Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Mar 05 2024 00:15:52 FOUS30 KWBC 050015 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 714 PM EST Mon Mar 04 2024 Day 1 Valid 01Z Tue Mar 05 2024 - 12Z Tue Mar 05 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF LOUISIANA AND MISSISSIPPI... A shortwave aloft is moving an area of confluence at 850 hPa eastward across LA/MS eastward with time. Afternoon and early evening convection has carved out a hole in the instability field, creating CIN and forcing convection to be elevated, helping to provide a focus poleward of a section of the old polar front returning from the Gulf of Mexico which has been reinforced by rain-cooled air. While mesoscale guidance continues to have less than ideal agreement, convective organization and progression thus far has allowed northerly solutions such as the 12z Canadian Regional to be less relied upon. A complication is the warming at 700 hPa acting to force convection northeast while confluence zone shift east...a net east to east-northeast motion to the rain area is expected. Precipitable water values are near 1.75", which are more than high enough for an excessive rainfall concern.=20 Effective bulk shear is around 40 kts, which has allowed some mesocyclones to form. Cell mergers, short periods of cell training, and mesocyclones have been the main source for the heaviest rainfall. Hourly rain totals to 2.5" and local amounts to 4" are possible within the risk areas. Kept the Slight Risk area back near the southwest MS/LA border where heavy rain has fallen and the ground should be fairly saturated. The heavy rain risk should slowly shift east and fade overnight, which appears to be due to shortwave progression weakening upper level divergence with time and warming 700 hPa temperatures eventually acting as a mid-level capping inversion and lowering the risk of heavy rain potential, despite persisting MU CAPE. Roth Day 2 Valid 12Z Tue Mar 05 2024 - 12Z Wed Mar 06 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHEASTERN LOUISIANA AND WESTERN COASTAL MISSISSIPPI... ....2030Z Excessive Rainfall Discussion... Overall...few changes needed. Still some question as to how quickly overnight convection will be fading as the Day 2 period begins at 12Z on Tuesday. So left the western portion of the Marginal Risk area in place. Otherwise the moisture transport from the Gulf ahead of the wave will be in place to locally enhance amounts. Overall forecast reasoning from before still is representative. ....0830Z Excessive Rainfall Discussion.,.,. In coordination with LIX/New Orleans, LA forecast office, a Slight Risk upgrade was introduced with this update. Ongoing convection across eastern Louisiana at the start of the Day 2 period early Tuesday morning will shift eastward along the Gulf Coast and the far northern Gulf through Tuesday morning as a potent cold front and much drier air races into Louisiana behind the front. The Slight Risk is highly dependent on flash-flooding-inducing rains having occurred overnight Monday night. By Tuesday morning, the last of the storms will move through ahead of the leading edge of cooler and drier air. Thus, the Slight Risk is very much a continuation of flooding potential into Tuesday from the bulk of rainfall on Monday, and really only is in effect for Tuesday morning. Once the drier air moves in, the flooding risk will end across southeast Louisiana. The Slight Risk area depicts the area likely to be wettest both from Monday and into Tuesday morning with straggling showers and thunderstorms, generally from New Orleans south and east. For the rest of the much larger Marginal Risk area, the storms in southeastern Louisiana in the morning will continue tracking east, with significant uncertainty as to how far north they will impact away from the immediate Gulf coast. Much of the guidance suggests the strongest storms will stay just off the coast, with limited impact for coastal communities from Mississippi to the Florida Panhandle. This was a large factor in keeping the area east of Mississippi in a Marginal Risk for now, as high FFGs should allow for effective absorption/runoff with only isolated flash flooding. Should the axis of strongest storms shift north this could change with future runs. Also worth factoring in is that most of the heaviest rains on Day 1/Monday are expected to shift south and off the coast, largely missing this area and not priming the soils for the heavier rains expected Tuesday. Overnight Tuesday night, much of the guidance suggests a surface low will form over AL/GA in the evening as a shortwave trough buckles the jet stream over the area. The low will more effectively draw the Gulf moisture along the coast inland and into Georgia. This will spike rainfall totals with more widespread 1 to 2 inch totals expected for the western two-thirds of the state and far eastern Alabama. Since most of this rain is expected overnight Tuesday night, instability will be quite lacking, though a few hundred J/kg of elevated CAPE may allow for some elevated convection. Regardless, the amounts of rain with lack of instability forecast are unlikely to result in more than isolated flash flooding impacts for this region. The topography of the southern Appalachians may locally increase flash flooding impact potential northeast of Atlanta. Wegman Day 3 Valid 12Z Wed Mar 06 2024 - 12Z Thu Mar 07 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE COASTAL MID-ATLANTIC THROUGH SOUTHERN NEW ENGLAND... 