Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Mar 26 2023 00:57:58 FOUS30 KWBC 260057 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 856 PM EDT Sat Mar 25 2023 Day 1 Valid 01Z Sun Mar 26 2023 - 12Z Sun Mar 26 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF THE SOUTHEAST... A Marginal Risk for excessive rainfall has been introduced across portions of the Southeast, mainly encompassing south-central Mississippi and Alabama (largely overlapping the Marginal Risk going into Day 2, though displaced a bit towards the southwest). CAMs have been very consistent in depicting convection imitation after 06z tonight along a stalled outflow boundary/frontal zone. A weak perturbation in the mid-levels looks to be responsible for this expected initiation, and little to no large scale ascent/lift looks to be present. The environment, however, will be very supportive of convective organization and sustainment as ML CAPE should range from 1500-2500 J/kg, precipitable water values look to range between 1.2-1.5 inches (between the 90th percentile and max moving average, per BMX sounding climatology), and deep layer (0-6 km) shear is on the order of 50-70 kts. Given the ample deep shear and somewhat curved hodographs, storm mode of deep convective activity should favor supercell structures (especially initially) with both splitting storms and localized training segments being possible. Supercell storms will likely be capable of 1-2"/hr rates, and 18z HREF neighborhood probabilities for 2" exceedance (through 12z) are as high as 15-30%, and 3" exceedance probabilities as high as 10%. These probabilities may be a bit underdone as well, considering that more recent HRRR runs (through 22z) depict the potential for localized 3" totals (and supercells will often exceed what the HRRR depicts as well, especially when associated hail production is at a minimum). Even so, localized totals of this magnitude will be fairly close to associated 3-hr FFG.. therefore the threat for any instances of flash flooding through 12z should remain rather isolated at best (and may not even occur if deep convection fails to initiate from the lack of large scale lift). Churchill Day 2 Valid 12Z Sun Mar 26 2023 - 12Z Mon Mar 27 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF THE SOUTHEAST... Multiple rounds of heavy rain are possible along and just to the north of a lingering quasi-stationary front extending from the Southeast to the central Gulf Coast States, which may prompt isolated instances of flash flooding. Initial convection across central AL and into east-central GA should be ongoing by 12z Sun. These storms by themselves will be capable of 1-1.5"/hr rainfall rates until modest 850 mb southwesterly flow weakens to below 30 kt by about 15z. The resulting cold pool should aid in suppressing the aforementioned frontal boundary closer to the Gulf Coast and setting the stage for the next round Sunday evening. Tropospheric precipitable water content per the 12z HREF is forecast to peak in the 1.5-1.8" range by Sunday evening in response to a surge of 850 mb moisture flux and overarching shortwave swinging from the southern Plains to the Ohio Valley. While not extreme, this moisture source would equate to around +2 standard deviations above climatology (per both the ECENS and GEFS) and into the 75th-90th percentiles. There will also be no shortage of instability, with a corridor of 2000-2500 J/kg MUCAPE Sunday evening along and just inland from the central Gulf Coast. Resulting convection is anticipated to initially develop to the southwest across LA and expand with the increasing 850 mb flow along the stationary front through southern/central MS and AL. Sufficient instability and southwest 850 mb flow running into the frontal boundary will support areas of organized convection (within a larger area of scattered shower/thunderstorm activity) and the potential for supercells, which in turn are likely to contain intense rainfall rates. The slow-moving frontal boundary will also allow for training cells or multiple cells to overlap areas that received previous rainfall. Rain Sunday morning across central AL could also prime soils before the evening convection rolls through and increase the potential flash flooding threat. 12z HREF neighborhood probabilities depicts the upper-bound of potential rainfall during this period by highlighting 15-20% chances for 12-hour QPF to exceed 5" across parts of central/southern AL. This is mainly due to FV3 and HRRR members, which are likely overdone spatially but not out of the realm of possibilities in a localized sense given the overall setup. A Slight Risk was considered during this update centered around parts of central AL, but opted for only an upgrade to a Marginal Risk given recent dry conditions (less than 2% 0-40 cm soil moisture percentiles per NASA SPoRT-LIS) and some lingering uncertainty on where exactly the overlapping heavy rain could set up. WPC QPF is in line with the latest HREF guidance in highlighting the potential for a swath of 2-4" total rainfall. Localized amounts over 4" are possible and would likely overcome even some of the more drier soils in the region. Will continue to monitor the need for an expanded risk area to the west across southern MS and the possibility of a targeted Slight Risk if CAM guidance shows more agreement. Heavy rain to the east into the Carolinas is expected remain fast-moving and limit any flooding impacts. Snell Day 3 Valid 12Z Mon Mar 27 2023 - 12Z Tue Mar 28 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ....2030z Update... No changes from the overnight issuance. Snell ....Northern California... The inherited Marginal risk area for coastal northern California north of the Bay Area, and portions of the northern Sacramento Valley was downgraded to a risk-free map with this morning's update. A vertically stacked low will drift off the coast of Oregon on Monday. The upper levels will be supportive of occasionally heavy rain, especially Monday night, as a 90 kt jet streak rounds the base of a highly positively-tilted upper level trough as it slowly moves south down the coast. This trough will bring with it a renewed push of cold air. Flow orthogonal to the coastal mountains will support locally heavier upslope precipitation as the trough, low, and its attendant surface cold front move south down the coast. With this synoptic setup in place, there are 2 primary factors that were considered in the ERO risk downgrade. First, the aforementioned cold air associated with the trough will hold snow levels to an average 1,500 to 2,500 ft above sea level range. Thus, a substantial fraction of the heaviest upslope precipitation into the mountains will fall as snow. Of course, any snowfall will not contribute to any flash flooding concerns. Thus, any potential flooding would be confined to the immediate coast and the lowlands. Further, the upper level pattern bringing cold Pacific moisture from the Gulf of Alaska into the Pacific Northwest, as opposed to tropical moisture, means there will be far less overall moisture available to fall out as precipitation. PWATs only very briefly exceed 1/2 inch with the heaviest precipitation Monday night. Secondly are the antecedent conditions. Of the entirety of the state of California, the area that has recovered the most from recent atmospheric river events has been the northern California coast. Soil moisture has fallen below 70% in this area, showing that much of the rainfall from recent weeks has had time to drain. Thus, FFGs in this area have recovered such that the chances this storm can produce enough rain to exceed those values are very low, especially considering the low snow levels converting most of the expected precipitation into higher elevation snow. In coordination with the MFR/Medford, OR and MTR/Monterey, CA forecast offices, all agreed the low snow levels will keep the flash flooding threat minimal. ....Southeast... Just as on Day 2/Sunday, the same stalled out frontal boundary draped from northern Louisiana into South Carolina will still be in place on Monday. The same scenario plays out again on Monday, where the weak frontal boundary is the only forcing for convection, despite a juicy airmass featuring PWATs above 1.5 inches and MUCAPE along the Gulf Coast getting up to 1,600 J/kg. So, in rinse-and-repeat fashion, thunderstorms are likely to develop again Monday afternoon, perhaps a little further west along the boundary, favoring the Louisiana-to-Alabama portion of the front. High FFGs will once again be the dominant factor preventing flash flooding, as the storms perhaps form around the front, but have little ability to consolidate. The strongest storms may cause briefly heavy rain, but those storms will be isolated enough that the likelihood of resultant flash flooding will be near zero once again. There is somewhat less confidence in the lack of flash flooding threat on Monday, primarily because the rainfall on Sunday will have possibly lowered FFGs in many of the same areas likely to see additional thunderstorm activity again on Monday. While the broad 5 state corridor from LA to SC is favored, where within those states those FFGs will be lowered will only really be known after Sunday's round of storms has taken its course. Thus, future forecast updates will need to evaluate how much overlap Monday's forecasted storms compare with Sunday's storms, of course taking into account just how much modification the FFGs have undergone after Sunday. Despite the potential for a reintroduction of the Marginal Risk at a later time, in coordination with BMX/Birmingham, AL, MOB/Mobile, AL, GSP/Greenville-Spartanburg, SC, CHS/Charleston, SC, and ILM/Wilmington, NC forecast offices, the Marginal Risk was also eliminated with this morning's update. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8nVoCpDb16PxAXKQnQ2Kzs2WtCqd8uxqTmbs_g5mY7ru= Wp8faxxPVU2Bf9HTg5jbdGFXmaDSO5A8Zjy_teuNFPwgQ0c$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8nVoCpDb16PxAXKQnQ2Kzs2WtCqd8uxqTmbs_g5mY7ru= Wp8faxxPVU2Bf9HTg5jbdGFXmaDSO5A8Zjy_teuNv3QXnJY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8nVoCpDb16PxAXKQnQ2Kzs2WtCqd8uxqTmbs_g5mY7ru= Wp8faxxPVU2Bf9HTg5jbdGFXmaDSO5A8Zjy_teuNyZ12KNY$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .