Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jan 12 2023 20:11:51 FOUS30 KWBC 122011 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 311 PM EST Thu Jan 12 2023 Day 1 Valid 16Z Thu Jan 12 2023 - 12Z Fri Jan 13 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHERN CALIFORNIA COAST... An amplified longwave trough approaching the West Coast will continue to provide favorable synoptic-scale lift associated with divergent flow aloft, WAA within the 850-500mb layer, and anomalous moisture within the atmospheric column. NAEFS depicts a large area of >90 climatological percentile mean specific humidity within all mandatory levels from 925-500mb from the North Coast of California on north to the Olympic Peninsula and Cascades this afternoon and into the overnight hours. The western third of Washington, in particular, also is likely to contend with >97.5 climatological percentile. This coincides well with an IVT >750 kg/m/s along the Pacific Northwest coast, which is also >99 climatological percentile. What all these situational awareness tools show is there is a deep reservoir of moisture to work with, as well as a strong current of air aloft delivering the moisture to the region. With the mean 850-300mb flow generally oriented SSW-NNE and the trough axis slowly moving east, this keeps most of the Pacific Northwest Coast and its bordering mountain ranges at greatest risk for flash flooding. The North Coast of California remains under a Slight Risk, largely due to antecedent soil moisture conditions being so saturated (NASA SPoRT-LIS shows soil moisture saturation >90 percentile into southwest Oregon). Rainfall totals here are likely to range between 1-2" (locally higher amounts possible), but it is farther north where the most anomalous moisture and best orographic enhancement will support heavier rainfall amounts. The Olympics are likely to see 2-4 inches on average with some areas potentially seeing isolated amounts >6 inches. Freezing levels are as high as 7,000' tonight, which means aside from the tallest peaks, much of the Cascades could contend with periods of rain atop some snowpack. In collaboration with SEW, went ahead and expanded the Marginal Risk to include portion of the northern Cascades. ....Ohio Valley... Ongoing rounds of strong-to-severe thunderstorms will continue to track to the north and east through the afternoon. Strong vertical ascent associated within the right-entrance region of a strengthening 250mb jet streak over the eastern Great Lakes will also feature a 50-60kt 850mb jet over the Ohio Valley. By this afternoon, IVT values ranging between 500-750 kg/m/s will set up from the TN Valley on north into the Upper OH Valley. Latest surface analysis also showed the trailing cold front still tracking through southwest KY/northwest TN as of 15Z. Initial segments of thunderstorms racing out ahead of the cold front have already triggered some areas of flooding in parts of the Ohio Valley, and with additional cells still to come along the cold front, there could be additional areas of flooding as well. One other consideration for the Marginal Risk was noting that 0-10cm soil temperatures in parts eastern OH, southwest PA, and northern WV are close to freezing. With such a cold surface, it can act as a hydrophobic surface win some instances. So despite that hourly rainfall rates may fail to surpass 1"/hr, rates of just 0.50"/hr could still lead to very localized areas of flash flooding, especially in poor drainage areas. ....Coastal New England... Farther up the East Coast, the amplifying upper trough with a lead shortwave disturbance tracking through the Mid-Atlantic Thursday night will promote strong vertical ascent in the upper levels of the atmosphere. As the surface low deepens over PA tonight, the low level jet along the East Coast will strengthen to as much as 70 kts. NAEFS shows >99 climatological percentile IVT values over eastern MA are aimed at the coastal plain of ME and foothills of southeast NH by 06Z Friday. Instability will be hard to come by, limiting top end hourly rainfall rates around 0.5"/hr in most cases. That said, frost depths in these areas range between 5-8 inches, making any moderate-to-heavy rainfall susceptible to run-off from the cold surfaces acting in a hydrophobic manner. In collaboration with GYX and CAR, introduced a Marginal Risk due to the potential for flooding from a combination of highly anomalous moisture and soil conditions acting to enhance the potential for excess run-off. Mullinax Day 2 Valid 12Z Fri Jan 13 2023 - 12Z Sat Jan 14 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN CALIFORNIA COAST AND ADJACENT MOUNTAINS... The upper trough responsible for the heavy rainfall in the Pacific Northwest on Thursday comes ashore along the West Coast on Friday. Strong vertical ascent over California is expected Friday morning as positive vorticity advection (PVA) and sufficient 700mb Q-vector convergence aloft makes its way over the California coast between 12-18Z. The IVT into California will not be as strong as what transpired on Thursday in the Pacific Northwest, but NAEFS still shows IVT values >400 kg/m/s south of the Bay Area oriented from the SSW-NNE. These values are still above the 90th climatological percentile through 00Z Saturday, while PWs largely range between 0.75-1.00". With the aid of 40-50kt 850-700mb winds, upslope flow into the coastal range and as far inland as the windward slopes of the Sierra Nevada can expect periods of heavy rainfall during the day on Friday. Rainfall rates >0.50"/hr are possible in areas where soils remain exceptionally oversaturated (NASA SPoRT-LIS 0-40cm soil moisture percentiles >98% for most of the Slight Risk region). The Slight Risk remains in place from just north of the Bay Area to just south of Eureka. Minor tweaks were made to the Marginal Risk to account for latest updates to QPF. The Olympics can expect another 1-3" of rainfall, which after Thursday's heavy rainfall means soils will be more sensitive for Friday's event. Maintained the Marginal Risk in the Olympic Peninsula but made some minor tweaks to account for the latest QPF issued this afternoon. Mullinax Day 3 Valid 12Z Sat Jan 14 2023 - 12Z Sun Jan 15 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN AND CENTRAL CALIFORNIA... By Saturday, the next atmospheric river (AR) is already on the way, delivering yet another round of heavy rainfall to California. A vigorous shortwave trough will be responsible for ushering in the next slug of anomalous Pacific moisture that contains 0.75-1.0" PWs. IVTs are likely to reach 400-500 kg/m/s (NAEFS showed these values are above the 97.5% climatological percentile) and will be aimed at southern California by Saturday afternoon. The heaviest rainfall arrives late Saturday morning and into the afternoon hours as the warm front pushes across the state, followed by the cold front later Saturday evening, which will act to cutoff the best moisture inflow. This AR is more progressive than some of the other ARs that have occurred in recent weeks, which should help to limit the extent of the flooding potential. All that being said, just about all of California; from the coast and both the Shasta and Sierra Nevada on south to the Transverse Range feature soil moisture percentiles >95%. Sufficient moisture and lift associated with this storm still supports a Slight Risk along the North Coast of California, as well as windward, upslope areas below 6,000' along the Sierra Nevada. Impacts as a result of Excessive Rainfall include areas of flash flooding, mudslides, rapid creek/stream rises, and urbanized flooding. It is possible, should rainfall amounts increase or initial environmental conditions worsen, that targeted Moderate Risk upgrades may be necessary in future forecasts. A Slight Risk could be warranted in future forecasts cycles along the Central California Coast or in the Transverse Range should projected rainfall totals increase. For now, maintained a Marginal Risk in these areas since there is some lingering uncertainty regarding rainfall amounts in these areas. Mullinax Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7EsEVyoaB7RtFH79YEOD80KTJZa9hcl2J7KrnJD3-ZLn= 8gr_yvP4oDeEHv398B2nhJIxgaJgD0c4wN6JoNHs1XC6Dcw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7EsEVyoaB7RtFH79YEOD80KTJZa9hcl2J7KrnJD3-ZLn= 8gr_yvP4oDeEHv398B2nhJIxgaJgD0c4wN6JoNHsCs_cU60$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7EsEVyoaB7RtFH79YEOD80KTJZa9hcl2J7KrnJD3-ZLn= 8gr_yvP4oDeEHv398B2nhJIxgaJgD0c4wN6JoNHslDAg518$=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) .