Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Mar 21 2023 20:29:30 FOUS30 KWBC 212029 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 428 PM EDT Tue Mar 21 2023 Day 1 Valid 16Z Tue Mar 21 2023 - 12Z Wed Mar 22 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS COASTAL AREAS OF SOUTHERN CALIFORNIA... ....1600 UTC Update... Morning radar showed a compact area of low pressure off the California coast moving northeastward. With the amount of on-shore flow accompanying the system and the depiction of rainfall rates/coverage on radar...opted to extend the Marginal Risk area northward a bit along the coastline. No changes were made elsewhere in California. The latest HREF has been advertising the potential for 1 inch per hour rainfall rates persisting into the 19Z or 20Z time frame before decreasing as the moisture plume shifts south and eastward. Models still show another wave of rainfall to develop and move inland across Southern California later. As a result...felt staying the course with a Moderate was still warranted (especially in urbanized areas)...as southwesterly low-level winds of 35 to 55 knots will continue to transport an airmass with precipitable water values of 0.50 to 1 inch (~3 to 4 sigma) onshore and into a region of complex terrain. Consensus depicts areal averages of 2 to 3 inches for the entire event...however hi-res guidance suggest isolated maximum amounts in excess of 6 inches could materialize with the potential for flooding being worsened by run-off off from nearby areas of steep terrain.=20 ....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. 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!9LpVrZzq0TqHFGCD-UObCViKZf0l2ETNToqlErquCQdN= K9o2zUjM-LaGNzh9I5olph5pQn06GFmvDSejKy2o0O-gtbI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9LpVrZzq0TqHFGCD-UObCViKZf0l2ETNToqlErquCQdN= K9o2zUjM-LaGNzh9I5olph5pQn06GFmvDSejKy2oHI1-3ms$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9LpVrZzq0TqHFGCD-UObCViKZf0l2ETNToqlErquCQdN= K9o2zUjM-LaGNzh9I5olph5pQn06GFmvDSejKy2oSHrYLQw$=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) .