Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Dec 02 2023 00:52:55 FOUS30 KWBC 020052 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 752 PM EST Fri Dec 01 2023 Day 1 Valid 01Z Sat Dec 02 2023 - 12Z Sat Dec 02 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL GULF COAST... 2100 and 0100 UTC Updates -- Have upgraded a portion of the Marginal Risk area to a Slight Risk along and near the central Gulf Coast. This is based on recent CAM trends, especially with the time lagged HRRR and even including the GraphCast Machine Learning output from the ECMWF. At 0100Z, we expanded the Marginal Risk a bit more across southern MS-AL based on the latest observational trends along with the 18Z HREF exceedance probabilities. The 18Z HREF guidance and recent HRRR runs continue to support the more enhanced (i.e. Slight) risk along/near the central Gulf Coast. Return flow east of a cold front over eastern Texas will continue to advect a warm/moist/buoyant air mass from the Gulf into the upper TX coast and southern LA today and the central Gulf Coast tonight as the front drifts east. PWATS of 1.75 to 2 inches spread across the central Gulf coast by this evening along with 1000-1500 J/kg MUCAPE. Meanwhile a southern branch of the upper SWly jet shifts over the Central Gulf coast, aiding lift in the right entrance region overnight. Redevelopment of showers and storms this afternoon should be near the Upper TX coast, then shift east through the night, reaching central AL by 12Z Saturday. Given the deep layer flow becoming nearly parallel to the frontal boundary, training storms can be expected near the Gulf. Some of the activity today/tonight will repeat over areas hit hard by heavy rain last night, like southwest and south-central LA where 3-6.5" fell. There is a risk for 2-4" from southwest LA through the central Gulf Coast with 12Z HREF probs 60-80% for >3" through 12Z Sat along the LA/MS/AL/FL Panhandle coast Hurley/Jackson Day 2 Valid 12Z Sat Dec 02 2023 - 12Z Sun Dec 03 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR THE FLORIDA PANHANDLE INTO LOWER SOUTHEAST ALABAMA... ....Gulf Coast and Southeast... Deep moisture will track northeastward along and ahead of a strong but slow-moving cold front over the east-central Gulf Coast. A SWly upper level jet streak will continue intensifying over a swath from Texas to the OH River Valley. Since mean deep layer flow will be nearly parallel to the front, there is a repeating thunderstorm risk, increasing the chances of flash flooding. Sufficient instability for rapid development (roughly over 750 J/kg) will remain near/just inland from the Gulf Coast, though increasing dynamics will allow some heavier rain to penetrate farther inland (mainly over GA) Saturday night. 12Z CAMs honed in on the FL Panhandle for the focus of convection both from late tonight pre-frontal activity moving east along the central Gulf Coast and a second round through much of Saturday with a risk for 3-6" rainfall along the FL Panhandle coast. This is where global models pool 2"+ PWATs. In coordination with WFOs TAE and MOB, a Slight Risk is raised for the FL Panhandle that extends into southeast AL. Organized convection like this is difficult to precisely place and the 18Z HRRR has come in with a more off shore solution on Saturday, so more remains to be seen with where the heaviest rain swath will be. ....Western Oregon into far Northwest California... The next atmospheric river shifts into the OR Coast Saturday afternoon with heavy rain and rapidly rising snow levels pushing to the Cascades through Saturday night. PWATs of 1.25", about 2.5 standard deviations above normal) arrive to the OR Coast by 12Z Sunday. The 12Z consensus among global and CAM guidance is for an additional 2-4" 12Z Sat-12Z Sun along the OR Coastal Range into far NW CA and along the OR Cascades. Isolated rapid flooding in sensitive areas supported by topography is possible, so the Marginal Risk is maintained. With the heaviest focus for central and southern OR, the Marginal Risk was removed from the Portland Metro. Jackson Day 3 Valid 12Z Sun Dec 03 2023 - 12Z Mon Dec 04 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE NORTHERN OREGON COAST RANGES AND CASCADES... A moderately-strong atmospheric river, centered on OR, continues to push into the Pacific Northwest coast Sunday before a brief reprieve behind a cold front Sunday night. 12Z global guidance consensus is for additional 2-4" of rain, with elevated snow levels of 6000-8000ft, over the northern OR Coastal Ranges and Cascades, extending into the WA Cascades. The combination of this prolonged heavy rainfall and warmer air/higher snow levels considerably increases the chances of rapid flooding and a Slight Risk has been raised for these portions of OR/WA in coordination with WFO PQR. With this heavy rain focus coming into a bit clearer view, the latitudinal range of the Marginal Risk was trimmed a bit south of the Canadian border in WA and from NW CA. Further refinement of these risk areas is anticipated as the amount of snow melt added to the rainfall comes into better focus. Will make a note here that an even stronger atmospheric river is progged to impact similar areas Monday night/Tuesday in what is a very active start December in the Pacific Northwest. Jackson Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6JNs3eCCwMGYYDoYcvTXjjuV6fC7BpsJr-M2hNSITmx9= wAQRuNftaHaAX4-ydh_HkzKsRcnyMDoGBTDAKP_NbdjljUE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6JNs3eCCwMGYYDoYcvTXjjuV6fC7BpsJr-M2hNSITmx9= wAQRuNftaHaAX4-ydh_HkzKsRcnyMDoGBTDAKP_NS4tNWJo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6JNs3eCCwMGYYDoYcvTXjjuV6fC7BpsJr-M2hNSITmx9= wAQRuNftaHaAX4-ydh_HkzKsRcnyMDoGBTDAKP_Nwux0HzY$=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. 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