Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon May 13 2024 00:11:30 FOUS30 KWBC 130011 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 811 PM EDT Sun May 12 2024 Day 1 Valid 01Z Mon May 13 2024 - 12Z Mon May 13 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN AND NEAR=20 CENTRAL LOUISIANA... An elongated mid- to upper-level low moving into the central=20 Plains will result in a downstream increase in broadly convergent=20 low- level wind speeds emerging from the Gulf of Mexico, noted by=20 850mb winds surging out of the S/SW as high as 20-40 kts by this=20 evening. This increases moisture and instability advection, as more favorable thermodynamics are drawn northward collocated with=20 MUCAPE reaching as high as 2000-3000 J/kg near a surface front=20 across TX/LA, and as high as 1000 J/kg farther north into the=20 Central Plains within an area of deformation around the elongated=20 upper level low. Various spokes of vorticity/shortwaves lift=20 northeast around the upper low, the upper jet streak positions=20 favorably to produce diffluence, and the warm front gradually=20 pivots northeast. This will result in two primary areas of heavy=20 rain and areas of potential flash flooding. Up north, hourly rain totals to 1.5" and local amounts to 3" remain possible (where a northeast adjustment was made to the Slight Risk area based on radar reflectivity trends and the 18z HREF probabilities). Down south closer to the Gulf Coast, totals up to 3" in an hour=20 cannot be ruled out near mesocyclones or any training segments.=20 Heavy rains will fall atop soils that are saturated from recent=20 heavy rainfall, especially in eastern TX, noted by NASA SpoRT soil=20 moisture reaching the 90-95th percentile. Instances of flash=20 flooding today still appear to be widespread enough for the Moderate Risk category; its areal extent has been pared back based on the 18z HREF and radar reflectivity trends. So far, 9C at 700=20 hPa appears to be acting as an effective capping inversion/=20 southward edge to the broader heavy rain threat, but temperatures=20 aloft are expected to cool somewhat both due to the convection=20 itself and upper level trough edging eastward. Convection moving=20 through LA has been aiding low-level convergence along the front=20 itself, which could make it a more effective focus, at least in the short term. Where the most pronounced training occurs, rainfall of 3-5" is probable, with maximum amounts exceeding 6" possible near=20 the TX/LA border where moisture convergence is maximized. Roth/Weiss Day 2 Valid 12Z Mon May 13 2024 - 12Z Tue May 14 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL GULF COAST FROM EASTERN LOUISIANA TO THE WESTERN FLORIDA PANHANDLE... ....Summary... Mid-level low moving from the Central Plains to the Upper Midwest Monday will remain amplified with 500-700mb height anomalies reaching as low as minus-2 sigma according to NAEFS ensemble tables. This will drive impressive height falls into an area of enhanced divergence downstream, producing robust synoptic lift aided additionally by a poleward arcing upper jet streak leaving favorable diffluence within both its RRQ and LFQ. This evolution will also result in increasing low-level flow emerging from the Gulf of Mexico drawing moisture and instability northward, with the attendant theta-e axis lifting as far north as the Great Lakes before rotating into a deformation axis across the Upper Midwest to cause a flash flood risk. Farther south, especially along and near the central Gulf Coast, shortwaves and accompanying MCS moving along a surface warm front will result in waves of heavy rainfall, and a more widespread and significant risk for flash flooding. ....Lower Mississippi Valley through the Central Gulf Coast... SW flow around the base of the mid-level trough will surge across the lower MS VLY and central Gulf Coast, with shortwaves embedded within the flow traversing west to east across the region. These impulses will create enhanced mesoscale ascent atop the low-level baroclinic boundary and band of frontogenesis associated with a surface warm front lifting slowly northward from the Gulf Coast. The guidance is in good agreement that two waves of heavy rainfall, one overnight Sunday into early Monday, and a second, more intense area Monday aftn/eve, will move along this front. However, there remains considerable uncertainty as to exactly where they will track due to the uncertainty in how the lead MCS will impact the advection of the front. The NAM/GFS are on the north side of the guidance envelope, and are likely too far north due to the typical southern trend bias which occurs with these warmer season events, and due to the general low-amplitude of the downstream ridge. This suggests the heaviest rain will occur near and along the Gulf Coast from eastern TX into the western FL Panhandle. With PWs exceeding 2" and MUCAPE reaching 3000-4000 J/kg, this will support rainfall rates of 2-3"/hr. The favorable orthogonal angle of the moist low-level flow into the front/remnant cold pool combined with boundary-parallel mean layer winds indicate an enhanced training potential of these rain rates, which supports the model trends of an axis of very heavy rainfall. In fact, recent HREF and RRFSE probabilities suggest a 60-80% of 5 inches of rain from eastern LA through the western FL panhandle, and there are even some lower probabilities for 8 inches. The EAS probabilities, while extremely impressive for 3"/24 hrs, are displaced just a bit north of the neighborhood probabilities, and this, combined with generally below normal soil moisture precludes an upgrade to the MDT ERO risk area at this time. However, after collaboration with WFO MOB, will continue to monitor the evolution of the guidance for a possible upgrade with later issuances as rainfall of this magnitude could result in widespread or significant flash flooding. ....Mid-Mississippi Valley and Upper Midwest... Have added a narrow SLGT risk for portions of the region from northern Missouri northeast towards Chicago, IL. There is good model agreement here that a focus of slow moving convection will develop within the impressive deformation axis. Prominent moist advection into this axis will result in enhanced convergence to drive ascent, occurring within a pronounced TROWAL reflected by strong mid-level theta-e ridging surging northwest from the Gulf Coast. This TROWAL will aid moisture and instability, with MUCAPE progged to exceed 750 J/kg, and PWs likely climbing to around 1.25", slightly above the 90th percentile, additionally reflecting the impressiveness of the thermodynamics. With strong ascent pushing into this environment, convection is likely to become widespread and heavy, with rainfall rates of more than 1"/hr likely noted by HREF neighborhood probabilities reaching 40-50%. These cells will move very slowly in this axis, and Corfidi vectors become increasingly aligned anti-parallel to the mean winds indicating backbuilding and training of these echoes. This could result in localized rainfall exceeding 3 inches as noted by HREF and RRFSE probabilities. This rain will occur atop soils with compromised FFG as low as 1.5"/3hrs, so scattered instances of flash flooding are possible. Weiss Day 3 Valid 12Z Tue May 14 2024 - 12Z Wed May 15 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE SOUTHEAST AND GULF COAST... 20Z Update... Changes to the previous forecast were minimal, and primarily cosmetic based on latest GEFS/ECENS probabilities, new WPC QPF, and updated NASA SPoRT soil moisture anomalies. The previous discussion remains well aligned with current forecasts as well, so no significant adjustments were needed to the ERO areas. The challenge for D3 will generally be focused from the western FL Panhandle through the coastal Southeast as heavy rain will move across regions that have been saturated recently. Fortunately, features should be more progressive by Tuesday so residence times of any heavy rainfall should preclude any risks above the inherited SLGT. The one caveat to this could be along the FL Panhandle where some training along the progressive cold front could occur atop areas that may get excessive rain on D2. If this comes to fruition, a targeted higher risk area may be needed, but at this time confidence in that evolution is too low for any upgrades. ....Tennessee Valley, Lower Ohio Valley, Mid Atlantic, and Southeast... While fairly transient, the models show a healthy dose of upper divergence/deep-layer ascent ahead of a mid-level trough, aided by the frontogenetic right-entrance region forcing from the upper jet streak lifting through the northern Mid Atlantic region. Moisture transport, IVT, and PW anomalies/percentiles remain rather robust during Day 3 per the ensembles/ESAT. In fact, 850-700 mb moisture flux sigmas climb to +4 to +5 eastern AL, southern 2/3 of GA, and into the Carolinas, accompanied by IVT as high as 1000 kg/m/s. Plenty of deep-layer instability will be present as well across the Southeast, particularly behind the warm front, as MUCAPE surges to nearly 2000 J/kg. While the ingredients are in play for heavy, potentially excessive rainfall (both dynamical and thermodynamical), the uptick in westerly MBE/Corfidi Vectors with time Tuesday, coinciding with a more progressive frontal progression over much of the region appears to be sufficient to simply maintain the Slight Risk of excessive rainfall. This idea matches the first-guess field from CSU which peak in the SLGT risk category as well. The one area that will need to be monitored for a potential upgrade to a Moderate Risk in subsequent outlooks will be across southern GA into northern FL, where the front and the 2.00+ TPW plume may get hung up a bit Tuesday night as the southwesterly low-level flow veers more westerly and aligns more parallel to the mean 850-300 mb flow later Tuesday afternoon into Tuesday night. At this point, Hurley/Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6A3-uYE5XjYLQQNTzq9wS66crl1qeJL8ATYJtMRvT1z5= 9-_nuuiHiBWp1lKC6aGcnfIkZvipP4ALa6RpbyAPIaWotdc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6A3-uYE5XjYLQQNTzq9wS66crl1qeJL8ATYJtMRvT1z5= 9-_nuuiHiBWp1lKC6aGcnfIkZvipP4ALa6RpbyAPO15OKEo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6A3-uYE5XjYLQQNTzq9wS66crl1qeJL8ATYJtMRvT1z5= 9-_nuuiHiBWp1lKC6aGcnfIkZvipP4ALa6RpbyAPmhGag5Y$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .