Extended Forecast Discussion NWS Weather Prediction Center College Park MD 303 AM EDT Wed May 31 2023 Valid 12Z Sat Jun 03 2023 - 12Z Wed Jun 07 2023 ...Broad multi-day thunderstorm and local heavy rain threat continues for parts of the Plains/Rockies... ...Overview... During the weekend into next week, models and ensembles continues to show a gradual retrogression of a strong upper ridge/high over the Upper Midwest and southern Canada as of early Saturday, supporting a persistent area of above normal temperatures from the northern Plains into middle/upper Mississippi Valley. Weak shortwave/upper low energy initially over the West through central/southern High Plains will weaken during the weekend but then a well-advertised southern stream Pacific low should track into California early next week. These features will maintain the potential for periods of convection over the Rockies and parts of the central-southern Plains, along with gradually increasing coverage from the Sierra Nevada into the Great Basin, but likely with somewhat lower rainfall totals than expected in the shorter term. Expect above normal temperatures over the northwestern states, though a front ahead an ejecting Pacific upper low may temper anomalies over the Pacific Northwest during the weekend. Downstream from the upper ridge, guidance continues to vary with two main bundles of energy forecast to drop southward within a general evolution favoring a steadily amplifying and deepening upper trough over the East. Details remain quite uncertain but expect associated fronts to produce rainfall of varying intensity and the deepening trough should increase potential for episodes of late day showers as well. ...Guidance Evaluation/Predictability Assessment... Guidance is still showing considerable difficulty in resolving details over the eastern U.S. within the overall pattern favoring increased troughing to the east of the central North America ridge. As was the case yesterday, individual solutions start diverging early in the period for where the upper low associated with the first shortwave may track during the weekend and first half of next week. The 00Z GFS has come in with the farthest west option thus far, over the Lower Great Lakes/Ohio Valley, while the 12Z ECMWF mean got no farther west than Nova Scotia. 12Z/18Z guidance on which the updated manual forecast was based generally ranged between Cape Cod and Nova Scotia for the westernmost point of the upper low's track (the 18Z GFS being the western extreme before the arrival of the 00Z run). The 00Z UKMET has shifted a bit westward but the 00Z CMC/ECMWF keep Cape Cod as the farthest west point of the track. The second and likely stronger shortwave dropping through eastern Canada into the Great Lakes/Northeast has plenty of spread as well, with a variety of ideas between a deep closed low and more open trough. The 00Z GFS has one of the more original evolutions seen thus far, pulling a small piece of the second shortwave off into the Great Lakes to merge with the first upper low. By next Tuesday-Wednesday, recent ensemble means are suggesting the deepening trough will be anchored by an upper low over or near Maine, which seems to be a reasonable starting point given the uncertainty. Meanwhile, the National Hurricane Center continues to monitor a disturbance over the Gulf of Mexico associated with a southern tier upper trough. These features should cross Florida during the weekend, though with some uncertainty in shortwave details for the upper trough. What happens to the trough after Sunday will depend on details to the north. The forecast farther westward is less chaotic in most respects. Guidance has been consistent in weakening initial shortwave energy/one or more upper lows over the West over the course of the weekend. There have been some recent trends toward a little farther eastward extent of height falls ahead of the upper low ejecting from the northeastern Pacific, with corresponding effects on the leading cold front and Pacific Northwest temperatures for a time. New 00Z runs show more divergence regarding whether some energy could linger over northwestern Washington/Vancouver Island. The majority cluster of guidance has held up well for the upper low expected to track into California next week, with most recent cycles having one or two stray solutions that ultimately trend closer to the mean. In this case the 00Z GFS has returned to consensus after the 12Z/18Z runs were on the fast side by Tuesday-Wednesday. Based on the array of 12Z/18Z guidance, the updated forecast used a composite of 12Z operational models on day 3 Saturday and then gradually increased 18Z GEFS/12Z ECMWF mean components so that there was an even model/mean mix by days 6-7 Tuesday-Wednesday. ...Weather/Hazards Highlights... The pattern evolution aloft over the western half of the country should keep a persistent threat for showers and thunderstorms over parts of the Rockies and Plains during the period, with diurnally favored showers/storms also likely to become more common with time from the Sierra Nevada-southern Cascades into the Great Basin as the southern stream Pacific upper low approaches/reaches California. Gradual weakening of upper impulses/lows initially over the West/High Plains should lead to somewhat lower maximum rainfall totals than expected in the shorter term, but localized flash flood potential will likely continue given wet ground conditions from recent or soon to occur rainfall. The Days 4-5 Excessive Rainfall Outlooks (covering Saturday through Sunday night) depict Marginal Risk areas across locations from the northern Rockies into parts of the southern Plains to reflect the persistent moisture/overall pattern that could still produce some areas of locally heavy rainfall. It will likely take into the shorter range time frame to resolve areas of more pronounced focus/confidence for any embedded Slight Risk areas. The Day 5 outlook incorporates the Sierra Nevada/Great Basin into the overall Marginal Risk area with moisture expected to increase across the region. Meanwhile the Day 4 outlook has added a Marginal Risk area to reflect the potential for some lingering heavy rainfall across the Florida Peninsula. Guidance is split on timing of the upper trough/associated surface reflection but rainfall from previous days would lower the threshold for any potential issues. Elsewhere, there is considerable uncertainty over what a low-predictability shortwave tracking underneath the upper ridge may do to promote convection over parts of the northern tier during the weekend, while rainfall details over the East continue to depend on uncertain specifics aloft. In general expect the potential for some showers/storms with a couple cold fronts dropping south during the period, with a potential trend toward diurnally favored pockets of rainfall in the Northeast as upper troughing deepens next week. Most of any moisture still crossing Florida during the weekend should progress offshore thereafter, though still with some lingering uncertainty given the guidance spread for the pattern farther north. The area of well above normal temperatures from the northern Plains through Upper/Middle Mississippi Valley into the central Appalachians as of Saturday should get steadily trimmed away on the eastern side as upper troughing develops over the East with increasing coverage of near to somewhat below normal readings. The weekend should start with decent coverage of highs 10-20F above normal, with some daily records possible, followed by most plus 10F or greater anomalies getting confined to the northern Plains by next Wednesday. The Northwest will be the other focus for very warm temperatures, with highs 10-20F or so above normal especially Monday-Wednesday. An ejecting Pacific upper low/leading front should temper the warmth during the weekend over western Oregon/Washington though. Clouds and periods of rainfall should keep highs 5-15F below normal over the southern half of the Rockies/High Plains for most of the period and the upper low reaching California will also bring a cooling trend there later in the period. 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices are at: 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