Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Mar 26 2023 15:16:39 FOUS30 KWBC 261516 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1116 AM 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 The probability of rainfall exceeding flash flood guidance is less than 5 percent. By Monday morning, most of the convective activity associated with the front in the Southeast will have drifted south of the main areas impacted by the strong storms today, i.e. closer to the Gulf Coast. With little in the way of overlap from Day 1 and good agreement overall that convective coverage and intensity will be greatly reduced as compared with Sunday's storms, a somewhat low confidence forecast of no flash flooding remains for this forecast update. However, should there be any northward trend with the forecast convection on Monday, then a Marginal Risk will be needed. Wegman 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 ADJACENT MOUNTAINS... ....Central California Coast... In coordination with the MTR/Monterey, CA and LOX/Los Angeles, CA forecast offices, the inherited Slight Risk area was expanded both north and south with this morning's forecast update. 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 3/4 inch, which is about 2 sigma above the climatological normal. This will bring with it a brief increase in snow levels as high as 7,000 ft 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 anywhere close to 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 expanded due to increasing forecasted rainfall through the Bay area and especially the mountains north of there, and expanded 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. ....Potential for Future Risk Changes... Despite the expansion of the Slight Risk area, at this point it appears further upgrades to the Slight Risk area are unlikely, as precipitation totals have remained fairly steady, and the lack of tropical moisture will preclude rainfall rates more typical of true atmospheric rivers. ....Gulf Coast through Coastal Georgia... The Marginal Risk area along the Gulf Coast is largely unchanged from the Day 4 update yesterday. There has been a small southward shift in most of the guidance, so the immediate Gulf Coast and points up to 100 miles inland remain in the Marginal Risk area for this update. The front will be in roughly in the same place Tuesday as it was on Day 2/Monday. Thus, expect Monday's rainfall to have lowered FFGs a bit in the Marginal Risk area by the time we get to Day 3/Tuesday. Forecasted rainfall is a little higher in this area than on Day 2/Monday, particularly around southern Mississippi. Thus, think continued convection in the same areas on Tuesday as on Monday warrants the continued Marginal Risk for the same stalled out front. ....Potential for Future Risk Changes... As with the current Day 1 update, if future model trends move higher for forecasted rainfall, it's very possible a Slight Risk may be needed for portions of the Marginal Risk area on Tuesday. Since a large component to any upgrades will be the rainfall distribution in this area on Day 2/Monday, confidence and current forecasted rainfall are low as to where any upgrades would be. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6k9X9c5urQLG_KpJEi9FUQDPPzQc0PlNAdr0WY0b8YpF= HYjcjZ2D3BhFk5g6IRiRp5_KsHm9mlu9XHuAWr8HC0SCRFs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6k9X9c5urQLG_KpJEi9FUQDPPzQc0PlNAdr0WY0b8YpF= HYjcjZ2D3BhFk5g6IRiRp5_KsHm9mlu9XHuAWr8H-9LkWW0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6k9X9c5urQLG_KpJEi9FUQDPPzQc0PlNAdr0WY0b8YpF= HYjcjZ2D3BhFk5g6IRiRp5_KsHm9mlu9XHuAWr8HRBjBUGA$=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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