Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Dec 30 2022 08:24:45 FOUS30 KWBC 300824 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 324 AM EST Fri Dec 30 2022 Day 1 Valid 12Z Fri Dec 30 2022 - 12Z Sat Dec 31 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN CALIFORNIA AND THE CENTRAL GULF COAST... ....California... An atmospheric river driving eastward along a 170 kt upper level jet is forcing light to moderate rain across portions of northern CA this morning. For the most part, this forcing will remain stationary, at least for the first half of the day. After that, a strengthening shortwave trough digging into the jet will both begin to weaken and shift it south overnight tonight. Since for most of the day the plume of moisture will be stationary, expect prodigious rainfall into the coastal ranges and the Sierras of northern CA. Rainfall totals of 4-7 inches are expected by Saturday morning in both mountain ranges, particularly between Eureka and the Bay Area and from Fresno north along the Sierras. Snow levels are not expected to move much today, hovering around 9,000 feet, meaning most areas will see rain except the highest peaks of the Sierras, where uplift may help locally lower snow levels. PWATs between 1.0 and 1.25 inches remain in the forecast with anomalies around +3 standard deviations above normal. As usual, the valley locations in between the aforementioned ranges will get significantly less rainfall due to downsloping off the coastal ranges. Finally, the most susceptible areas continue to be those areas high in soil moisture, burn scars, canyons, and urban areas. Changes from the inherited forecast were primarily to trim the southern portions of the risk areas...as the atmospheric river appears to be slower in beginning its move southward. Thus, rainfall amounts that could threaten flash flooding are less likely from Fresno south and over the Bay Area on Friday. ....Central Gulf Coast & Mississippi River Valley... In coordination with the MOB/Mobile, AL and LIX/New Orleans, LA forecast offices, trimmed the previous Slight to the south closer to the Gulf Coast. Guidance is nearly unanimous that thunderstorm activity capable of rainfall rates high enough to trigger flash flooding will be confined to the Gulf Coast, as little in the way of instability makes it very far inland. Further, the lines/cells of thunderstorms that are able to form are likely to be progressive moving towards the east, limiting the amount of rain any one location receives from any thunderstorm activity. The Marginal Risk area was maintained further north along the MS River into southern IN, but antecedent very dry conditions combined with thawing of the ground from the recent cold spell should allow the soils to effectively absorb much of the rainfall. As mentioned above, instability will be very lacking in this area, so cells or instances of heavier rainfall are unlikely in this area, further reducing the flooding threat. Wegman Day 2 Valid 12Z Fri Dec 30 2022 - 12Z Sat Dec 31 2022 ....THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR THE COASTAL RANGES OF CENTRAL CALIFORNIA, THE TRANSVERSE RANGES OF SOUTHERN CALIFORNIA, AND THE WESTERN SLOPES OF THE SIERRA NEVADA... ....California... The atmospheric river impacting much of northern California on Friday continues into New Year's Eve Saturday from the Bay Area east into the Sierra Nevadas. Unlike on Friday, by Saturday much of the heaviest rainfall totals are expected in the Sierra Nevada, and less-so into the coastal mountains. This is likely because the digging trough Saturday morning will place the Sierras and interior CA and far western NV more in the left exit region of the jet as it becomes much more amplified with time. Expect another 2 to 4 inches of rain across the northern Sierras Saturday morning before the rain associated with the AR finally begins shifting southward. Behind the atmospheric river, expect snow levels to rapidly fall to between 4,000 and 5,000 feet into New Year's night. This will change over any lingering upslope precipitation over to snow as it ends. The coastal ranges will see rainfall associated with the locally enhanced A.R. being helped by the digging trough being somewhat cancelled out by the increasingly fast southward movement of the moisture plume. The amplified nature of the moisture will direct the rainfall more south to north once it reaches SoCal on New Year's night. Therefore the Transverse Ranges will be favored by a short period of heavy rain during which 2-3 inches of rain are expected. The inherited Slight Risk along the coast was trimmed on the northern end as little lingering rain is expected on the northern CA coast, and also trimmed on the southern end to exclude the Peninsular Ranges. With the atmospheric river more directed south to north, the Peninsular Ranges will not effectively upslope the moisture...unlike the Transverse Ranges. The interior Slight Risk for the western slopes of the Sierras was left unchanged with this update.=20 ....Portions of the Southeast... The inherited Marginal Risk from the Florida Peninsula north to the southern Appalachians was removed with this update. Lingering rainfall over this area has trended downward quite significantly with the latest guidance, with most of the previous Marginal Risk area now expected to pick up less than an inch of rain. Wegman Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_dyQhz7LSX4Qiip4kyezyA5M4LWfXdsw63zARFYaOgJm= wuB8kauoL5ZAOjQintGDc0nV2I6d8wM458dVOEo7w0in6UI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_dyQhz7LSX4Qiip4kyezyA5M4LWfXdsw63zARFYaOgJm= wuB8kauoL5ZAOjQintGDc0nV2I6d8wM458dVOEo7knpf0Co$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_dyQhz7LSX4Qiip4kyezyA5M4LWfXdsw63zARFYaOgJm= wuB8kauoL5ZAOjQintGDc0nV2I6d8wM458dVOEo7Np9ajRc$=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) .