Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
004 FOUS30 KWBC 220043 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 743 PM EST Thu Nov 21 2024 Day 1 Valid 01Z Fri Nov 22 2024 - 12Z Fri Nov 22 2024 ...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHWEST CALIFORNIA... ...01Z Update... In coordination with MTR/Monterey, CA forecast office, the High risk area was expanded southward with this update to account for the heavy rains that have impacted much of Sonoma County. The plume of rainfall impacting much of northern California has set up a little bit south of where most of the HiRes guidance has suggested the heaviest rain would be. This has somewhat reduced the rainfall amounts in extreme northwest California and southwest Oregon, but consequently increased them today just north of the Bay area. Ongoing flooding across Sonoma County has closed roads along the Russian River west of Santa Rosa. Through tonight, a cold front associated with an approaching deepening low will both increase rainfall rates ahead of the front in the atmospheric river plume, but also pull the heaviest rain north and more parallel to the coast, to include far northwestern California and southwest Oregon. Thus, despite the southward adjustment today, the northern portions of the risk areas should still pick up a prolonged period of heavy rain with rates to 1 inch per hour through tonight. Wegman 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 early Friday morning, the attendant cold front and associated frontal band of heavy rain is expected to be near or just inland of the coastal CA/OR border. While the threat for rain rates greater than 0.5 in/hr will be quickly lowering for southwestern Oregon into northwestern CA, antecedent rainfall since Tuesday and localized potential for SW to NE oriented bands could support runoff from additional heavy rain with peak rain rates near 0.5 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 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- level lapse rates arrive beyond 00Z, the loss of daytime heating should further reduce any instability present. 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://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt