Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Mon May 15 2023 Valid 12Z Thu May 18 2023 - 12Z Mon May 22 2023 ...Warmer than average temperatures will persist across the Northwest... ...The south-central Rockies and Plains could see heavy rainfall Thursday-Friday... ...Overview... The overall weather pattern this week and into early next week should remain fairly stagnant across the country with an anomalously strong upper ridge over the West causing warm to hot temperatures there, while mean troughing is likely downstream across eastern North America. Individual shortwaves will support cold fronts that should push southeastward to the east of the Rockies and produce areas of showers and thunderstorms, including the possibility for some enhanced rainfall amounts potentially causing flash flooding in the south-central Rockies and Plains late this workweek. ...Guidance/Predictability Assessment... Model guidance remains quite agreeable with the large scale ridge in the West, with above average predictability. Details within the mean troughing downstream in the East still show some variability though. In particular, a compact upper low is forecast to drop southeast from south-central Canada Thursday and move eastward through the Great Lakes late week along with its surface low reflection and its accompanying cold front moving through the central U.S. and then the East. Overall, the 12/18Z model cycle slowed down with these features compared to previous cycles, which was reflected in the WPC 500mb heights and surface frontal charts. Within the slower trend, GFS runs as well as GEFS members were generally on the faster side of guidance compared to the ECMWF suite. But the 12Z and now the newer 00Z CMC were well on the slower side of the guidance envelope to the point where they could not really be used in the recent WPC forecast. Thus the forecast blend was based on the 12/18Z GFS and 12Z ECMWF and UKMET early in the forecast period that showed better agreement on timing, though these do show differences in the phasing of the upper low back into the mean flow late week (ECMWF maintains the separation longer). The incoming 00Z ECMWF continued the slower trend with slower movement than its previous run, but not quite to the slow extent of the CMC runs. Differences in the timing of the upper and surface low and frontal system continue into early next week and create a variety of solutions for when the front and precipitation ahead of it clear the East Coast. But the eastern Pacific also becomes an area of focus from a model diagnostics standpoint, with the potential for an upper low offshore of Canada to finally spin some energy/troughing toward the Pacific Northwest for some cooler temperatures with a cold front or two as the prevailing ridge weakens a bit and pushes eastward early next week. Some discrepancies remain in the model and ensemble guidance with timing and depth, but the main outlier in the 12/18Z cycle seemed to be the CMC again, which split some energy off and tracked it through western Canada while maintaining a West Coast ridge next Monday, out of phase with other guidance that progressed at least some semblance of the upper low eastward. Overall given increasing model spread, the forecast blend tended toward more GEFS and EC ensemble components through the mid to late medium range period, to about half the blend by the end. ...Weather/Hazards Highlights... A low pressure system moving through south-central Canada and across the Great Lakes will push a cold front into the central and then eastern U.S. during the latter part of the week into the weekend. The cold front looks to hang up a bit in the south-central Rockies on Thursday and Friday and produce upslope flow in an environment with 90+ percentile moisture anomalies. This would also be atop wet ground with high streamflows that could exacerbate flash flooding risk. Then farther east into the south-central Plains, the west-east orientation of the front and its possibly slow movement late Thursday could lead to training of storms with possibly high rain rates that form on the cusp of the instability/moisture near and ahead of the front. Thus the Slight Risk for Day 4/Thursday in the ERO will expand eastward into the Plains for this forecast cycle. One limiting factor there could be drier antecedent conditions though. The front will continue to shift south and east late week, with a Marginal Risk in the Day 5/Friday ERO from the south-central Rockies into the south-central Plains/Mississippi Valley. At this point it appears the front may begin to move faster for Day 5, precluding an embedded Slight Risk, as storms with high rain rates may not train as much. But a Slight Risk may need to be added in future cycles if heavy rain ends up falling over sensitive areas or the front stalls more. Meanwhile the cold front will move more progressively eastward across the Midwest/Ohio Valley on Friday with enhanced rainfall chances shifting into the East on Saturday along and ahead of the front, perhaps lingering along parts of the East Coast Sunday. Elsewhere, showers and storms may have some focus along a frontal boundary in the Southeast on Thursday in particular. With ample moisture and instability in place, there could be some localized heavy rain in and around southern Georgia/northern Florida, but amounts are not currently expected to surpass the high Flash Flood Guidance. Scattered showers and thunderstorms are also possible in the West at times late this week into early next week especially in the afternoons/evenings with a common diurnal cycle, as a monsoonal pattern sets up with anomalously high moisture remaining over a large portion of the western U.S. underneath the upper ridge. Coverage of the precipitation may not be terribly broad, but the details are yet to be determined. Temperature-wise, the persistent large scale western North America ridge will support warm to hot temperatures across most of the West. The Pacific Northwest is likely to be warmest on Thursday in the medium range period, with highs around 20 degrees above average before some gradual moderation late this week into early next week for areas west of the Cascades. However, interior areas like the Great Basin can expect the heat to persist into the weekend. Highs in the 90s there could be record-setting, while lows in the 50s and 60s could break records for warm minimum temperatures across much of the West. Highs above 100F are likely in the Desert Southwest, but this is not terribly anomalous for May. Meanwhile, the Northeast should see chilly temperatures Thursday morning, possibly approaching the freezing mark in interior areas and potentially setting a few records. Periods of rain and clouds and then post-frontal cooler air could lead to temperatures slightly below average in the Plains at times. 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, 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