Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Sat Jun 1 2024 Valid 12Z Tue Jun 04 2024 - 12Z Sat Jun 08 2024 ...Excessive heat likely to continue over southern Texas, while heat builds in the West for the latter half of next week... ...Overview... As the medium range period begins Tuesday, expect an upper-level shortwave to track across the north-central U.S. along with a surface low pressure/frontal system, spreading rain and thunderstorms to much of the Mississippi Valley. 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 atop Mexico and southern Texas Tuesday will expand to affect much of the western U.S. midweek and beyond, producing potentially hazardous heat. An eastern Pacific upper trough may come into the picture by next weekend but should still be well offshore as of Saturday. ...Guidance/Predictability Assessment... The dynamical and machine learning (ML) guidance broadly agrees on the upper pattern transitioning to a fairly amplified and more persistent ridge-trough configuration. The developing eastern trough should be composed of leading southern Canada/northern Plains dynamics and trailing Pacific energy streaming in underneath. However there still a number of discrepancies for important details such as the shape/orientation/location of the ridge and trough. First looking at the developing trough/upper low and trough, recent ensemble mean runs have been trending deeper and better defined with the upper low--which at least helps to increase confidence in the fairly deep operational guidance and ML models. There are differences in upper low track though, with the 00Z MLs on average a little south (more into the Great Lakes) of the 00Z/06Z dynamical guidance versus some of the new 12Z dynamical runs (especially the CMC which was previously fairly far south) adjusting northward into southern Canada through next Saturday. An additional complication introduced by the 00Z ECMWF and persisting into the 12Z cycle (now including the 12Z GFS to some degree) is the potential for trough elongation--yielding notably higher heights aloft over the Southeast, whether in the form of much weaker troughing or even ridging. The 00Z MLs were not very supportive of the ECMWF/12Z GFS ideas. Farther west, the 00Z MLs continued to support a ridge axis between the western ECMWF/ECens and eastern GFS/GEFS/CMCens with perhaps a partial tilt in the ECMWF direction. For the second day in a row, 6-hourly GFS runs have been steadily trending toward slower arrival of eastern Pacific troughing by late in the period so such a compromise still looks reasonable. Meanwhile the 00Z MLs also recommended slower northward progress (somewhat closer to latest CMC runs) of a southern stream upper low expected to be west of Baja California for most of the period and possibly lift northward by the end of the week. Most guidance comprising the favored blend reflected the faster progression but the ML trends will be worth monitoring. Guidance considerations based on the 00Z/06Z guidance led to a forecast update consisting of the ECMWF/GFS/CMC/UKMET in order of more to less weight through the first half of the period, followed by GEFS/ECens increasing incorporation reaching nearly half by next Saturday. ...Weather/Hazards Highlights... A vigorous shortwave and frontal system (anchored by southern Canada low pressure) reaching the Dakotas into the Upper Midwest on Tuesday will provide forcing and a focus for rain and thunderstorms as instability abounds ahead of it. Smaller scale shortwaves farther south should also track eastward and promote scattered convection. A broad Marginal Risk remains in place for the Day 4/Tuesday Excessive Rainfall Outlook along and a ways east/west of the northern three-fourths of the Mississippi Valley. Some locations within this area will be sensitive due to rainfall from prior days. While the combination of guidance clustering and QPF totals do not yet appear sufficient for one or more embedded Slight Risk areas, there are two regions of interest which at least are on the higher side of the Marginal Risk spectrum. One would be over the the Upper Midwest with the northern tier front/upper dynamics, while the second would be along and east/northeast of the Missouri/Arkansas border where weak shortwave energy may generate locally heavy convection. Higher resolution guidance may help to refine the potential threats over the coming days. Into Wednesday, convection will likely shift east ahead of the cold front into the Great Lakes region and Ohio and Tennessee Valleys, with a Marginal Risk depicted in the Day 5 ERO over those areas with some modest adjustments from continuity per latest guidance. By then there is notable spread for timing of a leading wave/warm front near along the East Coast, leading to the greatest uncertainty in specifics over the northern Mid-Atlantic. Faster timing of the UKMET/CMC and some ML models would lead to greater rainfall potential over the Mid-Atlantic relative to the GFS/ECMWF and most of their ensembles. For both days, the forward motion of the storms may be a limiting factor for flash flood potential. After early Thursday, progression of the front will take rain chances more to the East Coast/Northeast, with possible focus of heavier amounts from the northern Mid-Atlantic into the interior Northeast where the best forcing is. By the latter half of the week there is increasing uncertainty over the shape of the supporting upper trough. This 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 given the persistence of upper troughing. Expect some lingering precipitation over the Northwest on Tuesday followed by a drier trend as upper ridging amplifies over the region. Gusty winds are possible in the northern Rockies behind the strong southern Canada/northern tier U.S. surface low on Tuesday. Into late week, some return flow of moisture is possible across the southern/central Rockies and Plains, with one or more surface boundaries helping to focus showers and thunderstorms. Teleconnections relative to the forecast large scale pattern, as well as dynamical/ML models, support the general theme of rainfall potential over this region but confidence is currently low regarding details of coverage and amounts. 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. Next week will remain hot across South Texas westward through the Rio Grande Valley, where the combination of hot temperatures into the 90s and 100s as well as humidity will keep heat indices in the upper 100s, locally 110+. Temperatures nearing or exceeding 100F may stretch across the southern High Plains through Tuesday but should moderate somewhat by midweek behind a frontal passage. Then the next big concern will be increasing heat across the West as upper ridging builds in. Highs broadly will run 10-15 degrees above average, with even warmer anomalies of 15-20+ 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. Per the Climate Prediction Center, this hot pattern could continue into early next week. As for other aspects of the temperature forecast, some above normal temperatures are possible across the northern tier ahead of the front advancing eastward from the northern Plains. Cooler air behind the front may bring readings down to near or moderately below normal levels over an increasing area to the east of the Rockies or Plains during Thursday-Saturday. Rausch/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