Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Mar 12 2023 20:26:48 FOUS30 KWBC 122026 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 425 PM EDT Sun Mar 12 2023 Day 1 Valid 16Z Sun Mar 12 2023 - 12Z Mon Mar 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF NORTHERN CALIFORNIA AND EXTREME SOUTHWESTERN OREGON... ....West Coast... 16Z update... Scattered rain has been moving onshore from central California northward to southern Washington ahead of an approaching Pacific front. Rainfall intensities will pick up between 18Z to 22Z hours where they are expected to range from 0.25 to 0.75+ inch/hour (very localized rates over 1 inch/hour possible) and hold fairly steady for the duration of the D1 period. With these rates projected to occur over saturated areas from the previous AR event a few days ago, the risk for excessive rainfall and subsequent flooding will be elevated for this region. Given the latest trends in the hi-res guidance, the Slight Risk area was expanded west-east along the southern edge of the Slight over the Sierras, as well as, further south. Campbell The much-advertised next atmospheric river will be ramping up during the day as a deep positively tilted trough approaches the coast of the Pacific Northwest later tonight. Deep unidirectional west southwesterly flow ahead of the trough will continue draw plenty of moisture into northern California...with precipitable water values around 0.75 inches remaining consistent through the period. This flow will favor upsloping not just along the coast, but right up into the northern end of the Sacramento Valley as well. Meanwhile, snow levels will be largely steady in this region, holding between 5,500 and 6,500 ft. This is plenty high enough to result in rainfall on all but the highest peaks of the mountains in Siskiyou County. Continued snowmelt will also contribute to rising river levels and the excessive rainfall potential. The primary changes to the previous outlook was a subtle southward extension of the Slight Risk area along the coast. Rainfall rates of 0.25-0.5 inches/hour are expected...with 2 to potentially 3 inches of rainfall in most of the higher elevations, with 1/2 to 1 inch expected in the middle of the valley. Changes made to the Marginal Risk on Saturday area still looked reasonable with spaghetti plots of QPF still supporting the potential for an inch or more of liquid precipitation over an area with snow pack. As mentioned previously...the farther north you go, the less time snow levels will be that high, but nonetheless those rainfall amounts on snow could result in isolated flooding issues, particularly in urbanized areas and around burn scars. ....Southeast... 16Z update... The latest guidance is favoring better potential for excessive rainfall along the southern edge of of the Marginal Risk area as a line of convection progresses east/southeast through the evening and overnight hours. Hourly rates near/greater than 1.5 inches/hour will be possible with these thunderstorms. The southern end of the convection is expected to track across parts of Louisiana, southern Alabama and the Florida Panhandle, which are outside of the inherited Marginal Risk. Given the latest trends, the Marginal Risk was shifted southward and the northern edge was trimmed southward as the threat for excessive rainfall is not as great. Campbell There is a Marginal Risk of excessive rainfall over portions of southern Alabama and Georgia where the latest guidance has finally come into better agreement with the placement of the heaviest rainfall...presumably being focused on remnants of overnight convection and subsequent redevelopment. Corfidi vectors are close to parallel to the axis of the stalling front, and guidance is slowly catching on to this. Despite increasing QPF in the area...the Flash Flood Guidance is high so biggest concern is where cells with the highest rainfall rates train or in areas of repeat convection. Bann Day 2 Valid 12Z Mon Mar 13 2023 - 12Z Tue Mar 14 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHERN CALIFORNIA AND EXTREME SOUTHWESTERN OREGON... ....California... 21Z update... There are timing differences on where the low pressure will be located during this period which will contribute to how much/when precipitation the occur. The EC ensemble mean is the fastest to surge moisture on shore and shows a fairly good signal for exceeding 2-3 inches for parts of northern California and southwest Oregon. Other guidance that are that are a little slower still show the possibility of 2 to 3 inches with very isolated 4+ inches. The Slight and Moderate Risk areas were reshaped/expanded a bit to reflect this trend across the northern portions of the risk area.