Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Feb 04 2025 19:17:32 FOUS30 KWBC 041917 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 217 PM EST Tue Feb 4 2025 Day 1 Valid 16Z Tue Feb 04 2025 - 12Z Wed Feb 05 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL AND NORTHERN CALIFORNIA... ....16z Update... Slight risk areas remain largely on track for today with little=20 change to the meteorological reasoning from the previous discussion (see below). Additional adjustments to the Slight and Marginal risk areas were made based largely on the 12z HREF guidance, which did result in a tightening/reduction of the overall Slight risk area (mainly affecting coastal Humboldt county, being reduced to a Marginal risk). The Slight risk generally encompasses where 1-3"+ additional totals are most likely (as well as the highest risk for hourly accumulations of 0.5"+), and an additional internal 25%+ (high-end Slight risk probabilities) was introduced for portions of coastal Monterey county (with the prior 25%+ area for the northern Sierra foothills maintained for this update) where localized 3-5" totals (in association with terrain forcing) are most likely.=20 Otherwise, only minor adjustments were made to the surrounding=20 Marginal risk area. Churchill ....Previous Discussion... A large and energetic cutoff low that will remain largely stationary off the coast of Vancouver Island will be the upper level animus for the active weather ongoing across California today. An atmospheric river event that has been ongoing for the past several days will come to an end (or at least take a significant break) during this period. However, to get to the break, one last potent area of low pressure moving along the atmospheric river still has to move across California, resulting in another period of heavy rain. The cold air following behind its cold front will push the AR far enough south but also far enough away from the forcing to cause it to mostly dissipate tonight. The atmospheric river currently is located along the I-80 corridor from San Francisco through Lake Tahoe. It is expected to shift south to about a Monterey to Modesto line, leaving the I-80 corridor in periodic shower activity. It will not move much along the foothills of the Sierras, where upslope flow will keep the steady rain going well north of where the core of the atmospheric river is. From the Bay Area north along the coast, expect a break from the rainfall through until about 15Z/7am PST. That all changes beyond 15Z, when the aforementioned low begins to move ashore, resulting in steady rain for the almost the entire northern half of California. As in previous days, localized convective enhancement and convergence bands are expected to develop within the broader rainfall shield. This will be supported by a strengthening LLJ through the Sacramento Valley, which will intensify due to the approach of the southerly flow ahead of the low. Colder and drier air following behind the low will begin to clear the precipitation out from northwest to southeast starting around 22Z/2pm PST. Until the back edge clears, locally heady rain will continue, which will be enhanced by the passage of the surface cold front itself, bringing a brief period of heavy rain for most of northern California during the late afternoon and evening hours, potentially impacting this evening's commute. After about sunset or so, the entire atmospheric river will weaken, though the prevailing southwesterly upslope flow may continue much lighter rain into the Transverse Ranges, the southern Sierras, and eventually the Los Angeles Metro area. Despite the recent fires in that area, the rain should be light enough east of Santa Barbara County that there should not be enough to cause flooding concerns, so the light rain from the showers should be beneficial, if not nearly enough to dent the ongoing drought conditions in southern California. All of the guidance has shifted significantly northward with the track of the incoming low for the day today. This resulted in a northward shift in the ERO risk areas, with the Slight Risk being expanded well up the coast to Mendocino County and all of the northern Coast Ranges and trimmed out of the southern Central Valley. An internal higher-end Slight was added to most of the foothills of the northern Sierras since this area stands to pick up an additional 3-5 inches of rain on top of the foot+ they've seen the past few days. The ERO trimming was due to rain shadowing from the southern Coast Ranges. Unlike previous days, the southern Coast Ranges from the Santa Cruz Mountains south through the western Transverse Ranges should all get a healthy soaking today from the atmospheric river, though since those areas have seen significantly less rainfall in recent days compared to areas further north, have left most of these areas in the Marginal. Wegman Day 2 Valid 12Z Wed Feb 05 2025 - 12Z Thu Feb 06 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OHIO VALLEY... Strong southwesterly flow will advect abundant moisture northward into the OH Valley on Wed into Wed night, with PWs forecast to increase over the 90th percentile for early February. Increasing WAA, a mid level shortwave moving east within the quick zonal flow, and upper level divergence in the right entrance region of a jet to the north and left exit region of a strong jet over the MS Valley, will broadly support an expansion of showers across the region. Model guidance continues to indicate upwards of 500-1000 j/kg near and south of a northward moving warm front, thus expect we will see embedded thunderstorms as well. Given the forcing, moisture and instability forecast, do anticipate we will see some locally heavy rainfall rates materialize, especially by Wed evening into the overnight near the lifting warm front. Still some uncertainty on the location of this front, although models are in much better agreement tonight compared to last night. Precipitation north of the front will likely be either sleet or freezing rain, and thus the position of this feature will be important to delineate the northern edge of the flood risk area. The current consensus for the axis of heaviest rainfall is across portions of KY, southern IN/OH into much of WV. Some of these areas may start as sleet/freezing rain, but will transition to locally heavy rain as the warm front lifts north. Areal averaged rainfall is currently forecast in the 0.5"-1" range, although given the forecast instability, locally higher totals ~2" appear possible. Portions of central/eastern KY into WV have above average soil saturation and streamflows, and so this area is likely most susceptible to some flooding impacts if higher rainfall rates do materialize. Chenard 2100 UTC Update Only some minor changes made to the previous marginal risk outlook from the OH Valley into the Central Appalachians. While there is still some spread with respect to the qpf details day 2 across these regions, there is consensus for a tightening of the precip gradient with lesser amounts on the northern edge. We have subsequently adjusted the marginal risk approximately 40-50 miles to the south across southern IN and southern OH. Otherwise, no changes to the previous thinking with locally heavy rainfall across areas where soils are saturated or nearly saturated, resulting in localized runoff issues. Oravec Day 3 Valid 12Z Thu Feb 06 2025 - 12Z Fri Feb 07 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE OHIO VALLEY AND CALIFORNIA... ....OH Valley... Another round of locally heavy rain is expected Thursday across portions of KT/TN and WV along and ahead of an approaching cold front. Locally heavy rainfall should be ongoing at 12z Thursday across portions of WV, before a relative lull in activity by late morning into the early afternoon. However by later in the afternoon into the overnight hours another round of showers and embedded thunderstorms should develop near the cold front over KY and spread eastward into WV. Seeing enough instability forecast to support locally heavy rainfall rates. Some of this corridor already has above average soil saturation and streamflows, and there is also some overlap between this day 3 rainfall and the day 2 rains. Two day rainfall totals of 1-2" are forecast, but given the instability both days, some areas could locally exceed 3". This rainfall should be enough to result in at least some flooding impacts. ....California... After a one day break another system is forecast to move into CA on Thursday. This is expected to be a pretty dynamic system, with a strong mid level shortwave and upper level jet. These dynamics should support a short duration period of enhanced rainfall rates...although the quick forward motion of the system will limit overall rainfall magnitudes. Not seeing much in the way of instability currently forecast, and IVT and PWs are not overly anomalous with this system either. These limitations will be somewhat made up for by the stronger dynamics, however still not expecting this to be a high end event. Rainfall totals around 1" are forecast along coastal areas with 1-2" in the foothills of the Sierra (snow levels will be lower with this system limiting impacts). Given the saturated conditions over the region, this quick shot of locally heavy rainfall may still be enough to generate localized flood impacts, supporting the Marginal risk. The Marginal risk extends southward into Santa Barbara, Ventura and Los Angeles counties as well. Less rainfall forecast here, but there is an uptick in IVT into the terrain, and so would not be surprised if QPF trends up a bit as the event nears. Still not expecting significant rainfall, but possibly enough for localized impacts. Chenard 2100 UTC Update. No changes made to the previous marginal risk areas from the Central Appalachians into the Upper TN Valley. Still some north south spread in the latest guidance, but consensus for locally heavy total over saturated soils which may lead to localized runoff issues. No changes also made to the marginal risk area across coastal central CA, inland into the northern Sacramento Valley and upslope of the Central to Northern Sierra with the last in the current series of heavy precip events forecast to affect CA. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9Ru3u8N6q4gzK-A766p3b7EWawZQ8ozLXflb8Pf0dp7A= asR9srO19gaOtY4EEyOBMyjZuZD4E-16fwm55zcuzXST7pQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9Ru3u8N6q4gzK-A766p3b7EWawZQ8ozLXflb8Pf0dp7A= asR9srO19gaOtY4EEyOBMyjZuZD4E-16fwm55zcuqt0IAfI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9Ru3u8N6q4gzK-A766p3b7EWawZQ8ozLXflb8Pf0dp7A= asR9srO19gaOtY4EEyOBMyjZuZD4E-16fwm55zcuAHeH3Jg$=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) .