Extended Forecast Discussion NWS Weather Prediction Center College Park MD 228 AM EDT Tue Sep 28 2021 Valid 12Z Fri Oct 01 2021 - 12Z Tue Oct 05 2021 ... Heavy rain threat over portions of Texas late this week... ...Overview... The medium range period will begin Friday with a closed upper low centered over the Northeast, while ridging in the east-central U.S. separates it from mean troughing in the West. Moisture and a stalled frontal boundary ahead of a southern stream trough should produce heavy rainfall in the Southern Plains for the latter part of the week. By early next week, model guidance is coming into better agreement with shortwave energy over the north-central U.S. (underneath an upper high moving across western and central Canada), which should maintain a surface low pressure/frontal system across the Ohio Valley. ...Guidance Evaluation/Preferences... 12/18Z model guidance continues to show good agreement with the location of the closed upper low centered over southeastern Canada and spreading troughing to the Northeast through the end of the week, with perhaps some differences in the energy distribution potentially narrowing the low or not early on, and somewhat minor differences in the speed it lifts out by Monday/Tuesday. A multi-model deterministic blend worked well for this feature. In the West, troughing is expected for the end of the week in both the northern and southern streams. With the northern stream shortwave, model guidance is generally agreeable in its track passing through the Northwest Friday/Saturday, while splitting some energy off into the eastern Pacific offshore of California. The 12Z CMC is the exception, which once again spins most of its energy southward to create a closed low over Oregon for the weekend. Meanwhile with the southern stream trough, while there are some differences in its strength and whether or not a closed low develops briefly, no guidance stood out particularly as being an outlier, so a deterministic blend of models excluding the CMC was able to be used in the West through the end of the week. As the northern stream shortwave moves eastward early next week, and the southern stream shortwave lifts northeastward, they should generally produce lower heights/troughing in the north-central U.S. while an upper high slides atop them into Canada. Detail differences certainly remain, but the GFS runs have trended somewhat weaker with energy and a closed low there while ECMWF runs have trended somewhat stronger with energy in this area, meaning that for the 12/18Z cycle, their patterns matched pretty well. The latter part of the forecast period used a blend of the deterministic GFS and ECMWF and the GEFS and EC ensemble means to temper individual model differences. ...Weather and Potential Hazards... Portions of the Southern Plains can expect multiple days of widespread and potentially heavy rainfall for the latter half of the week, as the upper low in the Southwest exits into the High Plains and interacts with tropical moisture coming from the Gulf of Mexico. The specifics of rainfall duration and intensity remain somewhat uncertain given model differences in the timing and track of the upper low, but the current forecast calls for 2 of 4 inches of rain with locally higher amounts extending through a good portion of Texas. Periods of rain can be expected farther north across the rest of the Plains and the Midwest in the vicinity of the stalled front, with a tendency for the rain to slowly shift eastward into the Ohio Valley as the surface low pressure system stalls by early next week. Meanwhile, a round of showers is possible for the Northern Rockies/Plains on Friday as a front passes through, scattered showers and thunderstorms are possible for southern Florida through the period, and some moisture may return to the Four Corners states by Monday/Tuesday. Warmer than normal temperatures are forecast across the north-central tier of the CONUS through the end of the week, with highs in the 70s through the Midwest underneath upper ridging. On the other hand, cooler than average temperatures (especially highs) are expected for the Rockies into southern/central portions of the High Plains. Friday should be particularly cool over New Mexico/Colorado/Texas given the clouds and precipitation over the region. The West Coast can expect temperatures a few degrees above normal by the weekend, while slightly below average temperatures are expected for the Northeast with the lingering upper low. 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, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml