Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Mar 26 2023 20:05:46 FOUS30 KWBC 262005 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 404 PM EDT Sun Mar 26 2023 Day 1 Valid 16Z Sun Mar 26 2023 - 12Z Mon Mar 27 2023 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF CENTRAL ALABAMA AND GEORGIA... ....Southeast... Ongoing convection this morning has organized into a linear orientation along a draped frontal boundary and has already produced flash flooding across parts of GA ahead of a primary low amplitude shortwave. This initial area of thunderstorms containing 1-2"/hr rainfall rates, with locally higher amounts, should continue to push eastward across SC midday and lead to the potential for scattered flash flooding. This prompted the expansion of the Slight Risk eastward into central SC. Farther west, lingering scattered convection is likely throughout the day as persistent southerly low-level flow from the Gulf of Mexico and recharging instability fuel continued thunderstorm activity until a more organized round of heavy rain enters this evening. The overlap between the ongoing/morning heavy rain and the expected evening/overnight storms prompted the upgrade to a Moderate Risk for Excessive Rainfall from parts of central AL to central GA. A broad longwave trough over the Rockies will contain a potent shortwave to eject into the Mid-Mississippi Valley this evening an create an environment ripe for additional upper divergence and a sufficient low-level jet. PWATs of 1.5"-1.75" combined with around 2,500 J/kg MUCAPE and 40 kts of 850 mb inflow tonight is plenty for organized thunderstorm development, with an area to focus along a lingering frontal boundary. Some supercells are possible within a more broad area of thunderstorm activity, which will produce areas of particularly intense rainfall rates. Additional rainfall amounts along a narrow corridor within the MDT risk could approach 4" and occur over a similar area that has already received 2-4" this morning. 12z HREF neighborhood probabilities depict elevated chances for additional rainfall over 3" from southeast MS to central GA tonight. HREF also highlights much of the MDT risk area as having 40-60% chances for over 5" of QPF for the 24-hour period ending 12z Mon. If any of this overlap does occur overnight, significant flash flooding impacts are possible. There is the potential for convection to initiate farther to the south and closer to the Gulf Coast this afternoon along with the diurnal heating, which could drag a surface boundary and overnight convection further south. This would lead to less overlap and a lower threat of numerous instances of flash flooding. However, current thoughts are that the approaching shortwave, low-level jet, and current visible satellite trends supports solutions showing a heavy axis of additional rain over the MDT risk area. ....Southeast Missouri to Southern Indiana... A compact shortwave crossing the Midwest this afternoon will be associated with a slow-moving frontal boundary from southern MO to IN, which is likely to have an area of up to 500 J/kg of SBCAPE and 400 m^2/s^s of 0-3 km SRH to work with. A line of showers and thunderstorms is forecast to develop near the upgraded Marginal Risk area after 21z and could move in a linear fashion over similar areas given the frontal boundary and mean-layer wind profile appear nearly parallel. Rainfall rates will not be that impressive and likely remain under 1"/hr, but rainfall totals up to an inch could pose an isolated flooding/flash flooding risk, especially given this region experienced over 4" of rain not too long ago. Streamflows are running high and soil moisture percentiles are very high per NASA SPoRT. The antecedent conditions were the main driver for the upgraded Marginal Risk. Additionally, 12z HREF neighborhood probabilities do show up to 25% chances for 0.5"/hr rainfall rates between 00z and 06z Mon. Snell Day 2 Valid 12Z Mon Mar 27 2023 - 12Z Tue Mar 28 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF THE DEEP SOUTH AND NORTHERN CALIFORNIA... ....Deep South... By Monday morning, the same frontal boundary impacting much of the Southeast and Gulf Coast with scattered showers and thunderstorms will linger throughout the day and lead to additional chances for isolated flash flooding. Initial convection early in the time period is forecast to be ongoing across parts of southern AL and central/southern GA and could overlap with some areas receiving heavy rain today and tonight. PWATs will remain around 1.5" during this timeframe and 850 mb flow turning more westerly through midday will remain around 40 kts. This boundary is forecast to continue to be a focus for scattered thunderstorms from southern AL, southern GA, and the Florida Panhandle through the early evening hours. 