Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
936 FOUS30 KWBC 221900 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 200 PM EST Fri Nov 22 2024 Day 1 Valid 16Z Fri Nov 22 2024 - 12Z Sat Nov 23 2024 ...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHERN CALIFORNIA... ...16Z update... Areas of heavy rain continue to affect north-central California this morning with localized rainfall rates observed in the 0.5 to 0.7 in/hr range over the past 6 hours. 72 hour rainfall totals across the Coastal Ranges into the northern Sierra Nevada have resulted in double digit rainfall totals with at least one localized 20+ inch report in Sonoma County. Through today and tonight, the plume of IVT will continue to gradually sink south with possible brief stalling of the moisture axis in the vicinity of the San Francisco metro later this evening. Adjustments to the 16Z ERO were very minor and while there were small changes noted with the 12Z guidance, notably a little faster to advance precipitation toward the south, forecast reasoning below remains valid. Additional rainfall totals of 5-10 inches still look likely into the Sierra Nevada through 12Z Saturday and localized 3-6 inch totals for the Coastal Ranges in the vicinity of San Francisco Bay. See upcoming MPD #1165 for additional short term details. Otto ...previous discussion follows... The long duration, multi-day atmospheric river event that has been impacting northern California will finally begin to become more progressive and start to wane in intensity as a cold front just offshore of the West Coast advances inland and drives the axis of deeper layer Pacific moisture transport south and eastward. However, there will still be one more day of locally very heavy rainfall totals which are expected to be excessive and capable of driving locally significant flooding impacts. The 00Z HREF guidance supports some additional 5 to 10 inch rainfall totals for the 24-hour period ending 12Z/Saturday, with the heaviest totals expected to be over the windward slopes of the northern Sierra Nevada, and perhaps locally a few of the coastal ranges just north of the Bay Area where there has already been very heavy rainfall totals over the last couple of days. IVT magnitudes along and just south of the cold front will be rather high at the start of the period this morning, with magnitudes over coastal areas of northwest California upwards of 750+ kg/m/s, and these values should decrease slowly and settle southward toward 00Z. However, as the front gets into the Bay Area and the Central Valley by early this evening, there is some suggestion in the guidance that the front will slow down again just a bit. Coinciding with that will be arrival of a shortwave and associated upper- level jet streak (130+ kts) rounding the base of the deep offshore trough. This should foster a temporary resurgence/strengthening of the IVT values in the 00Z to 06Z time frame across the Bay Area and extending inland across portions of the Central Valley and foothills of the Sierra Nevada. Thus expectations are that heavy rains will overspread all of these areas along with some localized increase in rainfall rates this evening. In general, the rainfall rates will be capable of reaching 0.50" to 0.75"/hour across the coastal ranges around the Bay Area and also into the upslope areas of the northern Sierra Nevada. Given the additional rainfall amounts, and considering the antecedent conditions at this point, the Moderate Risk area is maintained across the northern Sierra Nevada, with a separate Moderate Risk introduced just north of the Bay Area involving Sonoma and Napa Counties. Other adjustments include extending the Marginal and Slight Risk areas a bit farther south to include the coastal ranges a bit south of the Bay Area and also more areas of the Central Valley. Flooding impacts will again be rather widespread and locally significant, with additional concerns for debris flows, rock and landslide activity, and flash flooding involving burn scar locations. In fact, the Park Fire burn area involving parts of Tehama and Butte Counties will need to be very closely monitored today for enhanced impacts given the amount of rainfall that is forecast here. Orrison Day 2 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... ...1900 UTC Update... Subtle changes made to the previous Marginal Risk area, mainly to trim off a portion of the westernmost foothills based on the latest deterministic and probabilistic QPF. 12Z HREF probs do show a more elevated risk of 0.50+ in/hr rates early in the period (i.e. 12-18Z Sat. Hurley ...Previous Discussion... At 12z Saturday the cold front and associated moisture plume will be dropping south across the central Sierra Nevada. By this time rainfall intensity will be decreasing, and the axis of better moisture will be dropping south of the areas hardest hit over the past few days. Thus the flood risk should be pretty low Saturday morning along this southward dropping front. Some weak post frontal instability is forecast which could allow for locally heavy showers in the wake of the front over the western slopes of the central Sierra Nevada. Not expecting anything too intense or widespread, however there is at least a chance that a few cells could drop a quick 0.5" or so of rain. This would be falling over increasingly saturated ground, and so a localized flood risk is possible. Thus we will maintain the small Marginal risk where a few heavier post frontal showers are possible Saturday. Chenard Day 3 Valid 12Z Sun Nov 24 2024 - 12Z Mon Nov 25 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTHWEST CALIFORNIA INTO FAR SOUTHWEST OREGON... ...1900 UTC Update... No changes were made to the previous Marginal Risk area, based on the latest (12Z) models and guidance trends. Hurley ...Previous Discussion... Locally heavy rain will be possible Sunday into Sunday night over northwest CA into far southwest OR. A front slowing near the coast will provide plentiful low level moisture convergence, and mid level shortwave energy should provide some additional lift as well. Moisture is a limiting factor for heavy rainfall with the better moisture plume situated well south and not really getting drawn into this system. Thus this is not expected to be a significant atmospheric river, and normally would not expect a system of this nature to pose a flood risk. However given the significant rainfall over the past few days, this additional rainfall may be enough to cause some localized additional flooding as the area will be more hydrologically sensitive than normal. Still some model differences in the exact evolution of this system and thus rainfall amounts this period are uncertain...but current indications suggest localized amounts of 2-3" will be possible. 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