Extended Forecast Discussion NWS Weather Prediction Center College Park MD 355 PM EDT Mon May 16 2022 Valid 12Z Thu May 19 2022 - 12Z Mon May 23 2022 ...Persistent record-breaking heat is likely in the south-central U.S. through this week with anomalous heat expanding into the East this weekend... ...Heavy rainfall and flash flooding possible near a strong northern tier system on Thursday... ...Overview... A digging trough over the West and rising heights across the East late this week will result in a more amplified pattern across the lower 48, yielding cooler temperatures and lowering snow levels in the West and a continued hot regime for the South and East. A closed low initially along the U.S.-Canadian border on Thursday could open up soon thereafter but either way a deepening low pressure system tracking from the northern tier/Upper Great Lakes into Canada will push a strong cold front eastward, bringing chances for heavy rainfall and severe weather to some areas from the eastern Plains/Mississippi Valley into the East. The upper trough will reach into the central and eastern U.S. during the weekend into the first part of next week (possibly becoming a little more shallow with time) while guidance has become indecisive with details upstream. A northward surge of tropical moisture will also increase rain/thunderstorm chances across Florida late this week and into the weekend. ...Guidance Evaluation/Preferences... Even in the first half of the period, guidance exhibits some noticeable spread and continuity changes for the initial upper low along the Canadian border and upper trough extending through the West. In general the 00Z/06Z model runs have trended toward an earlier opening of the upper low and thus far the 12Z runs have followed through on this change. Meanwhile recent GFS runs have been on the amplified side of the spread for the overall trough. The 06Z GFS was particularly strong with some of its trailing energy, ultimately leading to a slower trough than the majority of guidance and even a closed low over the Upper Mississippi Valley by early day 6 Sunday. The 00Z GFS was closer to consensus and the new 12Z run has returned to a more moderate feature. From the weekend into early next week the most prominent source of uncertainty becomes how quickly upper low energy and surrounding flow escape from the North Pacific (part of a longer term blocky regime over the Pacific and Bering Sea/Alaska). The 00Z ECMWF deviated significant from its prior run and most other guidance in being rather fast to bring the North Pacific energy into western North America. On the opposite side of the spectrum, GFS runs have tended to build the strongest upper ridge just off the West Coast by Sunday-Monday to yield more Rockies/High Plains troughing by next Monday. The 12Z GFS has tempered its late-period ridge-trough pattern a little bit. Diverse ensemble members average out to a modest ridge-trough pattern in the GEFS/CMCens while the ECens mean is nearly zonal. How the details from the Pacific into the West work out will eventually affect the forecast of the trough moving into the East, with the majority of guidance at least suggesting the eastern trough will become less amplified than seen in the 00Z ECMWF (with corresponding effects on surface frontal position). The new 12Z ECMWF has trended closer to what consensus exits for most aspects of the late-period pattern. Based on the review of 00Z/06Z guidance, the updated forecast started with the 00Z GFS/ECMWF/UKMET/CMC early in the period to represent latest trends for the northern tier system and to avoid the less confident aspects of the 06Z GFS. Question marks in both the GFS and ECMWF during the latter half of the period led to a rapid transition to 50-70 percent total weight of the 06Z GEFS/00Z ECMWF means, yielding a solution as close to continuity as could be hoped in light of the spread and variability of models/ensemble members. ...Weather/Threat Highlights... Initial upper ridging across the southern Plains and rising heights farther east over the Deep South will continue to promote a broad area of heat into Saturday. The greatest and most lasting anomalies should be over Texas (persisting from the short range time frame) where some locations could see highs up to 15-25F above normal, with numerous daily record highs likely. The heat across Texas should finally begin to moderate from north to south by next weekend an upper trough pressing through the West supports an arriving cold front. However, warm to hot temperatures will extend into the southeastern quadrant of the U.S. with potential for daily record highs, and warmer than average temperatures by 10-20F or so should expand in coverage across the Ohio Valley to Eastern Seaboard by later in the week. A warm front lifting through the Ohio Valley/Appalachians on Thursday will continue to provide a focus for showers and storms on Thursday, with a very localized flood threat particularly if heavy rainfall falls in areas that are already sensitive. Meanwhile, moderate to locally heavy rainfall is likely on Thursday in association with a deepening northern tier storm system that should lift away from the Upper Great Lakes on Friday. The system's evolution is somewhat complex and guidance is still adjusting with some specifics. Currently the best potential for flooding/flash flooding exists over parts of North Dakota and northern Minnesota where antecedent conditions are the wettest. A separate band of enhanced rainfall may exist farther south/southeast where greater instability exists. By Friday-Saturday, above average moisture and instability will allow for rain and thunderstorms to spread along the frontal boundary as it pushes through the Great Lakes/Ohio Valley/Mississippi Valley and then into the East. Locally heavy rainfall is possible but additional time is needed to resolve areas of greatest potential. The Storm Prediction Center highlights a threat for severe weather over some of the same areas with heavy rainfall potential along and ahead of the cold front Thursday and Friday. Behind this front, unseasonably cool temperatures will slide across the Northwest/northern Rockies on Thursday into the north-central U.S. Friday and Saturday, with highs generally 10-20F below normal. Parts of the northern-central High Plains could be colder than that on Friday. This cold spell should lower snow levels and possibly allow for some light snow to fall even in northern parts of the High Plains. The Northwest will be fairly dry late this week but may see some moisture return during the weekend or early next week, with uncertain flow details keeping confidence low for specifics of timing/coverage/amounts of any precipitation. The southern half of the West should see a warming trend expand from west to east Saturday-Monday with California seeing the warmest high temperature anomalies of plus 10-15F. Elsewhere, guidance agrees that tropical moisture should surge northward across the Florida peninsula late this week into the weekend, bringing a round of heavy rainfall to portions of the Sunshine State. Rausch/Santorelli 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 Hazards: - Heavy rain across portions of the western Great Lakes, the Upper Mississippi Valley, and the Northern Plains, Thu-Fri, May 19-May 20. - Severe weather across portions of the Corn Belt and Upper Midwest, Thu, May 19. - Severe weather across portions of the Central/Southern Plains, Mississippi Valley, into the Great Lakes, Fri, May 20. - Severe weather across portions of the northern New York, interior Mid-Atlantic, upper Ohio Valley, and the Southern Appalachians, Sat, May 21. - Flooding occurring or imminent across portions of the Lower Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains. - Flooding likely across portions of the Upper Mississippi Valley and the Northern Plains. - Flooding possible across portions of Interior Alaska. - Excessive heat across portions of central and southern Texas and the Lower Mississippi Valley, Fri-Sat, May 20-May 21. - High winds across portions of the Northern and Central Rockies, Thu, May 19. - Much below normal temperatures across portions of the Northern and Central Rockies into the Central and Northern Plains, Fri-Sat, May 20-May 21. - Enhanced wildfire risk across portions of the Southern Rockies, the Central Rockies, the Southern Plains, the Central Plains, and the Southwest, Thu, May 19. 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