Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Nov 21 2024 19:57:54 FOUS30 KWBC 211957 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 257 PM EST Thu Nov 21 2024 Day 1 Valid 16Z Thu Nov 21 2024 - 12Z Fri Nov 22 2024 ....THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHWEST CALIFORNIA... ....Northern California and Southwest Oregon... 16z update: Recent heavy rainfall across coastal ranges north of San Fran Bay in the vicinity of Point Arena have brought 0-40cm soil saturation ratios above 85 to 95%, well above the 95th percentile; so have expanded the categorical risk areas southward to account for higher probability for increased runoff given continuous orientation of the plume directed across this area for much of the remainder of the forecast period. Additionally across SW Oregon, expecting surge of warm air to infiltrate the region to bring freezing levels up to allow for increased rainfall totals across all but the highest peaks in the southern Cascade Ranges west...and have expanded the Marginal Risk areas a bit further northeast to account for this occurring mainly after 00z, as the return warm front lifts through. Gallina ----Prior Discussion---- Life-threatening flooding across coastal areas of northwest California is expected due to the very strong and long duration atmospheric river currently impacting the region which will continue through Thursday night. Dangerous flooding and debris flows are likely which will include rock and landslide activity along with a threat for burn scar flash flooding. The strong atmospheric river event into northern California will continue as deep layered southwesterly flow continues to the south of the northeast Pacific upper vortex. The anomalous deep layered southwesterly flow will support 850 to 700 mb moisture flux anomalies of 2 to 4+ standard deviations above the mean and IVT values in the 500 to 800 kg/m/s range across northwest CA. This will also be focused in close proximity to a quasi-stationary front draped in a southwest to northeast fashion across the region. However, the latest model guidance continues to advertise a strong vort max/jet streak rotating around the base of the northeast Pacific vortex by Thursday afternoon and this will support another rapidly deepening surface low moving northeastward along the front and to a position about 200 to 250 miles offshore of western Oregon by very early Friday morning. While this second rapid cyclogenesis event will not be as strong as its predecessor, it will help to begin to push the axis of strongest onshore flow/moisture transport back to the north. This will allow for heavy rains already impacting northern California to edge back into southwest Oregon. Rainfall rates are expected to reach 0.50" to 0.75"/hour at times which is well supported by the 00Z HREF guidance. The persistence of these rates are expected to support additional 24-hour rainfall amounts of 3-6" and isolated additional 24 hour totals of 6-10" across northwest California, and this will bring storm total amounts to as much as 12-16"+ by early Friday morning. Somewhat lower rainfall totals are expected for southwest Oregon, but an additional 3-5" of rain is forecast here by early Friday morning. Given the expected 2-day totals in the same region, the High Risk area is maintained with very little change. Significant flood risks will evolve today through Thursday night and early Friday morning as soils become saturated and streams and rivers continue to rise and overflow. Impacts will likely include debris flows along with rock and landslide activity. Additionally, there may be some burn scar flash flooding concerns as well given the relatively high rainfall rates over these very sensitive areas. The Park Fire burn area involving portions of Tehama and Butte Counties is one such location that may see impacts. Orrison Day 2 Valid 12Z Fri Nov 22 2024 - 12Z Sat Nov 23 2024 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHERN CALIFORNIA... ....20Z update... As a new surface low deepens ~300 miles west of the Oregon coast=20 early Friday morning, the attendant cold front and associated frontal band of heavy rain is expected to be near or just inland=20 of the coastal CA/OR border. While the threat for rain rates=20 greater than 0.5 in/hr will be quickly lowering for southwestern=20 Oregon into northwestern CA, antecedent rainfall since Tuesday and=20 localized potential for SW to NE oriented bands could support=20 runoff from additional heavy rain with peak rain rates near 0.5=20 in/hr well into the period. Farther south, very little change was needed for the Moderate Risk in the northern Sierra Nevada and what constitutes a higher end=20 Slight Risk from Humboldt County to just north of San Pablo Bay. Antecedent rains combined with rain rates of 0.5 to near 1 inch per hour will likely result in numerous flooding concerns, especially across burn scar locations. Otto ....previous discussion follows... The long duration atmospheric river begins to become more progressive and wane in intensity Friday as a cold front finally pushes the moisture axis southward. However still expecting rather widespread 0.25" per hour rates with this last push of rainfall, with localized 0.5" per hour amounts likely. These higher rates should only last for a couple hours during the morning over coastal areas, but should persist a bit longer in the Sierra Nevada. Overall expecting 1-3" of additional rain over coastal areas, with 3-6" over the northern Sierra Nevada. Warmer temperatures will keep snow levels pretty high such that even into the terrain areas, most of the precipitation is expected to fall as rain (aside from the highest peaks). By Friday conditions over much of the area will be saturated, and this last push of elevated rainfall rates on top of saturated ground will likely result in a continued flood threat...with additional landslides likely and a continued risk of burn scar flash flooding. Only minor changes were made to the inherited ERO areas. A MDT risk remains for the northern Sierra Nevada where the heavier rainfall will persist longer into Friday resulting in the aforementioned higher additional rainfall totals. A flooding risk is likely to be ongoing over coastal areas as well Friday morning, but should be a shorter lived threat with the higher rainfall rates coming to an end quicker...thus will continue to carry a Slight risk here. Chenard Day 3 Valid 12Z Sat Nov 23 2024 - 12Z Sun Nov 24 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SIERRA NEVADA... ....20Z update... Similar to the D2 outlook, the cold front will be near the northern end of the Marginal Risk area Saturday morning in the northern Sierra Nevada, advancing toward the southeast. The associated plume of IVT will be quickly weakening and moving south, but as discussed in the previous discussion, antecedent rainfall may leave a small area of the Sierra Nevada vulnerable to localized flash flooding from an additional 1-2 inches. Instability is forecast to be low during the daytime hours and fairly shallow. When steeper mid-=20 level lapse rates arrive beyond 00Z, the loss of daytime heating=20 should further reduce any instability present.=20 Will leave the Marginal Risk in place for now, but as the forecast window draws nearer and hires model guidance becomes available, the need for continuing the Marginal Risk can be reassessed. Otto ....previous discussion follows... At 12z Saturday the cold front and associated moisture plume will be dropping south across the central Sierra Nevada. Rainfall intensity will be dropping fairly quickly by this time and the prolonged atmospheric river event will finally be coming to an end. We will continue to carry a small Marginal risk over portions of the Sierra Nevada where an additional 1-2" of rain is possible Saturday morning. Typically rainfall of this magnitude would not be a concern. However, much of this risk area will have seen at least 3- 7" of rain total from the event, so even the modest additional rainfall Saturday morning could result in a flood threat, and thus continuing with a Marginal risk for now seems best. Although the risk is certainly lower than previous days, and quite possible this risk could be removed on future updates if additional rain after 12z Saturday trends down. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6NuHV_PXCs4gUtCu_4g5xY3IrsdepvjQnjul1zRxMQGN= NCQDknrUCiFly_GqNTX6yAJihp5IEkUTznC6591j9BX17_Y$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6NuHV_PXCs4gUtCu_4g5xY3IrsdepvjQnjul1zRxMQGN= NCQDknrUCiFly_GqNTX6yAJihp5IEkUTznC6591jzhQ3HSc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6NuHV_PXCs4gUtCu_4g5xY3IrsdepvjQnjul1zRxMQGN= NCQDknrUCiFly_GqNTX6yAJihp5IEkUTznC6591jrIMf7v0$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .