Preliminary Forecasts
Issued by NWS
Issued by NWS
066 FXUS02 KWNH 200704 PREEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 303 AM EDT Thu Jun 20 2024 Valid 12Z Sun Jun 23 2024 - 12Z Thu Jun 27 2024 ...Heat wave persists for the Mid-Atlantic Sunday with elevated heat over the South/Plains through much of next week... ...Heavy rainfall threat over parts of New England into Sunday... ...Overview... Guidance shows the upper pattern settling into a stable regime featuring a persistent upper high anchored over the southern Rockies and vicinity, with some ridging extending to the north, while the Northeast Pacific/West Coast and eastern North America see varying degrees of mean troughing. Individual shortwaves within the westerlies crossing the southern Canada/northern tier U.S. will support a series of progressive southern Canada surface systems and trailing fronts extending into the northern half to two-thirds of the lower 48. A leading system affecting the Northeast on Sunday may produce some areas of heavy rainfall, while the trailing front and another one behind it may produce episodes of convection over the central/eastern U.S. Expect hazardous heat to extend into Sunday over the Mid-Atlantic, with high heat indices likely across the South/Plains and Southwest for most of next week. There may be a rebound of some heat into the Mid-Atlantic around midweek. ...Guidance/Predictability Assessment... Latest dynamical and machine learning (ML) guidance agrees fairly well for the overall pattern but displays typical spread and run- to-run variability for the details. For the leading system over eastern North America, the 18Z GFS strayed to the strong/northwest side for the surface low while the new 00Z UKMET has come in on the slow side for the upper trough by Tuesday. Models are still having a tough time determining when there may be a low embedded within the upper trough, affecting the surface evolution. Upstream, latest guidance is holding onto recent trends toward progressive Pacific energy evolving into a southern Canada/northern tier U.S. shortwave that feeds into the eastern North America mean trough, while remaining upper low energy near/offshore British Columbia may combine with incoming North Pacific energy to produce a West Coast upper trough by next Thursday with an increasingly better defined leading surface system. Farther east by next Thursday, it will be worth watching dynamical versus ML guidance as the 12Z ECMWF and latest GFS runs are somewhat west of most 12Z ECMWF-initialized ML runs for the upper weakness that develops over and south of the Tennessee Valley. Based on 12Z/18Z guidance comparisons, the updated forecast started with a composite of 12Z models (18Z GFS looking too shallow with the eastern trough by Monday) for about the first half of the period. The the blend incorporated some 18Z GEFS/12Z ECens means while ultimately transitioning the GFS component toward the 18Z run by next Thursday (12Z GFS trending flat with the late-period eastern trough). ...Weather/Hazards Highlights... The southeastern Canada surface low and trailing front affecting the Northeast on Sunday and perhaps to a lesser degree into Monday will have the potential to produce some areas of locally heavy rainfall across the region. There is continued spread in the guidance for exactly how this system will evolve and thus for rainfall details. The Day 4 Excessive Rainfall Outlook covering Sunday-Sunday night maintains a Slight Risk area over parts of northern-central New England with modest adjustments based on latest model/ensemble guidance and first-guess fields. The surrounding Marginal Risk that extends as far south as the central Appalachians is also close to continuity and reflects the combination of potential for locally heavy activity and terrain sensitivity. The Day 4 ERO maintains a Marginal Risk area over far southern Texas where moist flow to the north of a second possible western Gulf surface low may produce some enhanced rainfall. In the day 5 ERO period (Monday- Monday night), guidance is starting to suggest an area of possible convection over the Upper Midwest/western Great Lakes ahead of an advancing warm front. Wet ground conditions due to prior rainfall favor starting with a Marginal Risk area, though confidence in details is somewhat below average for now. Anomalous moisture should persist over the Four Corners states, and Arizona in particular, during the Days 4-5 period and potentially lead to some locally heavy rainfall. However will await a more coherent signal in the guidance before depicting any risk areas. Parts of the Southeast/Florida may also see episodes of diurnally enhanced convection but ground conditions are on the drier side with FFG values quite high, so again favored no risk areas for the time being. Beyond early Tuesday, a central into eastern U.S. front will help to focus areas of showers and thunderstorms while portions of the South and the Rockies should continue to see diurnally favored convection. A late-period front reaching the Northwest may produce some light rainfall (as may a front on Sunday). The Mid-Atlantic should continue to have temperatures 10-15F above normal into Sunday with some daily records possible for max/warm min temperatures. The front progressing into the East will briefly suppress the heat southward next Monday but the hot temperatures should return northward toward midweek ahead of another front. Meanwhile a broad area of temperatures generally 5-15F above normal will extend from the South into the Plains as well as California into the Interior West. Near to slightly below normal highs over the Pacific Northwest should begin to expand southeastward by next Thursday. Best potential for maximum heat index values of 105-110F or so will be from the South into the southern half or so of the Plains as well as over the Desert Southwest. 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 $$