Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Sep 12 2023 00:14:26 FOUS30 KWBC 120014 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 813 PM EDT Mon Sep 11 2023 Day 1 Valid 01Z Tue Sep 12 2023 - 12Z Tue Sep 12 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT OVER PARTS OF THE NORTHEAST AND WEST-CENTRAL TEXAS ... ....01z Update... Changes to the inherited outlook were fairly minimal for this update, focusing mostly on the Northeast where locally significant to catastrophic flash flooding was ongoing at the time of writing (specifically in the vicinity of Springfield and Leomister, MA). Heavy rainfall in association with a quasi-stationary frontal boundary has likely already peaked, as SB CAPE has decreased by 400-1000 J/kg over the past few hours. Even still, locally significant flash flooding may continue into the overnight hours across eastern MA and into RI and surroundings, where the Slight Risk was maintained. The 12z HREF 40-km neighborhood probabilities for 3" and 5" exceedance are maximized across this region, ranging from 20-50% and 5-10%, respectively. The Slight Risk is considered to be higher-end across this area, whereas lower-end Slight Risk probabilities extend northeastward into coastal ME and southwestward into NYC. The surrounding Marginal Risk was also maintained, but trimmed based on the observational trends. Elsewhere, no changes were necessary for the Slight Risk across west-central TX (as the event is only just beginning to unfold), and the surrounding Marginal Risk was adjusted based on observational trends (trimming much of the northern and central extent). The Marginal across GA/FL was also trimmed based on trends, whereas the Marginal Risk across portions of the Midwest was able to be removed with long-duration additional totals of only an inch or so expected. Churchill ....16Z Update... In coordination with the Gray/Portland WFO, have expanded the Slight Risk across southern ME within the I-95 corridor. Latest 12z HREF neighborhood probabilities for 2"/hr or greater, as well as 24 hr ARI exceedance probabilities have increased into the 15-25% range, enough of a signature within the means to justify the upgrade. Have also adjusted the southern edge of the Slight over NNJ to align with the latest HREF blended mean and 2"/hr neighborhood probability at 15% or greater. This area has also been the target over the weekend for locally heavy rainfall with multiple FFW's issued. MLCAPE between 750-1500 J/kg will be found across NYC up through southern New England. The area of focus is leaning towards RI up through southern MA where the stalled frontal boundary bisects the area. This will be a focal point for convective initiation and training potential as storm motions lie parallel to the boundary. QPF maximum across all hi-res deterministic lies within that zone as well, so the trends have been consistent on that zone having the highest potential impact, outside the urban centers. The other area of focus remains over the southern plains where a round of nocturnal convection laid the framework for what will transpire later this afternoon and evening. A robust signal for more widespread heavy rain over west-central TX continues with impressive neighborhood probabilities for 1-2"/hr rainfall across portions of the eastern Permian Basin down into the Concho Valley. HREF blended mean QPF is now over 2" for a large portion of the area within the Slight Risk, even a touch further south along the lower Trans Pecos within the bounds of eastern Pecos and Crockett counties within MAF/SJT's respective CWA's. Consistent placement of the low-level theta-e ridge extending from the Edwards Plateau up through the Caprock aligns well with the forecast max QPF swath as storm motions will track right along and just to the end of that pronounced theta-e gradient. Only minor adjustments were necessary on the southern edge of the SLGT to attest for the latest HREF trends. Not much changes was necessary across the remainder of the D1 ERO coverage. Low-end MRGLs in place across the Midwest and FL where locally heavy rainfall is possible in each region, although rates will be the driver in FL while antecedent conditions and higher urban flash flood threat is the driver for the Midwest. Pretty consistent in successive runs for rainfall rates between 2-3"/hr during the peak of convective impact across FL. The main area of interest continues to be just south of Jacksonville along the I-95 corridor. The threat will last 3-6 hrs at most with convection weakening after sunset. Shortwave