####018008629#### FXUS02 KWBC 261859 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Fri Apr 26 2024 Valid 12Z Mon Apr 29 2024 - 12Z Fri May 03 2024 ...Potentially severe thunderstorms and heavy rain causing flash flooding are forecast to continue in the Lower Mississippi Valley Monday... ...Overview... Upper troughing will support a surface frontal system with widespread thunderstorms ahead of it across the central U.S. into Monday. Heavy rain is likely to cause flash flooding in the Lower Mississippi Valley in particular, while severe weather may also be a threat. The upper pattern looks to become more zonal after the early part of the week, with increasing uncertainty in shortwave/embedded upper low details and surface specifics from the Pacific eastward. For the Lower 48 there is at least a general theme of a leading system followed by another frontal system affecting the central-eastern U.S. by the latter half of the week, with both serving to focus areas of showers and thunderstorms. There is still a lot of uncertainty over the Northwest by late next week though. Near to above normal temperatures should prevail over much of the East into at least the southern half of the Plains through Intermountain West, while the Northwest and possibly other northern tier areas may see one or more days of below normal highs. ...Guidance/Predictability Assessment... Recent variability/trending and spread in the guidance leads to below average confidence in some important aspects of the forecast next week. Through the 00Z/06Z cycles, there were essentially two primary layers of uncertainty. The first involves the trough/upper low(s) over the Northwest U.S. and British Columbia as of early Monday. Over the past day or so the GFS/ECMWF and their ensemble means have trended toward greater progression of this feature whose energy could ultimately split into two pronounced upper troughs and/or lows. In contrast, the 00Z UKMET/CMC-CMCens clustered with what had been the prior consensus of holding the initial feature back near the Pacific coast while awaiting input from upstream Pacific energy. 00Z ECMWF-initialized machine learning models (MLs) agreed upon the more progressive theme, thus recommending exclusion of the 00Z UKMET/CMC from the forecast. However the MLs suggested that the upper energy could be more consolidated into one upper low (located near the northern High Plains as of early Wednesday) versus what the GFS/ECMWF were showing. Beyond this discrepancy, guidance diverged even more with respect to Pacific/western U.S. flow later in the week. ECMWF runs and the 00Z GFS along with the GEFS/ECens means brought a trough/possible compact upper low into the Northwest (with a lot of spread among individual ensemble members), but a slight majority of the MLs actually built a ridge over the West Coast by late week due to greater troughing over the Pacific near 140-150W. The 06Z GFS trended that way too, deflecting the late week upper low farther north into Canada. In light of guidance differences, ML suggestions, and continuity, the updated forecast used a blend of 00Z and 12Z/25 ECMWF runs and the 00Z/06Z GFS early in the period followed by a 40-50 percent weight of the 06Z GEFS/00Z ECens means with lingering weight of those GFS/ECMWF runs mid-late period. This solution yielded only minor nudges from continuity in this lower predictability regime. In the new 12Z guidance, the UKMET/CMC have trended significantly toward other dynamical guidance for progression of the initial Northwest U.S./British Columbia feature. This still leaves a difference between the dynamical solutions and more consolidated MLs. The CMC/UKMET also favor higher heights along the West Coast by late week while the GEFS mean still brings in a modest trough. ...Weather/Hazards Highlights... Ample moisture and instability will be present across much of the central U.S. into Monday ahead of a Plains upper trough and associated surface frontal system. The most likely focus for the heaviest convection and rainfall is across the Lower Mississippi Valley, where a Slight Risk is maintained in the Monday/Day 4 Excessive Rainfall Outlook. This may be a higher-end Slight Risk if the heaviest model QPFs of 3-5 inches with rain rates over 2 inches per hour end up verifying. By Tuesday the upper-level support will likely weaken, so overall expect lower rainfall amounts and rain rates. However, some additional convection could develop within an area of persistent above-climatology deep moisture, which could lead to flash flooding concerns especially if storms develop over areas that saw excessive rainfall amounts the previous day. A Marginal Risk remains a reasonable starting point to cover this threat on Tuesday/Day 5. Meanwhile, the models are starting to show more similar ideas for flow aloft to the north, leading to a front stalling over the Plains (with moisture increasing especially by Tuesday night) while awaiting upstream troughing/surface waviness. This has led to an increasing signal for locally heavy convective potential, currently with the best focus over or near eastern Kansas into western/northern Missouri. Thus the updated Day 5 ERO has introduced a Marginal Risk area over this region. There continues to be potential for locally heavy rainfall over other parts of the Plains/Mississippi Valley given recent behavior of guidance. By midweek and beyond, additional rounds of rain and thunderstorms are likely for the central/southern Plains and Mid/Lower Mississippi Valley (perhaps extending into nearby areas) as moisture surges ahead of a wavy frontal system. Elsewhere, one or more shortwaves/upper lows will lead to rounds of precipitation across the northern tier. In the Northwest, some higher elevation snow is possible with this activity, with the most widespread coverage on Monday before snow levels may rise even further. Mainly rain is expected farther east across the northern Plains to Midwest/Great Lakes/Ohio Valley and into the Northeast/Mid-Atlantic at times. Guidance is still in the process of trending, but suggests the potential for a period of enhanced northern Rockies/Plains precipitation that will be worth monitoring. At the same time, these trends are indicating less precipitation than previously forecast over the Pacific Northwest by late in the week. With the exception of the cooler West Coast and Northwest especially Monday-Tuesday, the bulk of the lower 48 will have near to warmer than average temperatures next week. An upper ridge over the East Monday-Tuesday should help produce temperatures around 10-20F above normal, with temperatures in the 80s possibly as far north as the Great Lakes region. Scattered daily record highs and perhaps more numerous record warm lows are possible. Temperatures are likely to gradually moderate but stay above average in the East until late next week, depending on frontal progression. Meanwhile, parts of the central U.S. will also see above normal temperatures with anomalies around +10F Monday-Wednesday before cooling. The Intermountain West to Rockies on the other hand should gradually warm and see its greatest warm anomalies Thursday-Friday. However note that recent guidance spread and variability yield lower than average confidence in the temperature forecast over the West, especially by the latter half of next week. Rausch/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 forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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 https://www.wpc.ncep.noaa.gov/#page=ovw $$