Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
639 FOUS30 KWBC 072008 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 408 PM EDT Tue May 7 2024 Day 1 Valid 16Z Tue May 07 2024 - 12Z Wed May 08 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE OHIO VALLEY, MIDWEST, AND NORTHERN PLAINS/ROCKIES... ...16Z Update... The SLGT risk across the Ohio Valley was removed with a broad MRGL now encompassing the area east of the Mississippi from previous forecast (Reasoning for the degraded risk in the "Ohio Valley" sub-heading below). The remainder of the forecast was still in-line with the previous thinking only relegated to minor changes within the confines of both MRGL risks in place over the Midwest and Northern Rockies. Probabilities still favor some minor flood potential across the Midwest and MT with the best HREF EAS signatures for at least 1" and 2" of precip actually located within those two zones. Hourly rates is the detriment to a higher potential with much of the rainfall more cumulative over a span of several hrs, and not just within a very short time with custom for the flash flood threat. Thus, maintained continuity with the risk and only made some adjustments to the bounds based on the recent HREF blended mean forecast. ...Ohio Valley... Initial wave of convection is currently pushing through the Midwest/Ohio Valley this morning with generally light to moderate rates running within the 0.25-0.5"/hr range at max intensity. The progressive nature of the precip will benefit the region with some rainfall, but limit the threat for flooding. The afternoon and evening will be the time frame of interest with appreciable instability on the order of MLCAPE reaching between 3500-4000 J/kg across the Mid-Mississippi Valley into the western 1/2 of the Ohio and Tennessee Valleys. Elevated PWATs between 1-1.5 deviations above normal favors a moist environment capable of locally heavy rainfall, but the pattern will remain more progressive north of the TN line as HREF probabilities for at least 1" of total precip remain within that 15-20% based on the EAS paradigm which is fairly low and bordering worthy of a SLGT risk when coupled with the environment. HREF neighborhood probability fields for hourly and 3-hourly rates are also low end with a 25-40% chance for at least 1"/hr and 2"/3-hrs respectively, but fades quickly towards <10% for higher thresholds of exceedance. This is not what we like to see for the greater risk areas which lead to a demotion back to area wide Marginal Risk to encompass the Ohio Valley. An area to monitor will be late in the period across TN as the flow becomes parallel to an expected stalled frontal boundary across the AR/MO into western and central TN. Some guidance has indicated a threat of training within the confines of the boundary during an ongoing nocturnal convective pattern. This is not reciprocated amongst all hi-res members, but the setup is possible. This would favor a locally better risk of flash flooding due to the training factor that is currently missing from the initial wave this morning/afternoon, and the initial convection later during peak diurnal instability. For now, maintained the MRGL risk with a targeted upgrade possible pending radar evolution, and any growing consensus in the short term for a better threat within the above area. Kleebauer ...Previous Forecast... ...Upper Midwest and Great Lakes... The impressive line of thunderstorms currently moving eastward across the Plains and Midwest will continue to lift north/northeast this morning into the Upper Midwest, generally working along the leading edge of the warm frontal passage and best isentropic lift. Moisture in the low levels will continue to rise through the period, with an axis of PWs > 1.25" expected from Minnesota southward into Illinois which will support isolated/embedded cores producing heavier rain rates. The latest hi-res guidance does suggest potential for some repeating/redeveloping convection as the flow becomes parallel to the storm motions over Wisconsin, which could produce isolated/streaks of higher rain totals. Altogether, the risk for flash flooding lies in the lower end of the range and the Marginal Risk looks good for this forecast update. ...Northern Plains/Rockies... The closing mid/upper level low over the Northern High Plains will linger through the period, with embedded vort energy pivoting/retrograding westward across the region. This should continue the forcing/lift across the region and with the anomalous moisture in place and orographic enhancement, an additional 0.5-1" is expected and this could cause additional flooding concerns. Taylor Day 2 Valid 12Z Wed May 08 2024 - 12Z Thu May 09 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF MID-MS VALLEY, TENNESSEE VALLEY, AND OHIO VALLEY... ...20Z Update... The Slight Risk area was maintained across the Ohio and Tennessee Valley`s with the core of the threat along the