Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Dec 30 2022 01:00:08 FOUS30 KWBC 300100 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 759 PM EST Thu Dec 29 2022 Day 1 Valid 01Z Fri Dec 30 2022 - 12Z Fri Dec 30 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE WEST, WESTERN GULF COAST, & LOWER MISSISSIPPI VALLEY... ....West... Flash flooding concerns return to the West Coast overnight as a low pressure systems and its attendant fronts move into the region. This round of precipitation will be a bit more substantial than the one this morning as frontal boundaries increase boundary layer moisture convergence, precipitable water values rise to 1-1.25", and inflow off the Pacific at 850 hPa rises towards 60 kts. Despite negligible instability, hourly rain totals are expected to peak in the 0.5-0.75" range with local amounts in the 3-4" possible, with the overall threat waning around 09z. Flash flooding is possible over well saturated soils, burn scars and perhaps urban areas. See Mesoscale Precipitation Discussion #1176 for more details of expectations through 09z. ....Lower Mississippi Valley/Western Gulf Coast... Maintained the previously updated excessive rainfall areas from the mid-afternoon amendment. Ample moisture, instability, and effective bulk shear exist for organized convection, which has manifested as occasional training bands and quasi-organized clusters. Hourly rain totals to 2.5" and local amounts to 5" have already occurred across portions of central to eastern TX and western LA, and repeated convective rounds are expected to persist beyond the end of the Day 1 period at 30/12Z -- each additional round could repeat similar hourly (up to 2.5") and local (up to 5") amounts. There is concern that somewhere along a narrow axis from roughly Hemphill TX towards Reston LA could see Moderate Risk impacts as additional convective rounds impact a region that has received 3-5" early, but the uncertainty as to how extensive of an area receives such impacts, as well as the narrowness of the axis of earlier heavy rainfall, has precluded an upgrade to a Moderate Risk at this time. Mesoscale Precipitation Discussions #1175 and #1177 can be consulted for possible impacts through 0429z and 0556z, respectively. Roth Day 2 Valid 12Z Fri Dec 30 2022 - 12Z Sat Dec 31 2022 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL OVER THE COASTAL RANGES OF NORTHERN CALIFORNIA, THE WESTERN FOOTHILLS OF THE SIERRA NEVADA, AS WELL AS PARTS OF THE CENTRAL GULF COAST AND SOUTHEAST... ....West... An on-going atmospheric river will continue to bring heavy rainfall across portions of the West Coast on Friday. A shortwave impulse embedded within an upper ridge will gather enough Pacific moisture to cause continued flash flooding concerns over coastal portions of northern/central California, including the Bay Area. Two rounds of rainfall associated with two separate low pressure systems are expected to occur on Friday and the early hours of Saturday morning, respectively. Rain rates of 0.1-0.50in/hr are anticipated with both rounds. NCEP models show Precipitable Water values between 0.75-1.25in are also expected with anomalies between +3 and +4. The highest rain rates and anomalies are expected along the northern coast and western foothills of the Sierra, where upslope enhancement should contribute to higher totals. 12Z guidance continued the wetter trend that was begun last night, therefore the northern California coast may see increase to between 2-4 inches and the Sierra foothills may accumulate between 3-6 inches by early Saturday morning. Western portions of Nevada, including the Reno metro area, have been included in the marginal risk area due to high snow levels and antecedent rainfall causing vulnerable surfaces to be more susceptible to runoff/flooding. The slight risk area mostly highlights places that are already high in soil moisture, burn scars and urban areas. ....Southern to Mid-Mississippi Valleys eastward into the Southeast... 12Z numerical guidance showed a stripe of moderate to potentially excessive rainfall continuing from the Gulf coast northward into portions of Mid-Mississippi Valley at the start of the Day 2 period at 30/12Z. There was reasonably good agreement with continuity and the only real change here was to embed a Slight Risk area that tended to straddle the Mississippi River given some elevated CAPE that lingered from the end of the Day 1 period. As the entire system translates eastward...there was a trend in the models to not bring as much rainfall northward as was shown by earlier guidance. This was supported by some of the high resolution/convective allowing models (even if those runs ended during the first half of the Day 2 period). The result was to remove the Slight Risk area from the northern portion of Alabama.=20 However, there were enough coarser-scale models with rainfall amounts of a couple inches in the area to warrant maintaining a Marginal Risk. Hourly rain rates ranging from 0.25-1in/hr may cause isolated instances for flash flooding. Higher flash flood guidance and low stream levels will mitigate a lot of the heavy rainfall impacts. Bann Day 3 Valid 12Z Sat Dec 31 2022 - 12Z Sun Jan 01 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF COASTAL CALIFORNIA AND THE FOOTHILLS OF THE SIERRA NEVADA... The latest guidance seems to be in reasonable agreement with respect to Friday's AR event over the northern/central coast of California sagging south through central California on Saturday and into southern Cali by the time New Year's Day rolls around on Sunday morning. A broad upper trough will amplify as it enters the West on Saturday...with the 29/12Z GFS still being a bit deeper and slightly more progressive than the 29/12Z EC with the amplification of this upper trough. The persistent and deep onshore flow should push precipitable water values to nearly an inch by late Saturday night/early Sunday morning...some 2 to 3 standardize anomalies above climatology for December. This leads to two day totals in northern California ending at 12Z Sunday in=20 excess of 4 inches...while the Sierra foothills are expected to receive over 7 inches. Inland valleys may receive a couple of inches as well. Parts of southern California are likely to receive between 1-2 inches of rainfall, which could be enough to cause flash flooding over burn scars and urban areas. The Day 3 areas remain unchanged from the prior issuance. There was less agreement in the 12Z guidance with respect to the QPF and resulting risk of excessive rainfall on Saturday across the Southeast U.S. as the entire system translates eastward. As a result...confined the Marginal Risk to a smaller area and confined between the Gulf of Mexico and the southern extent of the Appalachians. Confidence in this area seeing excessive rainfall is below average...but felt it was too early to remove the area entirely. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!69EdG6mSTUniPxyrRNCWlMeU-_vN4jbQ8KRLYaiVwxWj= zdHREGfrgAtCl7rl9wD9US_MGiBTDkkYiuq-_dqZFIjfFLI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!69EdG6mSTUniPxyrRNCWlMeU-_vN4jbQ8KRLYaiVwxWj= zdHREGfrgAtCl7rl9wD9US_MGiBTDkkYiuq-_dqZIG2SPtI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!69EdG6mSTUniPxyrRNCWlMeU-_vN4jbQ8KRLYaiVwxWj= zdHREGfrgAtCl7rl9wD9US_MGiBTDkkYiuq-_dqZm5SgmGE$=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) .