Extended Forecast Discussion NWS Weather Prediction Center College Park MD 316 AM EDT Sun May 29 2022 Valid 12Z Wed Jun 01 2022 - 12Z Sun Jun 05 2022 ...Multiple rounds of heavy rainfall/convection will focus across the south-central Plains near a front Tuesday-Thursday... ...Overview... The medium range period will begin Wednesday with a relatively deep upper low for early June atop south-central Canada, and troughing associated with this feature will gradually push a frontal system across the northeast quadrant of the country along with some showers and storms. The back end of this frontal system is forecast to slow, stall, and lift back north through the week, and will provide a focus for rounds of rain and thunderstorms that could cause flash flooding across the central/southern Plains. 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. Elsewhere, upper-level ridging is forecast to build across the interior West while an East Pacific upper low produces troughing along the West Coast. ...Model Guidance Evaluation/Preferences... Recent model guidance shows somewhat reasonable agreement that an upper low will be consolidating across south-central Canada this week, though with some notable differences in the 12/18Z cycle even as the period begins day 3/Wednesday. The 12Z UKMET in particular seemed unlike consensus where it showed separate energy to the east of the upper low over the Upper Great Lakes and not yet consolidated, which displaced the surface low to the southeast. The 12Z and new 00Z ECMWF are particularly strong with an upstream shortwave located near the central Rockies compared to other guidance, but still seemed acceptable to use in a forecast blend. Then by Thursday-Friday, the 12Z CMC drifts more southeastward with its centroid of the upper low than other models, producing more troughing in the Great Lakes/Ohio Valley compared to consensus and basically every ensemble member. The 00Z CMC looks to be in better alignment. The GFS runs and the ECMWF seemed to be aligned the best with each other and ensemble guidance, so favored those especially during the early part of the forecast. However, by the weekend, there are some considerable differences with the upper low--the 18Z GFS in particular starts to diverge as it merges the aforementioned upper low and another upper low that other GFS runs and other models show over far northeastern Canada/the western Atlantic. Ensemble means may also be merging two upper lows. But had to favor the ensemble means by over half by the end of the period regardless given the model differences, but maintaining a bit of 12Z GFS influence and more of the 12Z ECMWF, which seemed to be the best deterministic model proxy to consensus. Models stay more agreeable with troughing over the West Coast through the weekend despite lingering latitudinal differences with the East Pacific upper low, while ridging develops in the interior West possibly into the southern High Plains. Guidance varies much more considerably with the pattern over the Gulf of Mexico and Florida through the week with at least tropical moisture emerging from Agatha but also possibly energy from Agatha, which could combine with western Caribbean energy. Many models and ensemble members produce possibly tropical surface lows crossing the Gulf and the Florida Peninsula, but with timing differences on the order of days, and some linger energy and low pressure near the Yucatan/Bay of Campeche/western Caribbean. The WPC forecast at this point is on the slow and weak side of guidance, but with some ejection of low pressure northeastward into the Gulf by the weekend. This is most like the ECMWF in terms of the deterministic guidance. ...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 the Northeast and Mid-Atlantic Thursday-Friday. Rain and thunderstorms can be expected along this front, after some initial showers in parts of the Northeast 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 possibly Missouri. 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. There is increasing forecast agreement for tropical cyclone Agatha over the eastern Pacific to turn northeastward and move 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 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. 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. Tate/Kong 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