Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Mar 15 2023 00:59:09 FOUS30 KWBC 150059 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 858 PM EDT Tue Mar 14 2023 Day 1 Valid 01Z Wed Mar 15 2023 - 12Z Wed Mar 15 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS CALIFORNIA... While areas of moderate to heavy rainfall continue across much of the state of California this evening, the main change for this update was to remove the High Risk areas and to largely decrease the extent of the Moderate and Slight Risk areas. This is due to precipitation ending across much of coastal north-central California, as a distinct dry slot (as seen via GOES-West water vapor imagery) has enveloped a good portion of the state. Meanwhile, precipitation continues along upslope portions of the Sierra Nevada foothills, with one particularly impressive area of convection near Merced at this hour (with rainfall rates approaching 1"/hr, per MRMS estimates). This convective activity is expected to spread southward overnight, with much of the expected rainfall still to come for portions of Southern California (with widespread additional rainfall accumulations of 1-3 inches expected to be commonplace, and locally as high as 3-5 inches for portions of the Sierra Nevada foothills and 2-4 inches for the Transverse Ranges). While the coverage of rapid onset flooding impacts are no longer expected to be widespread enough to support a High Risk, there may still be significant impacts across the Moderate Risk areas (which includes the foothills of the Sierra Nevada, much of the LA metro area, as well as coastal portions of Southern California to as far north as Santa Maria and San Luis Obispo). Please see MPD #107 (and subsequent MPDs) for more information in the near-term. Farther to the north of the dry slot, areas of moderate to heavy rainfall also continue across portions of Northern California (primarily from Eureka eastward to Redding and surroundings). The dynamics of the low pressure system are primarily driving precipitation in this area (whereas the atmospheric river and the associated low-level moisture transport are largely driving heavy precipitation to the south). Another particularly impressive area of convection was ongoing this hour, just southeast of Redding (where rainfall rates are also near 1"/hr, per MRMS estimates). Additional rainfall totals here could be as high as 2-3 inches locally (particularly in the vicinity of the Sierra Nevada foothills), before gradually coming to an end later tonight. Please see MPD #108 for more information on the Northern California precipitation in the near-term. Churchill Day 2 Valid 12Z Wed Mar 15 2023 - 12Z Thu Mar 16 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN CALIFORNIA AND THE SOUTHWEST... ....2030Z Update... No noteworthy changes were made with this update. ....California... The tail end of today's atmospheric river (A.R.) will continue impacting far southern coastal California and the Peninsular Ranges early Wednesday morning. CAMs guidance is in excellent agreement that the heaviest rain will end by around 15Z. Thus, we are only looking at the final 2-3 hours of the event overlapping the Day 2 period. That said, being the tail end of the event, much of the storm's rain will have fallen prior to this time. However, the area may see as much as 2 inches of additional rainfall into the mountains. Fortunately, this area was largely missed by the previous A.R. event, and very little of the Peninsular Ranges have any snow cover on them, so snowmelt here will be insignificant. However, 2 inches in 3 hours at the tail end of tonight's rainfall may still cause flash flooding issues. Since this area of the California coast is the least flood-prone portion of the coast (which isn't really saying much), impacts are expected to be limited to flood prone creeks, dry washes and low-lying urbanized areas. Small areas immediately adjacent to the mountains may see more widespread impacts, but this area is not large or certain enough to draw a Slight risk. Thus, the Marginal risk remains a high-end on and a short-fused upgrade remains possible tonight as the impacts in this area are better known. ....Southwest... As the A.R. impacting California moves inland on Wednesday, its overall strength will diminish as the plume of moisture shifts south. However, enough atmospheric moisture will be available in combination with diurnal heating that widely scattered convection is forecast to break out Wednesday afternoon across the Marginal Risk area. With the convection taking advantage of anomalous moisture and MUCAPE as high as 500 J/kg, some of the stronger cells may produce localized heavy rain, which if that occurs over particularly sensitive slot canyons, burn scars, and dry washes, could result in isolated flash flooding. There is some uncertainty as to how numerous the convection will be, with more robust development likely causing enough convection to also potentially need a localized upgrade to a Slight risk. Since there is not good agreement on this scenario, however, the area remains in a high-end Marginal with this update. In a similar manner to the SoCal mountains, significant snow coverage across northern AZ is very limited, so snowmelt is unlikely to contribute much to any flooding concerns. Wegman ....Previous Discussion... ....California and the Southwest... While drier conditions are expected across much of California, a heavy to excessive rainfall threat may continue through Wednesday morning across portions of Southern California. The mid-to-upper level shortwave and associated surface low impacting California on Tuesday are expected to move east into the Intermountain West by early Wednesday. Onshore flow and deeper moisture back along the trailing cold front may continue to impact portions of Southern California, with additional heavy rains possible, especially in the upslope regions of the Peninsular Ranges. As the shortwave moves east, its associated frontal boundary and accompanying axis of deeper moisture are expected to move across the Great Basin and Arizona. This moisture interacting with favorable upper jet forcing may produce isolated runoff concerns, especially in the upslope areas of Arizona into portions of southern Nevada and southwestern Utah. Pereira =20 Day 3 Valid 12Z Thu Mar 16 2023 - 12Z Fri Mar 17 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF EAST TEXAS AND THE LOWER MISSISSIPPI VALLEY... ....2030Z Update... As the same shortwave trough causing today's A.R. tracks east into the Arklatex region, it will gather some Gulf moisture along with it. The air mass with the Gulf moisture will also feature significant instability, increasing to up to 2,000 J/kg by Thursday afternoon. Thus, showers and thunderstorms, some severe, are likely to break out across this region. Soils across much of the Slight risk area aren't saturated, but wetter than normal due to recent rains. At the surface, a strong cold front will rapidly dig southward in association with the upper level shortwave trough. Thus, the primary threat in this region will be severe weather. However, due to the Gulf moisture increasing PWATs ahead of the front to over 1.5 inches and the aforementioned instability, the strong to severe thunderstorms will be capable of locally producing very heavy rainfall, which in turn could result in widely scattered flash flooding concerns, particularly in the Ozarks, where topography could locally exacerbate flooding concerns. Fortunately, with the convection likely to track perpendicular to the motion of the front, training convection appears highly unlikely. Any flooding concerns further south and west over southeast TX will likely be in areas where prefrontal convection moves over the same area as frontal convection (likely with little to no break in between). Stronger storms may produce as much as 3 inches of rainfall, which may exceed 1 hr FFGs in northern areas. In the Marginal risk area across coastal southeast TX and southern LA, FFGs are quite a bit higher, and so convection able to produce rainfall rates that exceed FFGs are less likely, despite a high likelihood of periods of heavy rain with any storms. A small westward expansion of the Slight Risk was made with a bit of a slow-down (westward shift) in the guidance as to where heavy convection is expected to initiate. Wegman ....Previous Discussion... ....Eastern Texas to the lower Mississippi Valley... The mid to upper level trough moving east across the western U.S. Tuesday and Wednesday is expected to amplify over the northern Plains on Thursday, with a deep closed system developing over the Upper Midwest Thursday night into Friday. Strong southerly flow will begin to tap moisture moisture from the Gulf of Mexico, drawing it north ahead of the system's trailing cold front as it advances east from the southern Plains into the lower Mississippi Valley. Both the GFS and ECMWF show PWs climbing to around 1.5 inches along a southwesterly 40-50 kt low level jet centered across eastern Texas into Louisiana. This moisture is expected to interact with favorable mid-to-upper level forcing to support convection, with heavy rain rates developing across the region on Thursday. As the front begins to reorient west-to-east, training storms along the front may bolster the threat for heavy to excessive amounts across portions of the region. The overnight guidance showed a fair amount of spread regarding the location of heavy amounts and therefore forecast confidence is limited.=20 However, the overall consensus generally supported a southerly shift which is reflected in this outlook. Pereira Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_w8Bxfr8YuzIEExB09B6LcfAbyARxK6TbGs8KFwHO29C= 9p_ApAjGRkRSscg8RsjijTEPRam5JKX8mNP1if-luG2Onmg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_w8Bxfr8YuzIEExB09B6LcfAbyARxK6TbGs8KFwHO29C= 9p_ApAjGRkRSscg8RsjijTEPRam5JKX8mNP1if-ljPLVJF0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_w8Bxfr8YuzIEExB09B6LcfAbyARxK6TbGs8KFwHO29C= 9p_ApAjGRkRSscg8RsjijTEPRam5JKX8mNP1if-ldKOZBa0$=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) .