Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Fri May 12 2023 Valid 12Z Mon May 15 2023 - 12Z Fri May 19 2023 ...Some flash flooding potential could linger in/around Texas through the first half of next week... ...Early season heat wave to affect the Pacific Northwest... ...Overview... The overall weather pattern 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. This pattern will result in an early season heat wave 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 drop into the northern/eastern parts of the country. 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 short range period. ...Guidance/Predictability Assessment... Model guidance for the most part continues to be reasonably agreeable for the large-scale pattern of ridging in the West and troughing in the Northeast. At the start of the period Monday, models indicate that an upper low will be retrograding across the Northwest while weakening as the energy becomes embedded in the overall ridgy flow. There is also a general consensus for timing of a shortwave moving through the East, though the north-south placement varies enough to cause some precipitation forecast differences. But the main outlier for the 12/18Z model cycle was the 12Z CMC, which brought a shortwave into British Columbia that is strong enough to basically break down the ridge in the interior West by around Wednesday, with little support from the other models or ensemble members that show persistent ridging. The newer 00Z CMC trending back toward the latter provides confidence that the 12Z CMC was an outlier. Then by Wednesday-Friday, the 18Z GFS and 12Z ECMWF happened to agree fairly well on timing and placement of a potent shortwave coming around the southwestern/southern part of the trough extending from east-central Canada and its associated surface low. The 12Z GFS was a little faster. The incoming 00Z guidance is looking a little slower with these features tracking east, but agreeable in principle especially considering the typical spread for the late medium range. Thus the WPC forecast was based on a blend of the 12Z/18Z GFS and the 12Z ECMWF and UKMET early in the forecast period. For the mid-period the 12Z GFS and the UKMET were phased out in favor of the GEFS and EC ensemble mean guidance that were agreeable with the 18Z GFS and 12Z ECMWF. ...Weather/Hazards Highlights... Persistent moist inflow is likely to continue into the southern Plains through at least the early part of next week. Though rain amounts may not be as high as in the short term across Texas, another day of enhanced QPF is possible on Monday over what is likely to be very wet ground, so a Slight Risk of excessive rainfall/flash flooding will be maintained in this forecast cycle for Day 4/Monday across south-central Texas, with a minor expansion westward per newer model guidance. The surrounding Marginal Risk continues to encompass the relative majority of still diverse guidance for where heaviest pockets of rainfall may be, with the Marginal also extended westward into portions of New Mexico where a monsoon-like pattern starts to set up. By Tuesday and beyond this monsoonal type of pattern with moisture coming in underneath ridging aloft looks to persist across the Four Corners states and perhaps even farther north through much of next week, with typical diurnal enhancement. Moisture anomalies vary per model (with the ECMWF suite showing much higher anomalies than the GFS suite), but with instability in place there could be localized flash flooding. A Marginal Risk of excessive rainfall is planned for Day 5/Tuesday for the southern High Plains, though placement of heavy totals and flash flooding remains uncertain. Elsewhere, a round of rain is likely across the Mid-Atlantic and/or Carolinas on Monday with a shortwave impulse and surface front. Light rain is possible for the Northeast on Tuesday. An initially stalling surface front and another front approaching near/over the Gulf Coast could lead to scattered showers and storms across that area through much of the week. Then a low pressure system moving across southern Canada could sweep a front across the north-central U.S. late week, bringing potential for showers and storms ahead of it across the northern half of the Plains into the Midwest. Temperature-wise, a heat wave is forecast to persist from this weekend into the medium range period across the Northwest as large scale upper ridging prevails over western North America (though with a weakening embedded upper low settling over the region). Highs in the 80s and into the low 90s would be 15 to 20 degrees above average and could set records for many of the lower elevations in Washington and Oregon. Temperatures are forecast to remain 10-20F above average in the West through all next week, though lessening somewhat (below record levels for highs). However, many warm low temperatures could keep challenging daily and perhaps even monthly records for high minimum temperatures in the upper 50s and 60s. 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 not terribly anomalous for May). Periods of warmer than average temperatures should expand across much of the northern tier, though could eventually moderate closer to normal by late next week. The Northeast to Mid-Atlantic could see periods of slightly above and below normal temperatures varying depending on a series of frontal systems. Meanwhile the cloudy and rainy conditions should keep highs 5-15F below normal over the southern High Plains, perhaps gradually moderating through the week. 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