Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 11 2023 00:10:40 FOUS30 KWBC 110010 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 808 PM EDT Sun Sep 10 2023 Day 1 Valid 01Z Mon Sep 11 2023 - 12Z Mon Sep 11 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL PLAINS, AS WELL AS FOR MUCH OF THE MID-ATLANTIC AND NEW ENGLAND... ....Mid-Atlantic into New England... A very slowly drifting positively-tilted upper-level trough will continue east through the evening across the northern tier of the country (Northern Plains into the Great Lakes), notably digging southward into the Central Plains tonight. Meanwhile, a cutoff-low over the Tennessee Valley this morning has opened up into a trough and is tracking northeastward into the Northeast into the overnight hours. This pattern will continue to advect tropical moisture straight up the East Coast and into New England. The combination of large-scale lift in the atmosphere from the longwave trough over the Midwest and locally more significant added forcing from newly opened shortwave trough is resulting in a renewed round of showers and thunderstorms for much of the East Coast and Appalachians from NC northward. Recent rains from the past several days have primed the soils across much of this region, from the central Appalachians of the Virginia's through New England. Thus, yet another round of potentially heavy rain and training storms will only restart/worsen preexisting flash flooding all across the maintained (and somewhat trimmed on the southern extent) of the Slight Risk area. The highest probabilities for localized 3" exceedance exist across the updated Slight Risk area overnight, ranging from 20-50% (per the 12z HREF utilizing a 40-km neighborhood method). Convection is largely waning across the southern Mid-Atlantic (where a Marginal Risk was still maintained), but the newly opened shortwave is expected to continue to lead to renewed convection overnight from the northern Mid-Atlantic northward through much of New England. More recent HRRR runs are also supportive of this solution, depicting localized totals on the order of 3-6" through 12z. Notably, these totals are most tightly clustered across portions of far eastern PA into northern NJ and southern NY, which is also where the 12z HREF probabilities of 3" exceedance are maximized (with a 5" exceedance probability also as high as 20%). In this region, the Slight Risk is considered to be on the higher-end of the probability range (approaching Moderate Risk criteria, but still falling a bit short of an upgrade). Please see MPD #1055 (and potential subsequent MPDs) for more information. ....Central Plains... The Slight Risk area has been shifted a bit south from what was inherited, based largely on observational trends and adjustments in the new guidance. Convection has formed across southwest KS and surroundings this evening, focusing the most robust (supercell) storms as a strong cold front collides with a LLJ tracking northward out of TX. There is less forcing farther south into TX, as the focus for convection is more the dry line with less upper-level support (where a Marginal Risk was trimmed a bit on the southern edge due to the lack of robust convection). Any storms that form along the front will advect eastward across southern Iowa, gradually weakening with time (with expected totals remaining below concerns for excessive rainfall). This should favor perhaps a prolonged period of light to moderate rain. Antecedent conditions across much of the Plains into the Midwest have been bone dry, with soil moisture levels at less than 5% of both climatology and overall. Since the storms that get into Iowa will both be weaker and unlikely to train/repeat, we continue to think the large majority of any rainfall seen in Iowa overnight will be beneficial for the area, and flash flooding appears rather unlikely. Further, since much of Iowa is agricultural, even the bone dry soils should be less repulsive of moisture, especially since it won't be very heavy. Farther northwest into the High Plains, the Marginal Risk was maintained with any excessive rainfall likely limited to more robust supercell convection. Churchill/Wegman Day 2 Valid 12Z Mon Sep 11 2023 - 12Z Tue Sep 12 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF WEST TEXAS... ....2030Z Update... The forecast across the Great Lakes and Northeast remains on track with little variation in previous guidance. Main concerns will be heavy rainfall potential within urban corridors and areas where heavy rain has allowed for FFG's to be much lower than climo norms. Synoptic setup across the north remains relatively steadfast leading to only minor shifts in the bounds of the MRGL at best. After coordination with with the San Angelo WFO, a targeted Slight Risk was introduced for the D2 outlook. More can be found on the upgrade below... Kleebauer ....Northeast... A cold front sweeping across the region from far western NY and PA early in the day Monday will clear eastern Maine by Tuesday morning. The front will clear the anomalous atmospheric moisture from the Northeast. While the upper level trough over the Midwest will dig southward through the day, it will make relatively little eastward progress. Thus, in the upper levels, the unidirectional southwesterly flow will persist, but with little embedded forcing. As the moisture pushes off the coast, any resultant thunderstorms during the day Monday will progressively shift offshore. This will limit the potential for additional flash flooding. Any storms that form during the afternoon are progged by most of the guidance to remain widely scattered and in a line that gradually tracks east. This should limit the potential for flash flooding. ....