Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jun 23 2023 16:24:23 FOUS30 KWBC 231624 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1223 PM EDT Fri Jun 23 2023 Day 1 Valid 16Z Fri Jun 23 2023 - 12Z Sat Jun 24 2023 ....THERE IS MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF MONTANA TODAY AND TONIGHT... 16Z update... Upgraded portions of southern and southeast Wyoming to a Moderate risk after looking at the latest runs of the HRRR and the 12Z HREF neighborhood probabilities for rainfall exceeding flash flood guidance in the 1- and 3-hour time interval. While the signals had been there earlier...they increased with the 12Z run and were consistent with increasing coverage of convection over northwest Wyoming being earlier than earlier forecast. In addition...a shift of tails at between 2 and 3 was also a consideration. All this fits the synoptic patter which favors upper level difluence and an anomalously deep upper trough approaching the area.=20 Question was how far south and east to extend the Moderate risk area...with the signals become less clear. There was some indication that the risk may extend farther downstream than currently shown...but the prospect of repeat convection or training needed for flash flooding was not a certainty at this point. Felt the HREF probabilities provided enough support to expand the Slight Risk area a bit South Dakota and a small portion of nearby Minnesota.=20 The Slight Risk in Florida and the Marginal risk areas elsewhere were largely left unchanged. Bann ....Previous Excessive Rainfall Discussion... ....Montana into the Plains... Shortwave energy currently moving into Nevada early this morning will quickly lift northeast toward Montana by tonight. Strong/favorable setup for upper diffluence will be provided by dual left exit and right entrance region of the upper jets positioned over the Four Corners and southern Canada. Higher moisture seeping into Montana will reach upwards of 0.75", fairly modest at around 1.5 standard deviations above normal, but sufficient enough when combined with the greatest forcing and available instability expected. Based on the 00Z guidance and HREF probabilities, the best setup for widespread 1-2" totals and hourly totals as high as 1-1.5" will be across south-central Montana into southeast Montana. Localized 3"+ will be possible as indicated by the localized 20-25 percent neighborhood probabilities in the 00Z HREF, especially over southern Montana. A secondary area of heavier rainfall is expected across portions of the Plains, particularly southern South Dakota into far northern Nebraska, later this evening into early Saturday morning with a surging low level nocturnal jet expected as the aforementioned shortwave energy reaches the western Dakotas. Convection early this morning should lift northward into North Dakota followed by a relative lull this afternoon, before additional clusters of strong thunderstorms track through SD/NE. Training thunderstorms will be possible along with repeating rounds with the latest HREF probabilities suggesting isolated 2-4" possible across southern South Dakota between 00-12Z Saturday. Both of these areas have the setup, ingredients, and wetter antecedent conditions toward the higher-end of the Slight Risk range (15-40%) and if guidance continues to come together, a targeted upgrade to a Moderate could be possible across southern MT and/or southern SD. For the southern Plains, the typical afternoon showers and thunderstorms will redevelop again Friday afternoon, but since they will be a bit more isolated and less organized as the upper level flow diminishes, the flash flooding risk is expected to be more isolated as compared with previous days. ....Florida... Heavy rainfall continues early this morning across portions of the Florida Big Bend region as a steady fetch of deep Gulf moisture directs into the region. The environment will remain very conducive for heavy rainfall as PWs are near/above 2" and warm cloud depths very high. Little forward movement in the stalled frontal boundary is expected today/this evening, so additional showers and thunderstorms are likely to develop across portions of the FL Panhandle this afternoon then move eastward into the Big Bend and northern Florida region this evening. The Slight Risk area looks on track, highlighting the greatest risk area for additional heavy rainfall over areas that are increasingly very saturated due to recent heavy rainfall events over the past couple of days. ....Mid-Atlantic... The upper trough positioned across the Ohio Valley and Central Appalachians today into tonight will allow for nearly stationary boundary across the coastal areas to lift northward as a warm front through the Mid-Atlantic today. Expect the axis of deepest moisture to remain along/east of I-95 across the Mid-Atlantic, upwards of 2.0" per the latest guidance, while sounding profiles suggest a very favorable environment for efficient rainfall producing showers and thunderstorms characterized by tall/skinny CAPE profiles and higher warm cloud depths. Storms should continue lifting out of the Carolinas early this morning, reaching the Mid-Atlantic by mid-morning with additional development possible later this afternoon. Rainfall rates of 1-2"/hr will be possible in the strongest cores, which based off the latest hi-res guidance, could track through the urbanized I-95 corridor across VA into MD. Some of this rainfall could be intense, resulting in flooding of urban and other poor drainage areas. Further west toward the center of the upper trough across portions of the Appalachians, slower storms will pivot around the low and these slower storm motions also in the favorable environment for heavy rain could result in localized/isolated flash flooding. Taylor Day 2 Valid 12Z Sat Jun 24 2023 - 12Z Sun Jun 25 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR MUCH OF MINNESOTA AND THE EASTERN DAKOTAS... ....ND/MN area... A deepening shortwave trough is expected to track from portions of Montana and Wyoming early Saturday morning toward central Minnesota by Sunday morning. Ahead of that feature, southerly flow in the low levels will bring northward anomalously high moisture to the region, characterized by precipitable water values to near 1.5" or +2 standard deviations above normal. A stationary boundary is expected to remain draped across the region ahead of the main low pressure system, helping to focus the convection over a couple of days in the Northern Plains and portions of northern Minnesota. Rainfall amounts of 1-2" appear possible across the Day 2 Slight Risk area with upwards of another 1" or so in the preceding day which would help to saturate the soils some. As a result, isolated to widely scattered instances of flash flooding appear possible, particularly across the eastern Dakotas into much of western and northern Minnesota.=20 ....Northeast... The slow moving, stubborn upper level trough will inch forward during the day on Saturday, beginning in the Ohio Valley and reaching New England by Sunday morning. The combination of anomalous moisture (PWs up to 1.5"+), warm cloud depths, and skinny CAPE profiles, efficient rainfall producing showers and thunderstorms will be capable of producing localized heavy rainfall. With the mean flow remaining somewhat parallel to the overall storm motions, some repeating rounds will be possible, supporting isolated/localized instances of flash flooding. ....Southeast Florida... A deep axis of higher than normal moisture will position itself across South Florida this weekend and with weak flow and modest shear, higher coverage of afternoon/evening thunderstorms are expected to develop. The environment will support intense rain rates given the tall/skinny CAPE profiles and warm cloud depths. The latest guidance suggests the greatest convection may end up developing just inland away from the highly urbanized corridor, but there's enough ingredients and support in the various models to maintain the Marginal Risk in the ERO. Taylor Day 3 Valid 12Z Sun Jun 25 2023 - 12Z Mon Jun 26 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE GREAT LAKES...OHIO VALLEY...NORTHEAST...AND TENNESSEE VALLEY... ....Northeast... Deep southerly flow will remain in place across the Northeast, ahead of the approaching deeper closed low tracking into the Great Lakes, and with the anomalous moisture in place (upwards of +2.5 standard deviations) and expected daytime heating/instability, scattered to widespread thunderstorms are expected across mainly the interior Northeast. Mean flow will be relatively weak (under 10-15 kts) and warm cloud depths will continue to be relatively high, supporting efficient rainfall producing thunderstorms, supporting a localized/isolated threat of flash flooding. ....Great Lakes to Ohio Valley... The closed mid/upper level low is expected to be across portions of Minnesota at the start of the period (Sunday morning). Ahead of that feature, an axis of higher moisture is expected to lift northward and wrap into the system, characterized by precipitable water values as high as 1.0-1.25" across portions of Wisconsin into Michigan. While higher instability will be fairly limited this far north and the showers and thunderstorms are likely to be somewhat progressive, some locally heavy rainfall will be possible, particularly by the afternoon/evening hours during peak heating across Michigan. Much of the region has been considerably drier than normal however, and the rainfall will be more welcome than concerning, but any localized intense rain rates (approaching 1"/hr) over sensitive locations and totals up to 2" possibly could lead to flash flooding. Across portions of Indiana, Ohio, and northern Kentucky, the moisture and instability will be greater with convection expected to break out late in the day but quickly drop east/southeast. Localized 1-2"/hr rain rates will be possible. This region also has had some heavier rainfall over the last 14 days, with rainfall departures near to slightly above normal in spots across Ohio and Kentucky. ....Tennessee Valley... The main shortwave energy tracking across the northern tier will push a cold front southward across the Plains into the ArkLaTex region. The front will be increasingly oriented west to east across the region during the period as it stalls out. In the low levels, southerly flow will allow much above normal to pool along and ahead of that front with the latest guidance indicating upwards of 2.0" precipitable water likely across portions of Arkansas into Mississippi. Convection is expected to develop late in the period (mainly overnight) on the nose of the increasing low level jet, where 850 mb flow is expected to increase to around 30-35 kts, and then dive southeast into portions of Arkansas and Mississippi. There remains some spread among the model guidance on the location and intensity of the convective complex, but the environmental ingredients support intense rain rates and heavy rainfall totals over an area that has seen above normal precipitation over the last 14 days. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7i2csr-2YYVuRSyZP1l414TsoxvymDgTIJ9nnKd6D8yl= qchUWePukaarTTYV0f8Hwmkf0r9_c5A4xmn3wn5Cky47hPY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7i2csr-2YYVuRSyZP1l414TsoxvymDgTIJ9nnKd6D8yl= qchUWePukaarTTYV0f8Hwmkf0r9_c5A4xmn3wn5CtxmyuWo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7i2csr-2YYVuRSyZP1l414TsoxvymDgTIJ9nnKd6D8yl= qchUWePukaarTTYV0f8Hwmkf0r9_c5A4xmn3wn5CGTQdJ-U$=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) .