=20 Campbell There is a Moderate Risk of excessive rainfall over portions of northern California and a small portion of adjacent southwest Oregon and the next atmospheric river gets in full swing. There will be a dying low surface pressure system that moves ashore near the CA/OR border Monday morning which increases precipitation rates particularly in the northern end of the ERO risk areas for far northern CA and into OR. Rainfall rates could reach as high as 0.5 to 0.75 inches per hour, but will be steady and very persistent for much of the day, particularly into the middle of the Moderate Risk area of the northern CA coastal ranges and the far northern Sacramento Valley. By Monday evening, the cold front will have shifted all but some remnant shower activity into CA, and falling snow levels will end much of the flash flooding threat north of the border. Then a second and stronger low pressure system will approach the coast of northern CA Monday night, This low and its trailing cold front will provide the surface forcing for heavy rains farther south into central California Tuesday. But in the meantime, the higher rainfall rates will combine with upslope into the mountains and obviously very wet antecedent conditions along with fairly steady snow levels to result in numerous instances of flooding across much of the northern half of the state by Monday night. Snow levels will remain around 6,000 feet in northern California, though from San Francisco south, the slug of moisture will raise snow levels back closer to 9,000 feet, once again setting the stage for another round of very impactful rainfall on Tuesday. As the rain shifts south Monday night into the pre-dawn hours on Tuesday, the rain will impact increasingly sensitive portions of central CA that were hit hard by the rainfall on Friday and early Saturday. Thus, areas south of San Francisco's latitude will largely be spared the heavier rain until late Monday night, but it consequently won't take long once the steady heavy rain gets started for flooding impacts to resume. The snowpack across much of California's mountains has been primed due to yesterday's extreme rainfall event, so the combination of heavy rain in addition to a now saturated and primed snowpack and already full rivers will again result in impacts beginning much closer to the start of the heavy rains, and only worsen with time. ....Long Island and Surrounding Coastal Areas... 21Z update... Uncertainty remains on where the transition of rain and snow will occur, however the QPF has trended higher across far southern New York and northern New Jersey where flash flood guidance is lower from recent precipitation. The Marginal Risk was expanded west/northwest to account for this trend. Campbell There remains the usual uncertainty as to where the rain/snow line will set up, but those on the rainy side of that line have potential to see isolated flash flooding as rainfall totals around 1.5 inches are expected. This storm will be far more impactful on the winter/snowy side of the low. Bann Day 3 Valid 12Z Tue Mar 14 2023 - 12Z Wed Mar 15 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF CENTRAL AND SOUTHERN CALIFORNIA... 21Z update... There was a notable uptick in QPF across the southern Sierras and southern California that supported the expansion of the Moderate and Slight Risks further south and east. Model consensus has 3 to 6+ inches falling over the Sierras, the central and southern Coastline and points inland. This could quickly become problematic for areas that are still experiencing higher stream flow, nearly saturated soils and locations where the rain from the last event is trapped in the snowpack. Additional rain of this magnitude may result in scattered or widespread flood threats, especially if the snowpack gives way (rapid melt) to the added weight of several inches of rain. Campbell Heavy to excessive rainfall will be on-going as the Day 3 Excessive Rainfall Outlook period begins...with the focus gradually shifting southward in conjunction with a complex area of low pressure and an associated surface boundary and a persistent feed of deep moisture tied to the atmospheric river. Both the GFS and EC ensemble bring air with precipitable water values of 1.25 inches to the coast by the start of the period...which are in excess of 5 standardized anomalies..with moisture flux in the 850 mb to 700 mb layer exceeding 5.5 standardized anomalies by 15/00Z focused either side /north or south/ of the Transverse Ranges due to 850 mb onshore winds of 30 to 45 knots. Given the flow pattern...took the Moderate Risk as far south as coastal Santa Barbara county and into the ranges in Los Angeles county...while the Marginal extended as far east as San Bernardino mountains and the Santa Ana mountains. ....New England... 21Z update... The best location for rain, that's potentially excessive leading to a few flooding concerns, will likely be far eastern Connecticut, Rhode Island and eastern Massachusetts. The Marginal Risk was updated to remove southern Maine and southeast New Hampshire where it will likely be all snow during this period. Campbell Similar to the Day 2 discussion...There remains the usual uncertainty as to where the rain/snow line will set up as a complex area of low pressure makes its way north and eastward across parts of Southern New England. Those on the rainy side have the potential to see isolated problems with run off or ponding concerns where rainfall totals ranging from 1.5 to 2.0 inches falls. This storm will be far more impactful on the winter/snowy side of the low. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9SBJEhIHG-YpfSHyiCKDEES4IlI-jNyolX76NyELjh54= OIIh2SulQGooBb-6f1-UQGBDBCzqqHBWHIDeTchHHn1xp80$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9SBJEhIHG-YpfSHyiCKDEES4IlI-jNyolX76NyELjh54= OIIh2SulQGooBb-6f1-UQGBDBCzqqHBWHIDeTchHSfZzGn0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9SBJEhIHG-YpfSHyiCKDEES4IlI-jNyolX76NyELjh54= OIIh2SulQGooBb-6f1-UQGBDBCzqqHBWHIDeTchH-aMq0t0$=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) .