Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
181 FOUS30 KWBC 220125 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 925 PM EDT Fri Jun 21 2024 Day 1 Valid 01Z Sat Jun 22 2024 - 12Z Sat Jun 22 2024 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST SOUTH DAKOTA... NORTHEAST NEBRASKA... NORTHERN IOWA... SOUTHERN MINNESOTA... AND SOUTHWESTERN WISCONSIN... 01Z UPDATE... A primary area of concern for numerous flash floods during the evening and overnight remains across portions of the mid Missouri and upper Mississippi Valleys. A mid-level shortwave and right- entrance upper jet forcing is helping to organize convection along a well-defined, slow-moving front extending across the region. A deep moisture pool (PWs 1.75-2 inches), replenished by strong southwestern inflow, will support heavy rainfall rates, with redeveloping/training cells elevating the threat for heavy accumulations. Recent runs of the HRRR and HREF guidance show the greatest threat for heavy amounts centered over northwestern Iowa. HREF neighborhood probabilities indicate that overnight accumulations of 5 inches or more are likely across this region. This forecast axis is displaced slightly to the south of the previous night`s very heavy rainfall, however the most recent HRRR has shifted a little further north and current radar returns show convection once again developing over some of the same areas previously impacted, accentuating additional flash flooding concerns. Overall, the updated outlook is similar to the previous; however, the northern gradient was tightened some given the recent RAP analysis that shows much more stable air to the north. The Slight Risk was removed from southern New England. Storms that produced locally heavy rainfall and flash flooding across CT/RI are now moving east, with rainfall rates diminishing as they move into a more stable air mass. Scattered storms developing upstream ahead of the front dipping south across Upstate NY and southern New England have shown the potential to produce heavy rainfall rates across northeastern PA. Therefore, a Marginal Risk was maintained to reflect the threat for additional isolated runoff concerns. Elsewhere, made small adjustments based on recent hi-res guidance and observation trends. Pereira Previous Discussion... ...Mid Missouri Valley and Upper Mississippi Valley... The primary change with this outlook update is to shift the Moderate and Slight Risk contours south about 30-40 miles. An ongoing band of elevated convection from northern Nebraska (near VTN) to southern Minnesota (near MKT) seems likely to be a reasonable proxy for the northward extent of potential extreme rainfall today. The atmosphere to the south is rapidly becoming unstable, with abundant sunshine, and the dichotomy should reinforce the surface front. This afternoon, new convection is expected to focus closer to the surface front. Overall, the front is slightly further south than models were projecting in some of their previous runs. The new data seems to have led to convergence in the hi-res model solutions with reasonably good consistency on the projected location of the highest totals. There was some consideration given to a High Risk upgrade within the Moderate Risk area. The pattern over the region is classic for flash flooding with an extensive WSW-ENE oriented front, strong instability and anomalous moisture available to the south, and increasing low-level inflow from a southwesterly direction to favor backbuilding and training. One key lingering open question for a High Risk upgrade is the degree to which today`s heaviest rainfall will overlap the areas most sensitive from previous rainfall; most of the hi-res models are just to the southeast, but it is very close. The area of greatest concern is from YKN-EST, or from Yankton County, SD to Emmet County, IA, and about 25mi either side of that line. These areas are in sunshine at the moment (indicating the surface front may be nearby or even just north) and overlap sufficiently with the extreme rainfall footprint from the past 24 hours. So it is possible the 12Z hi-res models have slightly overestimated the cold pool from ongoing convection; if this is the case, a High Risk may be issued on an unscheduled update this afternoon. Convective trends will be monitored closely. Either way, the potential exists for significant and life- threatening flash flood impacts in the region, and people in the area are advised to closely monitor for warnings later today. ...Four Corners Region into New Mexico... The Slight Risk area was expanded to the west into portions of the slot canyon region of Utah. Model consensus is for another round of afternoon convection to develop in a fairly unstable air mass ahead of an advancing shortwave in portions of UT, W CO, and N AZ. Generally, one of the biggest differentiating factors between active flash flood days and inactive or less active days in the Southwest U.S. is instability, and models are showing fairly widespread areas of CAPE above 1500 j/kg by this afternoon. Despite some pockets of morning convection, there is enough sunshine to support widespread destabilization, and precipitable water values should be above the 95th percentile for this time of year. Convection may also be more organized than is typical, with some backbuilding, as southerly low-mid level flow will be relatively strong. The IVT is above the 99th percentile for this time of year. Further southeast into New Mexico, there is more cloud cover associated with a deep moisture plume from the remnants of T.S. Alberto, which has since dissipated. However, where breaks in the cloud cover can occur, enough instability should support renewed convective activity this afternoon and evening, and the deep moisture (as seen in the CIRA ALPW product) should support efficient rainfall. One of the better signals is, unfortunately, in the vicinity of the Sacramento Mountains where there are several large burn scars and the HREF has reasonably high probabilities of 1 inch per hour rain rates. GOES day cloud phase RGB does show some low level cloud cover beginning to scatter out, so some instability nearby may be able to develop. ...New York and northern Pennsylvania into New England... A Slight Risk was introduced over southern New York and the lower Hudson Valley into southern New England -- particularly Massachusetts, Connecticut, and Rhode Island. Precipitable water values approaching 2 inches (above 95th percentile) will support very high rain rates in scattered convection that is already developing across the region. For more mesoscale detail, please reference MPD #480 about to be issued. Across the Slight Risk area, the 12Z HREF probabilities of 2 inch per hour rain rates peak at about 20 to 40 percent