12z HREF neighborhood probabilities depict 20-40% chances for 6-hour QPF to exceed 2" across this part of the Southeast through 00z Monday night. This signal along with current rainfall trends aided in the decision to introduce a Marginal Risk here. The next in a parade of shortwaves to eject out of the western U.S. trough will aid in a separate area of developing convection across eastern TX that should push into LA and southern MS after 06z Monday night. The shortwave tracking through the Mid-Mississippi Valley (similar to today) will allow for additional upper divergence and an increase in southerly flow out of the Gulf of Mexico. However, upper-level ridging retrograding westward underneath this shortwave across the far southern Gulf of Mexico will allow for convection to develop farther west with this round. It's possible the upgraded Marginal risk for Day 2 may need to expanded farther into central TX if CAM guidance picks up on the potential for earlier/western convection on Monday. By 12z Tuesday, 12z HREF highlights very high probabilities along the southern TX-LA border for over 2" of rain in 6 hours, which could lead to isolated flooding concerns, even for areas with relatively high FFGs. ....Northern California... Rapidly deepening upper-level low and surface cyclone off the Pacific Northwest late Monday night will usher in the next atmospheric river to California towards the tail end of the Day 2 ERO. IVT values after 06z Tuesday increase to 500-600 kg/m*s north of the San Francisco Bay Area and will spread southward along the California coast on Day 3. Areas along the northern California coast will have the best chances to achieve higher than 0.5"/hr rainfall rates and the potential for isolated flooding or rapid runoff through 12z Tuesday, hence the upgraded Marginal Risk area. Snow levels will rapidly drop to around 3000 feet, limiting any snowmelt and runoff from the higher terrain. Snell Day 3 Valid 12Z Tue Mar 28 2023 - 12Z Wed Mar 29 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE CENTRAL CALIFORNIA COAST AND SAN FRANCISCO BAY AREA... ....California... Only minor changes were made to the previous risk areas, with the main expansion being to stretch the Marginal Risk to include Los Angeles county. An upper level polar cutoff low begins the day Tuesday morning centered off the Oregon coast, and tracks southeastward to be centered over the northern California coast by Wednesday morning. A 100 kt westerly jet streak wrapping around the low will direct a plume of Pacific moisture and precipitation into California as it tracks south. PWATs will approach 0.75-1.00", which is about +1.5-2 sigma above the climatological normal and IVT peaks at about 500-600 kg/m*s along central California. This will bring with it a brief increase in snow levels as high as 7,000 ft along coastal ranges during the day Tuesday. The attendant surface low will follow a similar track as the upper level cutoff low through this period, making it a vertically stacked low, that will result in largely unidirectional flow throughout the column south of the low. Since the vertically stacked low has polar origins, it will have plenty of cold air and a lack of plentiful Pacific moisture with it. Thus, this rainfall event is not expected to be as intense as the atmospheric rivers impacting the state in recent weeks. Nevertheless, the residual impacts from the atmospheric rivers remain across coastal central California, and the injection of up to 3 inches of rain into the coastal mountains are still expected to cause or worsen flooding impacts as that rainwater flows down the mountains and swells already high rivers. Thus, the Slight Risk was maintained due to steady forecasted rainfall through the Bay area and especially the mountains north of there, while also spanning south to include the westernmost Transverse Ranges of Santa Barbara County. Further inland, slightly lower snow levels, much higher mountains, and more recovery time from the last round of atmospheric rivers were factors that led to the continuance of the Marginal Risk, as much of the heaviest precipitation is likely to fall as snow. The highly favorable antecedent conditions for flash flooding remain a significant factor in the Slight Risk area. ....Central Gulf Coast States... The Marginal Risk area along the Gulf Coast was confined to the south and along the central Gulf Coast compared to the recent issuance in order to match recent guidance and the likelihood that intense rainfall rates will remain closer to the coastal regions. By this time, the lingering frontal boundary plaguing the Deep South will shift farther to the south and limit the available moisture/instability for areas to the north. Still, convection ongoing from Monday night moving into eastern LA and southern MS is likely to tap into 1.5" PWATs and have rainfall rates potentially exceeding 1"/hr. Urban regions and locations where rain has fallen over the last few days will be most at risk to localized flooding concerns. Snell/Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5c1Rj9UFIP3IJIR8EEYDgfvp1j-FfOu1E5V5y2epUyXA= ns1sFYiZf1egEwNELdW3SZXCm7tAoJY6bQPYAU2MVveQYfE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5c1Rj9UFIP3IJIR8EEYDgfvp1j-FfOu1E5V5y2epUyXA= ns1sFYiZf1egEwNELdW3SZXCm7tAoJY6bQPYAU2MUG5pmps$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5c1Rj9UFIP3IJIR8EEYDgfvp1j-FfOu1E5V5y2epUyXA= ns1sFYiZf1egEwNELdW3SZXCm7tAoJY6bQPYAU2MbRLmtnw$=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) .