Extended Forecast Discussion NWS Weather Prediction Center College Park MD 236 PM EDT Tue May 16 2023 Valid 12Z Fri May 19 2023 - 12Z Tue May 23 2023 ...Warmer than average temperatures will persist across the Northwest... ...Overview... It remains the case that the overall weather pattern this week should remain fairly stagnant into the medium range, as an anomalously strong upper ridge over the West causes warm to hot temperatures there, while mean troughing is likely downstream across eastern North America as multiple shortwaves and cold fronts move through. These should push southeastward 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 through Friday. Eventually early next week some upper troughing may come into the West, weakening the ridge somewhat while pushing it eastward. ...Guidance/Predictability Assessment... While recent model guidance continues to be agreeable with the western U.S. troughing late week, downstream there have been notable differences with the timing of a potent shortwave or compact upper low tracking near the Great Lakes with a surface low reflection and the associated frontal timing even by Day 3/Friday. It is still important to note that for the recent 12/18Z model cycle, the various model suites' ensemble members cluster with their deterministic runs without much overlap in the ensemble spread. The CMC/Canadian ensembles were on the slower side, the ECMWF/ECMWF ensembles in the middle along with the deterministic UKMET, and the GFS/GEFS runs on the faster side. The latest WPC forecast favored the 00 UTC ECMWF deterministic run and ensemble mean along with the 00 UTC UKMET, as they remained a good middle ground of the guidance and had similar timing to the previous forecast. However, guidance underdispersion seems to be gradually improving for the more recent model cycles. The CMC in particular has trend more progressive with the 12 UTC run more in line with the WPC preferred composite. The 12 UTC ECMWF/UKMET are a tad more progressive, but the 12 UTC GFS has not slowed. The progression of these features may also impact a shortwave farther south atop the Southeast or nearby western Atlantic and some potential surface low reflection. These relatively weak and small lows still have low predictability at this point, so additional refinement is likely. By early next week, there is some general agreement that an upper low offshore of Canada could 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. The details of this features such as the depth the energy reaches and the timing are more nebulous. But the ensemble means continue to have quite similar progressions and amplitudes and remain preferred. This solution also seems broadly reasonable downstream across the central and eastern states and acts to provide good product continuity versus chasing too much of any individual model detail. ...Weather/Hazards Highlights... A cold front mostly pushing southeastward does look to hang up a bit in the south-central Rockies from Thursday (short range) into Friday, with upslope flow on its backside in an environment with 90+ percentile moisture anomalies. Additional rainfall that occurs on Friday in that region would be atop wet ground with high streamflows that could exacerbate flash flooding risk, so a Marginal Risk was maintained in the Day 4/Friday ERO. Farther east, convection with possibly high rain rates could occur over south-central parts of the Plains and Mississippi Valley as well, leading to another Marginal Risk. The progressive nature of the front should not allow for too much training of storms, precluding any higher risk level. The cold front will move more progressively eastward across the Midwest/Ohio Valley on Friday. The southward digging of the trailing front through the Plains into Saturday may act to focus Gulf moisture and instability over the central Rio Grande Valley where a Day 5 ERO marginal risk area is planned to be added in a region with moist soils from recent rainfall. While the timing is still somewhat uncertain, it seems most likely that enhanced rainfall chances should also shift into the East on Saturday along and ahead of the front back up to the main low, lingering along parts of the East Coast Sunday. There may be some chance of locally heavy rain up the East Coast as Atlantic moisture and perhaps another surface system could enhance totals. Meanwhile, 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 monsoon-like pattern sets up with anomalously high moisture remaining over a large portion of the western U.S., first under the upper ridge axis and then as troughing edges into the Northwest. Temperature-wise, the persistent large scale western North America ridge will support warm to hot temperatures across most of the West. The Pacific Northwest could see above average temperatures lasting into late week, but moderated a bit compared to the heat in the near term, with temperatures looking closer to normal by Sunday. However, interior areas like the Great Basin may see their hottest temperatures of the week on Friday and Saturday, with highs in the 90s that could be record-setting. Low temperatures in the 50s and 60s could also break records for warm minimum temperatures across widespread areas of the West. The Northwest can expect moderating temperatures early next week while temperatures around 10-15F above normal shift into the north-central U.S. Farther south, periods of rain and clouds and then post-frontal cooler air could lead to temperatures slightly below average in the Plains at times. Tate/Schichtel 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