Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Thu May 18 2023 Valid 12Z Sun May 21 2023 - 12Z Thu May 25 2023 ...Threat of heavy rain increasing late next week over Florida... ...Warmer than average temperatures will push from the Northwest into the northern Plains... ...Overview... The medium-range period will begin this weekend with a low pressure wave exiting the Great Lakes as another wave exits the New England while taping into some subtropical Atlantic moisture. Meanwhile, persistent ridging in the West will shift eastward into the northern Plains by late next week as the next upper trough reaches into the Pacific Northwest. Monsoonal moisture will make its appearance over the central Rockies and the Great Basin. A lingering front across Florida will keep unsettled weather this weekend with some threat of heavy rain near the east coast but a better chance of heavy rain appears to be late next week as models show better indications that an upper trough/low will develop over the eastern Gulf of Mexico and interact with antecedent moisture associated with the lingering front over Florida. ...Guidance/Predictability Assessment... Model guidance this morning appears rather agreeable with each other in depicting the overall evolution of the synoptic pattern across the U.S. through the medium-range forecast period. Much of the disagreement appears to be centered around the next upper trough forecast to dig into the Pacific Northwest late next week. Over the central High Plains, the 06Z GEFS was fastest with the surface trough reaching into the region late next week, with the rest of the guidance being slower. Over the northern High Plains, latest model consensus has slowed down the northeastward progress of the low pressure wave late next week. This is reflected on the latest WPC forecast charts. ...Weather/Hazards Highlights... By Sunday a cold front will have cleared most of the East, with the exception of the Southeast, particularly southern Georgia into Florida. The front plus above average/ample moisture levels could spark thunderstorms with high rain rates. Marginal Risks are planned for the Day 4 and 5 (Sunday and Monday) EROs near the track of this front, first for southern Georgia and northern Florida on Sunday and for much of the Florida Peninsula on Monday. Though Flash Flood Guidance is relatively high, high rain rates could overwhelm particularly urban areas with potential for flash flooding. Meanwhile, scattered showers and thunderstorms are also possible in the West at times 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. as troughing edges in. One area that convection may be most common is across the southern High Plains through much of the week. Chances for showers and thunderstorms will increase across much of the Plains by Tuesday-Thursday ahead of the upper trough and a frontal system. Temperatures in the Pacific Northwest will have returned closer to normal by early next week, but for interior areas like the Great Basin, temperatures of 15-20F above average will be common through Sunday under the influence of the upper ridge/high. Temperatures in the 50s and 60s could break records for warm minimum temperatures across widespread areas of the West especially Sunday. Even with the arrival of cooler air associated with the next upper trough, temperatures are forecast to remain above normal into late week across the Northwest. Meanwhile, the warmth should shift east with time into the Northern Plains and eventually the Midwest (in moderated form) through the week, with some above normal temperatures remaining in California/Nevada though. Near to slightly below normal temperatures are possible across the southern tier with post-frontal cooler air and periods of rain and clouds with a tendency for an upper-level trough to develop over the eastern Gulf of Mexico and possibly trying to close off over Florida by late week. Interaction with antecedent moisture associated with the nearby stationary front would destabilize the atmosphere and deepen tropical moisture, leading to an increasing chance of heavy rainfall over Florida late next week. Kong/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