Extended Forecast Discussion NWS Weather Prediction Center College Park MD 303 PM EDT Fri May 31 2024 Valid 12Z Mon Jun 03 2024 - 12Z Fri Jun 07 2024 ...Excessive heat likely to continue over southern Texas, while heat builds in the West for the latter half of next week... ...Overview... The early part of next week will feature progressive shortwaves and frontal systems in the northern two-thirds or so of the lower 48, leading to precipitation chances in the Northwest on Monday and possibly multiple rounds of convection in the central and eastern U.S. through midweek. The upper pattern looks a little more blocky farther east, with a narrow ridge over the eastern Great Lakes/Canada and a North Atlantic mean low near the Canadian Maritimes. Meanwhile very hot and humid conditions should continue across Texas (especially southern areas) under the northern periphery of an upper ridge anchored over Mexico, to the south of the storm track. By the latter half of the week, expect a more amplified and persistent pattern to develop, as upper ridging expands from Texas into the Interior West (expanding the coverage of potentially hazardous heat), while an upper trough axis develops downstream in the East. ...Guidance/Predictability Assessment... The primary synoptic-scale questions arise by mid-late week as the amplified upper ridge-trough pattern takes shape. By early Wednesday the past couple days of dynamical and ECMWF-initialized machine learning (ML) models have been indicating a better ridge developing over California/Nevada and somewhat lower heights over the southern Plains versus what GFS/GEFS runs have been advertising. However the 12Z UKMET/CMC have nudged somewhat in the GFS direction. Later in the week, the ML models vary in their details but on average seem to favor a compromise between the western ridge axis of the ECMWF/ECens and eastern axis of the GFS/GEFS. The 06Z and 12Z GFS runs have been steadily raising heights over the West Coast versus the 00Z run so this compromise still looks good. Details of this ridge will also affect how much moisture could begin to lift into the West from lower latitudes, with the GFS most aggressive in that regard. Farther east, the ML models ultimately cluster a bit east/northeast of the 00Z ECMWF for the upper low that may reach Ontario or vicinity by late in the week. Only one of five ML models supported anything remotely like the 00Z CMC that dropped its upper low into the Tennessee Valley. The new 12Z CMC adjusted well northward by late week but is still a bit on the southeast side relative to most other solutions for the upper low. With the 00Z UKMET showing flatter flow than consensus across the northern tier, the updated blend based on 00Z/06Z guidance incorporated the 06Z GFS-GEFS/00Z ECMWF and a little 00Z CMC for about the first half of the period (GEFS helping to tone down some GFS track issues across southern Canada). Then the forecast transitioned to split ECMWF/ECens mean input and phase out the CMC, ultimately yielding a compromise with a very slight tilt to the ECMWF/ECens relative to the GFS/GEFS by next Friday. ...Weather/Hazards Highlights... An upper-level wave and atmospheric river coming into the Northwest will bring heavy rainfall continuing into Monday. In the Day 4 Excessive Rainfall Outlook, Marginal Risk areas remain in place for flooding concerns with the heavy rain. Snow levels may be lowering some by Monday in the Cascades and northern Rockies as the shortwave moves overhead, but there is still some concern for rain at elevations that still have a snowpack, possibly increasing the flooding threat. Latest guidance shows some precipitation continuing into Tuesday, but with a less pronounced moisture feed and less southward extent. Thus there are no risk areas over the Northwest in the Day 5 ERO. Expect drier conditions mid-late week as upper ridging builds. An initial low pressure/frontal system in the central U.S. and the next one coming in from the West, plus more diffuse shortwaves farther south, will lead to rounds of showers and thunderstorms over a broad portion of the central and eastern U.S. The focus for potentially heavier rain is rather uncertain for Monday as instability abounds in widespread areas, but there still seems to be a general model and ensemble signal for Missouri/Arkansas and surrounding states to maintain a Marginal Risk in the Day 4 ERO. A vigorous frontal system (anchored by southern Canada low pressure) reaching the northern Plains by around late Monday night may also produce locally heavy rainfall over North Dakota, but for now this activity appears sufficiently isolated and progressive to preclude any risk area in the ERO. By Day 5/Tuesday, the focus for convection is a little more clear as instability pools ahead of the aforementioned front. A Marginal Risk is still in place across northern and central parts of the Mississippi Valley Tuesday. Some locations within this area will be sensitive due to rainfall from prior days. Current detail/timing differences suggest it may take until closer to the event to determine whether there may be any merit to an embedded Slight Risk area. Continued progression of the Plains cold front should shift the best shower/thunderstorm focus into the eastern third of the country by mid-late week. Some areas of moderate to heavy rainfall will be possible but there is still considerable uncertainty over timing and location of highest rainfall totals. Additional weak upper-level energies from the Southern Plains to the Southeast will also keep up the potential for additional storms. A period of low level upslope flow behind the front reaching the southern Plains by Wednesday could also eventually help to develop some convection over parts of the southern Rockies/Plains. Also of note from the hazards perspective earlier in the week, the northern Rockies may see high winds on Tuesday behind the strong southern Canada/northern tier U.S. system. 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 early in the week but should moderate somewhat by midweek behind a frontal passage. Some above normal temperatures are possible across the northern tier at times. But the next big concern will be increasing heat across the West as upper ridging builds in. Highs broadly will be running 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 reaching over 110F in the Desert Southwest. Per the Climate Prediction Center, this hot pattern could continue at least into next weekend. 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