Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Dec 28 2022 23:35:42 FOUS30 KWBC 282335 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 634 PM EST Wed Dec 28 2022 Day 1 Valid 01Z Thu Dec 29 2022 - 12Z Thu Dec 29 2022 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Churchill Day 2 Valid 12Z Thu Dec 29 2022 - 12Z Fri Dec 30 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE COASTAL RANGES OF SOUTHERN OREGON, NORTHERN CALIFORNIA AND PORTIONS OF THE SOUTHERN PLAINS AND LOWER MISSISSIPPI VALLEY... ....Update... The 12Z suite of numerical guidance continues to show increasing flow off the Gulf of Mexico leading to one area of concern for excessive rainfall with the next system renewing the risk of excessive rainfall over the West Coast later in the day. Both areas were covered by Slight/Marginal Risk areas and only minor adjustments to the periphery was needed. Bann ....Excessive Rainfall Discussion... ....West... Flash flooding concerns return to the West Coast as a pair of low pressure systems arrive on Thursday. The first system will bring a round of rainfall to the coasts of the Pacific Northwest and northern/central California early Thursday morning. Rain rates should be between 0.1-0.25in/hr across the region with PWATs remaining modest at well below an inch. Forcing provided by a weak surface cold front won't help to enhance amounts, but higher totals will still be possible at higher elevations. The second round of precipitation will arrive with the entrance of another system later Thursday afternoon. Rain rates will be more substantial at around 0.5-1in/hr. PWATs between 0.75-1 inch should cause some flash flooding concerns over already well saturated soils, burn scars and in urban areas. ....Lower Mississippi Valley... An upper trough will move into the Great Plains on Wednesday leading to height falls and the return of southerly flow over the Gulf of Mexico and Mississippi Valley. Moist air advection will ensue with PWATs of between 1-1.5 inches flowing from the central Gulf coast up through the Mississippi Valley and into the Midwest. This moist/unstable airmass, overrunning a coastal warm front, a dry line and cold front to the west, will support convective initiation over the Gulf coast and inland areas, where rain rates may range between 0.25-1in/hr with locally higher amounts. The latest 00z guidance has come into better agreement with respect to the orientation and magnitude of the axis of greatest QPF in the Lower Mississippi Valley/Southern Plains. The latest QPF signal is a slightly more progressive one, which is reflected in the current slight risk area. The 00z NAM and UKMET appear to be the only outliers as the NAM is farther to the west with its axis and the UKMET is more progressive and wet than the deterministic and ensemble mean consensus. Kebede Day 3 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... ....Update... Except for some minor shifts in the guidance about placement of the rainfall axis in the Central Gulf coast and the Southeast, the previous forecast was standing the test of time there and in the West. There was no fundamental shift in the forecast reasoning for this cycle. Bann ....West... Heavy rainfall continues across portions of the West Coast on Friday. A shortwave impulse embedded within an upper ridge will gather enough Pacific moisture to cause 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 will occur on Friday and the early hours of Saturday morning, respectively. Rain rates of 0.1-0.25in/hr are anticipated with both rounds. PWATs between 0.75-1.25in are also expected with anomalies between +3 and +4. The highest rain rates and anomalies are expected along the western foothills of the Sierra where upslope enhancement should contribute to higher totals. 1-2 inches of rainfall are largely expected over coastal portions of southern Oregon and northern California, while 2-4 inches are likely over the foothills of the Sierra by Saturday morning. The slight risk area mostly highlights places that are already high in soil moisture, burn scars and urban areas. ....Central Gulf Coast & Southeast... An upper trough stationed over the Great Plains will continue focusing moisture from the Gulf of Mexico at the Central Gulf Coast and inland areas on Friday. There's reasonable agreement regarding the axis of heaviest rainfall (1-3 inches) setting up along the Gulf Coast from eastern Louisiana to the Florida Panhandle and southwestern Georgia. PWATs between 1.5-2 inches and hourly rain rates ranging from 0.25-1in/hr may cause isolated instances for flash flooding. The heaviest rates will likely be within convective cells. Fortunately, higher flash flood guidance and low stream levels will mitigate a lot of the heavy rainfall. Kebede Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4Xu1Rv0C3Srl03sKVldaGmaplN6hnoRKydEdV_wl_joi= Wa-xcZ3v1wlGExQs2TxeQGUD0pHJ5iMYicZzqLsqx1JqAR4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4Xu1Rv0C3Srl03sKVldaGmaplN6hnoRKydEdV_wl_joi= Wa-xcZ3v1wlGExQs2TxeQGUD0pHJ5iMYicZzqLsqttsvVho$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4Xu1Rv0C3Srl03sKVldaGmaplN6hnoRKydEdV_wl_joi= Wa-xcZ3v1wlGExQs2TxeQGUD0pHJ5iMYicZzqLsqi1259mQ$=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) .