Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Thu Jun 02 2022 Valid 12Z Sun Jun 05 2022 - 12Z Thu Jun 09 2022 ...Overview... As the medium range period begins Sunday, a tropical disturbance for which the National Hurricane Center shows a high chance of development should be east of the Florida Peninsula and pulling away from the U.S., but with some showers and storms possibly lingering over Florida and potentially up the Southeast coast depending on how close the low tracks to land. Meanwhile farther north, the lower 48 should see a fairly zonal pattern, but with shortwaves and surface frontal boundaries providing foci for thunderstorms and potential for heavy rainfall over much of the central U.S. and possibly into the East. ...Model Guidance Evaluation/Preferences... Model guidance generally agrees the tropical low will be centered east of Florida by Sunday morning, though its effects may be lingering. The 12Z CMC was a fast solution while the newer 00Z CMC has slowed down, even slower than other guidance, so the exact timing is still uncertain. The ECMWF has been persistently the deepest with the low, and thus shows more influence in the midlevels with a closed 500mb low at times. The EC has also indicated a slightly farther north track with the GFS runs farther south. A composite blend of the deterministic guidance seems to show a good compromise solution. Other than that feature, the pattern over the U.S. should be fairly zonal with shortwaves dominating the flow, spinning off from an upper low offshore British Columbia and another upper low initially in south-central Canada that will gradually lift northeastward through next week. Model guidance is reasonably agreeable with the overall pattern, and is even not too bad with individual shortwaves through midweek. Previous model cycles of the GFS in particular were showing some phasing of energy around the northern High Plains through Tuesday and beyond, but now the GFS/ECMWF/CMC are more agreeable with low amplitude ridging there Tuesday in between shortwave features. These shift eastward Wednesday-Thursday and the upstream shortwave does end up causing some differences in the depth of troughing across the east-central U.S. by Thursday, with the 12Z GFS among the strongest. Meanwhile models also show good consensus for southern tier ridging, with some typical timing and model variations with occasional closed highs. Thus a blend of the 12Z/18Z GFS runs and the 12Z ECMWF, CMC, and UKMET also worked well for the CONUS for the first half of the medium range period, swapping the UKMET for the ECMWF mean day 6 and adding in a small portion of the GEFS mean on day 7 as well. Overall a bigger percentage of operational models than normal could be maintained through the period with the reasonably good consensus. ...Sensible Weather/Threat Highlights... Though the bulk of the effects of the tropical disturbance will be in the short range period, some showers and storms could be present across Florida with lingering moisture and possibly reach the Southeast coast. Continued convergence across South Florida and the Keys could promote additional rain and thunderstorms there through at least the first half of next week, but scattered in nature. Meanwhile a weak to moderate atmospheric river this weekend aimed at the Northwest U.S. will increase precipitation chances there. This should be mainly rain as snow levels are forecast to be high, with chances for snow only in the highest peaks of the Cascades and northern Rockies. Parts of the central U.S. can likely expect the most impactful rain, however. Meandering frontal boundaries will provide areas of focus for enhanced rainfall with convection in a pattern with plenty of moisture. This looks to be most likely across northern/central parts of the Plains and into the Middle Mississippi Valley and possibly the Lower Ohio Valley. But unfortunately, models are quite variable with the placement of heavy rainfall beyond that, as the axes of enhanced rain will be highly dependent on where individual boundaries set up. The ingredients are certainly in place for heavy rainfall that could cause flash flooding, but the details of areas with the greatest threat could take into the short range to resolve, so held off on adding any Slight Risks to the experimental Day 4-5 Excessive Rainfall Outlook for now as they would almost certainly have to move around. Additionally, models have trended heavier with rainfall moving into the Northeast around Monday-Tuesday ahead of a cold front. Expect the general pattern for temperatures next week to be cooler than average in the northern tier and warmer than average in the southern tier. The northern Plains in particular could see highs 10-20F and lows around 10F below average with periods of troughing aloft. Meanwhile, much of the southern tier is forecast to see above normal temperatures by around 5-10F but reaching +15F anomalies in Texas for the first half of next week given an upper high aloft. The West Coast should be a bit cooler than normal over the weekend under the troughing influence, but moderating and warming to above normal in California and into the Great Basin and Southwest by around midweek. 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