Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Sat Apr 22 2023 Valid 12Z Tue Apr 25 2023 - 12Z Sat Apr 29 2023 ...Excessive rainfall threat to spread across The South Tuesday into Wednesday... 19Z Update: Most of the forecast is similar to the previous issuance from overnight, with the main changes being a small reduction to the overall rainfall expected across the Gulf Coast region and Southeast U.S. with the front likely being a bit farther south over the northern Gulf midweek. The Slight Risks areas on both Day 4 and Day 5 were maintained, but a little smaller in size to account for where the most concentrated model signal currently exists. Moist upslope flow across eastern Colorado in the wake of the cold front with 1-2 inch QPF in the form of a cold rain supports a small Marginal Risk area for excessive rainfall on Day 4, and heavy snow for the higher terrain of the Front Range. In addition, a Marginal Risk area was also added for Day 5 across south-central Texas for Day 5 that will be in close proximity to a dry line and a surface low dropping southward, focusing moisture inflow from the western Gulf of Mexico. In terms of the models, there was enough agreement through Wednesday to merit the use of a general model blend, and then increased use of the ensemble means to about 40% through the end of the forecast period while still keeping some previous WPC continuity. /Hamrick ------------------- ...Overview and Guidance/Predictability Assessment... Models and ensembles show a reasonably similar larger scale pattern evolution through medium range time scales, albeit highlighted by a tricky/transitional blocky flow focus from the northeastern CONUS through eastern Canada and the digging of a series of systems through the West that eject across the U.S. southern tier underneath the block. The WPC medium range product suite was primarily derived from best clustered guidance from the 18 UTC GFS/GEFS and 12 UTC ECMWF/UKMET/ECMWF ensemble mean along with the 01 UTC National Blend of Models. Predictability remains more suspect than normally would be the case as evident by differences with system timing/emphasis embedded within the southern stream flow and evolution aspects of a less defined than normal rex/omega style block. ...Weather/Hazards Highlights... Expect light rain chances will linger across the Northeast through early-mid next week. WPC forecast progs show system progressions/re-developments across the region into the Atlantic, but overall trends may prove slower given the complex/blocky nature. A main upper trough and surface frontal system is slated to dig slowly southeastward across the West Tuesday and spread cooled/unsettled conditions with a moderate precipitation/mountain snow focus through the north-central Rockies. Activity across mainly the central Rockies/High Plains may be locally enhanced by lower atmospheric upslope fetch and added moisture. Mid-late next week system ejections across the South/Southeast may have less certain lead impulses focus and emerging pattern favorable for moderate to heavy convective rainfall. Activity may be greatly enhanced across The South with multiple wavy/reforming fronts acting to focus Gulf of Mexico to Atlantic moisture and instability. The experimental WPC medium range Excessive Rainfall Outlook plans to show a "Slight Risk" area over the lower Mississippi Valley Tuesday and the Southeast Wednesday despite uncertainties with supporting waves. SPC suggests some potential for strong to severe thunderstorms. Midweek coastal low potential could bring heavier activity up through the Mid-Atlantic, but the deep solution of the 12 UTC Canadian seems overdone/too far north given blocking to the north. The latest 00 UTC Canadian has trended weaker and more offshore, but the 00 UTC UKMET now has a deepened low suggesting model sensitivity and lingering plausibility. Recent GFS/ECMWF runs and ensembles show greater likelihood for a more moderately developed low. Meanwhile upstream, the next main upper trough from the Gulf of Alaska and western Canada are then slated to dig east-southeastward across the Northwest and north-central U.S. mid-late next week. Run to run continuity is not stellar, but this general track would focus some light-moderate precipitation across the region in a pattern favoring development of much below normal temperatures with sharp southward frontal surge of cold Canadian air in the wake of system passage. Schichtel 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