Quantitative Precipitation Forecast
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878 FOUS30 KWBC 292000 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 400 PM EDT Wed May 29 2024 Day 1 Valid 16Z Wed May 29 2024 - 12Z Thu May 30 2024 ... THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHEAST TEXAS AND FAR WESTERN LOUISIANA AS WELL AS FOR EASTERN OHIO TO NORTH CENTRAL PENNSYLVANIA... ...16Z Update... ...Southeast Texas and Far Western Louisiana... Guidance remains in very poor agreement run-to-run as to the convective evolution expected across Texas and Louisiana today. Some of the guidance move the axis of heaviest precipitation hundreds of miles over a 6-hour update. That said, there is at least fair agreement on convection developing later this afternoon in and around Houston, which after getting hard-hit with heavy rain and thunderstorms just yesterday has reduced FFGs locally in addition to the urbanization. There are ongoing thunderstorms along the TX/LA border which are locally producing rates to 1.5 inches per hour over another very hard hit area over the past several weeks. For this region, the convection going on now may be the heavier of the 2 rounds, if the second round late this afternoon even reaches this area. The new Slight is both a blend of a multi- model average of where convection is expected to develop later today as well as low FFGs where river flooding remains ongoing. The low-end Slight upgrade was coordinated with HGX/Houston, TX; SHV/Shreveport, LA; and LCH/Lake Charles, LA forecast offices. The inherited Slight across North Texas was downgraded with this update in coordination with FWD/Fort Worth, TX forecast office. As mentioned above, guidance is all over the place as to how the convective evolution will play out this afternoon into tonight...however the consensus has trended quite a bit drier, with several of the CAMs, including the HRRR suggesting most of north Texas sees nothing at all. That is somewhat dependent on late night/early Thursday morning convection likely staying over the Panhandle and not reaching north-central Texas until the Day 2/Thursday period. Even then, the storms are expected to have developed into a fast moving squall line. ...Eastern Ohio to North Central Pennsylvania... A slow-moving upper low near the PA/WV/OH confluence at the time of this writing is producing a band of light to moderate rain with embedded heavier cores to 1.5 inches per hour. With diurnal heating under broken clouds south of this band, instability this afternoon is expected to increase to between 1,000 and 1,500 J/kg in a skinny CAPE profile. FFGs are low in the Slight Risk area, generally between 1 and 1.5 inches/hour. Given the expected increase in instability and the band of rain pivoting south with time into somewhat flashier areas of the Appalachians, potentially including Pittsburgh, think that these kinds of rates we have already been seeing over somewhat less-flash flood susceptible areas of northern Ohio will continue into eastern Ohio and northern Pennsylvania this afternoon. Thus, in coordination with PBZ/Pittsburgh, PA and CTP/State College, PA forecast offices, a Slight Risk upgrade was introduced with this update, roughly following the slow moving band of rain with embedded convection as it gradually pivots east. See MPD # 341 at wpc.ncep.noaa.gov/metwatch/metwatch_mpd_multi.php?md=0341&yr=2024 for more details. The surrounding Marginal was left unchanged with this update, but areas south of the Slight may see brief periods of heavier rain as stronger convection moves quickly across the area. Potential instability is higher given dry air aloft, but for that same reason, the convection is not expected to be widespread...instead more widely scattered in the form of quick-moving lines that move across the Mid-Atlantic. Further east, the band may pivot to more of a northwest-southeast orientation around the NYC area this evening. While brief periods of heavy rain are possible, the band will be moving northeast...or perpendicular to the axis of heavy rain, and therefore is not expected to persist in any one area for too long. Instability also decreases to around 500 J/kg by the time the rain gets to southern New England, so while portions of southern New England in the Marginal may see several hours of rain, it`s not expected to be particularly heavy, so only isolated flash flooding is possible. ...Northern Upper High Plains... No significant changes were made to the Marginal Risk area. The wave ejecting out of the Intermountain West may stall over the area, with the guidance trying to suggest the MT/ND border area under a higher threat. The area is generally flood resistant, so no changes were needed. ...Southeast Florida... No changes were made to the inherited Marginal Risk. As yesterday, slow moving convection developing along or interacting with the Atlantic sea breeze may result in slow or stationary convection developing along the highly urbanized urban corridor. This potential lends credence to the Marginal risk area. Wegman ...Previous Discussion... ...Texas... Multiple clusters of thunderstorms continue to progress to the southeast across TX with the latest surface analysis pin-pointing the stationary frontal boundary closer to the Gulf coast, extending back into south-central TX before wrapping back through west TX as a defined moisture gradient. All the convection earlier has allowed for the shift in the frontal boundary with lower theta-E`s positioned to the north of the front, a far cry from where we started 24 hrs prior. The upper level pattern will be progressing steadily eastward with a mid-level ridge positioned over the Continental Divide expected to shift over the plains thanks to a strong negatively tilted shortwave trough in the Pacific Northwest kicking the ridge downstream. The pattern evolution will bring about a slow moderation in the moisture field across the southern plains, but eventual southerly winds on the western fringe of the surface high focused east of the Mississippi will help advect a modestly unstable airmass poleward with a large theta-E ridge positioned east of the Rockies, tilting back into the southern plains. The stationary boundary over the south will slowly lift back north with diurnal mixing allowing for a distinct demarcation of the higher theta-E availability for the afternoon and evening hours. Small mid-level perturbations will eject out of the Front Range with another round of convection forming from WY down through CO/NM/TX and pressing eastward along the theta-E gradient. Local amounts of 1-3" could fall quickly within the environment established allowing for a broad MRGL risk area to encompass much of the central and southern plains. Remnant mid-level perturbations from this evenings convection will play a bigger role downstream into the Lower Mississippi Valley where a few rounds of thunderstorms could flare up with locally heavy rainfall in-of places like AR/LA/MS which is the reason for the eastward extension of the MRGL risk in place. The one area of focus will be across north TX where lower FFG`s, especially in-of the DFW metroplex will play a role in a higher flash flood potential given the antecedent conditions inherited from the previous period(s). The axis of the theta-E ridge will be positioned almost directly overhead of north TX with a tilt from northwest to southeast, bisecting the northern Concho Valley down into the region just southeast of DFW. There is a growing consensus for a stronger mid-level perturbation to initiate an area of convection upstream over southeast CO and the northern Panhandle and grow upscale, following the theta-E gradient to the southeast by the end of the forecast period. A modest signal for 1-2" of rainfall with locally as high as 4" is being depicted by select CAMs and deterministic allowing the ensembles to correlate with a heavy precip axis extending along and south of the Red River until you get just to the east of the metroplex. This total rainfall accounts for not only the anticipated complex of thunderstorms, but also any isolated convection that develops from afternoon destabilization, so the reflection of the QPF is a culmination of both convective modes. In either case, the lower FFG indices within the forecasted zone of heaviest precip was the greatest factor in the recent upgrade to a SLGT risk located over the aforementioned area in north TX. Other areas of note for flash flooding purposes include southeast TX with some guidance indicating some heavier cells developing along the stationary front which can always cause problems due to the focal point of the boundary helping with training cells. Local maximums of 2-4" were popping up across areas near and north of Houston, matching the ML output for a targeted area of convective interest given the synoptic pattern. Rates are not generously high and the recovery of FFGs across the region will mitigate a higher impact scenario, so the threat easily falls within the MRGL risk. Another area of interest is across the ...Montana and Northwest North Dakota... A fairly potent upper trough is currently analyzed over the Pacific Northwest with an axis of diffluent flow downstream into ID/MT causing waves of convection to form and move northeast over the topography of the Interior Northwest. The trough will pivot eastward over the course of the period and begin to take on a negative tilt indicating further strengthening which will be reflective of the evolving convective pattern downstream. A modest jet coupling over MT by the morning, onward signals some fairly significant large scale ascent entering into the northern High Plains across central and eastern MT extending as far east as the western Dakotas. A tongue of elevated theta-E`s will extend all the way to the Canadian border within developing surface trough to the lee of the Rockies. This will be a focal point for scattered thunderstorms to develop later this afternoon and push northeastward with the mean flow. Ample shear will be sufficient for a few relevant updrafts which will help strengthen some cells enough to produce some locally heavy rainfall within an axis of PWATs running 1-2 deviations above normal. The best chance for 1-2" of precip will lie within the confines of the surface trough positioned across eastern MT and northwest ND with the core of the jet dynamics reaching the region by the evening. Storms will be relatively quick moving which will limit the upper end potential for flash flooding, but there will be some cells capable for greater impacts with the latest HREF neighborhood probs for at least 2" running between 30-60% within the above zone. A MRGL risk was maintained from the previous forecast. ...Mid Atlantic and Northeast... Shortwave trough will pivot eastward from the Ohio Valley, eventually moving through the Northeast and Mid Atlantic by midday and beyond. Height falls present with the trough will provide a period of steep lapse rates within a zone of modest instability across the aforementioned area. Scattered showers and thunderstorms will maneuver overhead from OH/WV, across PA/MD/DE, eventually exiting to the northeast over NJ/eastern NY by later this afternoon and evening. Local maxes around 1-1.5" are forecast within any heavier cells over the above areas, so any areas that favor flash flooding in valleys or heavy urbanization will be subject to the potential. The MRGL risk from previous forecast was maintained and remains a lower-end of the threshold given the lack of deep layer moisture present ahead of the disturbance. ...Southeast Florida... Seabreeze propagation inland of southeastern FL will offer a period of locally heavy rainfall exceeding 2-3"/hr that could cause some flash flood potential within the urban corridor extending from Miami up through Fort Lauderdale. Latest HREF probabilities are running pretty high for at least 3" in spots within a short window between 18-00z, but the fields drop off to near 0% for at least 5", so the potential is likely capped. A low-end MRGL risk was introduced across the above area for the threat, but will likely remain at this lower risk given the ceiling for impact. Kleebauer Day 2 Valid 12Z Thu May 30 2024 - 12Z Fri May 31 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE CENTRAL AND SOUTHERN PLAINS... ...2030Z Update... No major changes were made to the inherited risk areas. The Slight risk was trimmed on the northwest side (central Kansas) due to lack of instability and very dry antecedent soil conditions, but also expanded to the south to include San Antonio and areas from west to northeast of Houston, as the area has already been hard hit and may be again. A higher-end Slight is considered for the area from about Childress, TX in the west to the southeast corner of OK in the east, and OKC in the north to DFW in the south. Expect 2 different lines of storms in this area Thursday...one during the morning from a line that will be somewhere near the southwest corner of Oklahoma, and races along the Red River, with a second similar strength line again Thursday evening. This effectively means that the day`s 24 hour totals will be effectively split half and half with 8-12 hours in between. This should greatly limit the flooding potential overall...but given the very wet antecedent conditions, still rises to a "higher-end Slight" threat level. Guidance remains in very poor agreement overall with where the lines track, how quickly they form, or how strong they get. Thus, the heavy rain footprint remains very big, extending from as far south as San Antonio all the way up to Kansas City and everywhere in between. Thus, unfortunately the Slight remains unusually large. This typically can indicate that within the Slight there may eventually be Moderate risk potential, but in this case it`s due to unusually high uncertainty. Should the guidance come into better agreement, the Slight will likely be able to be shrunk considerably. Wegman ...Previous Discussion... Longwave pattern will remain highly amplified with a strong mid- level ridge positioned along and east of the Mississippi River with a quasi-zonal pattern situated over the plains back to the western US. A negatively tilted trough over the northern Rockies will pivot into Canada with a few mid-level vorticity maxima traversing over the northern plains off the southern base of the shortwave trough. The combination of relative vorticity advection to the north and several mid-level perturbations ejecting eastward out of the zonal pattern to the south will create a large area of favored ascent within the central and southern plains on Thursday afternoon through the end of the period. A large surface ridge over the eastern CONUS will expand back west of the Mississippi with the backside of the ridge allowing for a funnel of elevated theta-E`s to migrate poleward into the plains. The combination of the enhanced instability fields and incoming shortwave advection will generate a prominent areal extent of convection from the Front Range all the way across towards the Mid and Lower Mississippi Valleys. Congealing cold pools across the plains will likely develop at least one MCS in-of the central plains leading to widespread heavy rainfall and increasing concerns of flash flooding. Other areas within the confines of the central and southern plains will have other opportunities just based off general convective evolution within a favorable environment. Guidance is beginning to key on a few areas of interest that could have greater prospects for flash flooding. The most notable is located within north TX through OK where the same stationary front that has caused so many issues the past few days will once again be a potential focal point for thunderstorm formations and storm motions anchored to the front and associated theta-E gradient. Storms are forecast to develop over northeastern TX the prior period and slide southeast through north TX with a correlating time frame that extends into the D2, so the first punch will come from adjacent periods. As that round of convection vacates, environment will once again destabilize with the stationary front remaining parked over the northern third of the state. Another complex is anticipated upstream, developing from quite a prolific mid-level shortwave that is within pretty much all guidance at this juncture. This will set off another round of thunderstorms with anticipated upscale growth as it plows east- southeast through the Southern Plains. Expect a long swath of heavy QPF across the eastern TX Panhandle all the way through much of OK and north TX within the path of the forecasted complex. 2-4" of rainfall is increasingly likely within a large chunk of the South Plains due to the complex in general, as well as other convection that spawns during the day. Environment is favorable given the anomalous PWATs running south to north along the western fringes of the ridge with anomalies generally 1-2 deviations above normal across TX up through the central plains, so the prospects for deep layer moisture presence to initiate the heavy rain threat is highly probable. Kleebauer Day 3 Valid 12Z Fri May 31 2024 - 12Z Sat Jun 01 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL AND SOUTHERN PLAINS THROUGH THE ARKLATEX AND LOWER MISSISSIPPI VALLEY... ...2030Z Update... As with Day 2/Thursday, uncertainty remains quite high as to the convective evolution on Day 3/Friday. However, there is somewhat better agreement on the whole Friday as compared with Thursday. The crux of the increased confidence is better run-to-run consistency in at least some of the guidance, indicating the Arklatex region as the bullseye for the heaviest rainfall. The guidance is a hair bit faster from inherited, so the Slight risk area was extended east and trimmed on the west side accordingly. Once again, the Slight was shifted south based on the latest guidance and typical guidance trends leading up to the event. The Arklatex region was highlighted for a higher-end Slight, and should guidance remain in reasonable agreement, could be upgraded to a Moderate given the very favorable antecedent conditions present in that area, as well as if guidance shows an increasing trend in amounts. Wegman ...Previous Discussion... Our very active pattern will continue into Friday with much of the residual convection over the southern and central plains shifting eastward towards the Mid and Lower Mississippi Valley allowing for a continued heavy rain footprint to extend further eastward. Several shortwaves will continue to eject out of the southern stream with a stronger, more amplified shortwave positioning itself over the central plains, likely from the organized convective pattern from the prior period. Surface low will advance east out of the plains as well with our stationary boundary finally lifting north as a warm front. Frontal positioning will play a significant role in where convection will spawn and maintain a focus over a general area that will enhance heavy rain and training prospects that would exacerbate flash flood concerns. As of now, the limited consensus is across the ArklaTex where a confluence of remnant of vorticity from previous convective output and the frontal boundary will likely be a primary focus for heavy rain in the beginning of the period, as well as later on as diurnal destabilization occurs and we get scattered thunderstorm activity the second half of the period. Another area of interest will be over central TX where a surface low will begin ejecting out of west TX with an increasing convergence signal occurring over the northern Hill Country, mainly along the western extent of the prevailing frontal boundary previously mentioned. A cluster of strong thunderstorms is forecast to spawn in-of the region with a focus of heavy rainfall near the initiation point and along the expected outflow boundary is gives off. Ensemble bias corrected data is pretty potent within the area it develops with 2-4" of rainfall likely in the vicinity of the cluster with some deterministic going even above 5" pending the evolution. QPF max is well defined over central TX up through the ArklaTex and Lower Mississippi Valley thanks to the expected pattern evolution. This was more than sufficient to maintain the previously inherited SLGT risk with some minor extension on the southwest edge into TX to account for some of the ensemble mean trends in the QPF field. Kleebauer 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