Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jun 02 2023 00:43:01 FOUS30 KWBC 020042 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 842 PM EDT Thu Jun 01 2023 Day 1 Valid 16Z Thu Jun 01 2023 - 12Z Fri Jun 02 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF EAST-CENTRAL MONTANA AND PORTIONS OF THE SOUTHERN/CENTRAL PLAINS... ....Central and Southern Plains... Deeper convection has been focused over portions of Oklahoma and southern Kansas this afternoon with a general lull in activity back over the Texas Panhandle where the environment has been overturned quite a bit. The latest 18Z HREF and most recent runs of the HRRR aren't keying on a lot of additional organized convection developing overnight with just scattered convection continuing across central Oklahoma into the Central Plains. The 18Z HREF shows low-end probabilities for 2"+ through 12Z (15-25 percent), centered across portions of OK/KS and an isolated signal across portions of MN/IA. The Slight Risk was removed out of the TX Panhandle where the signal for heavy rainfall overnight has dropped considerably, but was left largely in place for portions of OK/KS. ....Northern Great Basin through the Northern High Plains... The Slight Risk area was only minimally adjusted to reflect current radar trends and the latest model guidance for the overnight period. It lines up well with the 18Z HREF neighborhood probabilities for 12-hr QPF exceeding 2" (locally up to 40 percent chance) as well as the HREF EAS signal for 2"+ (central to northeast MT). Hourly rain totals may exceed 1" at times, especially later this evening with an increase in the low level flow and with the storm motions lining up somewhat parallel to the mean flow, a few repeating rounds will be possible. This area has been saturated recently, both by 24-hr rainfall that has been 2-4 inches according to MRMS, and 14-day rainfall that has been as much as 400% of normal, highest in MT. This has resulted in compromised FFG as low as 0.5-1"/3hrs, which the HREF indicates has a 30-50% chance of being exceeded. Elsewhere in the Marginal Risk area, generally isolated to scattered convection is expected through the rest of the evening with hourly totals between 0.5" to locally 1" that may lead to isolated instances of flash flooding. ....Florida Peninsula... Expecting a downward trend in deep convection over the next few hours with loss of daytime heating across the Florida Peninsula. HRRR and 18Z HREF probabilities suggest a few more hours of locally intense rainfall before transitioning to lighter showers and thunderstorms. The Marginal Risk was kept largely in place given some of the deeper moisture and transport loosely associated with Tropical Depression Two and the threat of some of the storms impacting areas that are more susceptible and vulnerable, like the areas that have already received heavy rainfall over the last 7-14 days. Taylor Day 2 Valid 12Z Fri Jun 02 2023 - 12Z Sat Jun 03 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE TEXAS PANHANDLE AND PARTS OF MONTANA... ....Southern Plains... Longwave trough across the Four Corners and a mid-level ridge expanding along the MS VLY will keep the Southern Plains entrenched within moist and confluent flow. This will produce yet another day of scattered to widespread convection, with organized thunderstorms producing heavy rainfall likely beneath shortwaves/vorticity impulses rotating through the flow and to the north. Within this pattern, 850mb inflow will persist from the SE out of the Gulf of Mexico, reaching 20-30 kts to push PWs as high as 1.25-1.5 inches, +1 to +2 standard deviations above the climo mean according to the NAEFS ensemble tables. This warm advection will also transport significant instability northward, with a ribbon of MUCAPE reaching 3000 J/kg progged to surge ahead of the dryline over NM. This environment will support rainfall rates that could easily surpass 2"/hr noted by HREF probabilities reaching 30% for this threshold. As the dryline wavers in the vicinity and shortwaves rotates out of New Mexico into the extremely favorable thermodynamics, there is likely to be rapid growth of convection both in intensity and coverage, from the High Plains of NM eastward through the Panhandle/Rolling Plains of TX. This is reflected by simulated reflectivity as well as moderate to high probabilities for more than 3" of rain in the GEFS and HREF, with low probabilities for 5" in the HREF also evident. Some of this higher rainfall could materialize as nearly aligned Corfidi vectors to the mean winds, despite being 15-20 kts, indicates that training or backbuilding into the better instability is likely before organization may drive an MCS to forward propagate later in the period. The concern across this area is not only that training of 2"/hr rainfall can result in runoff, but that this rain is going to be falling atop already primed soils. 14-day rainfall in the vicinity of the Panhandle has been 300-600% of normal, with heavy rain also occurring in the last 48 hours. This has led to high USGS streamflow and 3-hr FFG that is as low as 0.25" near Amarillo, and less than 1.5" in much of the area. Due to this compromised FFG, HREF exceedance probabilities reach as high as 80%, and it is looking more certain that problematic flash flooding will occur on Friday. For this reason, a MDT risk has been added for portions of the area, with subtle adjustments to the surrounding SLGT risk also created. ....Northern/Central Rockies and High Plains... A stagnant mid-level pattern driven by a longwave trough across the Intermountain West and a strong ridge centered across the Upper Midwest will produce continued impressive moisture transport through the Plains and into the Northern Rockies, leading to another day of widespread convection. Between these two features, 850mb winds emerging from the Gulf of Mexico will rise to 20-30 kts, driving elongated moisture transport vectors across the Plains and then rotating cyclonically into the Northern Rockies where convergence will occur. Additionally, this surge of warm moist air will drive a ribbon of MUCAPE exceeding 2000 J/kg as far north as SD, with 1000+ J/kg lifting into MT. The overlap of this moisture/instability will produce an environment favorable again for heavy rain producing convection as ascent through multiple embedded shortwaves and convergence along a front increases. As a surface front drapes slowly southeast through Montana at the same time a closed mid-level low pivots over the northern Great Basin, an axis of overlapped mid-level fgen and deformation is progged to pivot across central MT. This enhanced ascent will combined with modest upslope into the terrain and some isentropic ascent atop the surface front to produce an axis of heavier rainfall despite lower instability than points east. This is also a region that should feature slower storm motions than points east, with regenerating cells likely. The HREF probabilities indicate that rainfall rates have a 20-30% chance of exceeding 1"/hr across much of the area, reaching 60% or more in MT/northern WY. With training likely, this could result in 1-3" of rain in some areas, with more than 3" possible beneath that deformation axis reflected in HREF and GEFS probabilities as instability continues to resupply from the east. This rain falling atop soils that are already saturated from 14-day rainfall that is generally 150-300% of normal will likely result in at least scattered instances of flash flooding. With a significant event appearing likely in most of the available guidance, a MDT risk has been added for portions of north-central MT where ensemble probabilities are pretty well clustered, including HREF EAS probabilities for 2 inches reaching 30% and 3 inches as high as 10-15% overlapping 3-hr FFG exceedance of 70% or more. This was issued after coordination with WFOs TFX/BYZ/GGW, and this is the first MDT risk for this area of MT since at least 2016 according to the short climatology that exists at WPC. The surrounding SLGT risk was modified to account for these changes as well, and a wet/impactful day is probable for portions of the area on Friday. ....Central and Southern Florida... Tropical Depression #2, which is forecast to become a tropical storm by Friday morning, dropping southward across the northeast Gulf of Mexico should have little direct impact to Florida, but the attendant mid-level trough and increased tropical moisture will yield another day of slow moving, heavy rain producing, convection. High pressure to the north and a low-level trough across the peninsula will yield pinched easterly flow, converging with southerly flow from near the Keys along a weakening surface trough. PWs across the state will likely be around 1.75 inches near the low to the west, which will combine with MUCAPE as high as 1500 J/kg, highest across the southern peninsula, to produce a favorable environment for thunderstorms with heavy rain. Weak bulk shear and light 0-6km mean winds indicate that storms will generally be of the pulse variety and slow movers, with additional development likely during any storm mergers or along outflow boundaries. Rainfall rates will likely reach 1-2"/hr at times as noted by the HREF probabilities, producing rainfall that could reach 3-5" in isolated areas. At this time the greatest signal for any flash flooding appears to remain away from the urban corridors, so the MRGL risk was maintained. However, with heavy rain predicted on D1 as well, should this signal overlap more prominently with the D1 footprint, or move into more urban areas, a narrow SLGT risk could be needed with later updates. ....New England... A shortwave digging out of Canada will push southward across New England Friday evening, driving a back door type cold front into the Mid-Atlantic by the end of D2. The passage of this front into an unstable airmass characterized by MUCAPE of 500-1000 J/kg will likely result in scattered thunderstorms along and behind the boundary as it sinks southward into the evening. PWs are progged to exceed 1.25" in a narrow corridor across New England during this time, more than 1 standard deviation above the climo mean according to NAEFS, which will support heavy rainfall within the deeper convection. Coverage of thunderstorms should remain generally scattered, and cells will remain progressive on 15-20 kts of 0-6 km mean winds. The impressive thermodynamics will likely yield rainfall rates that will exceed 1"/hr at times, which could pose a flash flood risk, although extremely dry antecedent conditions across much of the northeast will be limiting. However, with HREF probabilities indicating the potential for as much as 3" of rain in a few areas due to several rounds of storms, the inherited MRGL risk was tailored only modestly to reflect the potential for isolated instances of flash flooding. Weiss Day 3 Valid 12Z Sat Jun 03 2023 - 12Z Sun Jun 04 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE SOUTHERN HIGH PLAINS THROUGH THE NORTHERN ROCKIES, AS WELL AS ACROSS SOUTH FLORIDA... ....Northern Rockies through the Southern Plains... The mid-level pattern becomes even more amplified Saturday as the expansive ridge over the Upper Midwest intensifies and retrogrades to the west. This will continue to squeeze moisture northward through much of the High Plains and back into the Intermountain West as far as the Northern Rockies. However, the expansion of the high and subtle weakening of the trough to its west should result in a somewhat less intense and less active day than Thursday or Friday. With weak impulses likely surging northward within the flow, it is possible some enhanced or more organized convection could occur anywhere within the broad MRGL risk which was just subtly adjusted from inherited. There is a subtle signal from the Big Bend of Texas northward through the Rolling Plains that a more pronounced rain event may occur noted by just a subtle disconnect between the QPF and GEFS probabilities, and this is also near where exceptional rain has fallen recently. However, confidence at this time is too low for any targeted SLGT risk areas, and the CSU first guess fields continue to indicate just a broad MRGL area. Otherwise, expect generally scattered thunderstorms with heavy rainfall, some of which may be slow moving across the northern tier. ....Southern Florida... The low pressure, which may a weakening TS or TD across the eastern Gulf of Mexico, will begin to dissipate Saturday as it continues to drop southward, before being ejected into the westerlies late in the forecast period as the parent trough weakens but elongates. While again no direct impacts to Florida are expected, the attendant tropical moisture pool noted by PWs of 1.5 to 1.75 inches will continue to plague the peninsula while a surface trough slowly skirts offshore to the east. Generally modest easterly flow will create slow moving convection once again, likely focused Saturday along the westward advancing sea breeze. Instability sufficient to support 1-2"/hr rain rates could again produce isolated instances of flash flooding, especially atop soils saturated from heavy rain D1 and D2, or along the urban corridor. Weiss Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9rZEz9mgwkkCt8qfe9H1KqoTqHjOexIdUtCywRJQfpt-= MtMBngP9zhPhDShEsX4p-7Izckw3tnyWt-totPcsmf7gT2k$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9rZEz9mgwkkCt8qfe9H1KqoTqHjOexIdUtCywRJQfpt-= MtMBngP9zhPhDShEsX4p-7Izckw3tnyWt-totPcs26UDtSM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9rZEz9mgwkkCt8qfe9H1KqoTqHjOexIdUtCywRJQfpt-= MtMBngP9zhPhDShEsX4p-7Izckw3tnyWt-totPcsSwlXnpU$=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) .