Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue May 23 2023 01:00:41 FOUS30 KWBC 230100 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 859 PM EDT Mon May 22 2023 Day 1 Valid 16Z Mon May 22 2023 - 12Z Tue May 23 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST AND SOUTHERN PLAINS... ....Southeast... Scattered convection was ongoing across the Southeast as of 00Z beneath a broad and disorganized upper trough containing a number of embedded vorticity maxima from the Lower Mississippi Valley to the southeastern U.S. coast. Precipitable water values remained high, ranging between 1.5 and 2 inches supporting rainfall rates of 2 to 3 in/hr, locally higher across the region. Stability has increased over eastern Alabama into most of Georgia behind a westward surging outflow boundary, extending from northeastern Mississippi to the Florida Panhandle at 00Z. Overrunning of the outflow boundary has supported the most intense cores on radar, some slow moving, over eastern Mississippi into southern Alabama and the western Florida Panhandle. Additional convection was noted over the Florida Peninsula but trends in infrared satellite showed warming tops as much of the region has been worked over by afternoon convection. Convection is expected to continue a gradual downward trend as overrunning gradually diminishes and nocturnal boundary layer stabilization occurs. However, a low end threat for flash flooding will remain over the Southeast in the short term with lingering convection and again overnight as southeasterly flow just east of an 850-700 mb circulation over Alabama draws low level moisture northward, with elevated instability supporting a possible flareup of convection near or just prior to daybreak across Alabama. The Slight Risk has been removed over northern Florida into Georgia/Alabama as the threat for additional heavy rain has diminished compared to earlier in the day. ....Southern to Central Plains... A few areas of convection remained from the Oklahoma Panhandle southward to the Rio Grande. An MCS appeared to be organizing over northwestern Texas with movement toward the southeast. Some increase in the low level jet is anticipated overnight over the southern High Plains with RAP forecasts of 20 to 30+ kt at 850 mb from the south. Localized 2-4 inch totals will remain possible, mainly near the advancing MCS with possible development along its southwestern flank overnight. In addition, slow moving storms near a mid-level vorticity max over southwestern KS will continue to pose a localized flash flood threat as the vort center drifts toward the ESE overnight, but eventually running away from the better instability to the west. Otto Day 2 Valid 12Z Tue May 23 2023 - 12Z Wed May 24 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST... ....2030z Update... Changes to the Day 2 outlook were fairly minimal this cycle, with no categorical changes proposed. The shaping of the Slight and Marginal boundaries was largely led by the addition of the complete 12z HREF neighborhood exceedance probability thresholds (mainly 5" and 3" thresholds for Slight and Marginal areas, respectively). The inherited risk areas barely had to be adjusted, after accounting for the new data. Overall thinking from prior discussion remains unchanged. Churchill ....Previous Discussion... ....Southeast... Heavy rainfall will be possible again across portions of the Southeast Tuesday into Tuesday with the stationary boundary remaining draped across the region. Moisture will continue to be above normal with the favorable easterly flow bringing in PWs 1.75"+ around the stationary boundary. Meanwhile, a weak mid/upper level shortwave will slide eastward across the Southeast, reaching the eastern Gulf and Florida Peninsula late in the period. This will bring another cold front southeast and this combination of lift, moisture, and peak daytime heating instability will be sufficient for widespread showers and thunderstorms. The ingredients will support intense rain rates, upwards of 2-3" in an hour at times, particularly across northern/northeast Florida into southern/southeast Georgia. Slow storm motions and colliding boundary interactions may also play a factor into the rain totals, which could top 2-4" in places for the period. Soils are likely to become increasingly more saturated by Tuesday thanks to a few days of heavier rainfall expected and this may lead to more scattered instances of flash flooding, particularly for the more susceptible locations and urban areas. ....Southwest OK through North Central TX... Another round of dry-line induced showers and thunderstorms is expected Tuesday