Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Mar 29 2023 20:31:48 FOUS30 KWBC 292031 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 431 PM EDT Wed Mar 29 2023 Day 1 Valid 16Z Wed Mar 29 2023 - 12Z Thu Mar 30 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF CENTRAL AND SOUTHERN CALIFORNIA... ....16z update... Marginal risk area expanded south along the southern portions of the Sierra Nevada foothills of Kern County. The latest model guidance suggest the potential for showers and thunderstorms to spread across much of the state this afternoon/evening with weak onshore flow. Isolated areas of moderate/heavy rainfall, particularly within thunderstorms may lead to runoff over currently well saturated soils. Kebede No changes were made to the Marginal Risk area inherited from yesterday's Day 2 ERO. The atmospheric river impacting California will slide southward into Southern California early on Wednesday and orient the axis of highest atmospheric moisture content (400-500 kg/m*s) orthogonal to the mountain ranges of SoCal. Snow levels are expected to be up to 6000 ft across SoCal as well, until crashing Wednesday night and remaining below 4000 ft for much of the state during the Day 2 period. Rainfall rates may briefly peak at 0.25-0.5"/hr while relatively quick moving across the southern part of the Marginal Risk, but saturated soils could support a localized flooding/rapid runoff threat. The second phase of the Marginal Risk (mainly across central CA, but eventually sliding into the Transverse Ranges) involves the vertically stacked strong polar upper-level low sliding across the region. A secondary convergence axis wrapped around this low along with steeping lapse rates and a subtle layer of instability (MUCAPEs up to 500 J/Kg) will contain scattered convective showers capable of containing localized heavy rainfall rates. Per the the global and CAM guidance, this convergence axis and areas of scattered showers starts near the central CA coast and Sacramento Valley this morning and towards midday, then will slide down the central CA coast and into the Transverse Ranges during the afternoon and evening. Total QPF amounts during this entire time period are not that impressive alone, however isolated 2-4" totals across the overly-saturated ground may lead to localized flooding concerns. Hurley Day 2 Valid 12Z Thu Mar 30 2023 - 12Z Fri Mar 31 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF THE UPPER MIDWEST... 21z update Minor changes were made to the Marginal risk area over the Upper Midwest as the latest guidance continues to tighten up the qpf gradient to the north a bit. The 12z ECMWF continues a trend of an elongated band of heavy rainfall organizing along a warm front extending from the North/Central Plains to the Upper Midwest. Kebede Only minor adjustments were made to yesterday's Day 3 Marginal Risk area, based on the latest guidance/trends. Vigorous deep-layer trough traversing the Great Basin and Southwest on Thursday will lift across the central-southern Rockies and High Plains Thursday night and early Friday. Increasingly difluent upper levels across the Upper Midwest during day 3 will make for a more favorable environment for deep-layer ascent, particularly Thu night as the the global models hint at enhanced right-entrance region forcing from the departing northern stream upper jet streak. A rapid increase in low-mid layer moisture transport from the western GOMEX is evidenced by both the GEFS and SREF, as 850-700 mb moisture flux anomalies climb quickly between 4 and 5 standard deviations above normal Thursday night and early Friday on the heels of robust SSW 850 mb flow of 50-60 kts. On the nose of that strong flow, enough instability and convergence will materialize to support deep convection that may become surface-based along the warm front across northeastern Nebraska and vicinity Thursday evening before migrating east-northeastward into a slightly more stable airmass. The orientation of convection could allow for spotty areas of convective training, with FFG thresholds generally around 1 inch/hr supporting isolated potential for excessive runoff. At this point, the main limitation to a higher excessive rainfall threat (i.e. Slight Risk) is the transient upper flow pattern, with the progressive closed 500 mb low and associated surface frontal features. Such a pattern, one without the benefit of more prolonged low-mid level frontogenesis, would limit the risk of cell training along and north of the axis of robust deep-layer instability (MUCAPEs reaching 1000-1500 J/Kg across eastern NE and western IA late Thu night/early Fri per the 00Z ECMWF). Hurley/Cook Day 3 Valid 12Z Fri Mar 31 2023 - 12Z Sat Apr 01 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF THE MID-SOUTH, TENNESSEE VALLEY, LOWER OHIO VALLEY and Upper Midwest... 21z update A Marginal Risk area was introduced over portions of central/southern Wisconsin into northern Michigan. This risk area is in the warm sector of a potent and dynamic mid-latitiude cyclone that will generate heavy snow over northern Wisconsin and much of the Upper Peninsula of Michigan. Anomalous moisture flow (+2 to +3 std) into a region of favorable instability (750-1500j/kg) supported by strong upper divergence and a deep closed 500mb low should be enough to produce showers thunderstorms capable of causing isolated flash flooding across the Upper Midwest. Higher soil moisture due to Thursday's rainfall event could pose problems over susceptible areas including creeks and rivers. The large Marginal Risk area over the Tennessee/Ohio Valley remains largely untouched with the latest guidance around where they were overnight. Kebede ....ArkLaTex and much of the Mid South into the Tennessee Valley and Lower Ohio Valley... The guidance is fairly well clustered with the timing of the compact mid-upper level trough traversing the Upper Midwest Friday night and early Saturday. The relatively swift progression of this feature will limit the excessive rainfall potential over the northern OH Valley and western Great Lakes; however, farther south, under a more confluent, zonal mid-upper flow, there will be a better chance of cell training along outflow-generated effective fronts oriented quasi-parallel to the deep-layer westerly flow. This scenario is a bit more difficult to glean from the guidance at this point (day 3 forecast), given the otherwise progressive storm track and fairly broad warm sector. Because of the lack of discernible low-level focus at this point from the models, at least one that can linger for a while, the outlook will continue as 'Marginal' for now, despite the highly-favorable deep-layer dynamic and thermodynamic parameters (including 850-700 mb moisture flux anomalies of +3 to +5.=20 Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8j4_cqjAgwT_PYrmsK2Qw3CmfvGtcDi9dEEd8UYoNMOX= qUblzrfXr_sdP1b8dsv5pOzVvLDY8jACx0zvEysC0mI8II4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8j4_cqjAgwT_PYrmsK2Qw3CmfvGtcDi9dEEd8UYoNMOX= qUblzrfXr_sdP1b8dsv5pOzVvLDY8jACx0zvEysCKBbKfPc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8j4_cqjAgwT_PYrmsK2Qw3CmfvGtcDi9dEEd8UYoNMOX= qUblzrfXr_sdP1b8dsv5pOzVvLDY8jACx0zvEysCEuNNveA$=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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