Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Thu Jul 13 2023 15:59:00 FOUS30 KWBC 131944 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 343 PM EDT Thu Jul 13 2023 Day 1 Valid 16Z Thu Jul 13 2023 - 12Z Fri Jul 14 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN AND NEAR PORTIONS OF CENTRAL MISSISSIPPI/WEST_CENTRAL ALABAMA AND SOUTHERN VERMONT... ....Northeast... Only minor changes were made across the Northeast and New England for this update as guidance continues to depict a strong line of thunderstorms capable of containing 1-2"/hr rainfall rates. For the most part this line will be progressive across northern New York and Vermont, but sufficient moisture and instability are likely to allow for quick 1-3" totals that exceed low FFG. For the Moderate Risk area in and near southern southern VT, there is potential for rounds of heavy rain to overlap very saturated terrain leading to greater chances for flooding impacts. This potential is particularly seen in 12z HREF neighborhood probs for 1-hr FFG exceedance centered over southwest VT by 00z tonight. The overall setup includes mid-level northwest flow will continue across the Great Plains and Mississippi Valley, lifting into more southwest flow across the Appalachians and Northeast as the broad and expansive mid-level trough centered near the Hudson Bay persists. This will provide an environment allowing waves of convection moving through the flow. Most of this development on Thursday will occur along a slow-moving cold front and leading pre-frontal trough. PWs will rise to 1.5-1.75" on W/SW 850mb inflow of 20 kts, with SB CAPE surging during peak heating south of the front to as high as 2500 J/kg. The Moderate Risk area has the potential for overlapping of sufficient moisture, instability, and mean southwest flow parallel to the associated frontal boundary promoting possible brief cell training. The training potential has diminished in recent guidance, but there still is potential for the QLCS to slow along the southwest flank from southern VT, western MA, and northern CT by tonight. Much of Vermont and New England remains vulnerable due to recent extreme rainfall, where areas of 300-600% of the seven day average rainfall has fallen. The above ingredients support 2"/hr totals, with locally higher amounts possible if training occurs. Within the Moderate risk area, expect several instances of flash flooding and impacts due to rapid runoff where terrain cannot adequately absorb heavy rain. ....Mid-South to Central Gulf Coast... A Moderate Risk remains over central MS and was expanded into parts of west-central AL due to recent radar and CAM trends. MRMS estimates 8-12" of rain already occurred this morning along the AL/MS border to the east of Meridian, AL. Very intense rainfall rates of 2-3.5" can be found within the training bands of convection, which can easily add up to over 5" total rainfall within an hour or two. Areas underneath these extreme rainfall rates can expected rapid inundation and significant flash flooding, even in areas where flooding is uncommon. See MPD 714 for more information. A previous MCV that was initially the catalyst for the efficient rainfall producing thunderstorms over MS and AL remains over the area as a separate shortwave approaches from the NW and is acting to further invigorate approaching updrafts. The low level inflow per JAN's 12z sounding is roughly double the mean 850-400 hPa wind, which is very efficient from a heavy rain perspective and generally a feature of warm core lows, with a freezing level up to almost 16k feet. Additionally, this part of the country typically has plenty of atmospheric moisture content, but PWs of 2-2.5" are into the 90th climatological percentile. It is expected for ongoing activity to continue over the current area of central AL/MS for at least a few more hours until the approaching shortwave from the NW shifts the activity further south this afternoon. As the shortwave continues progressing from the Mid-Mississippi Valley and LA, the threat for excessive rainfall during the afternoon and evening hours on Thursday is expected to shift towards the north-central and northeast Gulf Coasts. Here, the 12z HREF probabilities of at least 3"+ and 5"+ in 24 hours continue to support a Slight Risk, which was somewhat reconfigured based on the latest guidance. Some guidance hints at very heavy rain occurring over the western FL Panhandle tonight in sort of a similar fashion to the ongoing AL/MS activity, but there still remains far too much uncertainty regarding specifics in this region. ....Mid-Mississippi Valley through the Central Appalachians... Convective trends among the global models and CAMs has become a bit more organized and intense from the Ohio Valley into the central Appalachians. Although much of the activity Thursday, which should be widespread, will be generally progressive, effective bulk shear of 30-40 kts will allow for storms to organize into multicells and then train to the northeast, with additional enhanced ascent occurring where flow can favorably upslope into terrain. The favorable thermodynamics across this region will support hourly rain totals to 2", which through brief training could produce 2-4" of rain or locally higher amounts in some areas, which would be particularly problematic in the coal fields of WV and KY. Confidence in the exact evolution and development remains modest. Spotty areas have seen above normal rainfall over the past week in this region. The Slight Risk was maintained from the previous outlook. By the overnight hours, developing activity progressing over the Appalachians may make it into central VA (similar to recent HRRR runs). The airmass in place will support heavy rain as PWs increase to around 1.75", but too much uncertainty remains to warrant a risk increase. ....Central to Southern High Plains... Broad northwest flow on the upwind side of an expansive mid-level trough will drive multiple shortwaves down across the region on Thursday. These impulses will combine with modest upper diffluence within the tail of a departing jet streak to the east and convergence along a weakening front/surface trough moving across the area. A departing MCS in the morning could also produce some priming rainfall, but the greater risk for excessive rain will occur in the evening/at night. During that time, east-southeast low-level inflow will surge to 20-30 kts as a wave of low pressure develops across the High Plains along a stationary front over Colorado. This will draw PWs above 1.5 inches northward, with values as high as 1-1.25" in the High Plains of CO, combined with resupply of MUCAPE approaching 2000 J/kg. This moist ascent will drive a theta-e ridge northward, supporting expanding convection along a surface trough as another impulse moves overhead. Timing of the impulse and its affect on the impinging surface boundary is in question, and if they do not overlap the coverage of storms could be less. The Marginal Risk area captures the heavy rain signals seen in the 12z guidance, which remain scattered with not great agreement in placement. Hourly rain totals to 2.5" and local amounts to 4" are possible wherever cells can train/merge or mesocyclones can form. Some areas in the Marginal Risk have seen greater than 600% of their average seven day rainfall, so soils should be rather saturated. Slight Risk impacts cannot be ruled out. ....Near the southern AZ/NM border... Precipitable water values southern AZ remain 1.5"+ this period, with rather weak flow, implying slow cell motions for any thunderstorms that might develop. ML CAPE should rise to 1000+ J/kg. Temperatures at 700 hPa are near 15C around the time of convective initiation, implying modest mid-level capping. This should allow for isolated to widely scattered thunderstorms with locally heavy rainfall, like has been seen over the past few days. Snell/Roth Day 2 Valid 12Z Fri Jul 14 2023 - 12Z Sat Jul 15 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN NEW ENGLAND, THE MID-ATLANTIC, AND FROM THE MID-SOUTH TO THE GULF COAST... ....Northeast and Mid-Atlantic... The Slight Risk was mostly maintained, but expanded across southeast VA and into central NC with the 20z update. A weakening frontal boundary and pooling anomalous moisture creates conditions ripe for scattered instances of heavy rain and flash flooding potential. Continued troughing across the eastern Lower 48 due to a deep parent upper low over south-central Canada will be reinforced by shortwaves embedded within the cyclonic flow which pivot northward on Friday. One of these shortwaves is progged to lift across New England early in the period, helping to drive a weakening cold front eastward along the Mid-Atlantic and Northeast. Ahead of this cold front, a warm front is progged to surge north into Canada, leaving impressive thermodynamics within the warm sector across the Northeast. SW flow within this warm sector is progged to reach 20-25 kts, advecting anomalous PWs of 1.75" and MUCAPE of 1000-2000 J/kg northeastward. As a surface trough sharpens during the afternoon, coinciding with peak instability, widespread showers and thunderstorms should develop and then lift northeast along this trough with rain totals up to 2"/hr. Aligned mean winds indicate localized training is likely, lengthening the duration of rain rates which will otherwise be limited by 20-25 kts of storm motions, though erosion of instability should allow convection to shift somewhat with time. This training could result in an axis of 2" of rainfall, with local amounts of 4". The antecedent soils are saturated across New England. These vulnerable soils indicate that any heavy rainfall will quickly become runoff and may lead to renewed flash flooding, as well as an elevated potential near more urban areas. Although uncertainty remains with how convective evolution will occur Friday, the Slight Risk was adjusted to match the best 12z model consensus for heavy rainfall. Better certainty across New England will exist after early morning activity comes to an end, as any lingering mesoscale boundaries could focus heavy rain along the urbanized region between CT and central MA. It is this region where the 12z HREF highlights the best chances (40-60%) for at least 3" of rain and the potential for scattered flash floods. Farther south along the Mid-Atlantic coast and central NC. A pre frontal trough and lingering boundary from possible Thursday night activity will help spark slow-moving afternoon thunderstorms within a moisture rich environment. PWs of 2-2.4" and SB CAPE >2000 J/kg will allow for efficient rainfall. These storms will also occur within 20 kts of southwesterly low-level flow, possibly limiting their southeast progression enough to produce 3-5" rainfall totals. 12z HREF supports this idea with scattered 15-30% neighborhood probabilities for at least 5" in 6 hours, with the threat diminishing after 03z. ....Central Plains, Midwest, Mid-South and Central/Eastern Gulf Coast... The previous Slight Risk areas extending from eastern KS to the Gulf Coast were adjusted to highlight where confidence is highest for scattered instances of flash flooding and intense rainfall rates associated with slow-moving storms. The area extending from KS/OK to northern AR may have activity entering the area Friday morning that could warrant a targeted upgrade, but most flash flood risk throughout the Plains, Midwest, and Mississippi Valley is too uncertain or localized to maintain the Slight Risk at the moment. However, confidence is higher that intense rainfall may fall over saturated soils from parts of the Mid-South to the central/eastern Gulf Coast. For the Slight Risk area, a residual MCV that impacted parts of AL/MS on Thursday with significant rainfall is expected to drop southeastward overnight and center near the FL Panhandle on Friday. Even though this feature is expected to weaken overall, very high PWs and 2000-3000 J/kg MUCAPE when combined with weak steering flow will aid in localized heavy rain totals. The forcing for the eventual storms is most in question, whether it be the MCV itself or aided by lingering outflow boundaries or a sea breeze, but the overall environment and CAM guidance supports the SLGT designation. The ingredients suggest that hourly totals to 3", with local amounts to 6", would be expected. Some guidance shows the potential for local 8" amounts in or near the region, which cannot be ruled out given the history of this MCV as it has pushed from the Lower Mississippi Valley on Wednesday. With very saturated soils along the MS/AL border from 2-10" rainfall totals on Thursday, the SLGT was also extended in this direction to account for overlap with elevated 12z HREF probs for at least 3" of rain. Through the central U.S. and Midwest, thunderstorms will be driven by shortwaves and associated vorticity maxima rotating eastward in northwest to west mid-level flow, impinging upon an elongated cold front draped across the Mid-Mississippi Valley. The shortwave interacting with this front will drive impressive ascent into favorable thermodynamics characterized by PWs rising to ~2" co-located with a corridor of 2000-3000 J/kg MUCAPE. The 850mb inflow out of the SSW at 20-30 kts ahead of the approaching cold front from the NW will robustly resupply these thermodynamics into the area, and as effective bulk shear climbs to 50 kts Friday night, organized convection, possibly a potent MCS, will move across the area. Due to the inherent difficulty in forecasting the placement of these MCSs and the potential for fast forward prorogation, opted to go with a Marginal Risk rather than a broad SLGT and highlight the potential for localized flash flooding. If hires guidance becomes more aligned with an area that could see overlapping storms or training, a targeted SLGT is possible. ....In and near southern NM... Precipitable water values rise toward 1.75" in southern AZ and the low-level flow picks up out of the west-southwest as 700 hPa troughing develops across NM. However, temperatures at 700 hPa are on the rise, with the atmosphere modestly capped, which despite the available moisture should keep thunderstorm coverage isolated to widely scattered similar to previous days. A Marginal Risk of excessive rainfall was maintained to cover the potential for hourly totals to 2" where cells manage to merge and/or train. Snell/Roth $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .