Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Mar 23 2024 20:08:24 FOUS30 KWBC 232008 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 408 PM EDT Sat Mar 23 2024 Day 1 Valid 16Z Sat Mar 23 2024 - 12Z Sun Mar 24 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-ATLANTIC INTO NEW ENGLAND... ....16Z Update... Two main areas of concern this morning with regards to flash flooding concerns. The first is the larger of the two areas within the Northeastern U.S as our surface low continues to advance progressively northeast out of VA with the best low-level convergence pattern centered along the I-95 corridor. Heavy rainfall is pushing north of the Potomac near DC and will migrate to the northeast fairly quickly through the period with the heaviest rains located just to the north of the strengthening 850mb low. Rates have been capped between 0.25-0.75"/hr so far through the event, but a stronger convergence pattern north of PHI up into NNJ/NYC/CT will occur this afternoon with rates of 0.75-1"/hr increasingly likely between 18-22z as per the latest hi-res deterministic and accompanying HREF ensemble progs (See MPD #0110 for more details on the flash flood threat). This coincides with the strengthening 850mb low with moist LLJ inflow off the adjacent Atlantic providing the perfect pattern for a short term, heavy rain threat. The aforementioned area is where the higher end of the SLGT risk threshold will lie with progression into the Boston metro after 22z, ending later this evening. Additional totals of 1-2" are likely from northeast MD up through southern New England with a max of 3" possible within the zone mentioned above where the LLJ influence will be most prominent during the period. South FL continues to exhibit periods of heavy rainfall thanks to a shortwave migrating eastward along the cold front advancing through the region. Instability axis situated ahead of the boundary with general forcing allowed for a line of thunderstorms to advance into the south FL metro areas like Miami/Fort Lauderdale this morning with rates settling between 2-3"/hr at peak intensity. Rain rates have come down recently, but the next few hrs could see additional rainfall of 1-2" before the front clears the area and we go into a drying pattern. This will put an end to the threat for flash flooding and a future removal of the MRGL risk in place. Until then, wanted to maintain continuity based the latest radar trends and current MPD #0109 in place across the southern FL Peninsula. Removed the MRGL across northern CA due to the lack of sufficient rates and guidance trimming back on the total rainfall in the latest runs. Scattered to numerous showers with rates up to 0.25-0.3"/hr will still occur across southwest OR into the northwest CA coastal plain, but the threat of flash flooding will be below the necessary threshold to maintain the previous risk area. With agreement from both the Eureka and Medford WFO's, have removed the previous MRGL risk. Kleebauer ....Previous Discussion... ....Mid-Atlantic into New England... A wound-up low, associated upper level trough and shortwave, and 150 kt northwesterly jet streak have all aligned to result in a very active weather day along much of the East Coast this morning. Impressively deep moisture is being drawn northward up the Eastern Seaboard ahead of the well-defined low circulation. PWATs are above 2.25 inches around the Florida Keys, but for the Mid-Atlantic into New England, they'll exceed 1.5 inches. This moisture will provide ample support for widespread rain that is already evident on radar. The precipitation shield will continue pushing northeastward across the Slight Risk area through the morning. Across the lower Great Lakes, an impressive cold front supported by an even more energetic negatively tilted shortwave trough in the upper levels is already beginning to rapidly intensify a second jet streak that will roughly follow the US/Canadian border from NY to ME on the Canadian side. While sufficiently separate from the subtropical jet across the Southeast, the right entrance region of the jet and impressive baroclinic zone associated with the front will draw the aforementioned abundant moisture northward, clashing with the advancing cold air from Canada. This boundary will act as a focal line along which heavy rain (and snow on the north side) will develop and track along the boundary to the northeast. It's likely the heaviest rain will begin to form on the southwestern side of the Slight Risk area over eastern MD and the Delmarva, then progress up the coast. As it moves through the NYC area and into New England, the increasing baroclinicity will further enhance lift. Most of the CAMs suggest embedded convective segments and cells aloft across MD, DE, southeast PA, and NJ will coalesce into more of a line as they approach NYC and continue as a line of much heavier rain across southern New England. This heaviest line of rain will precede the back edge of the rainfall associated with the low's dry slot. Ahead of the line, convective cells aloft are likely, which will locally enhance rainfall rates. Fortunately they will be brief in any one area as any convection will be racing northeastward embedded within the broader precipitation shield. The combination of the off-and-on convective elements within the broader rainfall shield, followed by the line of much heavier rain will result in widespread storm total rainfall from MD through MA of 2 to 3 inches, with locally higher amounts. Urban areas along the I-95 corridor will be most susceptible to resultant flooding, with the highest risk from roughly Wilmington, DE through Boston. This area remains in a "higher-end Slight". The area of greatest concern within that corridor extends from NYC northeast through CT and RI, where a coastal front, upslope from the terrain of southern New England, and a well defined strong line of heavier rain will have already formed. Widespread urbanization in this area will contribute to lower thresholds for flooding. Within the line, rates to an inch per hour are possible, which following the entire morning of steady light to moderate rain should result in scattered instances of flash flooding, with potential for considerable flooding in the most susceptible areas. Little change in storm-total rainfall and the evolution of the rainfall across this region led to relatively little change in the forecast. Given somewhat dry antecedent conditions and widespread rainfall amounts largely unchanged, a Moderate was considered, particularly in the NYC to southern New England area, but the short duration of the heaviest rates and little change to the forecast precluded that. The guidance shifted a bit to the east across the Mid-Atlantic, so the Slight was removed from VA, and the Marginal was shifted a row of counties eastward. Otherwise there was very little change to the risk areas from NY into New England. The Marginal risk was expanded to include the entirety of coastal Maine with this update. The aforementioned convective line and the warm advection out ahead of it should allow for a transition of precipitation type for all of the coast from snow to mixed precipitation and over to heavy rain. Heavy rain falling on ice should enhance runoff, which will be offset by a lack of inflow from interior Maine due to all of the precipitation falling as snow. Given the uncertainty, the Marginal was hoisted but should conditions verify warmer, then Slight level impacts are possible as far north as Portland. ....South Florida... A Marginal Risk will remain in place for portions of southeast Florida from Ft. Lauderdale south and the Florida Keys. This Marginal is largely a precaution as the bulk of the rainfall has already occurred, but there remains some uncertainty as to whether the rain will be fully pushed off to the south and east by 12Z. The 00Z guidance has pushed south as compared with the 18Z guidance, which resulted in the heaviest rain remaining just south of the Keys, which should preclude any flooding. However there is not unanimous agreement among the guidance as to this scenario. There's better agreement that the heaviest rains will be clear of south Florida by the 16Z midday update. Wegman Day 2 Valid 12Z Sun Mar 24 2024 - 12Z Mon Mar 25 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA... ....20Z Update... Little change in the overall forecast for Sunday as increased onshore flow across southern CA will allow for a regional maximum in precip across the Peninsular Range down towards the San Diego=20 metro area. Guidance is fairly steady within the 1-1.5" range=20 across the interior with HREF EAS probabilities over 80% for at least 1"/24-hrs in a small zone in-of of the Peninsular Range.=20 Lower probabilities are located within the San Diego metro itself, but still non-zero around 10-20%. Primary concern will be terrain influences within the zone of heaviest rainfall, as well as poor drainage areas within the risk area that could be more prone to flooding. The MRGL risk was maintained. Kleebauer ....Previous Discussion... The vertically stacked low currently off the Washington/Oregon coast will have opened up into a vigorous positively tilted trough at the start of the period Sunday morning. The trough will track down the coast through Sunday evening as a northerly 150 kt jet streak approaches. The left exit region of the jet will align over southern California, while onshore west to west-northwesterly winds pump Pacific moisture into the Peninsular Ranges. Forecast rainfall Sunday afternoon and evening has been gradually increasing, with favored upslope west-facing slopes now expected to pick up around an inch of rain for the day. Thus, the Marginal remains in place with increased confidence for isolated flash flooding in the flash flood prone foothills of the Peninsular Ranges as well as poor- drainage locations in the San Diego metro. Wegman Day 3 Valid 12Z Mon Mar 25 2024 - 12Z Tue Mar 26 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER MISSISSIPPI VALLEY... ....20Z Update... The Slight Risk was maintained for the Lower Mississippi Valley for Monday as the overall synoptic pattern supports heavy rainfall thanks to improving dynamics and increased buoyancy ahead of a sharp cold front advancing east out of the plains. No major changes were made to the overall risk alignment, but a few places were added to the threat areas based off the latest ensemble trends and ML based guidance. The MRGL risk was expanded to the southwest to include the eastern section of the hill country to the east of I-35 as the latest ensemble trends show the tail end of the cold front tilting back to the southwest across south-central TX which would allow for trailing thunderstorms to propagate over the more flood-prone areas just to the east of the I-35 corridor. This also matches the GFS and ECMWF Graphcast ML models with the trailing convection idea that could spur trouble for localized flooding in that particular zone. The other area of interest was the expansion of the Slight Risk to the south over northern LA where deep-moist convergence along the advancing cold front into a well-defined instability axis will project a threat of heavy rainfall as ML and deterministic guidance paints upwards of 2" or more of precip within the corridor between I-10/20. Considering the stout meridional pattern expected over the central US, the trends for a=20 slower moving convective scheme is favored which would allow for a=20 greater threat of training. This is also agreed upon by the latest=20 ML total precip depiction, as well as the ensemble mean forecast=20 from the NBM and associated global ensembles.=20 Kleebauer ....Previous Discussion... A positively-tilted upper level longwave trough over the West will eject into the southern Plains Monday, becoming more neutrally tilted with time. Vigorous shortwaves will lead the reorientation of the longwave trough, with the northernmost one energizing a mature low and winter storm across the northern Plains. That low's trailing cold front will provide the forcing for convection across the Marginal and Slight Risk areas. Due to a second shortwave in the subtropical jet moving the base of the trough eastward, the trailing cold front will tap into increasing Gulf moisture in the lower levels, leading to the rapid intensification of a low level jet up the lower Mississippi Valley. Moisture-rich showers and thunderstorms will develop ahead of the cold front, resulting in a wide swath of 2+ inch forecast rainfall totals in the Slight Risk area from south-central Missouri across most of Arkansas and much of the northern two-thirds of Mississippi. Since these rainfall amounts will be associated with thunderstorms, it's likely they will fall in a short-time frame, which in turn will increase the flash flooding risk. The Slight Risk was nudged a bit towards the northwest and southeast to cover more of AR and MS, while trimmed across northern LA following small changes in the latest guidance from inherited. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-rX21tU8PiQlKuyPTlg3JpwJML-tiog3tz_h9vSB5g4z= Nxn3j2uYQg9MsZxBLM2yzFmVtGRObYqi-0WOLtnDGpfuwF8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-rX21tU8PiQlKuyPTlg3JpwJML-tiog3tz_h9vSB5g4z= Nxn3j2uYQg9MsZxBLM2yzFmVtGRObYqi-0WOLtnDVYZuroY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-rX21tU8PiQlKuyPTlg3JpwJML-tiog3tz_h9vSB5g4z= Nxn3j2uYQg9MsZxBLM2yzFmVtGRObYqi-0WOLtnD6z5CBiM$=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) .