Extended Forecast Discussion NWS Weather Prediction Center College Park MD 228 PM EDT Tue May 12 2020 Valid 12Z Fri May 15 2020 - 12Z Tue May 19 2020 ...Heavy Rain is Possible in Texas and the Western Gulf of Mexico Coast This Weekend... ...Guidance/Predictability Assessment... A blend of the 00Z ECMWF/UKMET was preferred for Days 3-5 with increasing usage of the 00Z ECENS mean and 06Z GEFS mean. Ensemble means were heavily used for Days 6/7, leaning toward the ECENS. The 00Z/06Z/12Z GFS is the most progressive with a trough exiting Pacific Northwest Friday/Day 3 which allows this trough to remain separated from a developing upper level low from northern Mexico shifting into Texas on Day 4 and stranding it over Texas into or through Day 7. This progression is faster than the GEFS mean. However, the ECMWF/UKMET are slower with the northern tier trough, allowing influence of it with the southern low, shifting it east to Louisiana on Day 5 before separating and retrograding back to Texas under the ridge Days 6/7. Given the propensity for the GFS to be too progressive with ejecting troughs, the ECMWF/UKMET along with ECENS and GEFS means were preferred. Given the potential influence of the northern trough and subsequent lack of steering flow under the ridge confidence in the position of the upper low developing over Texas is rather low at this time. ...Overview and Weather/Hazard Highlights... The Omega High over BC/eastern Alaska today will begin to break down Wednesday as the eastern low ejects across the US/Canada border, crossing New England Friday night. The low currently on the western side of the Omega, off the Pacific Northwest subduct the ridge Thursday night and eject east over the northern CONUS Friday through the weekend, crossing the Great Lakes Sunday night. As stated above the GFS is thought to be too progressive with this trough. This trough will interact with a wavy frontal boundary and then traverse the northern periphery of the eastern CONUS ridge. This setup will bring active weather to the northern/central Plains with potential for multiple waves of low pressure to move along a quasi-stationary front, eventually consolidating into a more organized frontal wave reaching the Great Lakes Sunday morning. Areas of heavy rainfall as well as strong to severe thunderstorms will be possible, although the specific areas affected will likely depend on convection-scale processes with low predictability in the medium range with the Northeast receiving active weather through the weekend. Rising heights over the southeast will bring above normal temperatures to the east-central CONUS through the weekend. A trough will amplify as it shifts east into Northwest Mexico on Thursday night, developing into a mid-level low as it tracks into southwest TX Friday night. This opens the Gulf and sets up a heavy rain threat for southern and eastern Texas through the weekend. There is considerable uncertainty given with southern Plains/Texas/West Gulf Coast heavy rain with unknowns about how much the northern trough will influence the movement of this trough. The ridge shifts east in the wake of the exiting trough, moving from the Intermountain West this weekend to the northern Plains early next week. Temperatures 10 to 20 degrees above average are expected. Farther west, a vigorous Pacific cold front is expected to push across the West Coast along with the height falls, bringing dropping temperatures and widespread precipitation (rain and high elevation snows) to areas from central California and the Pacific Northwest eastward into the northern Rockies. High temperatures Sunday to Tuesday are forecast to be 5 to 15 degrees below average across the West Coast and interior Northwest. Elsewhere, the trailing end of a stationary frontal boundary is forecast to linger in the vicinity of South Florida and the Bahamas into this weekend when an area of low pressure develops along the boundary over the Bahamas and moves northeastward out to sea. This boundary is expected to focus showers and thunderstorms, with the potential for locally heavy rains for the Bahamas Saturday before activity shifts farther out to sea. The National Hurricane Center and Tropical Analysis and Forecast Branch (TAFB) are monitoring this area for potential development as a sub-tropical system by this weekend. Jackson Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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