Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Mar 22 2024 15:42:42 FOUS30 KWBC 221542 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1142 AM EDT Fri Mar 22 2024 Day 1 Valid 16Z Fri Mar 22 2024 - 12Z Sat Mar 23 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE GOLD COAST AND THE FLORIDA KEYS... ....16Z Update... Limited changes necessary across the Marginal risk areas focused=20 over the Southeast/Mid Atlantic, as well as the northern CA coast.=20 Guidance is consistent in the placement and magnitude of the=20 expected rainfall from each disturbance with QPF maxima generally=20 positioned in similar locations when analyzing run-to-run trends. Northern CA will see its heaviest rainfall from now until early this evening before the primary moisture plume shoves inland in wake an advancing cold front. This will likely end the necessity of the risk area as the focus shifts inland with winter weather becoming the primary concern.=20 The heaviest rainfall across the Southeast and Mid Atlantic will=20 occur along the NC coast by the end of the period as a maturing=20 surface low will aid in projecting a strengthening 85H LLJ off the=20 adjacent Atlantic with modest theta-E returns centered over the=20 coastal Carolinas. Latest HREF probability fields indicate a nose=20 of elevated 1-2"/hr probability signals within the aforementioned=20 corridor which could lead to localized flooding within any training cells. Storm totals of 1-3" with locally up to 4" along the NC=20 coastal plain are plausible in the setup, but much of the rainfall=20 occurring over an area of higher FFG thresholds given sandier soils limiting runoff.=20 Across Florida, the trends have been consistent within the latest hi-res guidance on the southern FL Peninsula being the primary beneficiary of multiple waves of heavy rainfall that present totals breaching 5" over the course of the D1 time frame. The=20 Slight risk was expanded to the north over the southeastern FL=20 coast to align with the latest HREF blended mean QPF forecast and=20 accompanying probability fields signaling a threat for 2-3"/hr rates (25-45%) affecting the population centers from West Palm=20 Beach down to Miami through all the of the FL Keys. The lower FFGs=20 centered over the I-95 corridor will be the focus for the greatest=20 flash flood risk, but the FL Keys will have the greatest threat for the higher totals, so the entire area is well-within the criteria=20 for the SLGT risk being maintained from the previous forecast=20 cycle.=20 ....Previous Discussion... ....Southern Florida... Convection ongoing across the central and southeastern Gulf is tracking southeast towards southern Florida and the Keys this morning. The storms have formed along a robust moisture gradient, south of which PWATs are forecast to exceed 2.25 inches, which is impressive even by summertime standards. The storms are training across the Gulf, so the real question is where the line of storms sets up once they reach south Florida and the Keys. CAMs guidance continues to sag southward with where the heaviest storms will track, some even as far south as the north coast of Cuba. Despite this, the incredibly moisture-rich environment the storms have to work with, even by summer standards, should easily result in multiple inches of rain for the Keys especially, and depending on where the storms track could impact the Miami metro. Storm total rainfall amounts to 5 inches are forecast around Key West, closer to 3 inches around Key Largo, and 1-2 for Miami. Again these amounts are quite uncertain as any small northward deviation in the storm track could bring all of these numbers higher, while=20 continued southward shifts would lower them. The inherited Slight was trimmed out of all of southwest Florida, as the heaviest convection is most likely to miss that area to the south and west, and the swampy nature of the area should be able to handle the 1-3 inches of rain forecast since this will fall=20 over a longer time frame as outflow from the storms to the south=20 and west. The surrounding Marginal was left unchanged given the=20 inherent uncertainty as to where isolated flooding may develop. ....The Southeast... Cyclogenesis occurring ahead of a slow-moving but intensifying shortwave trough located in the "sweet spot" in between the left exit region of a 150 kt jet over coastal Texas and the right entrance region of a 140 kt jet along the US/Canada border from NY through ME will be the catalyst for heavy rain across the area. With the storm only just forming through 12Z Saturday, most areas should see a long-duration light rain, though with potential embedded convective elements. The most likely areas of isolated flash flooding will be into the southeast facing slopes of the southern Appalachians through the Great Smokies, where upslope will locally enhance rainfall rates, as well as along the Carolina coast where a plume of moisture to 1.75 inches PWAT will stream=20 northward ahead of the main low. The extra moisture in this plume=20 will locally enhance rainfall rates from Charleston, SC through the southern Outer Banks. Elsewhere in between, expect a rapidly=20 expanding precipitation shield that will move up the coast well=20 into New England by 12Z, as increasingly strong frontogenesis=20 ahead/northeast of the low focuses the precipitation shield all up=20 the East Coast. ....Northern California Coast... A strong vertically stacked will will approach the Washington/Oregon coast through today and into tonight. Already moving slowly, the low will turn northward up the coast tonight. The slowing eastward motion of the parent low will translate to a slowing eastward translation of the trailing cold front, the latter of which will be the focus for the heaviest rainfall=20 leading to potential isolated flash flooding. The area where the=20 cold front will be slowest moving is along the northern CA coast=20 and far southwestern OR, where coastal terrain will further enhance rainfall amounts. The inherited Marginal Risk for this area was=20 unchanged with this update as the terrain-focus of the heaviest=20 rainfall remains in excellent agreement. Wegman Day 2 Valid 12Z Sun Mar 24 2024 - 12Z Mon Mar 25 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-ATLANTIC INTO NEW ENGLAND... ....Mid-Atlantic into New England... Few changes were needed to the inherited Slight Risk across portions of the Northeast. Rainfall amounts are nominally higher from previous forecast, but there remains very good agreement that a coastal front will be enhanced by a strengthening southerly jet streak in the right entrance region along the coast. This will contribute to the already impressive frontogenesis across the northeast with 850 temperatures to -12C over the upper Lakes contrasting with +6 along the Eastern Seaboard at 00Z Sunday. This extreme temperature gradient will enhance the southerly flow ahead of it, which will force abundant moisture into the coastal front, extending the precipitation shield northward well away from the surface low, which will increasingly become secondary to the coastal front as far as the most impactful feature. Given the very cold air north of the front, there will be a stark rain-snow line across New England, with plowable snows expected north of the front, and up to 3 inches of rain to the south. As usual the bulk of the moisture will be on the warm side, which will align with the I-95 corridor from DC into southern Maine. Thus, expect a period where rainfall rates could approach 1 inch per hour along the urban corridor. Fortunately, HRRR guidance suggests that the period of time where rainfall rates could be that high will only be for a 1-3 hour time frame in any one=20 location, as the heaviest rainfall quickly races off to the north=20 and east. This will be the main limiting factor preventing more=20 widespread flash flooding. However, given soils that are only=20 beginning to green up across the area, much of the soils will be=20 unable to handle this amount of rain falling even this quickly, so=20 widely scattered instances of flash flooding are likely through the Slight Risk area. A higher-end Slight is forecast from Wilmington, DE through Boston. This area is where the strongest frontogenesis=20 will align with the leading edge of the moisture plume for the=20 aforementioned 1-3 hours at the ending of the event, and the=20 corridor where any potential upgrades to a Moderate are likely to=20 be focused. Despite the long-duration rain throughout the Slight=20 Risk, the relatively brief period of heaviest rains should preclude the need to upgrade at this time, but future shifts will evaluate. The primary changes from inherited were to include more of the northern Virginia suburbs of DC in the Slight due to lower FFGs in this area, as well as a significant northward expansion of the Slight from near Boston north through Portland, ME. In this area the rain-snow line will be the dominant factor as to which areas get potentially flooding rain versus plowable snow. ....South Florida... Lingering rainfall from the Day 1 period will be ongoing Saturday morning. The front causing the rain will shift eastward during the morning, ending the flooding threat rapidly from west to east. The Marginal risk was adjusted to closely match the Day 1 Slight area. Since the vast majority of the rainfall with this event will be over before the start of this period, the Marginal is mainly acknowledging continuing ongoing flooding, rather than any new excessive rainfall. Please see the Day 1 discussion for the fuller meteorological setup for this area. ....Northern California Coast... As the surface low off the Oregon coast moves inland and rapidly weakens, the overlying upper level low will slowly shift southward down the coast through Sunday morning. In advance of the low, rainfall will continue into the northern California coast, with another 1-3 inches of rain anticipated. These amounts generally remain below concerning levels, but given the similar amounts of rain expected in the Day 1 period, a doubling of that rain through Day 2 could again lead to isolated flash flooding concerns. The inherited area was nudged southward a row of counties on both ends, but is otherwise largely unchanged. Wegman Day 3 Valid 12Z Sun Mar 24 2024 - 12Z Mon Mar 25 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA... ....Southern California... Onshore flow ahead of a strengthening upper level shortwave diving southward parallel to the coast will keep a steady light to locally moderate rain ongoing across southern California, largely south of Los Angeles through the Day 3/Sunday period. Light rain will begin in this area in Day 2, but the bulk of the rain is expected on Day 3. Given the sensitivity of the soils and urbanization, a Marginal Risk area was introduced in response to increasing forecasted rainfall through the day Sunday. Rainfall amounts of around three- quarters of an inch may cause isolated flash flooding in the terrain east of San Diego, along with any poor drainage areas in the city and surrounding suburbs. The overall low-level windflow pattern from the northwest will prevent any tap of subtropical moisture, so the maximum amount of rainfall will be quite limited, so further upgrades in this area are not anticipated. ....Nebraska/Iowa... The inherited Marginal Risk area for eastern Nebraska and western Iowa was downgraded with this update. While moisture amounts with the developing storm are well above climatology, most of the guidance suggests the heaviest precipitation will be realized in the cold conveyor belt of the storm, and any locally heavy rainfall associated with embedded convection will be progressive.=20 Thus, the winter storm aspect of this low will be the far more=20 impactful. Further, antecedent conditions have been very dry over=20 this area, so the limited amounts of rain that are largely expected to remain under an inch should be well handled by the soils of=20 this area, precluding even isolated flash flooding. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6v-D-hnVclVdxTYfZFJt3yK7ogabkTCpMvQpDnNe49zG= YwiII2eWvKuHoZDK1RRUb9jHURqmvHwsXL9VxE3gqvun35M$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6v-D-hnVclVdxTYfZFJt3yK7ogabkTCpMvQpDnNe49zG= YwiII2eWvKuHoZDK1RRUb9jHURqmvHwsXL9VxE3gZy17Vls$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6v-D-hnVclVdxTYfZFJt3yK7ogabkTCpMvQpDnNe49zG= YwiII2eWvKuHoZDK1RRUb9jHURqmvHwsXL9VxE3gNJGmW6g$=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) .