Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Tue Jun 18 2024 Valid 12Z Fri Jun 21 2024 - 12Z Tue Jun 25 2024 ...Record-breaking heat from the Midwest to the East Coast expected to moderate by early next week, while some heat shifts south... ...Heavy rainfall threat from the northern Plains into Upper Great Lakes into Friday-Saturday... ...Overview... Latest models and ensembles continue to show the large scale pattern transitioning toward a more typical early summer regime late weekend and early next week, as initially strong ridging over the eastern half of the country gives way to a leading trough crossing the northern tier U.S. and southern Canada (reaching the East Coast by around next Tuesday) while a system reaching the Northeast Pacific and West Coast this weekend continues eastward thereafter. Although gradually weakening, the eastern upper ridge will still support a broad area of hazardous heat with potential for daily records extending from the Ohio Valley/Lower Great Lakes into the Mid-Atlantic Friday into the weekend. The northern Plains into Upper Great Lakes should see a continued threat for heavy rainfall through Friday-Saturday as developing northern Plains into Great Lakes/Ontario low pressure and associated fronts interact with anomalous moisture already in place. Some of this rainfall will extend into the Northeast. A trailing Northeast Pacific system will likely bring a well-defined front into the West by Sunday, continuing east thereafter. To the south of the warm upper ridge ultimately settling over the far southern tier, Potential Tropical Cyclone One (see NHC for updates) should dissipate over Mexico by Friday while NHC is monitoring potential for another feature over the western Gulf mid-late period. ...Guidance/Predictability Assessment... Model guidance remains agreeable with the main aspects of the pattern especially through the weekend. The upper ridge should begin Friday over the Mid-South to Mid-Atlantic, getting suppressed south with time as southern and northern stream energy phase to create a shortwave and likely small closed low near the Great Lakes by Sunday. This feature shows reasonably good agreement in the upper levels with dynamical and AI models, but at the surface there was some spread in the low strength and track. The 00Z ECMWF (deterministic) seemed to be an outlier with a strong surface low and northwest track compared to other dynamical/AI models that were farther southeast and weaker, but the 12Z EC has come into better agreement. The shortwave and the cold front associated with the surface low tracking northeastward should track through the Northeast Monday-Tuesday, with typical levels of spread. The next shortwave of note looks to come into the Northwest early next week, with some spread in its timing and depth as it tracks east near the U.S./Canadian border. There was a general trend toward the shortwave and the associated surface frontal system moving faster compared to the previous forecast. Then, attention turns to the western Gulf of Mexico where there is a possibility of a tropical system. Most deterministic models and many ensemble members show a low approaching north from the Bay of Campeche, but AI models show less potential. The WPC forecast used a multi-model deterministic blend early in the forecast period with inclusion of the GEFS and EC ensemble means by Day 5 and increasing their percentage to half by Days 6-7. ...Weather/Hazards Highlights... The most prominent focus for heavy rainfall during the period will be from the northern Plains into Upper Great Lakes during Friday- Saturday as upper dynamics emerging from western North America encourage northern Plains into Great Lakes/Ontario surface development. This system and associated fronts will interact with anomalous moisture that has been persisting over the region for multiple days. The Day 4 Excessive Rainfall Outlook for Friday maintains a Slight Risk area from parts of the Dakotas across Minnesota and the northern half of Wisconsin into the U.P. of Michigan (close to continuity). This could be a higher-end Slight across central parts of Minnesota into northwestern Wisconsin. The Day 5 ERO for Saturday will continue a Slight Risk area from northeastern Minnesota/northern Wisconsin into northern Michigan. Surrounding Marginal Risk areas account for less extreme but still locally significant rainfall potential over the northern Plains (Day 4) and over the Midwest along the trailing cold front (Day 5). Some areas across the northern tier will be sensitive to additional rainfall due to already wet ground heading into Friday. Elsewhere, the updated Day 4 ERO maintains continuity with a Marginal Risk area over parts of the southern Rockies/Four Corners region due to the abundant moisture likely from both the Pacific and the Gulf arriving, some forecast instability, and some model signals for locally enhanced rainfall. Guidance suggests that the greatest moisture anomalies should get pushed to the south and west during Saturday, with rainfall over the region likely to be lighter and more scattered, so a Day 5 Marginal Risk still does not seem needed. Meanwhile the Northeast continues to merit monitoring from Friday into the weekend. The combination of moisture and instability along with an east-west front settling over the region could support some locally enhanced rainfall rates and training/repeat activity. However antecedent conditions should be rather dry by that time, streamflows are already near to below normal, and model QPF is not exceptionally heavy thus far. Therefore the outlooks continue to depict no risk area at this time for Friday-Saturday. By Sunday (what is currently Day 6), forcing increases with a shortwave and cold front approaching, so this may warrant an eventual ERO risk. Continued progression of the low pressure system that should be near the Great Lakes as of early Sunday will bring the trailing cold front through the eastern U.S. and trailing back into the Plains by the start of next week, bringing areas of showers and thunderstorms of varying intensity. Another front reaching the Northwest this weekend may bring some light/scattered rainfall over northern areas. Continued progress of this front may generate some showers/thunderstorms over the central U.S. by the first half of next week. Some scattered diurnal convection may persist over the southern Rockies. Occasional showers/storms are possible along the Gulf Coast, but with increasing uncertainty over specifics of the Gulf of Mexico pattern at the surface and aloft by early next week keeping confidence low regarding any potential increase in moisture at that time. With a cold front reaching New England by Friday and providing a cooling trend there, the greatest temperature anomalies from late this week into the weekend should extend from the Midwest and Ohio Valley/Lower Great Lakes into the Mid-Atlantic and Northeast, with readings tending to be 10-15F above normal. This would translate to highs well into the 90s, along with lows in the upper 60s to upper 70s providing little overnight heat relief. Daily records for max/warm min temperatures will be possible within the above areas through the weekend, up through warm minimum temperatures early Monday (if they hold on through the calendar day). Meanwhile much of the West will trend warmer/hotter late this week into the weekend with highs reaching 10F or more above normal for a couple days or so. The front reaching the Northwest should bring cooler air to that region by Sunday-Monday, while some of the leading western heat should reach the northern-central High Plains at that time--connecting with lingering heat over parts of the Plains into the southern half of the East. Highs well into the 90s and high dewpoints in the Southeast will lead to heat indices around 105F, with widespread areas of major HeatRisk by early next week. Clouds and rainfall will support below normal highs over the southern Rockies/High Plains late this week. Tate/Rausch 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