Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 30 2023 00:50:58 FOUS30 KWBC 300050 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 849 PM EDT Fri Sep 29 2023 Day 1 Valid 01Z Sat Sep 30 2023 - 12Z Sat Sep 30 2023 ....A MODERATE RISK OF EXCESSIVE RAINFALL IS IN EFFECT OVER PORTIONS OF THE NORTHEAST... ....0100Z Update... Only minor adjustments were made to the D1 package across the CONUS, mainly to pare the western periphery of the Moderate/Slight/Marginal Risk areas across the Northeast and the Marginal Risk area over the eastern Dakotas and western-northern MN. This based on the latest observational and mesoanalysis trends, along with recent CAMs and the 18Z HREF exceedance probabilities. Hurley Previous discussion... ....Mid-Atlantic into Northeast... Heavy rainfall has impacted the western Long Island area into the NYC metro with area totals on the order of 4-6" with locally higher amounts located within the Brooklyn and Manhattan boroughs. 1-3" is very common in the surrounding boroughs and encompassing areas within the Lower Hudson, CT, and the northern third of NJ. Strong low-level frontogenesis located parallel to the NYC/NNJ/LI coast is analyzed on mesoanalysis and very well depicted within the banding structures persistent over the above locations. Cross-section analysis of both the 12z HRRR and 12z NAM nest show a rich moisture presence deep into the profile over northeast NJ up into western CT within a field of elevated Omega which aligns perfectly with the forecasted 1000-925mb frontogenesis fields. This is the main axis of concern through the rest of the D1 time frame where a north-south precip maximum will likely develop later this afternoon and evening. 12z HREF mean still shows a widespread area of 3-5" totals as far south as southern Monmouth county in NJ up through the Lower Hudson where terrain influences will aid in local heavy precip banding signatures as the strong U-vector wind anomalies provide upslope enhancement within the hills north of NYC across Orange and Putnam counties. Local totals exceeding 8" are increasingly likely within the Moderate coverage area, especially those areas that are already over 5-5.5" at this hour. 12z HREF neighborhood probability of >8" is up to 20-25% within NYC proper, a fairly robust signal in the means and the center of the MOD risk bounds. Our upper low to the west is slowly propagating through PA with center of circulation over the Laurel Highlands as depicted on WV satellite. The upper-level progression is important in the outcome of the downstream heavy rain threat as increased dynamics and textbook synoptic scale evolution will evolve over the course of the afternoon and evening, finally ending within the back end of the D1 period. ....Upper Midwest/Great Lakes... Shortwave energy propagating south-north into Canada will drive showers and scattered to isolated thunderstorm activity over the Upper Midwest today. Another inverted surface trough will direct and enhance anomalous moisture plume with PWATs of 1-2" into much of Minnesota and northwestern Wisconsin. Rainfall totals of 1-2 inches are possible over portions the Upper Midwest by tomorrow morning. ....Florida... The back end of a slow moving cold front will act as a focus for scattered to isolated showers and thunderstorms today. A Marginal Risk area was continued to account for any localized flash flooding concerns that may arise. Kleebauer/Kebede Day 2 Valid 12Z Sat Sep 30 2023 - 12Z Sun Oct 01 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE GREAT BASIN, NEW MEXICO AND FAR WEST TEXAS, UPPER MIDWEST, AND FLORIDA... ....20Z Update... There is little change to the previous MRGL issuances from the previous forecast cycle. Upper trough will dig across the western CONUS with downstream convective implications across the southern high plains, southern Rockies, and Upper Midwest. The main addition to the D2 ERO was the MRGL risk in place across the upper Midwest with primary focus on the U.P of Michigan and northern Wisconsin. More details on that threat below... Elsewhere, locally heavy rainfall will be possible across the Great Basin within a marginal convective environment. 12z forecast bufr soundings across several locations in NV indicate steep lapse rates and deep moisture presence that is typical under the center of a deepening mid and upper trough axis. PWAT anomalies are running between 2-2.5 devaitions above normal under the mean trough which coincides with the higher QPF outputs given by guidance. Both the HREF blended mean QPF and ensemble-bias corrected precip field indicate a widespread area of 0.75-1.25" of precip which would occur within a 3-6 hr time frame when convection develops and traverses the area. The consistently low FFG's in place over the interior west with the convective scheme forecast would lead to isolated areas of flash flooding, which would ultimately arise if the rates can exceed 0.5" or more per hour. Considering the dynamic nature of the setup, this is well within the cards, and accentuated within the core of higher PWAT anomalies present. West Texas and New Mexico will be under the influence of the difluent pattern on the eastern side of the mean trough. Modest instability over the terrain as indicated by HREF mean MUCAPE 500-1000 J/kg Saturday afternoon within a corridor of modest PWAT indices between 1-1.5 devaitions above normal lends credence to isolates flash flood concerns