Extended Forecast Discussion NWS Weather Prediction Center College Park MD 332 PM 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... Latest guidance continues to show good agreement for the large scale pattern while exhibiting some embedded detail differences/adjustments. One area of recent inconsistency has been with the cold front passing through New England around Tuesday. There was a brief period when the models were suggesting some wave development over/offshore New England or into Nova Scotia late Tuesday-Wednesday but now there is a better signal for a deepening wave farther northeast of New England by early Wednesday. Behind this front, guidance takes energy shearing into northwestern Canada and has varying ideas on how it could consolidate into a trough (with possible embedded low) that would reach somewhere between the northern Plains and Upper Great Lakes by day 7 Friday. The associated front should push into the lower 48 on a somewhat farther west path than its predecessor. The 06Z GFS was a bit on the western side of the spread with the core of the upper system but the new 12Z CMC ends up being as slow (with the 12Z GFS coming in just a tad faster than the 06Z version). Now the 12Z UKMET becomes a fast extreme by late in its run ending early Thursday. Meanwhile there are still notable smaller scale differences aloft across the central U.S. and into the Southeast, with effects on QPF placement and amounts. The low predictability involved with such features will minimize confidence in any specific solution. A composite of the 00Z GFS/ECMWF/UKMET/CMC provided the most coherent update to the forecast during the first half of the period, with a trend toward half models and half means (06Z GEFS/00Z ECens) accounting for the increasing detail uncertainty by the end of the period. ...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. Guidance still exhibits some spread but for now the relatively better signal for higher rainfall totals generally aligns with the current Slight Risk area so that remains unchanged this cycle. Likewise, the surrounding Marginal Risk area continues to reflect a more diffuse guidance signal for where heaviest pockets of rainfall may be, including the western part reaching 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 Great Basin 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/GEFS suite), but with instability in place there could be localized flash flooding. The Marginal Risk of excessive rainfall for Day 5/Tuesday over the southern High Plains reflects minimal change from previous cycle with some persistent hints of localized heavy totals and flash flooding potential but uncertainty in specifics. Toward the end of the week the latest guidance is suggesting potential for heavier rainfall over parts of the south-central Rockies/High Plains as a front drops into the region from the north, with low level upslope flow enhancing activity if the front reaches far enough south. 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 with the approach/passage of a cold front. An initially stalling surface front and then the trailing part of the Northeast front ultimately pushing south close to the Gulf Coast could lead to scattered showers and storms across that area through much of the week. It will take additional time to determine where the best focus for heavier rainfall may be. Late in the week a low pressure system moving across southern Canada and possibly as far south as the Great Lakes may bring 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, expect a heat wave 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 with highs decreasing a bit over western Washington/Oregon later in the week (below record levels for highs). Highs east of the Cascades could actually increase a bit toward the end of the week though, and possibly challenge record highs. Meanwhile, throughout next week many warm low temperatures could challenge 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). Warmer than average temperatures should expand across much of the northern tier early-mid week, followed by a cooling trend after frontal passage Thursday-Friday. 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 5-12F 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 early in the week, with more moderate readings likely thereafter. Rausch/Tate Hazards: - Heavy rain across portions of the Southern Plains, Mon, May 15. - Flooding possible across portions of the Upper Mississippi Valley, the Southern Plains, the Northern Rockies, the Northern Great Basin, and the Northern Plains. - Flooding occurring or imminent across portions of the Upper Mississippi Valley, the Middle Mississippi Valley, the Northern Plains, the Northern Rockies, and the Northern Great Basin. - Flooding likely across portions of the Central Great Basin, the Northern Plains, the Northern Great Basin, the Central Rockies, the Southern Plains, and the Southwest. - Much above normal temperatures across portions of California and the Southwest, Mon-Wed, May 15-May 17. - Much above normal temperatures across portions of the Pacific Northwest and Great Basin, Mon-Fri, May 15-May 19. 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