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 was expanded to include this area. Wegman ....Southern Plains... 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. All hi-res guidance is 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. 12z HREF guidance indicates a strong signal for locally heavy rainfall across portions of the western rolling plains of TX up into the adjacent Permian Basin. Neighborhood probabilities of 2"/hr or greater and 2-3"/3 hrs have increased substantially over the past few runs with a target of 50-60% encompassing the area between Sterling City down into the northern fringes of the Edwards Plateau. 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. The question of whether the heavy rainfall expected would be incapable of causing issues based upon how dry the soils are across the area, but with coordination with the local WFO in San Angelo there was enough of a prominent signal for flash flooding capabilities, especially within San Angelo proper and some of the towns within the confines of Sterling/Tom Green/Irion/Coke counties. Thus, have added a targeted Slight Risk over the aforementioned area. There is some potential for an expansion of the SLGT in future forecast packages, mainly up into the eastern Permian Basin, but with the initiation point being within the confines of the area and storm motions expected to press outside the zone fairly quickly, maintained the MRGL with agreement from the Midland WFO. It will bear monitoring pending trends in the hi-res blend mean and associated probabilistic indicators. Kleebauer Day 3 Valid 12Z Tue Sep 12 2023 - 12Z Wed Sep 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN NEW MEXICO... ....2030Z Update... Very little change to the the Slight Risk area across the Southwest U.S. A slight bump northward in the SLGT was administered given the trends in global ensemble guidance and NBM bringing a heavier QPF footprint to southern CO within the terrain. The axis of heaviest precipitation will lie along that coupled affect of upslope enhancement within the Sangre de Cristos and the progressive shortwave providing focused ascent within the terrain. Some deterministic outputs are fairly robust given the setup with local maxima approaching 1-1.5" for portions of the northern NM terrain, especially along and north of I-40. A secondary maxima is possible over the southern plains or high country of AZ, but the best Q-vector convergence is still placed over the northwest corner of NM for Tuesday, thus have kept continuity with the SLGT with a minor expansion based on trends in guidance. The synoptic setup for the Northeast continues to align with good model agreement at this juncture. Large scale ascent over the Northeast U.S will trigger another round of convective activity for areas that have been hit recently. Despite the consensus in the synoptic scale evolution and anticipated convective regime, forecast bufr and spatial sounding averages point towards modest storm motions that should limit the enhanced flash flooding concerns. That does not mean a more focused SLGT couldn't occur prior to the event, but for now, have kept continuity and will monitor hi-res probabilistic indices that may point towards a more widespread or impact focused threat. Kleebauer ....Previous Discussion... ....Four Corners Region... In coordination with ABQ/Albuquerque, NM forecast office, a Slight Risk upgrade was introduced with this update. 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. ....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. Despite the moisture recovery, it will not reach levels currently present over the area, and with some recovery time from Monday, the chances of flash flooding in the Northeast will be a little bit lower than today and previous days. Thus, the Marginal Risk inherited was largely unchanged. Any slowing of the overall pattern or more atmospheric moisture available to developing convection could result in a targeted Slight Risk upgrade over portions of the area with future updates. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5xrl0DTJjpRB1GVnprqqPAQp2eXlRdCOoaUZwJFQ7oTm= -Jet1PxQyJG_pxlEsxBq_yywbA6U6i2llc9cqDSVf_95cKA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5xrl0DTJjpRB1GVnprqqPAQp2eXlRdCOoaUZwJFQ7oTm= -Jet1PxQyJG_pxlEsxBq_yywbA6U6i2llc9cqDSVUuN67aY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5xrl0DTJjpRB1GVnprqqPAQp2eXlRdCOoaUZwJFQ7oTm= -Jet1PxQyJG_pxlEsxBq_yywbA6U6i2llc9cqDSVPiemH7A$=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) .