Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 24 2023 16:19:42 FOUS30 KWBC 241619 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1219 PM EDT Thu Aug 24 2023 Day 1 Valid 16Z Thu Aug 24 2023 - 12Z Fri Aug 25 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF THE FOUR CORNERS REGION AND NORTHEAST/OHIO VALLEY.. 1600 UTC Update -- A rather noteworthy adjustment to the Slight Risk area across the OH Valley/Northeast, based on several factors. Per collab with WFOs, have removed the Slight Risk area across central-eastern PA and all but far SW NY. Overnight/early morning MCS went tracked N-S farther west than previous HREF or NBM means would suggest -- aligning more so with the few CAMs (such as the ARW2) that were on the western edges of what has continued to be fairly significant guidance spread for a D1 forecast. Current (12Z) guidance continues to indicate quite a bit of spread in the QPFs, though with the shortwave trough departing east of the Canadian Maritimes, the mid/upper flow pattern in it's wake would be more conducive to a more eastern component in the next MCS track, or at least a split in the activity with a separate eastern cluster of convection pressing farther east across PA overnight and early Fri. However, feel this activity will be moving fairly swiftly, which combined with the relatively drier soils, believe a Marginal Risk is better suited across central-eastern PA. Therefore oriented the Slight Risk to align with areas that have bit hit by recent MCS activity (i.e. much wetter antecedent soils/higher soil moisture percentiles/lower FFGs), along with highest 1/3/6 hour QPF>FFG exceedance probabilities per the latest (12Z) HREF. Hurley Previous discussion below.. ....Lower 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 the western Great Lakes down through Lower MI into northeast OH into PA. 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 into the afternoon and evening hrs with expected initiation upstream over Central MI, pressing east and southeast along the general steering flow. 00z HREF mean 24 hr qpf signals a widespread area of heavy precip with 1HR HREF probs indicating a heightened risk of 1+" per hour rainfall rates with anything that does develop, as well as precip totals on the order of 1-2+" through the period. Convection will eventually grow upscale across northwest PA and northeast OH with forward propagation into PA and the southwest portion of NY state. There is some discrepancy in short range guidance on where the eventual complex will motion, but ensemble mean QPF orientation and general steering pattern would lend a greater signal for any complex to be positioned over the south shore of Erie down to northern WV on the southern extent and all points east thru PA. Given the antecedent conditions in place across northeast OH from this evening's convection and guidance pinning more convective precip for later today, have extended the SLGT further to the west across northern OH as the pattern favors another chance for flooding over areas that were impacting this past evening, especially towards Cleveland and surrounding suburbs. ....Southwest US... Remnants from Harold will 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 storm's remnants and seasonal monsoonal pattern will spread northward into the Four Corners with increased ascent focused near the mid-level vorticity maxima associated with what's left of the disturbance. Given the topographic elements and lower 1 HR FFG from all the recent rainfall, a SLGT risk continues across southern UT and western CO. =20 Kleebauer 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.. ....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. ....Mid Atlantic and Northeast 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 CENTRAL HIGH PLAINS/ROCKIES, TENNESSEE VALLEY, AND THE CAROLINAS.. ....Central Rockies across the Central Plains and Tennessee Valley.... Multiple shortwaves eject east and southeast from the Central Rockies across the plains with downstream propagation into the Tennessee 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 determinisitc guidance giving enough of a signature to multiple areas within the outlined risk to warrant the MRGL expansion. ....Carolinas... A stalled frontal boundary is forecast along the Carolina coast with focal area extending from Columbia, SC to the northeast up into northeastern NC. A ribbon of higher instability on the order of 2500-3000 J/kg will be focused along and east of the stalled front with deep, moist convective initiation expected during peak diurnal instability. Rainfall rates in excess of 1"/hr will be possible within the corridor outlined, along with slower moving thunderstorms that would allow for local QPF maxima.=20 Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9FHQkTjuzUyMKLL5w9bY_VvESlP6hqVEEURq3gbpAyZh= XrhliL0VBt-ZMYgN8asZdJ84bPGnSMdGQpT70GUFGF6bhv4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9FHQkTjuzUyMKLL5w9bY_VvESlP6hqVEEURq3gbpAyZh= XrhliL0VBt-ZMYgN8asZdJ84bPGnSMdGQpT70GUFFtR_UpM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9FHQkTjuzUyMKLL5w9bY_VvESlP6hqVEEURq3gbpAyZh= XrhliL0VBt-ZMYgN8asZdJ84bPGnSMdGQpT70GUFmkNzR_4$=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. 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