Preliminary Forecasts
Issued by NWS
Issued by NWS
002 FXUS02 KWNH 020659 PREEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sun Jun 2 2024 Valid 12Z Wed Jun 05 2024 - 12Z Sun Jun 09 2024 ...Excessive heat likely to continue over southern Texas especially through midweek, while heat builds in the West for the latter half of this week... ...Overview... Broadly, a western upper ridge and eastern upper trough pattern will set up later this week into next weekend. As the medium range period begins Wednesday, expect an upper low in south-central Canada along with a surface low pressure/frontal system to spread rain and thunderstorms to the east-central U.S. This shortwave trough will meet a somewhat blocky pattern farther east and slow/deepen over the east-central U.S. late week into the weekend, with rain chances gradually shifting toward the eastern U.S. along and ahead of the cold front. Meanwhile an upper ridge will expand from Texas to affect much of the western U.S. midweek and beyond, producing potentially hazardous heat. An eastern Pacific upper trough or two may approach by early next week but with uncertainty in timing. ...Guidance/Predictability Assessment... Model guidance remains in good agreement that an upper low in south-central Canada will produce a trough axis extending into the Midwest midweek. However, by Thursday-Friday, the model diagnostics process becomes much more challenging as the 12Z cycle of dynamical and AI/machine learning (ML) models diverge with the track of the upper low. Dynamical models from the 12/18Z cycle generally trended toward the low shifting/expanding eastward (not much southward) into late week, along with the AIFS ML model. Thus the associated troughing was not as deep into the Southeast as the previous 00Z model cycle showed. However, all other available AI/ML models that are EC based, as well as the GFS Graphcast, dove energy southeastward and formed an upper low in the east-central U.S. by Friday. This divergence of solutions does not inspire confidence in the forecast especially from Friday onward, despite the supposed deterministic model agreement. The ensemble means were a bit deeper with the troughing in the East compared to the deterministic models. It seemed prudent to lean toward these as a bit of a nod toward most ML models, and also to be closer to continuity. Thus for the WPC forecast, had to heavily favor the 18Z GEFS and 12Z EC ensemble means in the forecast, reaching half the model blend by Thursday and comprised the whole model blend over the weekend. Fortunately the 00Z GFS and CMC at least have trended farther south with the upper low center and the southern extent of its trough by late week. The 00Z ECMWF is less so, possibly in part due to differences in splitting energy into an upper low farther west in central Canada that was unlike other guidance. This ensemble mean-heavy blend also worked well for the pattern in the West. The upper ridge should be well established across Texas to the Four Corners states by Thursday, but then there are some differences in timing and placement of a southern stream upper low expected to be west of Baja California Wednesday-Thursday sneaking north or northeast for late week into the weekend. Meanwhile, a trough in the northeast Pacific continues to show spread in timing of its approach but also with the potential to break off an upper low diving south in the eastern Pacific. The variations in the deterministic and ML models yield low confidence so the ensemble means seemed like a good intermediate solution. ...Weather/Hazards Highlights... On Wednesday, a cold front anchored by a southern Canada surface low will be pushing through the east-central U.S., helping to focus rain and storms along and ahead of it. A broad Marginal Risk is in place for Day 4/Wednesday from the Lower Mississippi Valley into the Tennessee and Ohio Valleys toward the Great Lakes region, with a moist and unstable environment in all these areas for scattered convection that could produce heavy rainfall rates. Embedded Slight Risk(s) in future cycles cannot be ruled out, but the reasonably fast movement of the storms would be a limiting factor for flash flood potential. Some storms are possible into the Eastern Seaboard on Wednesday but without as much dynamical support as farther west. But by Thursday, the eastward push of the front and upper trough will shift the best rain chances into the Eastern Seaboard. Rainfall totals trended lower than the previous forecast, but still could be heavy enough to cause flooding concerns that could warrant a Marginal Risk in the Day 5/Thursday ERO for the interior Northeast and northern Mid-Atlantic near the best forcing and lower flash flood guidance. Uncertainties in the shape and axis of the supporting upper trough will affect the speed of the front and in turn intensity/duration of heaviest rainfall. Even behind the cold front, there may be diurnally enhanced episodes of rainfall in the northeastern U.S. into late week given the persistence of upper troughing. Farther west, some return flow of moisture (including perhaps Pacific moisture advecting around the ridge) is possible across the southern/central Rockies and Plains, with one or more surface boundaries helping to focus showers and thunderstorms. Scattered showers and storms may begin Thursday but increase in coverage and magnitude Friday into next weekend. Some scattered rainfall will also be possible late in the week over parts of the Interior West/northern Rockies, with details depending on the position of the upper ridge axis and how much moisture/shortwave energy may lift northward to the west of this axis. South Texas westward into the Rio Grande Valley will remain hot especially through Wednesday, where the combination of hot temperatures into the 90s and 100s as well as humidity will keep heat indices in the upper 100s to 110+. Some slight moderation of the heat indices is forecast later in the week in Texas. But the next big concern will be increasing heat across the West as upper ridging builds in. Highs broadly will run 10-20 degrees above average, with even warmer anomalies of 20+ degrees for the central California valleys and portions of the Great Basin. Temperatures should reach over 100F across the central California valleys, and near/over 110F in the Desert Southwest. Scattered daily record high minimum and maximum temperatures are possible. The higher anomalies look to drift into the Northwest for the weekend with some minor moderation of the temperatures in the Southwest. Meanwhile, most areas east of the Rockies (generally aside from Texas and Florida) should trend to slightly below normal behind the cold front. Tate 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 $$