Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Feb 04 2025 23:09:34 FOUS30 KWBC 042309 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 609 PM EST Tue Feb 4 2025 Day 1 Valid 01Z Wed Feb 05 2025 - 12Z Wed Feb 05 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL AND NORTHERN CALIFORNIA... A large and energetic cutoff low will remain largely stationary=20 off the coast of Vancouver Island. An atmospheric river event to=20 its south-southeast has yielded a surface low, which is crossing=20 northern CA at the present time. Its cold front will progress, pushing the AR far enough south but also far enough away from the=20 forcing to cause it to fade overnight. Localized convective enhancement and convergence bands are=20 expected to develop within the broader rainfall shield near and behind the front. Colder and drier air following behind the low=20 will begin to clear the precipitation out from northwest to=20 southeast shortly. Until the back edge clears, locally heavy rain=20 will continue. After local sunset, the atmospheric river will weaken, though the=20 prevailing southwesterly upslope flow may continue much lighter=20 rain into the Transverse Ranges, the southern Sierras, and=20 eventually the Los Angeles Metro area. Despite the recent fires in=20 that area, the rain should be light enough east of Santa Barbara=20 County that there should not be enough to cause flooding concerns,=20 so the light rain from the showers should be beneficial, though not enough to dent the ongoing drought conditions in southern=20 California. The ERO trimming was due to trends seen in radar reflectivity and the 18z HREF. Roth 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!-GA-rHbrUA4gU9skotQVr9GfM_VVWbBnICvFFNNdS7-1= 9ovpSk9jgmUeDbvjyu-f2uBun7re6iBlbZ0AK5xcpKQJeLM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-GA-rHbrUA4gU9skotQVr9GfM_VVWbBnICvFFNNdS7-1= 9ovpSk9jgmUeDbvjyu-f2uBun7re6iBlbZ0AK5xcsqHdG2k$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-GA-rHbrUA4gU9skotQVr9GfM_VVWbBnICvFFNNdS7-1= 9ovpSk9jgmUeDbvjyu-f2uBun7re6iBlbZ0AK5xc3gnpCgI$=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) .