Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Sep 17 2023 08:09:00 FOUS30 KWBC 170808 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 407 AM EDT Sun Sep 17 2023 Day 1 Valid 12Z Sun Sep 17 2023 - 12Z Mon Sep 18 2023 ....THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL IN EFFECT ACROSS PORTIONS OF THE MID-ATLANTIC, THE SOUTHWEST, AND NORTHEAST IL... ....Mid-Atlantic... A strengthening mid and upper trough traversing the Midwest will pivot eastward taking on a neutral tilt over the Ohio Valley with a more meridional flow in place along the east coast. Large scale ascent will be maximized from Georgia up into the Northeast U.S under the influence of an increasing jet max between 110-120 kts positioned over the Mid-Atlantic. Surface low development is forecast over GA later this morning, moving to the northeast off the Carolina coast the back-end of the period which will increase the expanse of the precip field over the western side of the surface reflection. A narrow corridor of instability will be positioned up the Blue Ridge and Appalachian front lending credence to convective impacts this morning and afternoon before the focus shifts further east within the western flank of our surface low. 00z HREF blended mean and bias-corrected ensemble both pinpoint an axis of 1-1.5" of precip across the terrain where FFG values are naturally lower and flash flooding concerns can crop up more easily. There has been some shift in the expanse of the heavy precip field with a wedge of higher QPF now positioned up through portions of eastern PA. This is likely in regards to a general expansion of higher PWATs advected northward ahead of the mean trough and co-located within a corridor of maximized ascent from the strengthening jet core. The one limitation of the setup that far north is the positioning of any appreciable instability as the main areas of MLCAPE exceeding even 500 J/kg are located along and south of the Mason Dixon line which is reflected within the deterministic QPF output from hi-res guidance. The highest totals within the blended mean and bias-corrected data sets are generally over the southeast and southern Mid-Atlantic where the instability presence increases flash flooding risks thanks to the convective nature of precip, as well as the focus for the western expansion of the precip field thanks to our developing surface low. HREF and global ensemble probabilistic signatures show the corridor from Savannah up through eastern NC as the best area to see 2-3" totals with local max around 4" across the coastal portions of the Carolinas where the surface low and convective concerns exist in a simultaneous fashion. As a result of all of the above, have expanded the MRGL risk for the region across all of the Mid-Atlantic up into eastern PA and the southern half of NJ with the western extent along the Appalachian front where convective concerns are greatest and FFG's remain within lower thresholds. ....Northeast Illinois into Northwest Indiana... A digging mid and upper trough axis is currently pivoting through the central Midwest with a surface low reflection positioned over southern WI and trailing cold front extending back into the central plains. A focused area of ascent is currently situated under a weak 850/700mb low reflection as indicated by the latest 02z RAP UA analysis and confirmed looking at the mid-level WV satellite imagery and radar composite. As the trough digs and pivots eastward, the low-level reflection around 850mb reaches its peak with most deterministic solutions closing off a weak 850mb and 700mb low just south of Chicago by the morning. As a result, increasing surface and low-level frontogenesis will be aligned from southwest to northeast across northeast IL into Lake Michigan which shows up very well within the RAP and 12km NAM forcing fields. Convergence signatures correlating with onshore flow off Lake Michigan and the aforementioned frontogen alignment will create a localized heavy precipitation field close to the lake shore with secondary banding like structures possible just away from the coastline within the greatest frontogen environment. This has lead to a sharp increase in 00z HREF precip fields within both the 3 and 6 hr windows between 12-18z today, dissipating as the best low-level forcing moves eastward under the advancing trough. After some collaboration with the local Chicago WFO, have agreed to place a targeted MRGL risk over the Chicago urban corridor and surrounding locales to account for the potential for local flash flooding threat generated from the small scale setup in place. Maximum QPF potential of up to 3" will be possible within any zone extending from the northern Chicago burbs down to Joliet and up into extreme northwest Indiana where some models bring locally heavy precip to areas just west of South Bend. These corridors are most susceptible to flooding due to sprawling urbanization factors and naturally low FFG thresholds.=20 ....Southeast Arizona, southern New Mexico, & Far West Texas... A weak impulse on the leading edge of an upper jet nosing in from the southwest will provide the necessary ascent over portions of the Southwest U.S later this afternoon and evening. Theta-E ridge aligned across southeast NM through the southern and central NM terrain into eastern AZ will provide a worthy boundary layer destabilization proxy for convective development within the zone of best upper forcing. A general 500-1250 J/kg of MUCAPE situated within the theta-E ridge and areal PWATs near 1 standard deviation above normal should provide the recipe for locally heavy rainfall and flash flood concerns given the convective nature of the precip within a highly terrain influenced area. Local burn scars and steep grade mountains within the Sacramento's and Colorado Plateau will only exacerbate the potential further. As a result, will maintain the previous MRGL risk issuance with little to no change to the general outlined area. Kleebauer Day 2 Valid 12Z Mon Sep 18 2023 - 12Z Tue Sep 19 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NEW ENGLAND... An increasing meridional component to the upper flow will continue on Monday as our strengthening upper trough continues its trek through the Northeast U.S. The increasing meridional flow pattern will advect a deep plume of tropical moisture northward along the Atlantic coastal plain into New England by the second half of Monday. Areal PWATs will shoot up to around 1-1.5 standard deviations above normal for a period of time before being swept eastward in wake of a cold front approach the end of the forecast period. Surface reflection along the Atlantic coast will continue to track to the northeast with deformation axis bisecting southern New England up through coastal ME. Typically a setup like this would not warrant a risk for flash flooding, or at least more widespread flash flooding due to forward speed and lack of convective nature of the precip field. However, recent rains over New England have created a much more favorable environment for local flood concerns, especially across eastern MA up through coastal ME where antecedent soil moisture is well above normal after a string of both convective and tropical impacts (Lee) within the past week. 00z HREF through the end of its run indicates a signal for locally heavy rains across southeast MA where low-level frontogenesis aligns within a zone of enhanced moisture on the western flank of our surface low. The result is multiple bands of heavy rain expected for places like Rhode Island up through western Boston and points southeast. 1-2" with locally higher amounts will be found across central and eastern MA up through ME as the surface low gains latitude and spreads its QPF footprint into northern New England, an area just now finishing up with impacts from Lee. The main change in this update is the western extent of the precip field trending drier as the progressive nature of the surface low and trough will kick the heaviest precip further east, leading to a slight adjustment in the MRGL risk. Kleebauer Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6EejrmOKyfVlmctCw8uTrqs3SNTEVh-i_VZwPz9_ikdB= QsGzjklhdGGIC9cFClswYpkwcfaZQnTpwX49xVSbS406Nnc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6EejrmOKyfVlmctCw8uTrqs3SNTEVh-i_VZwPz9_ikdB= QsGzjklhdGGIC9cFClswYpkwcfaZQnTpwX49xVSbbbVdqyQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6EejrmOKyfVlmctCw8uTrqs3SNTEVh-i_VZwPz9_ikdB= QsGzjklhdGGIC9cFClswYpkwcfaZQnTpwX49xVSbWLiSlGI$=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) .