Preliminary Forecasts
Issued by NWS
Issued by NWS
593 FXUS02 KWNH 040717 PREEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 316 AM EDT Mon Aug 4 2025 Valid 12Z Thu Aug 07 2025 - 12Z Mon Aug 11 2025 ...Major to Extreme heat threat for the Southwest to south- central Plains spreads to Upper Midwest/Great Lakes later week... ...Southeast U.S. Coastal Low in the wake of exiting Tropical Storm Dexter being monitored by the NHC... ...Overview... A pesky and cooling mean upper trough into the eastern third of the nation will be periodically reinforced by disturbances digging from the Plains/Midwest to the South/Southeast. This is all to the lee of an upper ridge anchoring over the Southwest and spread to the south-central High Plains. A wavy stalled front will support heavy rain threats over the Southeast into later this week, but as a Bermuda High begins to build westward, wet weather is also forecast to spread gradually up the eastern U.S. later week/next weekend, albeit with uncertainty of lift/expansion given variance with vorts/impulses embedded in the upper trough and uncertain interactions with a potential Southeast U.S. coastal wave/tropical development and track. Meanwhile, a series of amplified upper troughs/frontal systems progressing out from the Northwest to the north-central U.S. will fire multiple rounds of thunderstorms. ...Guidance/Predictability Assessment... The synoptic pattern will feature a dominant area of upper ridging from the Southwest to south/central U.S. with a main storm track shifted north/overtop along the U.S./Canadian border. Expect a series of amplified shortwave troughs will translate out through the northwestern U.S. over the next week. This pattern will keep precipitation chances generally limited to the northern tier of the U.S. along frontal waves ahead of the upper-troughs, as well as east of the upper-high over the southeastern U.S. to Mid- Atlantic where a lingering trailing frontal boundary remains under the influence of an upper-level weakness. There is one specific area of uncertainty with regards to the potential for development of a coastal wave/low pressure off the coast of the Southeast mid-late next week with significant impacts on precipitation chances. The National Hurricane Center has now included low probabilities for tropical development as well. Recent ECMWF/CMC and ECens mean runs offer potential for possibly more significant heavy rainfall along the central Appalachians, with the GFS trending significantly less robustly with coastal low development and track. Recent EC AI guidance/GFS Graphcast showed more QPF along the southern Appalachians with coverage varyingly more northward along the coastal Mid-Atlantic in a pattern with an elevated ceiling. Heading into the weekend, guidance still also tends to agree that additional upper-energy over the northeastern Pacific will help to strengthen the upper-trough over an unsettled Northwest and then bring this feature eastward as the upper-high over Southwest to south/central U.S. slowly weakens. However, there is disagreement in the progression, with recent GFS runs in particular more progressive over the north-central U.S. to south-central Canada. Most other deterministic and machine learning guidance remain slower. Accordingly, a growing contribution from the ensemble means was included by the weekend, partially at the expense of the GFS. ...Weather/Hazards Highlights... A rather expansive cool high pressure airmass will linger over the eastern U.S. over the next week. High moisture content and instability near a front on the leading edge of this airmass will focus some localized heavy rain to raise flash flooding risks given wet antecedent conditions. WPC Excessive Rainfall Outlook (ERO) Marginal Risk areas are depicted so far for Day 4/Thursday and Day 5/Friday from portions of the Southeast to south-central areas of the Appalachians/Mid-Atlantic given upper trough position and uncertain coastal wave/tropical development in addition to moist onshore flow from the Carolinas to the Appalachians/Mid-Atlantic. Farther west, shortwaves rounding into the eastern edge of the upper ridge from the Rockies/Plains are forecast to interact with a downstream frontal zone. Into late week, expect the northern Plains/Upper Midwest/Great Lakes to see periodic rain and storms. WPC Day 4/5 Marginal Risk ERO areas are depicted for Thursday and Friday given likely moisture/instability pooling with shortwave passages. Activity will spread more across the Great Lakes and Midwest later in the week. Meanwhile, the Four Corners states could see some increasing monsoon showers underneath the broad ridge, amid the relatively dry monsoon season so far. Elsewhere, some moderate rainfall chances may develop and slowly progress this weekend from over the Pacific Northwest under the influence of a cooling and unsettling upper-level trough. Cooler than average temperatures will continue on the cool side of the main frontal system for the central to eastern U.S.. Meanwhile across the Southwest, dangerous heat will build throughout the week with some location high temperatures as high as 110-115F, equating to major to extreme HeatRisk. Some relative relief may come next weekend as the upper-high begins to weaken. Temperatures will also be warming to above normal and offers some major HeatRisk areas over the southern and central Plains as the upper ridge expands, with pre-frontal heat spreading into the Upper Midwest/Great Lakes later week. Some record high temperatures there could be challenged and eclipsed into late week over the Southwest. Florida is forecast to see seasonable temperatures in the 90s, but with ample humidity allowing for some areas of Major HeatRisk there as well. Schichtel 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 $$