####018008604#### FXUS02 KWBC 290700 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Mon Apr 29 2024 Valid 12Z Thu May 02 2024 - 12Z Mon May 06 2024 ...Widespread rain and thunderstorms are likely in the central U.S. through at least Thursday... ...Overview... As the medium range period begins Thursday, mean troughing amplifying atop the Rockies will provide support for widespread convection as moisture and instability spread into the central U.S. and cause heavy rain and flash flooding. Lifting upper troughing moving east by Friday should push rain chances into the east- central and eastern U.S., though some may remain over the south- central U.S. this weekend near a lingering frontal boundary. Additionally, rounds of precipitation including some higher elevation snow are possible at times across the Northwest into the northern/central Rockies, but confidence in the details is still lower than desired given ample spread in the model guidance. ...Guidance/Predictability Assessment... Troughing across the West and ridging across the East will be the main theme early in the medium range period, but even Thursday- Friday there are some model differences within this overall pattern. The core of the upper trough/low should be located in the northern Plains/south-central Canada, but models show discrepancies on the southward extent of the trough as it pivots eastward. GFS runs (particularly the 12Z) and the 12Z UKMET dug the trough deeper than the 12Z ECMWF/CMC. Now the newer 00Z GFS is not as deep and is more agreeable with the EC/CMC, but the 00Z UKMET stayed deep and seems to be more clearly an outlier. Meanwhile a smaller shortwave/compact upper low upstream shows some differences Thursday too as it comes into the Northwest, which may play a factor in the evolution of the main trough downstream. This smaller feature's clustering in position seems better in the incoming 00Z model guidance. The ridging in the East seems pretty agreeable in the guidance. By late week into the weekend, attention turns to an upper low dropping southeast from the northeastern Pacific toward the West Coast. Recent models are now somewhat more agreeable in showing a midsize and closed upper low, rather than a huge upper low or just troughing/energy, which were both possibilities shown by the models a day ago. However, the position still varies greatly among the deterministic models and ensemble members and to some extent the AI/machine learning (ML) models. By early Saturday recent GFS runs have been on the southwestern side of the spread to varying degrees depending on the run, which then leads to the upper low farther south into California as it translates east early next week. The deterministic runs are generally deeper than the individual ensemble members, limiting confidence in the operational runs. Meanwhile the 12Z and now 00Z ECMWF do not dig the low as far south, taking it across the Northwest over the weekend and quickly into the northern Plains. The 12Z ML models were generally in between these southwest and northeast extremes with the feature. The 12Z CMC seemed like a more reasonable proxy for the low position as it was in between the two, somewhat like the ML models. There are also questions into early next week whether or not the feature stays separate or gets combined with yet more energy and troughing coming in from the Pacific. In all, the confidence for this feature is low and changes to the forecast are likely. The WPC forecast was based on a blend favoring the deterministic models early in the period, but quickly ramped up the proportion of ensemble means given low confidence in the deterministics. A split between the EC ensemble mean (which was more like its deterministic run in taking troughing east across the northern tier) and the GEFS ensemble mean (with troughing pulled more offshore) seemed to provide a reasonable middle ground. ...Weather/Hazards Highlights... On Thursday, ingredients for widespread rain and thunderstorms potentially causing flash flooding will be in place across central parts of the country. Lowering heights as upper troughing digs in the Rockies will provide lift and combine with ample moisture and instability. On Thursday model guidance already shows heavy rain totals of 3-5 inches in some areas and high rain rates are likely as well. A large Slight Risk remains in place for the Day 4/Thursday ERO for eastern parts of the Plains into much of the Mississippi Valley. Within this risk area, there may be somewhat greater potential around northeast Texas to eastern Oklahoma and western Arkansas--and this general area will likely be sensitive to additional rain due to wet antecedent conditions. There is potential for embedded upgrades within the Slight Risk depending on details of prior convection and how guidance consolidates for amounts/location of heaviest rainfall. The main change to the previous Slight Risk was to expand it northward into Wisconsin. There, morning rainfall is expected to be widespread and serve to wet the ground before a more unstable airmass comes in as a warm front lifts through the area for thunderstorms with possibly higher rain rates during the evening. By Friday, the cold front progressing east in the northern two-thirds of the country or so will transition rain chances more into the Great Lakes and Ohio/Tennessee Valleys, and toward the Appalachians to Eastern Seaboard on Saturday. However, the frontal boundary looks to linger longer or possibly lift across the the southern Plains and produce additional rounds of rain and storms through the weekend. A Marginal Risk is in place Friday across portions of the lower Mississippi Valley where there may be some heavier rainfall focus. Meanwhile, areas from the Northwest into the northern and central Rockies may see rounds of precipitation including snow in higher elevations through the latter part of the week into the weekend. On Thursday, the cooling associated with the upper trough could bring some early May snow into the northern and central Rockies. There is some chance that snow may make it into the Front Range of Colorado on Thursday. There is still quite a bit of model spread into late week and the weekend, which affects the precipitation forecast. Currently the best guidance cluster shows another round of precipitation in the northern Rockies Friday with a compact upper system crossing the area, while an upstream Pacific system renews precipitation of varying intensity across the Northwest from late Friday or Saturday through Sunday. The eastern and southern extent of the precipitation is uncertain and will be refined with time. The eastern third of the country will be warmer than average by 10-15 degrees through late week, with 80s reaching the Ohio Valley and Mid-Atlantic. Meanwhile cooler than average temperatures are forecast across the Rockies and parts of the Plains. These cooler temperatures should push east behind a cold front and help moderate temperatures a bit in the East, though could remain a few degrees above normal. Temperatures across the Rockies to Plains are forecast to warm up into early next week due to upper ridging ahead of the Pacific trough. The West Coast may become cooler than normal this weekend, but this will depend on how far east/onshore the Pacific trough reaches. 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 $$