in any given hour, and this would be enough to generate some flash flooding, particularly in urban areas. ...North Florida and Coastal Southeast... A Marginal Risk was introduced for the potential of localized heavy rainfall around the tropical disturbance nearby in the Atlantic. Hi-res models show localized rainfall in excess of 2 inches per hour in some of the convection today. Lamers Day 2 Valid 12Z Sat Jun 22 2024 - 12Z Sun Jun 23 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MIDWEST AND NEW ENGLAND... ...Upper Midwest and Western Great Lakes... It appears likely that organized convection will be ongoing at the start of the forecast period across the Upper Midwest, most likely from Central Wisconsin into Northern Iowa. The initial round of convection should continue to lift into northeast Wisconsin and northern Lower Michigan through the morning, with a secondary round of convection developing in the afternoon in central Wisconsin closer to an advancing surface low, and eventually progressing into northern Lower Michigan overnight. The multiple rounds of organized convection are producing a clear heavy rain signal in the hi-res guidance, particularly in northern Lower Michigan where the 12Z HREF has a maximum probability of 55 percent for over 5 inches of rainfall during this forecast period in the vicinity of Alpena. That is rare for the area, with a limited number of cases of 5 inches of rain in 24 hours, and an average annual recurrence interval of around 100 years. The Slight Risk was maintained across the region, but could be thought of as a "high end" Slight Risk at the present moment, and a Moderate Risk upgrade may need to be considered depending on convective evolution. Within the broader Slight Risk area, a second corridor of enhanced potential exists near the Wisconsin-Illinois border. Hi-res models are in reasonably good agreement about the placement of a band of thunderstorms in advance of the trailing cold front. The primary limiting factor for rainfall amounts and flash flood potential may be that this cluster or line of thunderstorms could be more progressive, limiting the amount of time it spends in any one location. Some hi-res models, such as the 12Z WRF-ARW2, showed a period of greater training, but that is only a possibility. ...New England... Portions of the region were upgraded to a Slight Risk, primarily in New Hampshire, Massachusetts, eastern Connecticut, and Rhode Island. The persistence of a plume of high precipitable water values approaching 2 inches (above the 95th percentile) and moderate to strong instability should support organized convection with relatively high rain rates. These could exceed 2 inches per hour at times, and the best signal for that is where the Slight Risk was drawn. However, flash flooding cannot be ruled out in the broader Marginal Risk area which encompasses more of the region. ...Portions of the Southwest... The upper-level trough over California into the Southern Great Basin will weaken as a ridge expands westward. An area of anomalously high precipitable water values will, however, expand farther to the north and west, encompassing much more of southern and central Arizona and continuing in New Mexico. The primary uncertainty will be how much heating can occur, as there can often be thicker mid-high level cloud cover with such a deep moisture plume. Given that instability tends to be a big differentiating factor for flash flood activity in the region, this is not a trivial uncertainty. Therefore, the Marginal Risk was generally maintained for now. ...Far South Texas... The next surge of high PW values to the north of low pressure moving east to west across the southwest Gulf of Mexico/Bay of Campeche will move into South Texas day 2. Model consensus is for the heaviest precip to remain across northeast Mexico, with far South Texas on the northern edge of this precip. A Marginal Risk was maintained from the previous issuance given potential for locally heavy rainfall amounts across regions that received heavy rains recently from Alberto. ...Portions of southern Georgia into North Florida... The surface low approaching the Southeast coast should weaken further as it pushes into south central Georgia. Simulated radars from hi-res models show potential for some slow moving cells in the vicinity of this weak low level circulation, which will be accompanied by an axis of above average PW values. Locally heavy precip totals possible near this weak low level center, southward into North Florida where a marginal risk was maintained from the previous issuance. Lamers/Oravec Day 3 Valid 12Z Sun Jun 23 2024 - 12Z Mon Jun 24 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN NEW YORK STATE INTO NORTHERN NEW ENGLAND... ...Northeastern US... A shortwave trough, with corresponding height falls and a deepening surface low, will advance toward the Northeast U.S., shifting the plume of anomalous precipitable water values more squarely into the region as well. Model consensus is strongest for heavy rainfall in northern New England and Upstate New York closer to the nose of the LLJ and northern periphery of the expanding warm sector. The Slight Risk was maintained in these areas, with a Marginal Risk existing elsewhere further south along the cold front. One limiting factor overall may be a tendency for forward progression for the convection, but enough instability and deep moisture should exist to support 1-2 inch per hour rain rates in some of the stronger and more organized convection. ...Portions of the Southwest... The Marginal Risk was maintained across the region on Day 3, largely due to very anomalous precipitable water values that will remain above the 99th percentile in some areas. One notable change for Sunday relative to Saturday would be continued advancement of the higher PWs into southern California. At this point, the Marginal Risk mostly does not include California due to a weak QPF signal, but this may need to expand into those areas in future outlooks. The primary uncertainty continues to be with the potential for some areas of mid-high level cloud cover that may restrict instability, overall thunderstorm strength, and therefore put a ceiling on potential rain rates. ...South Texas... PW values 2-2.5" and sustained east southeasterly low level flow is expected to persist for most of the upcoming day 3 period across northeast Mexico into South Texas. Similar to the day 2 period, model consensus is for the heaviest totals to be across northeast Mexico with far South Texas on the northern edge of this precip. Given no big changes overall to the pattern, the marginal risk was maintained over far South Texas for isolated runoff issues. Lamers/Oravec 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