Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Dec 09 2022 20:21:59 FOUS30 KWBC 092021 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 321 PM EST Fri Dec 09 2022 Day 1 Valid 16Z Fri Dec 09 2022 - 12Z Sat Dec 10 2022 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM FAR SOUTHWEST OREGON INTO NORTHWESTERN CALIFORNIA... 16Z Excessive Rainfall Forecast Discussion... Made a few minor adjustments to fit the boundary of the Marginal Risk area to the latest QPF with an eye on snow-levels. Latest operational guidance still shows 850 mb to 700 mb moisture flux values ramping up during the day and then pushing southward along the coast with time (mainly during the latter part of the period). The 09/12Z HREF probabilities of rainfall exceeding 0.5 inches per hour develop over the southwest corner of Oregon around 10/00Z and then exceeds 50 percent of occurrence within 3 to 4 hours before the area expands south- and eastward along the far northern California coast overnight. As a result, the forecast reasoning remained unchanged from the earlier outlook and the outlook had no significant changes. Bann 0830Z Excessive Rainfall Discussion... Model guidance remains consistent with the arrival of an atmospheric river event into coastal Oregon and northwest California after 0000 UTC Sat ahead of the amplifying mid to upper level trof off the Pac NW coast. 850-700 mb moisture flux anomalies of 2 to 2.5+ standard deviations expected to sink southeastward across these areas. HREF hourly probabilities of ..50"+ are high...60-80%... in this anomalous moisture flux axis..especially after 0500 utc Sat. Probabilities for 1"+ hourly amounts are much lower, topping out at near 30% in very small areas 0600-1200 utc Sat. The axis of anomalous onshore flow will be fairly progressive to the southeast. This and only slightly above average precip values over the past week for northwest California will keep the risk level at marginal at best. There were no significant changes from the previous outlook for this period. Oravec Day 2 Valid 12Z Sat Dec 10 2022 - 12Z Sun Dec 11 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG THE COASTAL RANGES OF CENTRAL CALIFORNIA TO THE SAN RAFAEL RANGE... ....Coastal California... Introduced a Slight Risk area basically from the Santa Lucia Range southward to the Transverse Range in Southern Caifornia as a deepening upper trough directs an atmospheric river southward along the coast on Day 2. Global models still show strong west southwest low level onshore flow supporting 850-700 mb moisture flux anomalies 3 to 3.5+ standard deviations across the risk areas...and 09/12Z HREF probabilities remain high...60-90%...for 50"+ hourly amounts along the central CA coast for much of the day (with some enhancement in rainfall rates as low-level lapse rates steepen in response to cooling aloft from the afternoon into the evening). The probabilities remain in the 60-90% range as the system builds into parts of southern California during the evening and overnight. Deterministic QPF numbers from WPC increased into the 2 to 4 inch range...which certainly allows for locally higher amounts...and has the potential for flooding where the National Water Model shows above to much above normal stream flow (e.g. around Santa Barbara) and in burn areas. Bann ....Far northeast Texas, southeast Oklahoma into central Arkansas... 2030Z Excessive Rainfall Discussion... Except for a slight southward expansion of the Marginal Risk area in deference to the low-probabilities of 1+ inch per hour south of a front dropping southward...the Marginal Risk area was unchanged from the 0830Z issuance. 0830Z Excessive Rainfall Discussion... A marginal risk area was added across portions of the Southern Plains toward the Lower Mississippi Valley for training potential Saturday along and south of the cold front expected to push east southeastward across this region. Southwesterly low level flow expected to strengthen early day 2 across the Southern Plains, with the potential for training of cells through Saturday afternoon in what will be an axis of PW values 2 to 3 standard deviations above the mean. In area of training, .50-1"+ hourly amounts possible with potential for more than one round of convection to affect a region... In areas of training, isolated runoff issues possible. Oravec Day 3 Valid 12Z Sun Dec 11 2022 - 12Z Mon Dec 12 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHWESTERN CALIFORNIA... 2030Z Update... No changes were needed to the previously-issued Slight Risk area.=20 Guidance remained consistent in the big picture and the forecast reasoning remained unchanged. Bann 0830Z Excessive Rainfall Outlook Discussion... The axis of anomalous 850-700 mb moisture flux anomalies will continue its southeastward push day 3 into Southern California.=20 Values of 3-3.5+ standard deviations above the mean will push across Southern California during the first half of the day 3 period, supporting heavy rainfall potential from the Peninsular Range westward to the coast and to the south of the eastern portions of the Transverse Range. Overall, there is good model agreement for heavy rains across these areas, with consensus for areal average 1 to 1.5"+ amounts with max totals of 2-3" in the favored terrain regions of the Transverse and Peninsular Ranges.=20 These rains will support localized runoff issues, especially across the numerous burn scar regions of southwestern California. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9FUiXpAx_Ub363Co8vp0LvDB1WSLv4-O0YeO0B6ZUQZV= _L-9tYgksJfGKO-imAuf1kHS8mE2wolGqUJm5Tr_msDHM3o$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9FUiXpAx_Ub363Co8vp0LvDB1WSLv4-O0YeO0B6ZUQZV= _L-9tYgksJfGKO-imAuf1kHS8mE2wolGqUJm5Tr_fHXlHIs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9FUiXpAx_Ub363Co8vp0LvDB1WSLv4-O0YeO0B6ZUQZV= _L-9tYgksJfGKO-imAuf1kHS8mE2wolGqUJm5Tr_gMfu4ZU$=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) .