Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Mar 21 2023 08:30:27 FOUS30 KWBC 210830 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 429 AM EDT Tue Mar 21 2023 Day 1 Valid 12Z Tue Mar 21 2023 - 12Z Wed Mar 22 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS COASTAL AREAS OF SOUTHERN CALIFORNIA... ....California... Model guidance continue focus a strong AR into central/southern California and downstream into Arizona as a closed low approaches the California Coastline. Low level, southwesterly winds of 35 to 55 knots will direct PW values of 0.50 to 1 inch (~3 to 4 sigma) onshore and forced up and over the terrain. Local enhancement will drive higher rainfall rates, nearing 0.75 inch/hour, along coastal SoCal and the adjacent Transverse and Peninsular Ranges and to a somewhat lesser degree to the southern Sierra foothills. Consensus depicts areal averages 2 to 3 inches, however hi-res guidance suggest very localized maximums of 7 to 10 inches could material for Southern California. These rain amounts are forecast over locations that are already experience high streamflows and near soil saturation. The strong wind associated with this system will also increase the chance for downed trees and debris that may clog storm drains in the many urbanized portions of the Moderate Risk area. Rainfall amounts in the densely populated valleys and coastal areas will generally range between 1.5 and 2.5 inches. When adding runoff from the steep nearby mountains where rainfall totals may exceed 4 inches, numerous flash floods are likely. The timing of the heaviest rain also favors added impacts, as they will likely be ongoing during the Tuesday morning commute. The inherited Risk Moderate and Slight Risks were broadened a bit to the north and east. Further north into central California the inherited Slight and Marginal Risks are largely unchanged. This is largely due to the heaviest rain impacting southern areas, and snow levels may never rise above 5,000 feet for the entire event. Thus, despite obviously favorable antecedent conditions, the lower forecasted rain totals and much of the precipitation falling as snow farther down the mountains will greatly limit the flash flooding threat with latitude.=20 Campbell ....Central Arizona... The abundant moisture from the AR 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. Minor reshaping of the Marginal and Slight Risks were needed to reflect the latest QPF trends and WPC forecast amounts. Campbell 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... Rain associated with the 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 will keep much of southern California and central Arizona sensitive to additional precipitation. With soils likely reaching or exceeding saturation and with stream flows already forecast to be high, there will be a continued threat for excessive rainfall and localized risk for flooding. Therefore a Marginal Risk for both locations was raised for this period. Campbell 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... 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!_swmlnCLf37lu5BNFlmRe6BTaUfvzzUT81iOJll0HDzP= sjCyOmucbLh-XikzwT32bcF3RrjbyzWfE9ryrhJuXHiRvyg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_swmlnCLf37lu5BNFlmRe6BTaUfvzzUT81iOJll0HDzP= sjCyOmucbLh-XikzwT32bcF3RrjbyzWfE9ryrhJu8hr-OGo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_swmlnCLf37lu5BNFlmRe6BTaUfvzzUT81iOJll0HDzP= sjCyOmucbLh-XikzwT32bcF3RrjbyzWfE9ryrhJuDs8aA8g$=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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