Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 25 2023 00:50:21 FOUS30 KWBC 250050 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 849 PM EDT Thu Aug 24 2023 Day 1 Valid 01Z Fri Aug 25 2023 - 12Z Fri Aug 25 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF THE FOUR CORNERS REGION AND EASTERN GREAT LAKES/UPPER OHIO VALLEY.. ....Great Lakes and upper Ohio Valley to the Northeast... Northwesterly flow around the broad anti-cyclonic ridge axis over the central US will remain steady with prevailing steering pattern focused from Southeast WI through Lower MI, OH, and western PA/NY. Environment across the aforementioned area will be ripe with PWAT anomalies 1-3 standard deviations above normal and SBCAPE on the order of 3500-4500 J/kg allowing for sufficient buoyancy through the overnight. Given the antecedent conditions in place across northeast OH from activity last night and the risk for additional heavy rain, have maintained the Slight Risk from Detroit through central/eastern OH/western PA, and northern WV which is featured in recent HRRRs. Activity has developed across the lower LP and southeast WI with lake breezes allowing locally stationary activity. This will continue as a cold front continues to drift south overnight through northern IL/IN and western OH where the Marginal Risk was expanded through. Still expecting fairly progressive activity tonight over eastern PA through the northern Mid-Atlantic where the Marginal Risk is in place. ....Southwest US... Remnants from Harold are just north of the Four Corners continue to maneuver around the western flank of the broad ridge axis centered over the plains. Enhanced low-mid level moisture flux stemming from the remnants and seasonal monsoonal pattern over the Four Corners will continue to have increased ascent focused near the mid-level vorticity maxima. Given the topographic elements and recent rainfall, a Slight Risk continues across southern UT and western CO.=20=20 ....Central Appalachians down through the Upland Carolinas... Farther downstream of the ridge and downstream of the southern Appalachians has been a multi-cell cluster over west-central NC. This is picked up fairly well by recent HRRRs which have it progressing down stream through central SC overnight. The strong CAPE gradient (higher to the west into the ridge) and PW approaching 2" means localized flash flooding may continue through the overnight despite 3hr FFG generally around 4 inches. A Marginal Risk is added through this area. ....North-Central Great Plains... The top side of the ridge features westerly flow and activity that has developed over northern Neb will continue progressing east overnight as seemingly indicated well by recent HRRRs. Adding a Marginal Risk along the SD/Neb border then east to the center of the IA/MN border seems reasonable given a strong CAPE gradient into the ridge to the south that is parallel to the Wly mean flow and the frontal boundary just to the south. Repeating heavy thunderstorms are possible which may produce enough rainfall to top the 2.5" 3hr FFG which is common over the new Marginal Risk area. Jackson Day 2 Valid 12Z Fri Aug 25 2023 - 12Z Sat Aug 26 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS THE CENTRAL HIGH PLAINS/ROCKIES.. 2000 UTC Update -- Few changes made to the Day 2 ERO, based on the latest guidance/trends. For the Slight Risk across portions of the central Rockies into the Front Ranges and High Plains, brought the area slightly southward based on the non-NAM guidance trends (especially the CAMs, but even the 12Z ECMWF came a bit farther south with the heavy rainfall footprint). Within the Slight Risk area are the 12Z HREF's highest probabilities of the 24hr QPF exceeding 3" (40-60%). The HREF also shows 1+ inch/hr rate probabilities peaking around 40-50% after 19Z over northern portions of the Slight (NE CO/SE WY), with 60-80% probs over SE CO given greater available instability (1000-1500 J/Kg surface-based CAPE per the HREF). Hurley Previous discussion below... ....Southwest to the central High Plains.... Mid-level ridging across the plains will continue aid in tracking remnants of Harold as it moves back eastward across the northwest flank of the ridge axis centered over the plains. Increasing mid-level forcing aloft over northern CO and southern WY will be a key reason for widespread precip initiation over the front range on Friday afternoon into Saturday. At the surface, sfc cyclogenesis over the lee of the Rockies will generate an easterly, upslope component across the front range in CO with precip enhancement located along the higher terrain. PW anomalies on the order of 2-4 standard deviations above normal place a favorable environment for heavy rainfall within any convective development. A standard 1-2" with locally higher amounts in the Foothills will be common during the D2 period enhancing localized flooding potential. Further