Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Dec 10 2022 23:04:55 FOUS30 KWBC 102304 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 603 PM EST Sat Dec 10 2022 Day 1 Valid 2300Z 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 WESTERN TRANSVERSE RANGE... ....Northern to Central California Coast, western Transverse Range... An atmospheric river event is sinking southeastward down into central California coast on the southeast side of the amplifying mid to upper level trof off the Pac NW coast. A well defined band of heavy precip likely to sink southeastward along and ahead of the associated cold front in the region of strong south southwesterly low level flow, followed by more showery conditions post frontal. 850-700 mb moisture flux anomalies in this onshore flow axis expected to remain in the 3 to 4+ standard deviations above the mean. While the front and associated strong low level moisture flux axis will be pressing fairly quickly southeastward, there will be a period of heavy rains with model consensus for heavy precip totals along the northern to central California coast to the western portion of the Transverse Range.=20 The slight risk area was drawn where HREF neighborhood probabilities were high...50-90%...for 3"+ totals this period.=20 Some of the northward portions of the area were removed during the 16z update due to radar trends. Model consensus is for 2 to 4" in the slight risk area, with max totals 5 to 7" in the coastal range from Monterey south to near San Luis Obispo. Hourly rainfall rates likely in the .50"+ range with HREF probabilities high in the anomalous moisture flux axis...60-90%...but drop off significantly for 1"+ totals, illustrating the hourly precip limits due to the fairly quick southeastward push.=20 ....Northeast Texas into southern Arkansas... Showers and thunderstorms have been trying to train in a west southwest to east northeasterly direction along and behind a surface cold front across North TX, with occasional and isolated hourly rain totals of 1-2". The polar front responsible for this activity is sinking southward, and 18z HREF guidance indicates that the spotty heavy rainfall across northern TX should wane as instability/ML CAPE values decline while the front shifts south towards a better instability pool located along sections of the Upper and Middle TX coasts. An organized thunderstorm west of Houston is a sign of things to come, as hourly rain totals to 2" have been noted with this right-moving mesocyclone. With time, the polar front sags southward into the southern convective area north of I-10 in East TX which should lead to a magnification of rain totals and areal extent of heavy rainfall. Cell training, mesocyclones, and cell mergers would be the main culprits for heavy rain production. The ingredients available support hourly rain totals to 2", and the 18z HREF has a ~50% for 5" amounts between Saturday afternoon and Sunday morning -- local amounts of 4-6" are expected. Since the area has been quite dry lately, figured a broadening of the previous Marginal Risk area down into portions of Eastern TX north of I-10 made more sense than an upgrade to a Slight Risk, which was coordinated with the EWX/Austin-San Antonio, HGX/Houston, and LCH/Lake Charles LA forecast offices. There could be isolated spots north of Houston TX where Slight Risk level impacts occur, but not a large enough area to depict within the probabilistic graphic. Roth/Bann/Oravec=20 Day 2 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... On-going outlook remains on-track as an atmospheric river associated with an amplifying large scale trough continues to spread southward on Sunday and Sunday night/early morning. Made a couple of small edits...but the forecast reasoning was the same as it was for the 0830Z ERO. ....0830Z Excessive Rainfall Outlook Discussion... The day 1 atmospheric river event affecting northern to central California coastal sections will continue to press southeastward day 2, with the axis of anomalous 850-700 mb moisture flux values pressing into Southern California. 850-700 mb moisture flux anomalies of 4+ standard deviations above the mean will push across Southern California during the first half of the day 2 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. Similar to the day 1 period, this anomalous moisture flux axis will be fairly progressive to the southeast, limiting hourly amounts to 50"+. There continues to be 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. These rains will support localized runoff issues, especially across the numerous burn scar regions of southwestern California. Oravec Day 3 Valid 12Z Mon Dec 12 2022 - 12Z Tue Dec 13 2022 .....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN PLAINS... ....2030Z Update... The potential for excessive rainfall will be shifting out over the Central High Plains as the large scale trough makes its way eastward across the Great Basin and crosses the central Rockies.=20 The on-going Marginal Risk area still fits the best overlap of instability with QPF model consensus. Still think that convection will be moving quickly and will maintain the Marginal Risk area. Bann ....0830Z Excessive Rainfall Outlook Discussion... The strong closed low moving inland into California and the Great Basin on day 2 will push across the Great Basin, Central Rockies and into the Central High Plains by day 3. South southwesterly low level flow expected to strengthen significantly on the southeast side of this closed low through much of the Southern and Central Plains, with 850-700 mb moisture flux anomalies 3 to 5 standard deviations above the mean across these areas. Favorable upper difluence, strong frontal/pre-frontal convergence in this axis of strong moisture flux will support increasing precipitation coverage day 3 across much of the Southern and Central Plains.=20 The marginal risk area was confined to the southern portion of the model qpf consensus region where instability will be the greatest and possibly support an area of convection that could produce isolated short term precip totals of 1"+. At the moment, it appears any convection that does form along and ahead of this front will be fairly quick moving to the east, keeping the risk level at marginal. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!99tNd-n6EuIYVI0rCQtkrNCKcriSOxMKi07hg4UJbo9H= _wSneGebF6ilmqP53nYyjbYnYkhZ8sGUFT8Ml2x3ywJUSDQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!99tNd-n6EuIYVI0rCQtkrNCKcriSOxMKi07hg4UJbo9H= _wSneGebF6ilmqP53nYyjbYnYkhZ8sGUFT8Ml2x3Lq3YtQI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!99tNd-n6EuIYVI0rCQtkrNCKcriSOxMKi07hg4UJbo9H= _wSneGebF6ilmqP53nYyjbYnYkhZ8sGUFT8Ml2x3wgnKALg$=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) .