Extended Forecast Discussion NWS Weather Prediction Center College Park MD 353 PM EDT Sat May 28 2022 Valid 12Z Tue May 31 2022 - 12Z Sat Jun 04 2022 ...Multiple rounds of heavy rainfall/convection focus across south-central Plains near a stationary front next week... ...Overview... The medium range period will likely begin with an amplified trough and a low pressure system exiting the northern Plains into south-central Canada next Tuesday. The associated trailing cold front will gradually push east and southeast across the northern tier states and further into the Plains against a warm ridge over the eastern U.S. forecast to be gradually eroded by the front through the end of next week. Mountain snows initially over the northern Rockies and gusty winds/convection over the northern Plains will give way to rounds of thunderstorms over central/southern Plains where the front is forecast to become nearly stationary. Meanwhile, thunderstorms should progress through the mid-Mississippi and Ohio Valleys, the Great Lakes, and then into the Northeast as the week progresses. Farther south, tropical moisture can be expected to increase across southern Florida through the medium-range period as a subtropical jet is forecast to interact with moisture coming up from tropical cyclone Agatha currently located in the eastern Pacific. ...Model Guidance Evaluation/Preferences... The abrupt model trend in the past couple days toward a more amplified upper trough over the northern Plains for early next week has stabilized, although the most recent model runs still tend toward a more closed off system, with the associated lead front pushing faster toward the east across the upper Midwest. The GFS and GEFS show their typically faster solutions with subsequent eastward progression of the front across the Northeast by next Thursday. By late next week, deterministic model solutions begin to diverge regarding the timing and amplitude of a frontal wave near New England coast. Meanwhile, models indicate reasonably agreeable timing of the arrival of the next wave of Pacific moisture into the Northwest later next week despite quite a bit of model spread with the latitude of the associated offshore upper low. Over Florida, models are now more consistent with bringing tropical moisture northeastward from the eastern Pacific where tropical cyclone Agatha is currently located. The GFS again favors faster solutions with this tropical moisture feed whereas the ECMWF and CMC indicate a slower and more consolidated cyclonic gyre moving across Central America. Thus the WPC medium range forecast was based on a multi-model and ensemble blend of 40% from the 06Z GFS/GEFS, 40% from the 00Z ECMWF/EC mean, and 20% from the 00Z CMC/CMC mean, transitioning to mostly the ensemble means by Day 7. ...Sensible Weather/Threat Highlights... The surface low pressure system lifting into central Canada beginning Tuesday will send a cold front across the north-central U.S. and across the northeastern quadrant through the latter part of the workweek. Rain is possible first across North Dakota and the Upper Midwest to Great Lakes along this front Tuesday and then shifting eastward across the Ohio Valley and into the Northeast/Mid-Atlantic by Thursday, after some initial showers in parts of the Northeast Tuesday-Wednesday near a backdoor front. However, the back end of the main front is forecast to move more slowly through central and southern parts of the Plains and Mississippi Valley, and will provide a focus for rain and thunderstorms for multiple days around Tuesday-Thursday. Flash flooding is possible especially where storms repeat over the same area, which at this point appears most likely to occur around Kansas, Oklahoma, and Missouri, but with uncertainty in the details beyond that. Elsewhere, there is increasing forecast agreement for tropical cyclone Agatha over the eastern Pacific to turn northeastward and move across Central America over the next few days. Scattered showers and thunderstorms are expected to become more frequent and widespread across southern Florida later next week although the specifics are not clear at this time. Factors that influence the timing and amounts of rainfall will include (a) the interaction of a subtropical jet with tropical moisture coming from Agatha, and the cyclone's subsequent track and intensity after crossing Central America, (b) the proximity of an upper low forecast to gradually gain tropical characteristics near and northeast of the Bahamas midweek, and (c) possible interaction with a tropical wave moving into the western Caribbean Sea. The eastern U.S./Mississippi Valley upper ridge will promote above average temperatures by 10-20F over the Midwest/Ohio Valley and parts of the Northeast/Mid-Atlantic Tuesday-Wednesday, with some daily record high maximum and minimum temperatures possible. The backdoor front reaching New England and possibly as far south as the northern Mid-Atlantic would bring a cooler trend to these areas though. Meanwhile upper troughing will bring very cool temperatures to areas from the Intermountain West through northern/central Plains through midweek with highs forecast to be 10-20F or below normal. The flattening pattern aloft will push the initial Plains cold front eastward while leading to a moderating trend for below normal temperatures over the central U.S., with mostly single-digit negative anomalies by Thursday-Friday, though around 10F below normal in rain-cooled Kansas and Oklahoma. The East should see shrinking coverage of warm temperatures on Thursday and especially Friday-Saturday. The West Coast and into the Great Basin could flip back to warmer than normal temperatures by around 5-15F Wednesday and beyond, followed by a modest cooling trend on Friday-Saturday along the immediate West Coast with the arrival of a Pacific front. 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