Mid-Mississippi out towards the tri-state MO/IL/KY borders with a primary QPF max focused within western KY and points east/southeast over the KY/TN border. An expansion further to the west and southwest into AR and southeastern MO was executed given the trends in recent guidance for the initiation of convection within the confines of the aforementioned areas. The SLGT was also expanded into eastern TN and south into northern MS/AL. The setup remains the same based on the evolution, but there was a growing consensus in a higher impact being felt further south compared to previous forecast issuance based on both probabilistic output, as well as the ensemble QPF footprint for the period. A higher risk across portions of KY and TN was entertained given the focused QPF max and higher HREF EAS probabilities for precip exceeding 1" (70-90%), and 2" (35-60%), with a lower end probability depiction for exceeding 3" (10-15%), centered over western KY into south-central KY and northern TN. The question becomes more of how the overnight convection this evening evolves as the positive feedback mechanism will drive the location of the stalled frontal boundary that will become a primary feature for enhanced low-level convergence and training within the warm sector. Some guidance has trended south with this feature which would play a pivotal role in the location of the primary QPF max for the D2 period. The 12z HREF mean was still positioned over that western KY corridor with an expansion in the higher means further east, so that remains the focus for this forecast. If consensus grows for that general area, there is an opportunity we could see an upgrade to a MDT risk, but we want to make sure there is continuity run-to- run for the placement of the areal QPF maximum for an upgrade since the risk also implies higher confidence. Regardless, this is a, "higher-end SLGT" for a large portion of the Ohio and Tennessee Valley`s with locally significant impacts possible within the confines of the warm sector. Elsewhere, limited changes to the MRGL risk across the Midwest with the anticipated inverted trough under the main upper circulation. Locally heavy rain with totals between 1-2" will be common within the confines of the MRGL risk area with a large bullseye of 50-70% EAS probability for at least 1" and a precipitous drop off to 10-20% for upwards of 2" within the same bounds. This caps the potential overall, especially considering rates generally maxing out around 1-1.5" just based on neighborhood probabilities across northern IA, southern MN, and far southeastern SD. Instability is generally meager, as well as PWAT anomalies being only slightly above normal for climo. Thus, the potential is fairly capped with a MRGL risk more than sufficient for the setup in question. Kleebauer ...Previous Discussion... ...Mid-MS Valley to Ohio Valley... The strong closed low from today/tonight is expected to open up into a large positive tilted trough axis stretching from the Rockies to the Great Lakes. Embedded within that, pieces of shortwave energy will ripple through the Plains to Ohio Valley and be the primary focus for forcing in the mid/upper levels. At the surface, a low pressure will deepen/organize in response to the large scale forcing and track through the Mid-MS Valley to Ohio Valley, pulling a warm front northward into the Ohio Valley by the afternoon. In the warm sector, the airmass will be characterized by dewpoints well into the 60s, perhaps into the low 70s, across the region and along with PWs rising well above 1.5", this should be more than sufficient moisture to work with. And with the increasing height falls aloft and steepening lapse rates, a large area of favorable instability is expected, reaching 1500-2000+ J/kg MUCAPE. With daytime heating and convective temperatures reached, rounds of strong to severe thunderstorms are expected from the Ohio Valley initially that then build/grow along the advancing frontal boundary southwest across the Mid-MS Valley. Compared to earlier issuance, the northern edges of the ERO outlines have been trimmed back some as the latest guidance suggests the warm front may not lift as far north, stalling along the Ohio River. This may keep the greatest threat for training/intense thunderstorms south into KY and into TN tied to the better instability. This is where there is the greatest overlap of forcing, moisture, and instability and the setup for training convection that pushes the ERO risk toward the higher-end of the Slight Risk range. This also lines up also where the antecedent ground conditions are wet with recent heavy rainfall events giving lower/reduced FFGs and more vulnerability to flash flooding. ...Upper Midwest... Compact area of deeper convection is expected to develop along an inverted trough underneath the core of the upper low in the Midwest that will be capable of producing localized heavy rainfall and flash flooding. A narrow axis of higher moisture is expected, perhaps above 1-1.25" from far southeast South Dakota through Iowa and southern Minnesota. Given this greater moisture, broad forcing, and steepening lapse rates, pockets of slow-moving and pivoting showers and thunderstorms are likely. Rain rates may approach/exceed 1-2"/hr at times and while isolated, there is at least some flash flood risk so the Marginal Risk covering the area looks good. Taylor Day 3 Valid 12Z Thu May 09 2024 - 12Z Fri May 10 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST U.S AND LOWER MISSISSIPPI VALLEY.... ...20Z Update... The main shift in the D3 was a general movement to the west for the previous SLGT risk issuance based on the latest ensemble QPF trends, as well as NBM probabilities for exceeding 2" within the period. There`s still some discrepancy amongst global deterministic in the positioning of the cold front related to the setup across the Southeast US with the GFS further north in placement of the front and ECMWF further south, closer to the Gulf Coast. Latest ensemble bias corrected QPF is a touch further south compared to previous runs, but still within the field of error at this lead. QPF maximum has shifted a bit further west into LA/MS with a bullseye of 2.5-3" at a 72 hr lead. The setup is fairly primed for some localized impacts over areas that have been hit with a lot of rain recently. The ejection of the shortwave out of the sub-tropical jet is still pretty well forecast with cell initiation out across east TX. The forward propagation of the shortwave and the positioning of the boundary will be the primary focal points for the setup, and so far with have growing consensus on the speed of the shortwave and some split in frontal placement. Regardless, the setup will be conducive for locally heavy rainfall with QPF max of up to 5" possible within a zone of training potential along and just south of the cold front on Thursday evening. A SLGT risk was maintained for continuity in the potential and expanded a bit on the northern and southern edges to account for the uncertainty in the evolution of the boundary. There are no changes to the northern MRGL across the Mid-Atlantic and Northeast as the thermodynamic environment remains favorable for scattered convection capable of heavy rainfall within the bounds of elevated PWATs focused over the area. A split in ML guidance and primary deterministic add some uncertainty in the risk for placement of the heaviest precip potential, so maintained continuity given the likelihood of low-end MRGL somewhere between the Mason Dixon and Upstate NY later Thursday. Kleebauer ...Previous Discussion... ...Southeast... The strong spring system over the Central U.S. will begin to elongate/stretch in a lengthy, positively tilted shortwave trough by the start of the forecast period and sink southward toward the region. Earlier convection from the previous day is likely to push outflow boundaries and a cold front through the region while another embedded shortwave trough is expected to move through in the southern stream. Altogether, this should put the region in a favorable region of forcing for ascent. Deeper moisture is expected to pool along/ahead of the surface boundaries and will be characterized by PWs greater than 1.5", which is around 2 std above the climatological normal for this time of year. With sufficient instability developing that will overlap with the better forcing and moisture, rounds of thunderstorms are expected to develop along the boundary before slipping south/southeast toward the Gulf Coast. By this point in the storm system evolution, the flow is likely to be more parallel to the overall storm motions and this could favor some repeating rounds or training convection particularly over portions of MS/AL. The environmental ingredients will support intense rain rates over 1-2"/hr at times and this could bring scattered instances of flash flooding and for this update, the Slight Risk inherited looks good was largely unchanged from the previous forecast. ...Central Appalachians to the Northeast... Low pressure is expected to track from the lower Ohio Valley to the eastern Great Lakes over the forecast period as shortwave trough energy aloft and height falls over the region. A strong southwesterly flow in the low levels will bring higher moisture, characterized by dewpoints in the upper 50s to lower 60s and PWs above 1", especially from the Central Appalachians and southern areas of the NOrtheast. With the uncapped and warm/moist environment, there should be scattered to numerous showers and thunderstorms that develop that will be capable of producing isolated/localized rain rates in excess of 1-1.5"/hr at times. This could cause isolated flash flooding, particularly for some of the areas that have seen recent heavy rainfall and more saturated ground conditions as well as urban locations. Taylor Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt