Prognostic Meteorological Discussion
Issued by NWS
Issued by NWS
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870 FXUS02 KWBC 231858 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Sun Jun 23 2024 Valid 12Z Wed Jun 26 2024 - 12Z Sun Jun 30 2024 ...Hazardous heat possible for the southern tier through the period and expanding northward into the central Plains/Mississippi Valley and Ohio Valley late week... ...Overview... An upper level ridge will meander around southern parts of the nation through the week, leading to multiple days of above normal to possibly hazardous heat. To the north, one shortwave through the northern tier will amplify somewhat as it crosses the Midwest/East mid- to late week. The next system reaching the West Coast around Thursday should push a defined surface low pressure system into the northern Plains by late week. A third shortwave looks to enter the West again next weekend. ...Guidance/Predictability Assessment... The latest suite of guidance continues to offer good agreement on the large scale, and typical uncertainty in the details that could have impacts on sensible weather/QPF. Models continue to offer good agreement on the first shortwave tracking from the Great Lakes midweek into the Northeast late week. There were a bit more differences with the next shortwave moving through the northern U.S. by Friday and the associated surface low, especially with the 00Z CMC, but still minor and within typical spread for Day 5. Details on energy through south-central Canada reinforcing this trough as it moves into the Great Lakes and Northeast next weekend remain uncertain and would impact overall timing. Models even show reasonably agreeable timing on the next trough approaching the Northwest by Sunday, though this may change in future cycles. The updated WPC forecast used a blend of the GFS, ECMWF, CMC, and UKMET for the first half of the period amidst sufficient model agreement. Transitioned to 50 percent of the ensemble means in the blend (with the ECMWF and GFS) late period to mitigate some of the model differences which will take additional time to resolve. Overall, this maintained good agreement with the previous WPC forecast. ...Weather/Hazards Highlights... A cold front moving into the East on Wednesday and copious moisture and instability in a typical summertime convective pattern will support widespread showers and thunderstorms from parts of the Lower Mississippi Valley into the Northeast. There remains enough model QPF discrepancies to preclude any sort of Slight Risks, and the front should be fairly progressive as well, limiting the overall threat. Still, given moisture and instability, high rain rates are likely so a broad Marginal Risk seemed reasonable for the Day 4 Excessive Rainfall Outlook. The front should push offshore of the Northeast on Thursday but linger in the Southeast. Along it, scattered thunderstorms are expected combining with diurnally enhanced convection over Florida. Dry conditions/high FFGs continue to preclude any excessive rainfall risk areas. Scattered thunderstorms over Florida look to continue through late week and the weekend. Another low pressure system will reach the northern Plains by Thursday, bringing a renewed threat for heavy rainfall across the north-central Plains to Upper Midwest. A broad Marginal Risk is in place there for Day 5/Thursday, with an embedded Slight Risk. There is some uncertainty in where the heaviest rain will occur. Recent model guidance trended toward showing the highest QPF farther south into Iowa compared to areas just north from far eastern Nebraska and southeast South Dakota and near the Iowa/Minnesota border that have seen extremely high rainfall totals recently. The updated Slight Risk encompasses both these farther north (wet antecedent conditions) and south (better model consensus for heavy amounts) areas to cover the threat. A heavy rainfall threat may emerge in the Great Lakes and Middle Mississippi Valley Friday and Saturday as the front presses farther southeast. Anomalous moisture with widespread precipitable water values over the 95th percentile will persist over the southwestern U.S. through Wednesday and slowly lessen later in the week. This should support monsoonal-type showers and thunderstorms with heavy rain. Marginal Risks remain across the Four Corners states and vicinity for the threat of flash flooding both Wednesday and Thursday, with burn scars and slot canyons particularly vulnerable. Though perhaps not everywhere in the quite broad Marginal Risk areas will see heavy rain, slow-moving convection is a concern for these areas and warrants the risk depiction in the ERO. The main change to the previous issuance was to have the Marginal Risk cover the slot canyons of southern Utah that are quite sensitive to rainfall. One uncertainty for any upgrading to any Slight Risk (on Day 4/Wednesday when moisture levels are highest) is whether cloud coverage could limit instability and limit rain rates. Farther east into the central High Plains, a stalling front combining with this moisture advection could lead to heavy rain in a possible MCS there on Wednesday night, so the Marginal Risk on Day 4 encompasses that area too. Coverage of storms may decrease by Friday with a break in the anomalous moisture, but increase once again especially in New Mexico over the weekend. The focus for hazardous heat by midweek will be across the south- central Plains/Mississippi Valley and parts of the Southeast, where a long duration heat wave will be ongoing and heat indices near 110F for some is leading to widespread major to extreme HeatRisk. The heat across the Southeast should briefly moderate on Thursday with troughing moving through, but the upper ridge will build back into the region and northward into the Mississippi and Ohio Valleys late week, bringing several days of above normal temperatures (both daytime highs and overnight lows) from the south-central Plains eastward. Temperatures may be a few degrees above average in the Desert Southwest, equating to highs 105-115F. Meanwhile the main area of cooler temperatures will be across the northern tier, where upper shortwaves and surface fronts provide some relief. Tate/Santorelli Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw $$