Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Mar 03 2023 08:31:13 FOUS30 KWBC 030831 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 330 AM EST Fri Mar 03 2023 Day 1 Valid 12Z Fri Mar 03 2023 - 12Z Sat Mar 04 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM NORTHEASTERN ARKANSAS NORTHEAST INTO SOUTHEASTERN MISSOURI, SOUTHERN ILLINOIS, AND CENTRAL INDIANA... A complex synoptic and mesoscale pattern will continue to evolve overnight and throughout the day Friday into early Saturday morning as a potent, negatively tilted mid to upper-level shortwave moves northeastward from the Southern Plains towards the Midwest and eventually the Northeast. At the surface, an area of low pressure will better organize/consolidate over the ArkLaTex early Friday morning and deepen rapidly as the system occludes, moving northeastward through the Middle Mississippi and Ohio Valleys and eventually to the Lower Great Lakes by Saturday morning. A cold front will sweep eastward across the South and Tennessee/Ohio Valleys as the warm sector expands northward behind a warm front pushing across the Ohio Valley, Appalachians, and Mid-Atlantic. Deep moisture transport northward from the Gulf will be quite robust with a strong 65-75 knot low-level jet leading to anomalously high PW values upwards of 2-3 standard deviations above the mean, particularly where convergence is maximized along the track of the surface low/occluding front through the Middle Mississippi and Ohio Valleys. Multiple areas of heavy rainfall are expected as storms progress to the northeastward throughout the day, with the focus on the Mississippi and Ohio Valleys. In coordination with MEG/PAH/ILX/IND/LMK, a Moderate Risk has been introduced for portions of the Lower/Middle Mississippi and Ohio Valleys from northeastern Arkansas to central Indiana. While CAPE values will be relatively low ( ~ 500 J/KG ) for the expectation of heavy rainfall rates, the combination of strong mid to upper-level dynamic forcing, the rapidly deepening cyclone, convergence along the occluding surface fronts, and significantly anomalous surface moisture will compensate to produce a period of intense rainfall following the surface low track. Multiple Hi-Res guidance members including the HRRR, FV3-LAM, and HRW-ARW2 have a strong signal for areal average rainfall totals of 3-5 inches, with the bulk of this rainfall occurring over a relatively short frame given the system's rapid movement to the northeast. HRRR max hourly rainfall totals confirm the expectation of rainfall rates of 1-2" per hour. In addition, clusters of storms forming along and ahead of the surface fronts will have the tendency to train over the same regions as they track to the northeast. The expectatioin is these rainfall totals will easily exceed FFG values given already wet antecedent conditions and additional rainfall that will likely already have occurred overnight with convection extending north-northeastward along and to the north of the warm front moving into the Ohio Valley. While intense downpours and heavier areal average rainfall totals of 1-3" will continue further eastward into the Upper Ohio Valley later Friday afternoon, the decreasing coverage of advance rainfall along with the continued rapid system movement will lessen the areal coverage of impactful heavy rainfall in the Slight Risk area compared to further west. A couple rounds of rainfall are likely for the Mid-Atlantic both with a band of rain lifting northward ahead of the warm front and later along and to the east of the Appalachians as the cold front pushes through late Friday afternoon and into the early overnight hours Saturday. Some locally heavy rainfall totals of 1-2" will be possible as storms tend to track northeastward along the front, particularly for upslope favored areas along the Appalachians. There is less confidence in the coverage of impactful rainfall further south into the Tennessee Valley given the progressive nature of convection developing along the surface cold front. However, the Hi-Res guidance still does hint at the potential for some localized heavier downpours and with wet antecedent conditions in place, the Marginal Risk was maintained. Putnam Day 2 Valid 12Z Sat Mar 04 2023 - 12Z Sun Mar 05 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR COASTAL AREAS OF SOUTHERN OREGON AND NORTHERN CALIFORNIA... The previous forecast remains on track. A Marginal Risk was maintained for the coast of southern Oregon/northern California as some locally heavy rainfall of 1-2", potentially 3", remains possible at lower elevations along the terrain of the immediate coast. The wet antecedent conditions/pattern recently over the region will locally enhance the chance for an instance or two of minor flooding. Putnam ....Previous Discussion... The busy pattern in the Northwest looks to continue as another deep upper low develops off the Pacific Northwest coast late Friday into Saturday. A shortwave on the south side of the mid-level closed low is likely to impact the coast of OR into northern CA at the start of the Day 3 period (Saturday morning) with a round of locally heavy rain. A second round of heavy rain is anticipated in the 00-12Z Sunday window, tied to another mid-level shortwave rounding the base of the closed low. Each shortwave is forecast to be accompanied by a surge in 850 mb winds of 40 to 50 kt, with wind direction ranging from S to W over the period, impacting the lower elevations of the Coastal Ranges. While precipitable water values do not look to exceed 0.5" along the coast and snow levels will be near 2000 ft ASL, lowering into Sunday morning, a localized threat for flooding will exist given the potential for locally heavy rain from the two rounds of precipitation. Steep lapse rates will be present from 850 mb to 500 mb (or above 500 mb given a low tropopause height) which should promote a showery regime with embedded heavier cores. 24 hour totals of 1-2 inches are expected along the coast but with localized totals near or just above 3 inches possible. Recent rain/snow has helped soil moisture to be above average for the region and WFO EKA noted minor flooding on some area roadways, so the additional rainfall from Saturday into Sunday could renew localized flooding concerns. As the event draws nearer, future forecast guidance may support adjustments or complete removal of the Marginal Risk area. Otto Day 3 Valid 12Z Sun Mar 05 2023 - 12Z Mon Mar 06 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Putnam Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!46AqohHEdeyF1nYfOmYDHMKQNzge72a-Fa5EAJ85Ts4u= h_PiUzaIvbH_3haKBrt8brR9H28SBwfEKUAxl7e_DmTmM30$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!46AqohHEdeyF1nYfOmYDHMKQNzge72a-Fa5EAJ85Ts4u= h_PiUzaIvbH_3haKBrt8brR9H28SBwfEKUAxl7e_0AWr3Hs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!46AqohHEdeyF1nYfOmYDHMKQNzge72a-Fa5EAJ85Ts4u= h_PiUzaIvbH_3haKBrt8brR9H28SBwfEKUAxl7e_IMrloD8$=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) .