Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Mar 27 2023 15:49:03 FOUS30 KWBC 271548 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1148 AM EDT Mon Mar 27 2023 Day 1 Valid 16Z Mon Mar 27 2023 - 12Z Tue Mar 28 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF SOUTHEAST ALABAMA, FAR SOUTHWEST GEORGIA, AND THE FLORIDA PANHANDLE... ....Southeast and Gulf Coast... The main frontal boundary responsible for the active weather across much of the Deep South and Southeast the past few days will press southward towards the Gulf and Atlantic coasts today and tonight, as scattered showers and thunderstorms along and ahead of the front lead to additional chances for isolated to scattered flash flooding. Current convection stretches from eastern LA to the South Carolina Lowcountry on a gradual southward trajectory. These storms also continue to contain localized rates up to 2"/hr, but are shifting out of more vulnerable areas of GA and AL, thus the removal of the previous Slight Risk and adjusted Marginal Risk along the northern periphery. PWATs are currently between 1.5-1.8", which will support the potential for storms to contain intense rainfall rates. In fact, the 12z sounding from LIX reported a PWAT of 1.8", which is above the daily max per SPC's sounding climatology. A limiting factor could be the reduced 850 mb flow out of the Gulf of Mexico, which should drop below 30 kt later this afternoon. However, the frontal boundary is forecast to continue to be a focus for scattered thunderstorms from southern AL, southern GA, and the Florida Panhandle through the evening hours. Rainfall from Sunday has lowered FFGs across southeast AL and along the border with FL, which was also a consideration along with current radar trends for the upgraded Slight Risk here. 12Z HREF neighborhood probabilities depict 20-30% chances for 6-hour QPF to exceed 3" across the highlighted Slight Risk area through 06z Tuesday. 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 Tuesday. The shortwave tracking through the Mid-Mississippi Valley (similar to the one yesterday) will allow for additional upper divergence and an uptick in southerly low-level flow/moisture transport 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. Between 06-12Z Tuesday, the 12Z HREF probabilities have decreased some, but still indicate scattered 20-30% probabilities along southeast TX and southern LA for over 2" of rain in 1 hour, 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 (modest/compact) Atmospheric River to California towards the tail end of the Day 1 ERO period. 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 2. Areas along the northern California coast will have the best chances to achieve 0.5"+/hr rainfall rates per the 12Z CAMs, and thus the potential for isolated flooding or rapid runoff through 12Z Tuesday. Snow levels will rapidly drop to around 3000 feet, limiting any snowmelt and runoff from the higher terrain. Snell Day 2 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, including a slight eastward expansion of the Slight Risk area to include the southwest portion of Napa Valley along with a little more of the eastern Bay Area. 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 upper level 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 those ARs linger across coastal central California, evidenced by the high soil moisture percentiles (>80%) within the top 100 cm per latest NASA SPoRT, and the injection of scattered 3-5 inch rainfall totals per the CAMs into the coastal mountains are 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-Eastern Gulf Coast Region and Parts of the Southeast... The Marginal Risk area along portions of the central-eastern Gulf Coast was expanded eastward across North FL, southern GA, and the southern tip of SC based on the latest guidance trends. By Tuesday evening, as the main shortwave to the north traverses the Mid Atlantic Region, the surface cold front will get more of a southerly push through North Florida. Beforehand however, pre-frontal convection will capitalize on decent deep-layer instability (CAPEs 1000-2000 J/Kg) along with PWs ~ 1.5 to 1.6 inches in supporting hourly rainfall rates of 1.5 to 2 inches. Available CAMs for Day 2 (NAM CONUS-Nest and FV3) both show localized rainfall totals of 3-4+ inches, which given the current FFG, would pose a marginal/isolated risk for flash flooding, especially over urban areas. Hurley/Snell Day 3 Valid 12Z Wed Mar 29 2023 - 12Z Thu Mar 30 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6gNLegqMeK8Gq2ZqsLpkJSlt6vb0M_vw9pP7JAQWfTUL= A7gVZm7RQfAxTbiKHmnjJEHLPgdHfXyT-eOnIKi2Oa7dl7c$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6gNLegqMeK8Gq2ZqsLpkJSlt6vb0M_vw9pP7JAQWfTUL= A7gVZm7RQfAxTbiKHmnjJEHLPgdHfXyT-eOnIKi21xsT5O4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6gNLegqMeK8Gq2ZqsLpkJSlt6vb0M_vw9pP7JAQWfTUL= A7gVZm7RQfAxTbiKHmnjJEHLPgdHfXyT-eOnIKi27yl1Us4$=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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