across the Midwest will continue to track eastward with the back-edge of the rainfall moving into IL currently. Haven't been too impressed with the rates over the affected area, but there's still some non-zero signatures for locally heavy rainfall between Chicago up through central MI, thus kept continuity and can reassess in later updates. Kleebauer ....Previous Discussion... ....Southern Plains into the Southern-Central Rockies... A sharp cold front moving through the southern plains will intersect a developing LLJ and stout theta-e ridge extending up into the TX Caprock. The CAM guidance continues to be in agreement on the initiation of a cluster of thunderstorms across portions of west TX, propagating southeast around the eastern fringes of a mid-level ridge centered over the southwestern U.S. The convergence of the frontal boundary and axis of instability created from the LLJ will lead to an uptick in areal convective coverage with enhancement from a mid-level vorticity maxima sliding southeast out of NM. 00Z HREF guidance continues to show a strong signal for locally heavy rainfall across portions of the Rolling Plains of TX into the Concho Valley and northwest portion of the Hill Country. 00Z HREF neighborhood probabilities 2-3"/3 hrs peak between 60-70% within the Slight Risk area tonight (between 03-09Z). The heavy rain axis being depicted aligns well with a 1-1.5 standard deviation above normal PWAT anomaly that is likely the result of the convergence signature along the cold front pressing south within the LLJ axis being forecasted across all guidance, including global deterministic. Per collaboration with WFOs LUB, MAF, and SJT, have expanded the Slight Risk area from yesterday's Day 2 ERO to encompass more of the Rolling Plains and Permian Basin, again given the guidance trends. Deterministic QPFs continue to indicate a heavy rainfall footprint in this area, perhaps with a little less spread compared to previous runs. The CAMs show the potential for localized totals between 3-6". ....Parts of the Northern Mid Atlantic into New England.... SSW to SW flow within the low-mid layers persists today as shortwave energy pivoting into the Upper Midwest amplifies the longwave trough. This will keep the surface boundary relatively stationary along the Mid Atlantic-Northeastern Seaboard. While a gradual reduction of TPW is expected during the period, sufficient daytime heating should boost mixed-layer CAPEs into at least the 1000-1500 J/Kg range, which given the favorable thermodynamic profile, should allow for more numerous showers/tstms to fire along the surface front during the peak heating hours and continue into at least early evening. Latest (00Z) HREF neighborhood probabilities of at least 1-2"/hr rainfall rates peak between 60-80% between 20-03Z. Given the recent heavy rainfall over the past 24-72hrs, per collaboration with WFOs OKX, PHI, ALY, and GYX have hoisted a Slight Risk area, coinciding with the uptick in deterministic and probabilistic QPFs (especially among the CAMs). This aligns with the lower FFG, and higher 0-40cm depth soil moisture percentiles from NASA SPoRT (80-90+ percentiles). Within the Slight Risk area, the latest HREF probabilities of 3 hourly QPF exceeding 3hr FFGs tops out between 40-60% between 21-03Z. CSU UFVS verified first guess fields also support a Slight Risk within this region. ....Midwest... Strong forcing from a digging longwave trough over MN/WI will approach lower MI on Monday. The northeastern-most extension of the southern Plains LLJ will move into lower MI in advance of a cold front associated with the upper level trough. The LLJ will advect some moisture into MI, which will be lifted by the approach of the trough. The guidance shows an extensive area of light to moderate rain with embedded convective elements will move over Lower MI. Isolated flash flooding is possible, particularly in any urban areas and other low FFG portions of lower MI as a result. A small portion of northwestern Lower MI near Traverse City was hit hard by recent rainfall, and the additional rain expected Monday may allow for additional flash flooding, so the Marginal Risk continues to incorporate this area. ....Florida and Southeast Georgia... Weak upper level forcing on the tail end of the upstream mid-upper trough will provide sufficient deep layer bulk shear (~25kts between 0-6km) for more widespread/organized, clustered convection during the peak heating hours this afternoon, then continuing into the evening. The activity will no doubt be aided by the convergence of Atlantic/Gulf breezes. 