to the east along the CO high plains and along the KS/NE border, area convergence along a cold front stemming from high pressure building south over the northern plains in tandem with the surface low moving northeast will create a ribbon of precip enhancement Friday evening into Saturday morning. Given the prevailing mid-level flow pattern, energy from Harold is forecast to eject eastward across the above area prompting a line of precipitation to develop and slowly press southward. Local precip maxima from deterministic models and signals at the tail end of the 00z HREF EAS probabilities show a potential for locally heavy rainfall with enhanced rates of up to 1"/hr and totals near 1-2". Given the signature, have expanded the SLGT into the aforementioned areas with highlight over northwest KS and southern NE. ....Northeast and Mid Atlantic out to Kentucky... Area low pressure will track to the northeast from the Great Lakes area on Friday with increased mid-level difluence out ahead of a stronger shortwave trough digging eastward out of Ontario. First round of rainfall will stem from area convection moving northeast out of the northern Mid Atlantic, eventually exiting off southern New England. A second round of convection will develop near a warm front pushing northward along the coast with a trailing cold front pressing east along the base of the surface low. General instability axis will preside within the warm sector from the Mid Atlantic up into New England with PW anomalies a solid 1-2 standard deviations above normal, especially across New England where the heavy precipitation signature is most prominent. Convective initiation will occur up and down the east coast Friday afternoon and press eastward through the evening hours. Any thunderstorm will be capable of producing locally heavy rainfall with rates up to 1"/hr. Southern New England up the Maine coast will be the area of interest for any potential upgrades in future forecasts.=20 A secondary area of focus back into eastern KY will transpire with diurnally driven convective initiation in the afternoon within a deep, moist environment in place. PW between 1.75-2" will be common in a corridor from Louisville and points east. Given the terrain influences and expected higher rates from area convection, have expanded the MRGL to include the area outlined above. Kleebauer Day 3 Valid 12Z Sat Aug 26 2023 - 12Z Sun Aug 27 2023 ....A MARGINAL RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS THE SOUTHERN-CENTRAL ROCKIES AND CENTRAL PLAINS EASTWARD INTO PARTS OF THE LOWER MISSOURI AND MID MISSISSIPPI VALLEYS... 2000 UTC Update -- Based on the latest guidance/trends, including the amplifying mid-upper level trough across the Northeastern CONUS as the broad upper ridge gets impinged over the Central Plains-Mid MS Valley (while becoming centered over the southern Rockies), have removed a portion of the Marginal Risk area over Parts of the Mid MS Valley into the Lower OH Valley and northern TN Valley. Meanwhile, deterministic and probabilistic QPFs have come up across more of NM into eastern portions of AZ, and as such have expanded the Marginal Risk over that region. Lastly, have removed the Marginal Risk across the Lower Mid Atlantic Coastal Region. The 12Z guidance cam in much drier/lighter with the areal-average QPF, and while expect at least some scattered organization along/ahead of the advancing W-E surface front, possibly enhanced by sea breeze boundaries, the latest exceedance probabilities from the GEFS and ECENS would support a sub-5% (though still non-zero) 40km neighborhood probability of 3-6 hour QPF exceeding current FFG. Previous discussion below... ....Southern-Central Rockies, Central Plains, Lower Missouri Valley, Mid Mississippi Valley... Multiple shortwaves eject east and southeast from the Central Rockies across the plains with downstream propagation into the Mid MS Valley. Continued axis of well-above normal PW indices on the order of 1-2 standard deviations will be co-located within a corridor of higher theta-E's promoting a higher than normal potential for localized flooding within any convection development. Guidance is not keying on any particular region at this time, but the ingredients for locally heavy rainfall are present with deterministic guidance giving enough of a signature to multiple areas within the outlined risk to warrant a MRGL area. Hurley/Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5VxPtcu5N0BDYcdAJLV_4K4YQipBkW1-LTMwEa8oiOvK= byggvUtxE98VzvgZNByDE39yTLrfAln5ABirGkrY2OylRnc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5VxPtcu5N0BDYcdAJLV_4K4YQipBkW1-LTMwEa8oiOvK= byggvUtxE98VzvgZNByDE39yTLrfAln5ABirGkrYyiFH6p4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5VxPtcu5N0BDYcdAJLV_4K4YQipBkW1-LTMwEa8oiOvK= byggvUtxE98VzvgZNByDE39yTLrfAln5ABirGkrY6VT9m3M$=20 $$ =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .