Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
137 FOUS30 KWBC 250033 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 733 PM EST Sun Nov 24 2024 Day 1 Valid 01Z Mon Nov 25 2024 - 12Z Mon Nov 25 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF NORTHERN CALIFORNIA INTO FAR SOUTHWEST OREGON... ...01Z update... No changes from previous forecast. Pereira ...16Z update... No significant changes were needed for this update and forecast reasoning described below remains valid. The 12Z hires guidance showed the greatest threat for heavy rainfall arriving between 21-00Z, and continuing until 03-06Z associated with the low level wave west of the northern California coast. A localized excessive rainfall threat will continue through 12Z Monday and given 850-700 mb winds will waver between SSW and S over the next 12-24 hours, the highest rainfall totals should tend to be favored into the terrain of southwestern Oregon with the highest rainfall totals given the track of the low level wave. Spotty 1-2 inch totals through 12Z Monday are expected from northern California into southwestern/western Oregon, with perhaps an isolated 3 inch total from northern CA into coastal Oregon. Otto ...previous discussion follows... Locally heavy rain will be possible today and especially tonight over portions of northern CA into far southwest OR as an offshore shortwave and attendant wave of low pressure arrives near the coastal ranges. Generally the IVT magnitudes out ahead of this surface wave will be modest with values increasing to as high as 250 to 400 kg/m/s. However, with a front nearby and favorable upslope flow/orographic ascent involving the coastal ranges, there will likely be some concentrations of heavier shower activity with rainfall rates capable of occasionally reaching into the 0.25" to 0.50"/hour range. Some pockets of heavier rain may also make it into the far northern Sacramento Valley and adjacent foothills of the northern Sierra Nevada and Shasta/Siskiyou ranges. The 00Z HREF guidance supports some spotty 1 to 3 inch rainfall amounts going through early Monday morning, and thus this is not expected to be a particularly heavy event. However, given the significant rainfall and lingering hydrological impacts from the most recent atmospheric river, these additional rains may result in at least some localized runoff problems and flooding. Orrison Day 2 Valid 12Z Mon Nov 25 2024 - 12Z Tue Nov 26 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF CALIFORNIA AND SOUTHWEST OREGON... ...20Z update... For northern California into southwestern Oregon, there were no changes needed to the previous Marginal Risk and an additional 1-2 inches (localized 3") still looks reasonable for the favored southwest facing terrain for the 24 hour period. The greater heavy rain footprint still looks to setup from the south-central California coast, eastward into the southern Sierra Nevada, on Monday. Moderate westerly flow perhaps exceeding 40 kt from the west at times is forecast from the Santa Lucia Range into the southern Sierra Nevada. The flat mid-level flow will limit 700-500 mb lapse rates and instability with negligible values forecast by the 12Z model consensus from Monday into Tuesday. With the addition of the hires window through the period, the lowest model QPF still shows 3-5 inches over the 24 hour period with a blend of the 12Z HRRR, NAM_nest and ARW indicating 3-7 inches into the Sierra Nevada below snow levels and 3-5 inches for the Santa Lucia Range. Snow levels will likely rise up to between 9-10 kft per the 17Z NBM but a lack of instability is expected to limit peak rainfall rates to near 0.5 in/hr. Collaboration with WFO HNX led to the maintenance of a Marginal Risk for the San Joaquin Valley into the Sierra Nevada. Otto ...previous discussion follows... Showery conditions will likely continue across portions of northern CA and southwest OR Monday into Monday night as a deep layered low and shortwave energy moves inland. Certainly not a classic setup for flooding with limited moisture and dropping IVT values... however this area remains more hydrologically sensitive than normal following the significant atmospheric river event the past few days. Additional rainfall over far northwest CA into southwest OR is upwards of 1-3" in the guidance, with isolated totals of this magnitude also possible over the western slopes of the northern Sierra Nevada. A more significant plume of moisture/IVT will move into portions of southern/central CA...generally from portions of San Luis Obispo and Monterey counties into the western slopes of the southern/central Sierra Nevada. IVT values are forecast to be around 500-600 kg/ms, which at this latitude is starting to approach climatological max values for late November. PWs are also forecast to increase to around the 99th percentile for late November, and thus expecting plenty of moisture and upslope flow to allow for decent rainfall totals. Not expecting any instability, so rainfall intensity will be solely driven by the aforementioned moisture transport and upslope enhancement...and these factors alone are unlikely to support rainfall rates high enough for true flash flooding. However we will have an extended period of time with elevated IVT values, and so a prolonged steady rainfall could eventually result in some localized minor flood concerns. Overall expecting 1-3" of rain over the coastal ranges in San Luis Obispo and Monterey counties (with localized totals towards 4" possible)...with totals ranging from near 1" in the eastern valley locations to 2-4" over the western Sierra below snow levels. Chenard Day 3 Valid 12Z Tue Nov 26 2024 - 12Z Wed Nov 27 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL AND SOUTHERN SIERRA NEVADA... ...20Z update... Model differences remain in the 12Z guidance with respect to the timing of a shortwave moving inland from the California/Oregon border although the GFS has trended faster (closer) to the remaining model consensus. While the GFS remains heaviest across the central and southern Sierra Nevada for the 24 hour period ending Wednesday morning, it was considered less likely to verify. Lingering heavy rain from Day 2 will continue into early Tuesday but the consensus is for weakening IVT into the terrain as the shortwave trough to the north continues to move inland. The non-12Z GFS consensus is for 3-5 inches for the central/southern Sierra Nevada over the 24 hour period, but those values may be too low at least locally considering the coarser resolution of the available guidance for these longer lead times. Depending on future guidance, an possible upgrade to Slight on Day 2 may require an upgrade on Day 3 as well given the event will be ongoing at 12Z Tuesday. Otto ...previous discussion follows... The atmospheric river event described in the day 2 discussion continues into Tuesday over portions of the central and southern Sierra Nevada. There is a bit more model spread by this time with regards to the persistence of the IVT and strength of the mid/upper forcing. A majority of the 00z deterministic models have trended a bit weaker and faster with the shortwave energy pushing across CA and into NV...and this trend would result in less additional QPF over central CA on day 3. However the 00z GEFS and recent NBM probabilities suggest there is still a chance for a slower/stronger wave resulting in more robust rainfall amounts. This will be the 2nd consecutive day of steady rainfall for these areas, and so do expect some increased hydrologic sensitivity that could result in at least some localized minor flooding issues. When summing the two days of rainfall together, the latest WPC deterministic QPF is calling for around 2" in Fresno to Visalia, increasing to 3-6" in the foothills/western slopes of the southern and central Sierra Nevada. Given the magnitude of IVT forecast even even forecasting 1-2" spilling over to the east of the crest towards Bishop and Independence. These forecast values average around a 2yr ARI for 48 hr amounts, also supportive of a localized minor flood threat. As mentioned above, there is both higher and lower end potential with this forecast rainfall. The higher end scenarios could even result in Slight risk level impacts, while the lower end outcomes would be a minimal flood threat. So will need to continue to monitor trends, and for now still think the Marginal risk is appropriate. 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