00Z HREF probabilities of >2"/hr rainfall rates peak between 40-60% from 20-03Z across (mainly) interior portions of the northern-central FL peninsula. Anticipate isolated or localized instances of flash or urban flooding with these slow-moving cells before quickly becoming downdraft-dominated and weakening. Therefore, have included a Marginal Risk in the Day 1 ERO. Hurley/Kleebauer Day 2 Valid 12Z Tue Sep 12 2023 - 12Z Wed Sep 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF NEW MEXICO, EASTERN AND CENTRAL PORTIONS OF ARIZONA, AND SOUTHWEST COLORADO... ....2030z Update... After coordination with the Grand Junction WFO, the main update to the D2 period was an extension of the Slight Risk into southwest CO given the latest 12z CAMs and hi-res ensemble trends for heavy rainfall focused across the terrain in the southern portion of Grand Junction's CWA bounds. There was a distinct uptick in the neighborhood probabilities for 1-2"/hr or greater across the mountains north of Cortez/Durango in CO which lies within a lower FFG footprint given the steep terrain influences. Global deterministic output still pinpoints an axis of well-above normal PWATs, 1-2 standard deviations above climatological norms within the confines of the SLGT risk area pointing to a recipe for heavier rainfall over the inter-mountain west. Large scale ascent will be maximized across the Southwest U.S over the course of the afternoon and evening hours putting the timeframe within a general maximum for diurnal instability. With the latest ensemble means and deterministic outputs conveying a sharp increase in convective potential and associated QPF over northern NM, AZ, and southern CO, will keep continuity on the remaining bounds of the inherited SLGT and only provide the small extension into CO. The Mid-Atlantic and Northeast continues to be a late-afternoon and evening flash flood risk as large scale ascent within an increasing area of mid-level difluence will initiate a corridor of convective development along and east of the Appalachian front with greatest coverage focused over MD into eastern PA. Storm motions will be relatively quick which will limit the ability for a higher risk, however the antecedent conditions stemming from the last several days of rainfall has primed the region encompassed creating a higher than normal threat given the synoptic setup. Maintained the size and orientation of the MRGL as the outlined area remains the best probable location for elevated flooding concerns given the latest 12z HREF blended mean and bias-corrected ensemble guidance for QPF. Kleebauer ....Four Corners Region and Southwest... In coordination with ABQ, EPZ, FGZ, PSR, AND TWC, the Slight Risk area was expanded south and westward to include southern NM and eastern-central parts of AZ. Have also expanded the Marginal Risk area to capture more desert areas toward the AZ-Mexico border, as the pattern and thermodynamic profile (sufficiently high mixed-layer CAPEs) would support isolated-scattered convection coming off the terrain and persisting across the deserts. Southeasterly low-level flow will advect Gulf moisture northwestward out of TX and into NM on Tuesday. Meanwhile a passing longwave trough will move eastward across the Plains, with a weak jet streak developing along the base of the trough. This will contribute broad-scale lift over NM. Further, monsoonal moisture from the Gulf of California will track northeastward into the desert Southwest. The clash of these air masses as well as a stalling out cold front over the Plains will result in widespread showers and thunderstorms across much of the Four Corners region. Further west over UT/AZ, fast storm motions and somewhat drier antecedent conditions should keep any flash flooding isolated. Meanwhile NM is both a bit wetter, and expected to pick up additional rainfall on Day 2/Monday, such that by Tuesday, the rainfall resulting from the aforementioned forcings should result in more widely scattered flash flooding. Flooding is most likely over any burn scars, slot canyons, and in the Sangre de Cristo Mountains of north-central NM. ....Mid Atlantic into the Northeast... Moisture recovery along the coast ahead of another strong cold front at the leading edge of energetic forcing associated with an advancing longwave trough/upper low over the Great Lakes will result in another round of showers and thunderstorms over much of the Mid-Atlantic and Northeast Tuesday. However, given the increased amplitude of the upstream upper trough (as it takes on a more neutral-negative