Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
543 FOUS30 KWBC 010828 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 428 AM EDT Wed May 1 2024 Day 1 Valid 12Z Wed May 01 2024 - 12Z Thu May 02 2024 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL AND EASTERN TEXAS... ...Southern Plains... Latest hi-res ensemble and associated CAMs have come into better agreement on the development of a complex across TX with origins out in west TX near the dryline, growing upscale through the evening with aid from a well defined LLJ and shortwave progressive within the sub-tropical jet over the region. Given consensus now favoring at least 2" of precip over a large expanse of central and eastern TX with much higher embedded amounts given the convective nature of the precip and anomalous tropospheric moisture (2-2.5 standard deviations above normal PWATs), have allowed for an upgrade to a Moderate Risk over the aforementioned areas. This is now the primary period of interest with a shift away from the previous MDT that was forecast for the now D2 time frame. The reasoning for the shift was due to the progressive nature of the convective pattern within the latest CAMs suite, as well as some instance within the ensemble bias corrected mean QPF footprint. Previous forecast iterations were more robust for the now D2 time frame, but the addition of CAMs guidance and the ensemble have allowed for a shift in the timing of when the heaviest precipitation would occur, as well as the general locations. 00z HREF EAS probabilities for at least 2" of rainfall are now upwards of 40% across a large swath of central and east TX, a good portion now overlapping with the area that saw extreme rainfall totals over 6" from this past weekend. The conjunction of heavy rain over those areas impacted and forecasted rates exceeding 2"/hr will likely force easier flash flooding potential due to the moist antecedent conditions in place. Also of note is the area over central TX where despite missing the previous rainfall, the flash instances are climo favored due to the soils and high runoff potential. With coordination with the local WFO`s involved within both TX and LA, the MDT risk was shifted to account for the abrupt change in convective timing and impacts. ...Midwest/Central Plains... A closed upper low is currently analyzed over Big Sky into the southwestern Canadian Provinces. A robust mid-level vorticity maxima will pivot around the base of the mean trough carved out by the upper low with sights on the central plains and adjacent Midwest allowing for rapid convective development downstream with a corridor of heavy rain forming later this afternoon through the remainder of the period. Guidance has trended to a favored heavy rain footprint across northern KS through eastern NE with the heaviest rain with a swath of 2-4" of rainfall over the course of this evening into early Thursday AM. 00z HREF EAS probabilities for at least 1" have become most aggressive in-of southeast NE which matches well with the current ML output from the past several runs. This makes sense synoptically given the best mid-level forcing will be downstream of the approaching vort max in an axis of diffluence perfectly aligned overhead. Rates between 1-2"/hr will be common within the areal extent of convection leading to some local totals between that 2-4" marker forecast. This should be sufficient for some higher flash flood potential within larger urbanized settings and areas that see the higher rates in question. The progressive nature of the precip will limit the higher end impacts, so the previous SLGT risk forecast will suffice despite the alignment synoptically. Kleebauer Day 2 Valid 12Z Thu May 02 2024 - 12Z Fri May 03 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE MISSISSIPPI VALLEY EXTENDING FROM THE NORTHERN MIDWEST DOWN TOWARDS THE GULF COAST... ...Midwest... Previous forecast still has merit with little deviation from past issuance. Thus, maintained continuity from the last discussion with some minor changes to details within the discussion below.... Overnight convection from this evening will continue to march northeastward as the congealed cold pools from the initial cell mergers will advance through the central Midwest with sights on northern IA into WI and MN on Thursday morning. The complex will eventually peter out as it moves northward into a less favorable environment which will put an end to the flooding threat as we move into the afternoon. Later in afternoon and evening, a cold front will propagate to the east out of the central plains with a redevelopment of convection over eastern KS, IA, and northwest MO. This will allow for the secondary peak of flash flooding potential as thunderstorms migrate through the mid and upper Mississippi Valley, impacting some areas that were just affected in the last 24 hrs. With the lower FFGs expected, the threshold for flooding will be lower than climatological norms for some areas within the expected secondary convective regime. Totals are currently within the 1.5-3" range on guidance which is significant enough to warrant at least the current SLGT risk. ...Southern Plains and Lower Mississippi Valley... As stated previously in the D1 outlook, our complex of thunderstorms in TX will migrate eastward into the Lower Mississippi Valley with heavy rainfall likely within the western half of LA into southern AR. The setup has now trended towards a "front loaded" forecast where the primary rainfall in the period that would induce flash flood concerns will occur during Thursday morning and early afternoon before dissipating due to the loss of sufficient upper forcing. The back end of the latest CAMs and associated HREF signal the opportunity for upwards of 2-3" of rainfall during that short time frame Thursday AM which will be enough to cause problems given the overlap from the previous weekend event that dropped copious amounts of rainfall over southeast TX into western LA. 00z HREF blended mean has an areal extent of 1.5-2" with probability matched mean upwards of 3" along the TX/LA border. HREF neighborhood probability for rates exceeding 2"/hr are also upwards of 30-40% within the same areas, correlating with the forecasted rainfall in that 2-3" zone. The setup really comes down to the timing of the complex in TX as to whether a reintroduction of a MDT risk will be needed after it was downgraded and shifted one period earlier. This will be assessed in future forecast updates as we closely monitor radar and hi-res guidance trends over the course of today and overnight into Thursday. For now, the SLGT risk will remain with wording of it being on the higher end of the risk threshold. Further north across KS and OK, the cold front to the north will move south with a push from high pressure nosing in through the plains in wake of the low pressure moving to the north. An area of convection will likely form over the central plains and move south with the boundary leading to more locally heavy rainfall in association with the area of convection. The threat is less aggressive compared to what is expected further south, but the chances are still prevalent. The SLGT risk was maintained from the previous D3 connecting the two primary areas of interest to the north and south. Kleebauer Day 3 Valid 12Z Fri May 03 2024 - 12Z Sat May 04 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF TEXAS, THE LOWER MISSISSIPPI VALLEY, TENNESSEE VALLEY, CENTRAL PLAINS, AND THE PACIFIC NORTHWEST... ...Southern Plains and Southeast... A multitude of shortwaves will ripple through the flow out of the subtropical jet with convective development across portions of TX through the Lower Mississippi Valley up into the Tennessee Valley. The convection over TX will have the best potential for flash flood concerns due to the regional instability and alignment of a stalled frontal boundary forecast across the central and eastern portion of the state with the dryline positioned over west TX. QPF distribution is scattered in nature with the maxima shifting all over the place pending deterministic. Considering the spread, the MRGL risk inherited was maintained, but there is potential for an upgrade within the two area boundaries due to the features being a focal point for training convection and surface convergence. Further east, widely scattered thunderstorms will produce heavy rainfall at times with the best opportunity occurring earlier in the period when the strongest shortwave progresses through the region(s). Ensemble bias corrected QPF is consistent with a maximum across LA which will have gone through some significant precip prior, so that will be another area to monitor in time for potential upgrades. Forecast QPF is still marginal for any upgrade potential, but CAMs will shed more light as we move closer to the time period of interest. ...Central Plains... A fairly robust mid-level perturbation will eject east-southeast out of WY/CO with convection initiating over the Front Range, eventually congealing into a MCS as it migrates eastward through southern NE into KS. Models are in agreement of at least some modest QPF with 1-2" scattered across the aforementioned areas and QPF maxima running upwards of 3" at this time. These MCS setups are notorious for localized flooding concerns, so wanted to make sure to add a Marginal Risk area to cover for the potential. As we move closer, the MRGL risk will likely be shifted around to account for the trends in guidance, especially within D1 when the CAMs and HREF provide more favor. ...Pacific Northwest... A relatively weak IVT pulse will enter coastal OR and northwest CA by the second half of the period with locally heavy rainfall expected through Saturday morning. IVT index is running at an "AR 1" which signals a weaker atmospheric river event, but one that can still produce modest rainfall totals to the coastal plain. Ensemble trends have come up a bit from previous forecasts with QPF running between 1.5-2.5 over the span of 12 hrs (00-12z Saturday), with a bit more as we head into D4. As a result, kept continuity with the MRGL risk from previous forecast package. 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