Extended Forecast Discussion NWS Weather Prediction Center College Park MD 335 PM EDT Sun May 29 2022 Valid 12Z Wed Jun 01 2022 - 12Z Sun Jun 05 2022 ...Multiple rounds of heavy rainfall/convection forecast to focus across the south-central Plains near a front late Tuesday-Thursday... ...Late week heavy rain threat increasing over southern Florida ahead of a tropical system... ...Overview... A cold front extending from a deep upper low over south-central Canada is forecast to become nearly stationary across the south-central Plains where multiple rounds of convection with heavy rainfall are forecast to impact the area from late Tuesday to Thursday. Additional rounds of convection/heavy rain are possible over the central Plains by the weekend. The front could trigger areas of heavy rain over interior New England and the Mid-Atlantic Wednesday and Thursday. Meanwhile, 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. Elsewhere, upper-level ridging is forecast to build across the interior West while an East Pacific upper low produces troughing and unsettled weather into the Northwest. ...Model Guidance Evaluation/Preferences... Model guidance this morning are in reasonably good agreement with one another regarding the synoptic evolution across the U.S. through the medium-range period. The 00Z ECMWF was notably slower than the rest of the guidance regarding the southward progression of the cold front moving through the central Plains on Wednesday into Thursday, resulting in the associated heavy rainfall axis being noticeably farther to the north. On the other hand, the 00Z EC mean was farther south with the frontal placement. Thus the slow ECMWF solution is not preferred. Across the Northeast, models continue to trend toward a flatter flow regime as a frontal wave forms just off the northern Mid-Atlantic coast and moves out to sea. Along the West Coast, models are in good agreement with the timing of the arrival of the next round of moisture late this week. The EC solutions favor a reinforcing surge of moisture into northern California and northward during the weekend, a scenario not as prominently indicated in the GFS, GEFS, as well as the CMC solutions. By next weekend, ensemble means are in general agreement for a warm front to lift northward into the central Plains as cooler air edges southward into the northern Plains. Farther south, models continue to show more consistency with bringing tropical moisture northeastward into southern Florida from the eastern Pacific late week into the weekend with the approach of a tropical system. The recent CMC solutions were the fastest among the guidance, taking the center of this system across the Florida Peninsula over the weekend. 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, discounting the 00Z CMC by Day 5. The results are comparable to previous WPC forecasts. ...Sensible Weather/Threat Highlights... A surface low well into central Canada by Wednesday will send its trailing cold front across the Great Lakes and Ohio Valley Wednesday and then across the Northeast and Mid-Atlantic Thursday-Friday. Rain and thunderstorms can be expected along this front where locally heavy rain is possible over interior New England and the Mid-Atlantic. However, the trailing portion of the main front is forecast to move much more slowly through central and southern parts of the Plains, 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 southern Kansas to Oklahoma. Uncertainty remains with the axis of the convection each day/night, but a Slight Risk of excessive rainfall in WPC's experimental day 4 outlook seemed warranted given moisture and instability will be abundant along with a focus for lift with the front in place, and considering antecedent conditions could be quite wet with Tuesday's rain. Some additional rounds of heavy rain is possible on Thursday slightly farther south across Oklahoma. 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. This scenario will help spark additional rounds of enhanced rainfall over the central Plains during the weekend. There is increasing forecast agreement for tropical cyclone Agatha over the eastern Pacific to move northeastward across Mexico over the next few days. Scattered showers and thunderstorms are expected to become more frequent and widespread across southern Florida later next week. 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 some tropical characteristics near and northeast of the Bahamas midweek, and (c) possible interaction with a tropical wave moving into the western Caribbean Sea. 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. Rounds of rain are possible in the central U.S. next weekend, but with considerable uncertainty in the details at this point. Upper ridging ahead of the central U.S./Canada trough/low will promote above average temperatures by 10-20F over the Midwest/Ohio Valley and parts of the Northeast/Mid-Atlantic Wednesday, with the exception of parts of New England due to the backdoor front. Meanwhile upper troughing will bring continued cool temperatures across the Rockies and central High Plains through midweek with highs forecast to be 10-20F or below normal. The flattening pattern aloft will push the cold front eastward, 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 into the weekend. The West Coast and into the Great Basin can expect warmer than normal temperatures by around 5-15F Wednesday and beyond, followed by a modest cooling trend on Friday-Sunday 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