Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM 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... 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 model guidance continues to be agreeable with the western U.S. troughing late week, downstream there are still 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. For the 12/18Z model cycle, the various model suites' ensemble members cluster with their deterministic runs without much overlap in the ensemble spread (they are underdispersed). The CMC was on the slower side, the ECMWF in the middle along with the deterministic UKMET, and the GFS runs on the faster side. The WPC forecast favored the ECMWF deterministic run and ensemble mean along with the UKMET, as they were a good middle ground of the guidance and had similar timing to the previous forecast. The incoming 00Z model cycle may offer some slight model convergence through late week, as the CMC and ECMWF sped up a tad while the GFS slowed. However, the 00Z ECMWF does eject more quickly by late Saturday-Sunday than expected. 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. However, one preference was away from the ECMWF runs that had high QPF reaching onshore of the Carolina coasts, which seems unlikely, and in fact the most recent EC adjusted by putting heavy QPF offshore. 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 had similar progressions and the 12Z ECMWF seemed to match their timing/track the best out of the 12/18Z deterministic models. For these reasons, the WPC forecast had a very ECMWF-heavy blend along with some UKMET for the first half of the period (other than reducing the ECMWF component with the Southeast QPF late week), and maintained an ECMWF/ECens mean majority through the medium range timeframe along with some GEFS mean for the latter part of the period. ...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. While the timing is still somewhat uncertain, it seems most likely that enhanced rainfall chances should shift into the East on Saturday along and ahead of the front, lingering along parts of the East Coast Sunday. There may be some chance of locally heavy rain somewhere in the East 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 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