Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Sat May 13 2023 Valid 12Z Tue May 16 2023 - 12Z Sat May 20 2023 ...Warmer than average temperatures will persist across the Northwest... ...Overview... The overall weather pattern through next week will consist of an anomalously strong upper ridge over the western half of the U.S. and into western Canada that will be persistent aside from an upper low gradually shearing out/weakening atop the Northwest through around midweek. This pattern will maintain warm to hot temperatures across the Northwest and to a lesser extent over other parts of the West. Downstream, mean troughing is likely across eastern North America, with individual shortwaves supporting cold fronts that should push southeastward to the east of the Rockies and produce areas of showers and thunderstorms. Meanwhile moist inflow continues into the southern Plains, maintaining some risk of excessive rainfall there, though the threat of widespread flash flooding should decrease through next week given less upper-level support compared to the near term and short range period. However the south-central Rockies/High Plains could see an increase of rainfall around the end of the week. ...Guidance/Predictability Assessment... Latest models and ensembles still show some detail differences and adjustments within the agreeable large scale pattern. With the details of the strong upper trough crossing the Northeast around Wednesday becoming more stable, the primary large scale focus from the model diagnostics perspective is the ultimate track and evolution of a potent shortwave/possible embedded upper low along with a surface low reflection reaching south-central Canada by early Wednesday and continuing farther eastward thereafter. The 00Z/06Z guidance as a whole generally nudged toward somewhat faster system/frontal timing versus the previous majority, with the 00Z CMC being a slower holdout. New 12Z models thus far show improved clustering, both in terms of a surface low track near Lake Superior by early day 6 Friday as well as for timing with the CMC adjusting faster. Some typical differences remain with an average providing a reasonable reflection of consensus. Meanwhile, lower predictability small-medium scale issues continue to plague the forecast from the southern half of the Plains eastward. In the latest model runs through 12Z, the CMC/UKMET and to some degree 00Z ECMWF suggest a weak upper trough/low reaching Texas in some fashion by the start of the period on Tuesday and then progressing eastward while the GFS suppresses/shears out this feature in favor of a compact central Plains into Southeast feature. The 12Z UKMET offers potential for some interaction among the two. A blended approach yields a discernible but still conservative southern stream shortwave. Guidance comparisons led to updating the forecast with a 00Z/06Z operational model composite on days 3-4 Tuesday-Wednesday, followed by removal of the slow 00Z CMC for the northern tier system by day 5 Thursday and than some incorporation of the 06Z GEFS/00Z ECMWF means (plus splitting GFS input among the 00Z and 06Z runs) by days 6-7 Friday-Saturday. ...Weather/Hazards Highlights... Persistent moist inflow is likely to continue into the southern Plains through much of next week. Though rain amounts should not be as high as with recent events and in the short term across Texas, some rainfall continuing over what is likely to be extremely wet ground has led to Marginal Risks in the Excessive Rainfall Outlook continuing into Days 4-5/Tuesday-Wednesday, with some eastward shift for Wednesday compared to Tuesday. There is still a fair amount of model spread with placement of any heavier totals that may not be resolved until the short term or near term, depending in part on the evolution/progression of features aloft. Based on the relative clustering among non-GFS solutions for a possible upper trough/low over Texas, expect the updated Day 4 ERO to reflect somewhat of an eastward/northeastward shift and expansion of the Marginal Risk area over Texas while the plan for the Day 5 ERO is an eastward expansion of the Marginal Risk (while maintaining the original area as-is) to include the Gulf Coast region based on the potential for some areas of heavy rainfall near/ahead of the upper feature. There could also be the potential for some areas of enhanced rainfall to extend farther eastward through the Southeast coast along a wavy front (and other dropping down from the north to replace it). For now the uncertainty over this region is greater than desired for adding to the Marginal Risk area as well. Returning westward, another uncertainty is the extent and magnitude of locally heavier rainfall extending back through New Mexico and Arizona. Latest ECMWF/GFS runs have essentially flipped as far as which reflects the heavier maxima, but remaining models and ensemble probabilities suggest lighter activity. Mixed messages between model/ensemble guidance versus an overall background of anomalously high precipitable water values within a monsoon-like pattern that sets up have favored not yet reflecting any risk areas in the ERO but requires continued monitoring. Areas of rain farther north in this monsoonal type of pattern with moisture coming in underneath Great Basin ridging aloft are also uncertain in terms of placement and amounts. But by the latter part of next week, a low pressure system moving across southern Canada and possibly as far south as the Great Lakes should bring a cold front across much of the central U.S., serving to focus rain and thunderstorms along and ahead of it with slightly more predictability, with rain chances gradually progressing south and east with time coincident with the front's movement. This front and trailing high pressure could bring some upslope flow over parts of the south-central Rockies/High Plains, enhancing potential rainfall amounts there. The Northeast could see a round of rain with a leading cold front on Tuesday but then is likely to see dry conditions until moisture is drawn into the East late week ahead of the next cold front that will be approaching from the central U.S. Temperature-wise, expect warm to hot temperatures to persist across the West as large scale upper ridging prevails on the whole. The Northwest (the Pacific Northwest into the Great Basin) is likely to see the highest temperature anomalies of 10-20 degrees or so above average persist through much of the week, though with perhaps some slight moderation late week for the Pacific Northwest while areas farther inland may trend slightly warmer. The highest potential for record highs has now phased into the short range period, but record high minimum temperatures in the 50s and 60s could continue to be widespread through mid- and late week across the West, perhaps even challenging monthly records. The warming trend over the northern Great Basin by Friday-Saturday may once again reach record highs though. Farther south, highs in the 90s are likely in the California Central Valley with highs above 100F in the Desert Southwest (though the latter is less anomalous for May). Warmer than average temperatures should expand across much of the northern tier early-mid week, followed by a cooling trend Thursday-Saturday after the frontal passage. The Northeast to Mid-Atlantic should see variable above or below normal temperatures depending on frontal progression. Currently anticipate the most notable anomalies being on the cool side of normal by 5-15F over the Northeast during Wednesday into early Thursday after a Tuesday frontal passage. The cloudy and rainy conditions over the southern High Plains should keep highs 5-15F below normal especially for the first half of the week, followed by a brief rebound close to normal. Frontal passage and a rebound in clouds/rainfall may drop the region's highs below normal again by Friday-Saturday. 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, 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