tilt), the more favorable deep-layer support will contract farther west toward the trough. This will lead to less numerous showers and thunderstorms across the Mid Atlantic and Northeast compared to the past few days, and therefore at least some moisture recovery over most areas. As such, and based on the guidance trends (both deterministic and probabilistic), have cut back considerably with the Marginal Risk area from yesterday's Day 3 ERO. Hurley Day 3 Valid 12Z Wed Sep 13 2023 - 12Z Thu Sep 14 2023 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS AND NORTHEAST... ....2030Z Update... The axis of heaviest QPF across the southern plains continues to shift a bit more south compared to the last few periods of global deterministic and accompanying ensembles. There's a greater focus now across the TX Panhandle down through the Caprock and western rolling plains, within the confines of where impacts will be felt over a succession of days prior. There is some disagreement on exactly where the heaviest stripe of heavy rainfall will occur within the newly adjusted Slight Risk, however the moist environment and convective premise within the bounds of the risk will be sufficient for where ever the axis of heaviest rain will occur. Bias-corrected ensemble mean shows a wide swath of 0.75-1.5" in total for D3 which is a pretty aggressive considering the lack of greater hi-res influences at this range. As we move closer to the event, it's possible we could narrow the footprint of the SLGT as we pinpoint the proxy of convective propagation across the southern plains. Not much change within the synoptic evolution of the setup across the Northeast U.S. Increasing mid-level ascent from a pivoting trough across the Great Lakes will provide plentiful forcing to initiate a widespread swath of heavy precipitation over the Northeast, especially within NY state and New England. Trended the eastern flank of the Slight Risk a touch further eastward to account for ensemble QPF shifts from the previous cycle of guidance. Remainder of the forecast is still on track and any expansions of the MRGL will be pushed to future outlook forecasts pending output from hi-res deterministic and correlated ensemble means. Kleebauer ....Previous Forecast... ....Southwest and Four Corners Region into the Southern Plains... Slight/Marginal ERO area on Day 2 shifts a bit farther east by Day 3, as the next upper shortwave drops across southern CA and the western Great Basin. Compared to Sunday's Day 4 ERO, the Slight Risk area was pulled a bit farther south across the Red River Valley and Cap Rock-Rolling Plains of southern OK and northern TX. This was based largely on the guidance (QPF) trends, and with the anticipation that much of the soil will see sufficient moistening over the next couple of days prior to Day 3 (12Z Wed-12Z Thu). ....Northern Mid Atlantic into Eastern New York and New England... The aforementioned upper level trough, continuing to take on a more negative tilt with time, will pivot across the northern Mid Atlantic and Northeast Regions late Wed-Wed night. Strengthening deep-layer forcing (Qs convergence) ahead of the trough will likely get a boost from a 80-100kt upper jet streak on the eastern flank per the models (enhanced Qn convergence and low-level FGEN). After somewhat of a respite Tue-Tue night, this would support more widespread convection breaking out Wed-Wed night along and ahead of the advancing surface cold front. Not quite yet within the window of the higher-resolution CAM guidance, the models nevertheless continue to indicate a heavy rainfall signal across northern NJ and eastern NY into much of New England. The Slight and Marginal Risk alignment correspond to the latest deterministic/probabilistic QPF and guidance trends. Expect TPW values to peak aoa 1.75" prior to FROPA, and considering the degree of deep-layer forcing and recent heavy rainfall (lower FFG/increasing soil moisture percentiles), the Slight Risk noted from yesterday's Day 4 ERO would appear to be on track, albeit with a minor westward shift. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_fsMt9inWtb6nrzZ88X2J-UTCaSuwLP-EWXdSxfN2rOJ= tem9pf1HCvxlwhQf3PMmVqenj6dm0gIv58NPH1M_5ACQmfs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_fsMt9inWtb6nrzZ88X2J-UTCaSuwLP-EWXdSxfN2rOJ= tem9pf1HCvxlwhQf3PMmVqenj6dm0gIv58NPH1M_-0fOF4k$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_fsMt9inWtb6nrzZ88X2J-UTCaSuwLP-EWXdSxfN2rOJ= tem9pf1HCvxlwhQf3PMmVqenj6dm0gIv58NPH1M_FmVc9P0$=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) .