Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 AM EDT Wed May 17 2023 Valid 12Z Sat May 20 2023 - 12Z Wed May 24 2023 ...Warmer than average temperatures will persist across the Northwest through this weekend... ...Overview... Strong upper ridging across the West will continue to cause warm to hot temperatures there through the weekend, before an upper trough finally shifts into the Northwest next week and shifts warmer than normal temperatures into the north-central CONUS. A monsoonal pattern will be in place across the West with anomalously high moisture underneath the ridge, with scattered showers and thunderstorms likely. Farther east, a cold front will push a round of rain through the East this weekend, and then additional frontal systems may spark showers and thunderstorms across Florida in particular through much of next week. ...Guidance/Predictability Assessment... While recent model guidance continues to be agreeable with the western U.S. ridging late week, downstream there are still timing differences 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/Saturday. Through the 12/18Z model cycle, GFS runs and the GEFS ensemble members remained faster than the overall now better clustered ECMWF, CMC, and UKMET. Leaning more toward the latter cluster seemed prudent as the majority solution and as its timing was fairly consistent with the previous forecast. The incoming 00Z models look pretty similar on timing to the previous runs, with the GFS staying on the faster side. The progression of these features may also impact a shortwave farther south atop the Southeast/Mid-Atlantic or nearby western Atlantic and some potential surface low reflection into Saturday. These relatively weak and small lows still have low predictability at this point, so additional refinement is likely. In the West, energy spilling in from the northeast Pacific could finally spin some troughing toward and into the Northwest through the first half of next week. For the most part there are some fairly typical model differences with the timing and depth of this troughing. The main outlier appeared to be the 12Z GFS, which progressed stronger energy more quickly eastward by Sunday-Monday compared to other guidance, producing troughing over the north-central U.S. by Tuesday-Wednesday out of phase with the other guidance. The 18Z GFS seemed reasonable though, though farther east it may be maintaining too much separation in upper troughing in the Northeast by around Tuesday. There are some variations in the pattern in the East next week dependent on timing and depth of shortwaves moving through the mean troughing pattern. The GEFS and EC ensemble means at least were fairly similar with the pattern across the country, though the EC mean maintained a bit deeper trough in the East Tuesday-Wednesday compared to the GEFS mean. Thus the WPC forecast was based on a blend favoring the 12Z ECMWF with lower proportions of the 18Z GFS, 12Z UKMET, and 12Z CMC early in the period. Then introduced and increased the GEFS and EC ensemble mean proportions in the blend amid increasing spread in the individual models, reaching a little over half in the blend for the mass fields by the end of the period. ...Weather/Hazards Highlights... A cold front is forecast to track through the East this weekend and produce widespread rain along and ahead of it. There is a chance of locally heavy rain across the Northeast on Saturday as Atlantic moisture and perhaps another surface system could enhance totals, but flash flooding is not expected to be a big threat due to the ongoing dryness in the region. The back end of the front stalling near the central Rio Grande Valley could focus moisture and instability there that could produce some heavy rain rates atop moist soils from recent rainfall. A Marginal Risk was maintained for parts of southwestern Texas for this forecast cycle for Day 4/Saturday. Meanwhile, scattered showers and thunderstorms are also possible in the West at times this weekend into 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. Chances for showers and thunderstorms may increase in the Plains by Tuesday-Wednesday. Meanwhile multiple frontal systems could move through the southeastern U.S. including Florida, which could provide focus for ample moisture and enhance daily diurnally-driven showers and thunderstorms. Coastal areas of the Pacific Northwest can expect temperatures to remain warmer than normal by around 10-15 degrees on Saturday, before moderating to around normal next week as upper troughing and a cold front or two move in. Interior areas like the Great Basin are likely to see above average temperatures by 15-25F underneath the upper ridge axis, with highs in the 90s that could be record-setting on Saturday. Low temperatures in the 50s and 60s could also break records for warm minimum temperatures across widespread areas of the West over the weekend. Then upper troughing coming into the Northwest should shift warmer than average temperatures eastward into the north-central U.S. through Monday-Wednesday, moderating a bit with anomalies around 10-15F above normal. Meanwhile farther south, post-frontal cooler air and periods of rain and clouds could lead to slightly below average temperatures for parts of the Plains at times through early next 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