Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Mar 22 2023 00:34:03 FOUS30 KWBC 220033 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 833 PM EDT Tue Mar 21 2023 Day 1 Valid 01Z Wed Mar 22 2023 - 12Z Wed Mar 22 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS CENTRAL ARIZONA AND COASTAL AREAS OF CENTRAL AND SOUTHERN CALIFORNIA... ....0100 UTC Update... Overall, made only minor updates with no significant changes to the previous outlook. Previous Discussion... ....Central and Southern California... The plume of deeper moisture that brought locally heavy precipitation to portions of Southern California has now shifted farther south into northern Baja California. While drier air to the north offered a short reprieve, shower and thunderstorm activity supported by colder air and energy aloft is beginning to move into Central and Southern California. This activity is expected to continue through the evening and into the overnight, with additional locally heavy amounts expected along the favored terrain. Localized rainfall rates of 0.5 in/hr are expected, fueling scattered additional totals of 1-2 inches along the Central to Southern California coast. Given the showery, progressive nature of these storms, guidance is suggesting a more isolated excessive rainfall threat. This along with rainfall totals from the initial round that were generally less than expected, opted to remove the Moderate Risk with this update.=20 However, the Slight Risk was left largely unchanged, noting that instances of flash flooding are considered possible through the evening and into the overnight, especially near burn scars and other sensitive areas. ....Central Arizona... The abundant moisture from this atmospheric river will usher in heavy rains and mountain snow to the state. With the snow levels hovering near 8,000-9,000 ft and the progressive nature of the heavy rain will limit total rainfall, and therefore reduce resultant impacts. Areal averages of 1 to 3+ inches are expected before the cold front quickly pushes through. No modifications to the Marginal and Slight Risk areas were needed after the minor reshaping done to the areas overnight and given consistent QPF from the models and from WPC. Pereira/Bann Day 2 Valid 12Z Wed Mar 22 2023 - 12Z Thu Mar 23 2023 ....THERE IS A MARGINAL FOR EXCESSIVE RAINFALL ACROSS SOUTHERN CALIFORNIA AND CENTRAL ARIZONA... The models still suggest that rain associated with this AR will persist during this period as the upper trough tracks from southern California into the Desert Southwest. Although rain rates will not be as impressive as those expected for the day 1 period, new accumulations of 1 to 2 inches (mainly focused in/near the terrain) that fall on areas that are already hydrologically sensitive to additional precipitation will keep the potential for excessive rainfall going over portions of southern California and central Arizona on Wednesday. Bann Day 3 Valid 12Z Thu Mar 23 2023 - 12Z Fri Mar 24 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM THE SOUTHERN PLAINS TO THE OHIO VALLEY... ....2030Z Update... Models continue to agree on the idea of a stripe of moderate to potentially excessive rainfall extending from the Southern Plains into the Ohio Valley...but the spread in the model guidance remains large in terms of placement. Made a couple of minor adjustments on the southwest end and the northeast end of the Slight Risk areas but left the core of the Slight Risk area largely in tact. Much of the forecast reasoning below remains valid. Bann ....0830Z Excessive Rainfall Discussion... The cold front associated with the AR during the D1 and D2 periods will be tracking across the central and southern states while shortwave energy ejects further north across the Plains. This will result in a long fetch of moisture stream in along and ahead of an elongated front, spanning from the Great Lakes region and trailing southwest to Mexican border/southern Desert Southwest. This boundary will provide a focus for showers and thunderstorms to develop and track eastward. The best forcing and lift will likely align near the Ozarks and central Mississippi Valley as this is where the probability of exceeding 2-3 inches is the greatest. Model consensus show areal averages of 2 to 4 inches. A broad Slight Risk was inherited from the experimental Day 4 and still encompassed where there will be an elevated threat for excessive rainfall and isolated to scattered flooding. The Slight spans from the Oklahoma/Texas border to western Ohio. A Marginal Risk encloses the Slight, stretching from northeast Texas to western Pennsylvania. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_RHB9DDz26XQ73DfbIiCksHlTSyg8mjcMGyZcmw8dSUp= RrBQbMbkQi3Y1LnIe_4NOeINO3u31ZTjZlLagXAOx8G94pc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_RHB9DDz26XQ73DfbIiCksHlTSyg8mjcMGyZcmw8dSUp= RrBQbMbkQi3Y1LnIe_4NOeINO3u31ZTjZlLagXAOpMkmi0g$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_RHB9DDz26XQ73DfbIiCksHlTSyg8mjcMGyZcmw8dSUp= RrBQbMbkQi3Y1LnIe_4NOeINO3u31ZTjZlLagXAO7XgVTPE$=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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