2030Z Excessive Rainfall Discussion... ....Mid Atlantic Coast through Portions of Southern and Eastern New England... Introduced a Slight Risk of excessive rainfall over a portion of the coast Mid Atlantic and New England as low pressure continues to spread rainfall north and east on Wednesday and Wednesday night. Rainfall amounts in the 12Z suite of guidance has increased along an axis from coastal New Jersey across parts of Long Island to eastern Massachusetts. The area over portions of Southern New England already have high stream flows as a result of a several recent soaking rains recently...indicative that any additional rainfall will become run-off quickly and result in flooding. The 12Z ECMWF now has more QPF than most of the high resolution guidance from late Wednesday into Wednesday night/early Thursday...which limits confidence in its rainfall amounts. But the upward trend in rainfall amounts was consistent in other guidance. But with precipitable water values forecast to exceed the 95th percentile by Wednesday evening in this corridor and the magnitude of convergence along a boundary within this corridor...an upgrade was warranted. ....Southern California... Low pressure approaching the central California coast will spread moisture and increasing coverage of rain across southern California during the day on Wednesday that persists into Wednesday night. While amounts don't look to be blockbuster in nature...the area has not had much time to recover from recent soaking rainfalls which makes the area more susceptible to run off and flooding concerns than usual. Bann 0830Z Excessive Rainfall Discussion... A developing surface area of low pressure that will be over northern Georgia Wednesday morning will track northeastward up the Eastern Seaboard through Wednesday night, and will be centered off the Delmarva by Thursday morning. A plume of moisture with PWATs of 1.5 to 1.75 inches originating from the Gulf will lead ahead of the low center, generally tracking up coast. As the low moves towards the Delmarva Tuesday night, it will increasingly interact with an increasingly colder air mass behind a potent cold front along the St. Lawrence River to northern Maine. The strong southerly flow ahead of the low bringing the very warm and moisture-rich air mass into the cooler air to the north, the strengthening low, and interaction with the Catskills and the mountains of southern New England will locally enhance rainfall amounts in this area, including through NYC and Long Island. Rainfall amounts from the Jersey Shore to Boston will range between 1.5 and 2.1 inches. While there is not expected to be much instability associated with the warmer air mass and moderate rain, these rainfall totals expected over a 12-18 hour period will be enough to cause isolated flash flooding issues for the urban areas of NYC and the nearly saturated soils and high river levels of southern New England. Thus, while rates are expected to be underwhelming, the volume of water is concern for isolated flash flooding. The Marginal Risk area was shifted 100-150 miles east from the inherited forecast with this update. While a soaking half to 1.25 inches of rain are expected from the Carolinas north through upstate New York, a lack of instability causing stratiform rain, fast movement of the low, southerly flow limiting upslope enhancement, and somewhat limited interaction with the cold front until the rain moves into New England should limit rainfall rates and overall totals. Thus, the flash flooding risk was determined to be very limited west of the Delaware River. The highest rainfall totals and greatest threat for flash flooding will be east of the Delaware River. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9K3Y0pWyXokaRjUkS-jOR06TK775Fx-_RfSrvzG3I3QC= JC_98wYg3mBbmPIswr9hTQ_Nt-6eIVMuz7OX5A1GNLaT2Sc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9K3Y0pWyXokaRjUkS-jOR06TK775Fx-_RfSrvzG3I3QC= JC_98wYg3mBbmPIswr9hTQ_Nt-6eIVMuz7OX5A1GdvkWteA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9K3Y0pWyXokaRjUkS-jOR06TK775Fx-_RfSrvzG3I3QC= JC_98wYg3mBbmPIswr9hTQ_Nt-6eIVMuz7OX5A1Gt_HiFmA$=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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