Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Oct 05 2023 23:59:42 FOUS30 KWBC 052359 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 757 PM EDT Thu Oct 05 2023 Day 1 Valid 16Z Thu Oct 05 2023 - 12Z Fri Oct 06 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Central Gulf Coast... The mesoscale guidance has some signal for 2-3" of rainfall starting in the next few hours near Slidell which propagates east-northeast into southern Mississippi. This is on the north side of a pool of 500-1500 J/kg of MU CAPE in a diffluent area southeast of the base of an upper level trough. The region has been quite dry this week, so outside of an urban areas that might be impacted, there doesn't appear to be much risk of flash flooding since it would be more helpful than harmful. While the threat is non-zero, didn't feel it rose to the 5% neighborhood probability threshold. Should convective trends prove otherwise later this evening and overnight, the situation could be handled by a mesoscale precipitation discussion. Roth Day 2 Valid 12Z Fri Oct 06 2023 - 12Z Sat Oct 07 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST... ....2030 UTC Discussion... Given decreasing spread in the model QPF typical of decreasing lead-time, a part of the Marginal risk area was trimmed away...leaving the area along and immediately ahead of the cold front. Left a lobe of Marginal Risk area extending towards New York City given the ECMWF (and to a much lesser extent by the UKMET/CMC) showing at least some convective potential right over a region that has been hydrologically sensitive since heavy rainfall nearly a week ago. Latest QPF also allowed of the Marginal from Far South Texas.=20 Much of the previous discussion was still valid in terms of the large scale synoptics and antecedent conditions. Bann ....Previous Discussion... ....Upper Ohio River Valley into the Northeast... A strong, broad high amplitude trough will be transitioning to a much narrower but also stronger/deeper trough as an upstream shortwave over the Dakotas at the start of the period digs into the base over the Central Great Lakes/Ohio River Valley through D2 reaching over -2 standard deviation from the mean. Downstream a similar strong blocking ridge over SE Canada into the Subtropical Western Atlantic (enhanced by T.S. Philippe) will result in slowing eventual negative tilting of the trough into the Allegheny Plateau by the end of the day. This will provide solid upper-level evacuation/outflow, while concurrently slowing eastward progression of the cold front and strengthening low to mid-level flow. Moisture will be modest with 1.25-1.5" which is above normal but only about 90th percentile with southerly winds in the 25-35kt range with very limited instability (generally below 500 J/kg). The the prolonged persistence and flux will bring IVT values in the 400-500 kg/m/s range which is not bad given proximity to some orographic ascent as well. Greatest duration of moderate rainfall/training will exist across western to central NY and while areal averaged rainfall will probably not get much over 1", swaths of 2-3" are possible and may result in widely scattered exceedance of FFG in the region consistent with the Marginal Risk category. Hi-Res CAMs and some global guidance hint at some steepening of lapse rates due to mid-level drying in the post-frontal regime, but given the upslope component and lingering low level moisture, instability of 250-500 J/kg is most common across the Upper Ohio River Valley. Isolated narrow core thunderstorms may exist and given the deep layer steering may have a potential of short-term training. Given naturally lower FFG over E PA, N WV and SE OH, have pulled the Marginal Risk back a bit to account for this potential. ....Hudson Valley, Long Island, and Western New England... Guidance continues to remain uncertain on placement and scale of potential south to north training of shallow thunderstorms tapping sub-tropical moisture from the approach of what is to evolve (sub-tropical or extra-tropical) of T.S. Philippe, resulting in a low confidence forecast. The ECMWF continues to be most consistent for a few runs with the greatest rainfall totals but has been on a slow westward trend toward the other camp of guidance. However, there are some signs of a similar band or bands within other models as well, including some of the longer duration Hi-Res CAMs like the FV3, CMC-regional and Nam-Nest...however, the overall the signal is weaker, more inconsistent run to run. Environmentally, the region is much cooler and less moist than last week but a similar evolution appears to be unfolding, as favorable frontogenesis on an effective warm front from the approaching large scale trough will increasingly see strong moisture flux convergence nearly perpendicular to the north-south boundary as the warm conveyor belt from Philippe fluxes increased moisture up to 1.75" total PWat with 10-25kts from the east (depending on the model run). Weak instability may allow for shallow warm cloud thunderstorm/rainfall processes allowing for quick 1.5-2"/hr totals but the overall coverage width of updrafts and density of convective cells are likely to be a limiting factor given weak instability of up to 500 J/kg. Still, this remains a low confidence forecast, but there does appear to be at least a threat of excessive rainfall with this setup especially in the vicinity of the area affected last week. Though the ECMWF trend in magnitude is coming down with localized 2-3". If placement consistency continues and is confirmed with the full suite of Hi-Res CAM guidance at 12z, an increase in excessive rainfall category may be required, especially if the guidance locks in on SE NY, CT, W MA where soil saturation remains above 70% which is generally about the 80-90th percentile for early October.=20 Interests in the region should have increased attention to these trends as the risk may increase quickly if confidence is bolstered with model agreement.=20=20 ....Far South Texas... No significant change in forecast guidance suite with respect to far southern Texas; 00z guidance and HREF probability suite continue to suggest post-frontal convection to linger in the vicinity of the lower Rio Grande Valley with best potential for heaviest rainfall at the far southeastern portion of the Valley and just off-shore. So will retain the conditional Marginal Risk again for this cycle, though tendency for the front to be a bit further south remains and the Marginal may be removed in subsequent updates. Gallina Day 3 Valid 12Z Sat Oct 07 2023 - 12Z Sun Oct 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST... ....2030 UTC Excessive Rainfall Discussion... Tightened up the western boundary of the Slight Risk somewhat but largely left the Slight Risk over Maine pretty much in-tact from the earlier issuance. The concern has been as much about the moisture streaming northward from Philippe and interacting with a cold front as it is about the energy or remnants of Philippe itself. The large scale synoptics will be undergoing a sharp change with the formation of a deep layered cyclone over eastern North America...resulting in strong mesoscale forcing and increasing upper level divergence in response to the development of a negative tilted trough axis. The resulting low level winds accelerate to between 45 kts and 65 kts and draw with precipitable water values in excess of 1.5 inches inland. The uncertainty is least over Maine where 1 to 3 inches...with isolated maximum rainfall of 5 inches...is most likely given the prolonged period of moisture flux off the ocean. Areas farther inland have more uncertainty...how much/low long there will be instability to support higher rainfall rates...when dry air sweeps eastward in the wake of the upper level negative tilted trough and the magnitude of the cold advection. The 05/12Z UKMET and 05Z/12Z CMC both maintained max QPF amounts in excess of 4 inches due to a deformation zone that they set up in parts of the terrain in western New England. The presumption is that these amounts may be overdone but at least suggestive that subtle changes in model guidance could still yield important QPF differences with implications to excessive rainfall. Bann ....0830 UTC Excessive Rainfall Discussion... ....Northeast... At the start of the forecast period (07.12z), a south to north band of shallow thunderstorms may be training/ongoing along an effective deep layer frontogenetical band in proximity to the Hudson Valley and Western New England as the warm conveyor belt associated with deep sub-tropical moisture associated with Philippe is fluxed westward into deformation axis. To compound the issue, the northern stream shortwave trough will be transitioning from negative tilt to closed low over the Mid-Atlantic; resulting in the deep layer pivot to maintain strong forcing ascent across the Green Mountains into Western NY throughout the morning into the afternoon. Strong moisture flux with nearly 150-180 degrees of convergence of 30-40kts of 850-700mb flow combined with favorable orographic ascent of the warm moist conveyor belt should result in prolonged moderate to occasionally heavy showers into the afternoon hours supporting 24hr totals of 2-4"across VT into W NY, perhaps locally higher in best facing orography; this while scattered showers/totals of 1-3" are more likely further south as the stronger westerly flow undercuts the deeper closed cyclone.=20 Timing/placement of the remnants of Tropical Cyclone Philippe will surge the warm conveyor belt toward Cape Cod by afternoon and while longitudinal placement remains uncertain, the trend toward a stronger closed low over the Mid-Atlantic is supporting a slight westward trend of the axis deep tropical moisture and stronger low level winds (50-60kts) further west potentially grazing the Cape but central Maine should feel the brunt of the prolonged moisture flux resulting in a secondary south to north axis of enhanced rainfall. IVT values 500-750 kg/m/s given 1.75-2" of total PWATs and broad 45-65kts of LLJ southerly flow generally for 12hrs starting around 07.21z, with some localized rates of .5-1"/hr within the core of the conveyor belt resulting in spots of 3-5" by the end of the day 3 period. Both areas are likely to see scattered incidents of flooding, perhaps isolated rapid inundation flooding given the aforementioned potential of rain-rates.=20=20 Between the two axes, there is solid model agreement that the mid-level drying is expected and generally aligns with southeastern New England. As such, a Slight Risk of Excessive Rainfall remains in place across these axes based on fairly solid model agreement in the global guidance suite with small adjustments for better agreement in the placement of the dry slot. An upgrade to Moderate Risk may be necessary based on how rain falls through the day 2 period and potentially further saturates areas for the day 3 period.=20 Gallina Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6o5Ju58j0zKLkZXzcn52sBdFiCvHREDPfYrHGcAkjrjb= PikHUhbKKa9BTAefkK3YdHltAeqixcDuV6mRlOAL1Win1FI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6o5Ju58j0zKLkZXzcn52sBdFiCvHREDPfYrHGcAkjrjb= PikHUhbKKa9BTAefkK3YdHltAeqixcDuV6mRlOALKARHZjY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6o5Ju58j0zKLkZXzcn52sBdFiCvHREDPfYrHGcAkjrjb= PikHUhbKKa9BTAefkK3YdHltAeqixcDuV6mRlOALuu_2U1g$=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. 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