Extended Forecast Discussion NWS Weather Prediction Center College Park MD 308 AM EDT Mon May 30 2022 Valid 12Z Thu Jun 02 2022 - 12Z Mon Jun 06 2022 ...Late week heavy rain threat increasing over southern Florida ahead of a tropical system... ...Overview... The medium range period will see a fairly stagnant mid-upper pattern as a closed low spins across south-central Canada, sending periods of low amplitude troughing across the central and eastern U.S. along with surface fronts that could provide foci for convection. Meanwhile, tropical moisture is expected to increase through the period across southern Florida streaming northward from tropical cyclone Agatha currently located in the eastern Pacific. Energy from Agatha may combine with additional Caribbean energy in a large and complex area of low pressure near the Yucatan to form a tropical system in the northwestern Caribbean or Gulf of Mexico late week, but with high levels of uncertainty at this time. Elsewhere, upper-level ridging is forecast to build across the interior West while a northeastern Pacific upper low produces troughing and unsettled weather into the Northwest. ...Model Guidance Evaluation/Preferences... Recent model guidance is in reasonably good agreement for the overall synoptic pattern at least in the northern stream late week into early next week. The centroid of the central Canada upper low shows fairly good consensus between the 12Z/18Z GFS and 12Z ECMWF and UKMET, while the 12Z CMC sunk the low a bit farther south. The 00Z CMC has come in with somewhat better alignment. The details of troughing stemming from the low and thus placement of the surface fronts and axes for rain/convection are more uncertain. A multi-model blend of mainly the deterministic guidance, though with less influence from the CMC and increasing influence from the GEFS/EC ensemble means later in the period, seems the best way to handle the smaller-scale differences for now. This also worked well for the upper low spinning offshore British Columbia and its troughing influence into the West Coast. GFS runs have been more aggressive than the ECMWF/CMC at a shortwave ejecting into the northern Rockies by next Monday, which may be too fast. More notable differences continue to be seen farther south with tropical moisture and energy that could affect Florida in particular. Models continue to show the potential for energy in the Bay of Campeche/Yucatan/western Caribbean to consolidate into a forming tropical system with a general track northeastward. However, there are ample differences in deterministic models and ensemble members with particularly the timing of this ejection northeastward (on the order of days), but also with its strength and track. GFS and CMC runs have both tended to be on the fast side of the envelope, and with the GFS on the southern side along the Florida Straits and the CMC more north crossing Florida and making its way into the western Atlantic. The ECMWF on the other hand has been on the slower side and more favored, including the 12Z run, though notably its new 00Z run has sped up considerably with its low track, though not to the speed of the GFS/CMC runs. The WPC forecast has continued to take a conservative (in strength) and slower approach with the low at this point and will continue to coordinate with the National Hurricane Center for updates. ...Sensible Weather/Threat Highlights... A cold front is forecast to press across the East late this week, with rain and thunderstorms expected along it. While there is some uncertainty with the placement of heaviest rainfall, it appears that areas of the eastern Mid-Atlantic could see locally heavy rainfall and an isolated flash flood threat Thursday. However, the trailing portion of the front is forecast to move much more slowly through southern parts of the Plains and cause multiple days of convection from the short range period lasting into Thursday and possibly Friday. Flash flooding is possible especially where storms repeat over the same area, and with likely wet soils by the time the medium range period begins Thursday, this will lessen the amount of rain needed to get flash flooding. The best chance for flash flooding issues looks to be across Oklahoma and northern parts of Texas. By the weekend, the front is forecast to lift northward as a warm front while cooler air from Canada is forecast to edge southward into the northern Plains. These fronts will continue to spark additional rounds of enhanced rainfall across the central U.S. this weekend, but with low confidence in the details. There is increasing forecast agreement for tropical cyclone Agatha over the eastern Pacific to move northeastward across Mexico over the next couple of days. Scattered showers and thunderstorms are expected to become more frequent and widespread across southern Florida later this week, first with the moisture from Agatha streaming northeast and then with the potential for energy consolidating into a possible tropical system tracking northeastward. The timing and amounts of rainfall will be dependent on the interaction of a subtropical jet with tropical moisture coming from Agatha, then any remaining energy stemming from Agatha after crossing Mexico and the possibility it could combine with tropical wave energy over the western Caribbean to produce a tropical cyclone. Additionally, rain chances are expected to increase in the Northwest during the latter half of the week given the trough axis near the West Coast and a couple of frontal systems. The Southwest should remain dry and hot though. A relatively flat pattern aloft compared to the more amplified short range period should produce lesser temperature anomalies late this week into early next week. Cooler than average spots will be across the central/southern Plains Thursday into Friday with rain cooled air, but should switch to above normal in Texas by about 10F by early next week. Periodic troughing in the northern tier should lead to slightly below average temperatures by 5-10F there. The western U.S. could be warmer than normal by 5-10F late in the workweek but cooler temperatures are forecast by the weekend under the troughing influence. Warm temperatures across the Southeast Thursday will moderate closer to normal late week behind a cold frontal passage. 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