Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Sep 10 2023 20:08:36 FOUS30 KWBC 102008 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 407 PM EDT Sun Sep 10 2023 Day 1 Valid 16Z Sun Sep 10 2023 - 12Z Mon Sep 11 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF THE CENTRAL PLAINS AS WELL AS MUCH OF THE MID-ATLANTIC AND NEW ENGLAND... ....16Z Update... Little change in regards to the previous Slight risks with some minor adjustments on the eastern bounds of each respective slight. Plentiful cloud cover across the Mid-Atlantic and Northeast will keep the thermodynamic aspect of the forecast a bit more tame compared to the last few days but rich moisture positioned across the Appalachians and points to the east provides the necessary element for any convection to potentially drop locally heavy rainfall. The three areas of interest today are; across southern New England where the best instability will be focused over the northeastern U.S, the Appalachian and Allegheny fronts over PA/WV/VA where FFG's are very low after successive days of heavy rainfall, and the central plains around western KS where a cold front will lead to a strong convergence signal in conjunction with a developing LLJ extending north into the plains. There is a higher confidence in heavy rainfall over KS/OK border extending into the northeast corner of the TX Panhandle. This is where convergence is forecast to be maximized and the 12z HREF along with several deterministic place the heaviest rain for later this evening. The 12z HREF probabilities for >2"/3 hr period also has quite a robust signal for each respective area with 40-60% chances during the peak of forecast impact and >3"/ 3 hr signals are non-zero for the same locales. Given the limited change in synoptic evolution and a strong signal for flash flooding within the SLGT risk bounds, will maintain continuity and make any adjustments based on observational trends. Kleebauer ....Previous Forecast... ....Mid-Atlantic into New England... A very slowly drifting positively tilted upper level trough will continue east through the day today. Meanwhile a cutoff low over the Tennessee Valley early this morning will quickly open up into a trough and track northeastward into the Northeast through tonight. 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 localized added forcing from the cutoff low turned shortwave trough will result 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 in many regions, from the central Appalachians of the Virginias through New England. Thus, yet another round of potentially heavy rain and training storms will only restart/worsen preexisting flash flooding all up and down the Slight Risk area. The Slight Risk area was expanded southwestward into portions of southwestern VA and southern WV in coordination with RLX/Charleston, WV and RNK/Blacksburg, VA forecast offices. This area has been hard hit for multiple days with flash flooding from repeating thunderstorms, and while overall coverage of storms should be lesser today than previous days, the aforementioned antecedent soil conditions will support lesser amounts of rain resulting in flash flooding. The axis of heaviest rainfall has shifted a bit to the east further north into the Jersey shore, NYC and Long Island, and southern New England, so the Slight Risk area was also expanded eastward to include those areas. Expect higher amounts of instability from onshore flow off the warm waters of the Atlantic into NJ, NY, and southern New England, which will support stronger storms which will also repeat/train in the unidirectional southerly flow. Finally, the Slight Risk area was also expanded to include nearly all of northern New England except portions of the eastern Maine coast. Thunderstorms tracking northeastward ahead of the longwave trough will move over the same areas that were hit with storms in previous days. While the storms may weaken as they leave the coast into northern Maine, antecedent soil conditions favor lower FFGs in far northern Maine, which could result in flash flooding even that far north, especially into tonight. ....Central Plains... The Slight Risk area is largely unchanged from inherited. Guidance has come into better agreement that western KS will be the focus for the strongest storms as a strong cold front collides with a LLJ tracking northward out of TX. Forcing will be lesser into TX as the focus for convection is more the dry line with less upper level support, so the Marginal Risk down along the Caprock remains in place. The Slight risk area was trimmed out of Colorado as the strongest and most persistent storms look to form east of there. Further, the Marginal Risk was trimmed out of Iowa entirely with this update. Any storms that form along the front will advect eastward across southern Iowa, gradually weakening with time. 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, think the large majority of any rainfall seen in Iowa today will be beneficial for the area, and flash flooding appears highly 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. 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 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8T9DztflZUE1B-2eBk2AZ0774fc8N6fhzJE_Ytvri6ry= 2UoAl1glPJw0nrz8779-xpuc0O9d9ZxDlQHvTgvTFYaesCc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8T9DztflZUE1B-2eBk2AZ0774fc8N6fhzJE_Ytvri6ry= 2UoAl1glPJw0nrz8779-xpuc0O9d9ZxDlQHvTgvTJr90ego$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8T9DztflZUE1B-2eBk2AZ0774fc8N6fhzJE_Ytvri6ry= 2UoAl1glPJw0nrz8779-xpuc0O9d9ZxDlQHvTgvTC285_JY$=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) .