners region, and the largest 500-hPa height anomalies centered near the northern Great Basin. The CMCE solution features a stronger mid-level ridge than yesterday, more like the ECENS, which changed little. Both show positive 500-hPa height anomalies >+18 dm at the start of week-2. The GEFS mean is also fairly amplified (+12 to +18 dm), but is weaker than the other two ensemble means. Given the shift in the CMCE today, the stronger mid-level ridge is the favored solution. This feature is expected to weaken only slightly and drift slowly northward or northwestward through the period, which keeps mid-level ridging over most of the West during all of week-2. This set-up is favorable for the development and persistence of excessive heat in a large part of the CONUS from the High Plains to the Pacific Coast, potentially bringingmuch of this broad area its first heat wave of 2024. Very warm to excessively hot air would already be in place as the period begins, and the uncalibrated National Blend of Models (NBM) shows high chances for temperatures to approach 105 deg. F in the central California Valley on the first day of week-2. Somewhat lower 500-hPa heights may impinge on California and some adjacent areas for the rest of the period, quickly reducing the high excessive heat risk there, and eventually ending the slight excessive heat risk along most of the California coast and part of southwestern Arizona around the middle of week-2. The GEFS, CMCE, and ECENS all show cooler air slowly becoming entrenched in this area as the period progresses. As the largest 500-hPa height anomalies drift northward thereafter, the greatest excessive heat risks follow suit. A broad area across the interior West, especially the lower elevations, have the best chances to record exceptionally high temperatures, and a moderate risk for excessive heat is posted here. With the expected persistence of the mid-level ridge, the moderate risk has been extended through all of week-2. There is more uncertainty in higher elevations and in areas farther to the east, somewhat removed from the greatest 500-hPa height anomalies, so a slight risk of excessive heat is posted across this large area, covering almost the entire western half of the CONUS, for the entirety of the period. The ECENS and CMCE uncalibrated output shows chances occasionally approaching 50 percent for temperatures to reach 100 deg. F as far north as the Snake and Columbia River Valleys, and adjacent lower elevations. In addition, most models (particularly the ECENS) show abnormally high humidity entrenched across the West during week-2, which could generate higher heat indices than usual given the same temperatures. Farther east, a frontal boundary is expected to pass through south Texas before the week-2 period, leaving near normal temperatures across the region as the period gets underway. This should keep excessive heat at bay until the last few days of week-2, when increasingly hot air is expected to move back into the region, creating a slight risk of excessive heat there late in the week. There is a risk for rapid onset drought (ROD) during early to mid June for southern Texas. The ROD hazard is based on 30 to 60-day precipitation deficits, low soil moisture, and likelihood of periods of above-normal temperatures along with the lack of a strong wet signal. The GEFS and ECENS PETs continue to indicate a 20 to 40 percent chance for temperatures to exceed the 85th percentile each day during the period. This signal extends farther north than yesterday, spreading a slight risk of excessive heat into portions of Georgia and South Carolina as well.. Abnormally high 850-hPa temperatures are forced into the region on the backside of a mid-level ridge, and persist through the forecast period. The ECENS shows daily high temperatures hitting at least 90 deg. F every day at most sites, with temperatures remaining 1.5 to nearly 4 deg. F above normal through almost the entire period. A few inland areas could reach 95 to 100 deg. F on a few days, with the best odds shown by the GEFS members. The climatologically-high humidity will exacerbate the effects of the heat, generating heat indices well into the 100s. Heat and humidity has the best chance of bringing excessive heat to the interior central Florida Peninsula, where a moderate risk of excessive heat has been posted for the entirety of week-2. Guidance shows slightly cooler weather across south Florida, where a moderate excessive heat risk was posted yesterday. Outside the central Florida Peninsula, where there is a little more uncertainty, a slight risk of excessive heat is posted for the entire period. In addition, the heat may combine with subnormal precipitation over the next few weeks to promote rapid drought development across part of the central Florida Peninsula. As for precipitation, the uncalibrated ECMWF and PET guidance is somewhat drier than yesterday, but enough members in the ECENS, GEFS, and CMCE show a frontal complex dropping moderate to heavy rainfall as it moves through early week-2 to maintain a slight risk of heavy precipitation over part of the Northeast for the first day of week-2. Guidance is in good agreement that this system will move faster than expected yesterday, exiting the northeastern CONUS quickly after week-2 gets underway. Multi-model ensemble means are in very good agreement that a deep 500-hPa closed low will be located over the Gulf of Alaska by the start of week-2. This anomalous mid-level trough is likely to enhance onshore flow to Southeast Alaska where above-normal precipitation is favored. The GEFS, ECENS, and CMCE PETs depict probabilities of about 20 percent for exceeding the 85th historical percentile, but actual 3-day precipitation amounts remain below 2 inches precluding the posting of a heavy precipitation hazard. FORECASTER: Rich Tinker $$