Preliminary Forecasts
Issued by NWS
Issued by NWS
781 FXUS02 KWNH 190701 PREEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Wed Jun 19 2024 Valid 12Z Sat Jun 22 2024 - 12Z Wed Jun 26 2024 ...Record-breaking heat from the Midwest to the East Coast expected to moderate early next week, while some heat shifts into the South/Plains... ...Heavy rainfall threat from the Upper Great Lakes into northern New England Saturday-Sunday... ...Overview... Guidance still shows the upper pattern settling into a more typical early summer regime with upper ridging settling across the far southern tier while a leading northern tier upper trough reaches the East by the start of next week and upstream Pacific energy progresses along/north of the Canadian border. These northern features will support the two primary frontal systems of interest during the period. The eastern system will bring a heavy rainfall threat from the Upper Great Lakes into New England during the weekend and by early next week will help to moderate the area of hazardous heat extending from the Midwest into the Mid-Atlantic through Sunday. However heights aloft will tend to remain somewhat above normal otherwise, ultimately leading to fairly broad coverage of above normal temperatures over the lower 48. To the south of the southern tier upper ridge, Potential Tropical Cyclone One (see NHC for updates) should dissipate over Mexico before the start of the forecast period while NHC is monitoring potential for another western Gulf feature that could bring additional rainfall into far southern Texas. ...Guidance/Predictability Assessment... Dynamical and ECMWF-initialized machine learning (ML) guidance agrees in principle on the overall forecast pattern, but there are important detail differences yet to be resolved. On average the guidance has recently trended a little more open with the upper trough supporting the system tracking east/northeast from the northern Plains Saturday onward. A mean of ML solutions would continue to support an average of operational guidance, though notably the MLs have not favored the deep side of the guidance per some recent ECMWF runs. Meanwhile latest trends have gone in the way of some earlier GFS runs upstream, with a greater signal toward a more sheared presentation of incoming Pacific energy as it continues across the northern tier U.S. and southern Canada. This leads to a faster trend for the Canadian surface low and trailing front into the CONUS. By next Wednesday the 12Z ECMWF seems to stray on the fast side with bringing in the next larger scale feature (some combination of lingering Northeast Pacific upper low and upstream energy). Operational guidance has trended in the way of prior MLs in suggesting whatever energy/surface low pressure there could be over the Gulf would be more suppressed, given a stronger southern tier upper ridge. 12Z/18Z guidance comparisons led to starting the forecast with an operational model composite early in the period, followed by some incorporation of the 18Z GEFS and 12Z ECens/CMCens means later in the period as details become somewhat more uncertain. ...Weather/Hazards Highlights... Developing low pressure tracking from the northern Plains/Upper Midwest toward New England/southern Canada along with its associated fronts will interact with moisture that has been over the Upper Midwest for a number of days to support an area of heavy rainfall potential during the weekend. The Day 4 Excessive Rainfall Outlook for Saturday made modest adjustments to the existing Slight Risk area covering northern parts of the Great Lakes/Midwest based on latest guidance. The surrounding Marginal Risk area remains fairly close to continuity over the Midwest through central Great Lakes, but parts of the Northeast has been added based on recent faster trends for potentially significant rainfall. By Day 5, uncertainty over the details of surface low pressure complicate the forecast somewhat. Currently the best signal for heaviest rainfall exists over northern New England so a proposed Slight Risk area covers this region of relatively higher probabilities. A Marginal Risk area extends southwestward into the central Appalachians, incorporating guidance signals for some locally heavy rainfall and typical terrain sensitivity. Elsewhere, guidance is not unanimous on the magnitude but in general is suggesting the flow of enhanced moisture into far southern Texas to the north of another possible area of western Gulf low pressure during the weekend. Thus the Days 4-5 EROs have introduced a Marginal Risk area, given QPFs in some models and wet antecedent conditions. Meanwhile some scattered diurnal convection may persist over parts of the Four Corners states. Anomalously high precipitable water values suggest some activity could be locally heavy but so far guidance has not been quite organized enough to merit a risk area. Parts of the Southeast/Florida may also see locally heavy rainfall but again without enough organization to favor a risk area. During the early-middle part of next week the next frontal system reaching the Plains/Mississippi Valley may produce showers and thunderstorms of varying intensity. Locations from the Midwest into Mid-Atlantic should continue to see temperatures 10-15F above normal through the weekend. These anomalies 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). The front progressing into the East will suppress the heat southward Monday-Tuesday. The warm sector ahead of the next frontal system will spread very warm to hot temperatures into the West during the weekend and continuing into the Plains next week, while readings remain somewhat above normal back through California into midweek. By Sunday-Wednesday expect highs well into the 90s and high dewpoints over the South, plus some 100F+ readings over the central/south-central Plains, leading to increasing coverage of maximum heat index values in the 105-110F range across these regions (with HeatRisk in the major to locally extreme category). 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 $$