into Tuesday night, a bit displaced to the east compared to today's expected convection. The latest guidance shows a better/strong low level jet upwards of 30-35 kts at 850 mb along with a large pool of higher moisture across the region. The 00Z guidance remains on track for showing potential of 1-2" areal average totals in the risk area with some signal for higher amounts possible and the Marginal Risk was only minimally changed for this update. ....Intermountain West... A very slow moving longwave trough over the West Coast will direct a large stream of moisture northeastward in advance of the trough. Embedded within the trough is an upper level low that will be over the BC/AT border north of WA/ID. A shortwave disturbance pinwheeling south of the low will approach the intermountain west Tuesday. This will locally increase the forcing over the Marginal Risk area over northeast NV, northern UT and southeastern ID. Anomalous moisture will still be in place, with PWATs up to 0.75 inches are over 3 sigma above normal. This will support training storms tracking northeast from NV through UT and into ID. The storms that are forced by the local mountains may stay more stationary, which will locally increase the flash flooding potential. ....Northern Plains... Scattered to numerous showers and thunderstorms are likely to develop across portions of North Dakota into northern Minnesota Tuesday evening as a cold front approaches from Canada and intercepts a low level southeasterly jet of 25-30 knots that will be advecting deeper moisture into the region. Localized 1-2" totals are expected based on the latest forecast guidance and although the areal coverage of heavy rainfall isn't expected to be high, the storms that do form will be capable of producing intense rain rates that could be great enough to cause isolated instances of flash flooding.=20 Taylor Day 3 Valid 12Z Wed May 24 2023 - 12Z Thu May 25 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHERN MAINE AND EASTERN COLORADO INTO WESTERN KANSAS... ....2030z Update... No significant changes were made to the Day 3 outlook for this update. Both Marginal Risk areas were adjusted based on the new 12z guidance, which resulted in small expansions to both. The more significant of the expansions was across the Front Range, where the Marginal was expanded a bit northward based on some guidance initiating convection farther north. Overall thinking from below remains unchanged. Churchill ....Previous Discussion... ....Colorado into Kansas... Southwesterly flow across the Rockies is expected as the central U.S. ridges builds further and a closed low moves onshore the West Coast. Embedded shortwave energy will pass through the southwesterly flow with one wave likely to reach the Central Rockies Wednesday into Thursday. Meanwhile, low level southerly flow across the Plains ahead of the approaching trough will bring higher moisture northward into eastern Colorado and western Kansas. Convection is likely to initiate off the higher terrain then move eastward into the Plains, interacting with any residual surface boundaries. The sufficient instability and moisture combined with the signal for a few repeating rounds suggests an isolated risk of heavy rainfall (1-2" hourly totals) that will be capable of producing flash flooding and a Marginal Risk was introduced this cycle. ....Maine... A deepening trough crossing the Hudson Bay region is expected to become negatively tilted and close over eastern Ontario by late Wednesday into early Thursday. A sharp cold front will slip southeastward into the region with an area of low pressure developing along the front. The strong frontogenesis and mid/upper level diffluence is likely to lead to an axis of heavier precipitation from the northern portions of New Hampshire into northern Maine along the Canadian border. Moisture return isn't overly impressive and there's likely to be limited instability (perhaps up to 500 J/kg), nonetheless guidance shows fairly strong signals for 1-2" totals over the period which could be in a narrow axis over a relatively short period of time leading to isolated instances of flash flooding. Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5sdDDy75zy5eM_wlxT3Q2-8b62VybtQwuGYqzE0hSbcr= XsBfABhlXCZ8C8q8lDblJ85zZW-QEgdYpJWfO4jXfmC00tU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5sdDDy75zy5eM_wlxT3Q2-8b62VybtQwuGYqzE0hSbcr= XsBfABhlXCZ8C8q8lDblJ85zZW-QEgdYpJWfO4jXfJFTJUM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5sdDDy75zy5eM_wlxT3Q2-8b62VybtQwuGYqzE0hSbcr= XsBfABhlXCZ8C8q8lDblJ85zZW-QEgdYpJWfO4jXOqJslP4$=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) .