Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Mon May 08 2023 Valid 12Z Thu May 11 2023 - 12Z Mon May 15 2023 ***Periods of heavy rain expected for portions of the southern Plains and also the northern High Plains and vicinity*** ...Synoptic Scale Overview... An amplified upper level trough with an accompanying surface low will be making weather headlines across the Rockies/Plains to end the week as the main upper low tracks from the central Rockies to the Dakotas. There will likely be an enhanced surge of Gulf moisture northward that will fuel scattered to numerous showers and storms with heavy rainfall potential across much of the Plains. An upper ridge over the east-central U.S. ahead of the Plains storm system will likely break down going into Sunday and beyond with a trough likely building southward from eastern Canada across the northeastern quadrant of the nation. Much of the southern/eastern U.S. will remain unsettled under this pattern. Meanwhile, a ridge will significantly amplify across the west, bringing anomalously warm, near record-tying/breaking highs to portions of the Pacific Northwest this weekend and into early next week. ...Guidance/Predictability Assessment... The model guidance shows the upper-level pattern over the CONUS during the forecast period (12Z THU 11 MAY - 12Z MON 15 MAY) beginning with a southern stream trough moving northeastward over the Plains before related energy retrogrades westward as a steep ridge/closed high builds to the north over the western CONUS into western Canada. Both the deterministic guidance and ensemble means are in good agreement in the overall evolution of this transition, with differences mainly related to subtle small-scale differences in the embedded energy weakening/transitioning westward and potentially lingering over California/the Southwest. Both the GFS and ECMWF also show good run-to-run consistency over the last few cycles. One other area where the solutions diverge a bit is with respect to a closed low developing over southeastern Canada and shortwave energy rotating around the low, which may impact precipitation chances and temperatures over the Great Lakes/Northeast. The guidance shows the progression of one particular shortwave/surface cold front will also dictate precipitation chances and temperatures into the Southern Plains/Southeast late in the period. A general model blend of the deterministic guidance between the 00Z ECMWF/CMC/UKMET and 06Z GFS has good agreement/ensemble support with a resultant forecast that remains very similar to the previous WPC blend. A small contribution from the means is included to replace the time-limited UKMET mid- to late forecast period, which helps to smooth out naturally increasing small-scale deviations in the solutions with increasing forecast lead time. ...Weather/Hazards Highlights... Scattered to numerous showers and thunderstorms will likely develop across much of the Plains states going into the latter half of the work week, as moisture advection from the Gulf of Mexico increases ahead of a slowing frontal system over the central/southern High Plains and the upper trough/low approaches from the West. The Day 4 (12Z Thursday-12Z Friday) Excessive Rainfall Outlook maintains a Slight Risk over parts of eastern Montana into northeastern Wyoming and western portions of the Dakotas. Guidance continues to indicate the likelihood for widespread heavy rainfall in a favorable upslope pattern over wet ground conditions due to moderate/heavy rainfall the prior day. Additional moderate to heavy rainfall may spread eastward into the Northern Plains following the surface low track on Friday. The Day 5 (12Z Friday-12Z Saturday) Excessive Rainfall Outlook maintains a Slight Risk across portions of central/southern Texas where there is a strong signal for potentially significant rainfall of several inches. Additional rounds of moderate to locally heavy rainfall look probable into the weekend as the surface boundary stalls over the region, raising concerns over flash flooding given increasingly saturated ground conditions. Shower and storm chances will also spread into the Ohio Valley, Appalachians, and Midwest into the weekend as the frontal system slowly pushes eastward. As the upper low crosses the Great Basin and northern Rockies, there should be sufficient cold air aloft to support some late season higher elevation snow, especially for portions of the northern/central Rockies. In terms of temperatures, highs will likely be running 10 to 15 degrees above early May averages from the Upper Midwest to the Northeast states to close out the work week, and then trending cooler going into the weekend as an upper-low over Canada slides southward and multiple cold fronts look to effect the region. Warmth will build across the Pacific Northwest by Friday and especially into the weekend with highs reaching 20-25+ degrees above mid-May averages across much of Washington and Oregon. Some record-tying/breaking highs into the mid-90s look possible. Lows may be a little less extreme but could still challenge a few daily records for warm lows by the weekend. Highs will likely be slightly below average the Intermountain West in association with the upper trough through Thursday before a moderating trend commences. Overnight lows over those areas early in the period should be closer to average while areas east of the Rockies should see mostly above average lows with increased cloud cover and humidity. Putnam/Hamrick 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