from diurnally driven convection. Theta-E advection into eastern NM is fairly prolific for this time of the year, putting the area within a more late summer-like environment which is consistent with heavy rainfall potential. Main areas of concern will continue to be terrain favored regions and local urban corridors. Florida will remain locked into a persistent pattern of elevated PWATs and focused ascent along and south of the frontal boundary drapped over central FL. Primary focus will be within the coastal urban corridors. ....Upper Midwest... Energy ejecting to the northeast out of the central plains will move overhead of the northern fringes of the Midwest by Saturday evening. At the surface, a frontal boundary will lift north in the form of a warm front with positioning to the south of Superior after 00z Sunday. Modest instability across northern WI into the Michigan U.P will create an environment favorable for convective development within a core of elevated PWATs approaching 2-2.5 deviations above normal. The combo of ascent from weak vorticity advection and surface convergence along the warm front will be plentiful for convective initation along the southern lake shore. 12z HREF neighborhood probability is fairly robust for local heavy rain signatures with 10 ARI exceedance probability running between 20-30% over northern WI and even a max of 45% showing up within the means between Sault Ste. Marie and Marquette. Considering the signal for locally heavy rainfall within a convective scheme rooted in a very anomalous, moist environment, went ahead and added a Marginal Risk encompassing higher QPF areas forecast within the HREF blended mean and WPC forecast QPF.=20 Kleebauer ....Great Basin/Nevada... A deep upper-trough will transport a modest plume of moisture into the West on Saturday. A cold front associated with a deepening low pressure system will act as a focus for convection over the Great Basin in particular. Some parts of central Nevada, southeastern Oregon and southwestern Idaho could see rainfall rates in excess of 0.25"/hr. Ample forcing over elevated terrain could lead to more efficient/enhanced rainfall. Between 0.5-1" total areal rainfall is forecast over 3-4 hour period therefore, localized flash flooding is our primary concern. ....New Mexico/west Texas... Convection supported by an upper trough to the West and shortwave energy downstream from it will support another localized flash flood threat over parts of eastern New Mexico and into west Texas. Decent instability, modest moisture flux and low level jet may produce bursts of heavy rainfall over our marginal risk area. Urban areas and dry washes are most vulnerable of flash flooding. ....Florida... A slow moving shortwave trough will allow for the continuation of scattered to isolated thunderstorms across the central/southern peninsula on Saturday. Some cells may produce localized heavy rainfall within the marginal risk area. Antecedent conditions will also contribute to the sensitivity to any isolated heavy rainfall threats. Kebede Day 3 Valid 12Z Sun Oct 01 2023 - 12Z Mon Oct 02 2023 ,...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF FLORIDA AND THE SOUTHERN HIGH PLAINS OF NEW MEXICO AND TEXAS... ....20Z Update... No changes to the previous forecast over FL as persistence in the overall synoptic scale pattern will lead to another risk of locally heavy rainfall along and south of a slow moving cold front within a very anomalous tropical environment. An additional Marginal Risk was added across the southern high plains...More on this threat discussed below... ....Southern High Plains... Another round of convection is anticipated on Sunday afternoon and evening thanks to a persistent difluent pattern aloft on the eastern flank of a deep, longwave trough centered over the Great Basin. A strengthening theta-E ridge will extend up from TX into eastern NM providing a focal point for another round of scattered thunderstorm activity across the southern Rockies over into the southern high plains in NM and TX. General instability forecast is favorable with global deterministic around 750-1250 J/kg MLCAPE centered within the theta-E tongue. Lower FFG's are anticipated after the first round of convection on Saturday, so the expectation is for flash flooding concerns to continue in similar areas affected prior, as well as the focal point within a zone of terrain over northern and eastern NM. As a result, a MRGL risk was added to the forecast to convey the threat. Kleebauer ....Florida... Shortwave energy present across the state of Florida will support isolated flash flooding concerns on Sunday. Scattered to isolated thunderstorms will continue to develop along a quasi-stationary front draped across the southern peninsula. There will be plenty of instability in place so there's always the risk of some cells over performing with efficient rainfall rates exceeding flash flood guidance. Kebede Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7cNpgRJ5-L6d8zB2LadN-2LQEJgq1OIy58ESxLhlZIR3= THyK8w_7Vo17MYVnEAN-1nglq3zgUTlJcHpw1Ts2nEh4XFQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7cNpgRJ5-L6d8zB2LadN-2LQEJgq1OIy58ESxLhlZIR3= THyK8w_7Vo17MYVnEAN-1nglq3zgUTlJcHpw1Ts2JJendLU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7cNpgRJ5-L6d8zB2LadN-2LQEJgq1OIy58ESxLhlZIR3= THyK8w_7Vo17MYVnEAN-1nglq3zgUTlJcHpw1Ts28MZoMC8$=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) .