Preliminary Forecasts
Issued by NWS
Issued by NWS
336 FXUS02 KWNH 170659 PREEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Tue Jun 17 2025 Valid 12Z Thu Jun 19 2025 - 12Z Mon Jun 23 2025 ...First significant, dangerous heat wave of the season expected across much of the central/eastern U.S. beginning late this week/weekend... ...General Overview... A significant, dangerous heat wave is expected to start the Summer across much of the central/eastern U.S. late this week, through the weekend, and into next week as a strong upper-level ridge begins to build in. Storm track north of the ridge may bring MCS activity and heavy rainfall/flash flooding from the Upper Midwest/Great Lakes to the Interior Northeast Friday-Saturday. A deep-upper trough over the West will bring cooler, below average temperatures, with eventual lead frontal system development over the Plains bringing a renewed chance for thunderstorms and potential heavy rainfall/severe weather threat to the central/northern Plains and Midwest early next week. ...Guidance/Predictability Assessment... Model guidance remains in excellent agreement with the depiction of the large scale pattern through the medium range period. A qausi-zonal 500mb flow pattern late this week into the first half of the weekend is forecast to become increasingly amplified through early next week, setting the stage for a high confidence, significant heat wave from the Plains to the Mid-Atlantic. Most guidance remains well clustered depicting the last in a series of upper-waves in the zonal pattern with leading frontal system pushing east off the East Coast and into the Atlantic Friday. The latest 12Z CMC is much faster and the 18Z GFS a bit slower compared to the 12Z GFS/ECMWF/UKMET, which are also supported by the ECMWF AIFS/GraphCast, GFS Graphcast, and ensemble means. Then, all guidance shows that the pattern will begin to amplify, with an upper-ridge initially over the southwestern U.S. progressively building/expanding north and eastward through the weekend and into next week. Meanwhile, an upper-low anchored over the northeastern Pacific will drop southward, with deepening upper-troughing overspreading the western U.S. Most of the uncertainty is with regards to upper-level shortwave(s) and leading surface frontal system development over the Plains as the upper-trough makes slow eastward progression through the weekend, with at least a couple rounds of cyclogenesis over the Plains possible. The ECMWF, Graphcast GFS, and means favor one main surface low, while the latest 12Z runs of the GFS, CMC, and ECMWF AIFS/Graphcast, especially the 18Z GFS, suggest at least one initial upper wave will lift a system northward ahead of the main development focused over the Plains. Other than these specifics though, guidance shows the main, larger- scale features of the pattern with upper-ridging in the eastern U.S. and upper-troughing in the western U.S. will remain through the period into early next week, though expect a general weakening of the upper-trough with time. The updated WPC forecast uses a blend of the 12Z ECMWF/UKMET as well as the 12Z/18Z GFS but not the CMC given the other guidance is generally well clustered. The 12Z GFS is eventually favored over the 18Z as its depiction of leading wave energy ahead of the upper- trough is more similar to other available guidance while the 18Z GFS is significantly more aggressive in deepening a lifting surface low. A contribution from the means is steadily added up to 45% of the blend as uncertainties with these smaller -scale details over the central U.S. grow. ...Weather/Hazards Highlights... Late week/weekend will bring a pattern change as an amplifying upper-level ridge over the central/eastern U.S. shifts the storm track northward and brings a period of drier conditions to many after unsettled weather much of the week. Embedded impulses within the upper-ridge will trigger convection, potentially organized, across the Upper Midwest/Great Lakes Friday (day 4) and Great Lakes/Interior Northeast Saturday (day 5). A Marginal Risk ERO is in place for both days as a strong low level jet and high precipitable water values (up to 2 standard deviations above the mean) support the threat of heavy rainfall and at least isolated flash flooding. An embedded Slight Risk looks possible based on forecast QPF values with more confidence in areal placement of this organized convection. Daily showers and thunderstorms are also expected in vicinity of the Gulf Coast as a frontal boundary lingers in the region and with possible influence of tropical moisture for the western Gulf Coast/south Texas. This influx of moisture also looks to bring precipitation chances further northwest into western Texas and the southern high Plains by late weekend/early next week. Upstream, the energetic approach of a deep upper- trough/Pacific system and northern stream into the West will bring increasing precipitation chances to the Pacific Northwest and northern Rockies/Plains Friday through the weekend in unsettled flow with moderate to heavy rainfall potential. A Marginal Risk ERO has been added for Friday (day 4) for portions of north-central Montana given the presence of strong, very moist upslope flow to support heavy rainfall. Eventual lead frontal system development over the Plains will bring a return of thunderstorm chances to portions of the northern/central Plains and Midwest Sunday and particularly Monday- Tuesday, with potential for more heavy rainfall/flash flooding as well as severe weather. The first significant heat wave of the season is expected across the central Plains by Friday, the Middle/Upper Mississippi Valley by Saturday, and the Lower Great Lakes/Ohio Valley/Mid-Atlantic by Sunday thanks to anomalous upper-level ridging building overhead. Summer heat will also begin to intensify across the Lower Mississippi Valley and Southeast as the ridge expands eastward. Widespread Major to Extreme Heat Risk (levels 3 and 4/4) is forecast, indicating an intensity of heat that is dangerous to anyone without adequate cooling or hydration, as heat indices climb into the 100-110 degree range, potentially higher. In addition, muggy overnight lows in the mid- to upper 70s will bring little overnight relief from the heat. Unfortunately, this heat wave looks to persist through at least the middle of next week. Highs will begin to trend cooler and below average by as much as 10-20 degrees across portions of the Pacific Northwest by Friday and expand into the Great Basin and northern Rockies/High Plains by the weekend as an upper level trough and associated surface cold front from the northeast Pacific begin to overspread the region. Putnam 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 $$