Quantitative Precipitation Forecast
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248 FOUS30 KWBC 132000 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 400 PM EDT Thu Jun 13 2024 Day 1 Valid 16Z Thu Jun 13 2024 - 12Z Fri Jun 14 2024 ...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN FLORIDA... ...16Z Update... A SLGT risk was issued across portions of the central Midwest through the Mid-Mississippi Valley for later this evening. Advancing cold front will move south-southeast out of the northern Midwest later this afternoon with an increasing convergence pattern along and ahead of the front. Ahead of the boundary, some significant increase in areal theta-E`s will prompt an improving environment for convection and heavy rain potential. Soundings out of KILX this morning were approaching the 90th percentile for PWATs with a value of 1.37", and only expected to increase prior to the incoming convective event. Latest 12z HREF neighborhood probabilities have increased from prior runs for the upper thresholds of >3" and >5" with the former running between 25-40% in a broad area from the IA/MO border across into northern IN. 5" probabilities are running between 10-15% across central IL which is the consensus for best area to see the max potential rainfall. Totals of 2-3" with upwards of 6" are going to be littered across the outlined area within the SLGT as discrete supercell development ahead of the front potentially garnering some scattered heavy rain chances even prior to the upscale growth pattern along the boundary as it slowly moves southward. In coordination with the central Midwest WFO`s involved, the SLGT risk was agreed upon given the favorable environment and forecasted probabilities. The High Risk across FL was maintained from the previous special update as the area from Fort Meyers/Naples over to the other coast near Miami/Fort Lauderdale will see another round of significant rainfall to exacerbate the remnant flooding in place from the past few days. The highest threat aligns with the I-75 corridor across into the urban areas extending from just north of FLL down into Miami proper. Latest 12z HREF probabilities correlate well with the expectation for at least 2-3" as the fields are both respectively above 90% for the neighborhood probability, coast-to-coast along the southern peninsula. Even more impressive is a swath of 20-35% EAS probabilities for at least 3" running along the aforementioned area which given the conservative nature of the EAS field is statistically significant. The High Risk aligns very well with the EAS field in question adding confidence to the issuance of the risk. The rest of central and southern FL will see scattered instances of significant rainfall with totals between 2-4" with locally as high as 8" within the SLGT/MDT risks surrounding the High risk area in place. Heaviest rainfall will occur this afternoon through the evening before some marginal clearing of precip overnight. ...1425Z Update... After collaboration with the local Miami WFO, have opted to upgrade a portion of the southern FL Peninsula to a High Risk to account for significant impacts expected with the next round of heavy rainfall over extremely sensitive areas that include the urban corridor in Southeast FL through the I-75 corridor over Alligator Alley. Additional totals of 2-5" are likely with locally as high as 10+" in isolated locations along the southwest coastal areas extending through the interior of the Peninsula. Considering the ongoing flooding with many reports of standing high water and the historically low FFG indices for all 1/3/6/12 hour intervals, the prospects for significant impacts will continue through the afternoon and evening. Thus, the High Risk was issued to account for the general expectation. Kleebauer ...Previous Discussion... ...Florida... Several inches of rain focused over South Florida yesterday which has left soils water logged and have lowered the 1/3/6 hr FFG to as low as 0.25 inch. The latest hi-res guidance show another very heavy band developing potentially over the same areas by late morning and into the afternoon hours. It will take very little additional rain to aggravate ongoing flooding in rural and urban locations. A Moderate was raised for a majority of the southern counties which includes the Miami metro where areal averages with be around 3 inches with isolated maximums up to 8 inches possible per hi-res guidance. Totals in the periods prior will be running between 3-12" for an areal average across the above corridor with some maxes approaching 15" in the areas that have been hit the hardest over successive days. Considering the repeated nature of the heavy precip, expectation is for more flash flooding to arise late Thursday morning through the afternoon as diurnal destabilization within the convergent axis provides another round of significant rainfall for those that have seen a prolific amount already. ...Midwest... Shortwave over the Northern Plains will pivot around the northern periphery of the mid-level ridge located across the Southwest U.S with sights on the central Midwest by Thursday afternoon. At the surface, a cold front will be pressing south-southeast over portions of the Central Plains through the Mid-Mississippi Valley. The coupling of the two distinct features in the midst of a prolific thermodynamic regime located over the above area (MLCAPE exceeding 4000 J/kg) will help initiate a robust line of convection with cores capable of locally enhanced rainfall as they move southeast with the forecast steering flow. The Marginal Risk was maintained to allow for more consensus with guidance as they get inside 24 hrs of the event. While there continues to be some variance in where the convection will be, consensus favors a line from the borders of Iowa/Missouri/Illinois to east/northeast of Chicago where areal averages of 1 to 3 inches are forecast. The environment may be conducive for locally significant rainfall in the strongest convective cores, particularly if backbuilding cells become anchored to the front. Campbell/Kleebauer Day 2 Valid 12Z Fri Jun 14 2024 - 12Z Sat Jun 15 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHERN FLORIDA... ...20Z Update... The update for the SLGT risk across South Florida can be found in the sub-heading below.... Elsewhere, other areas of note include the Central High Plains and the Northeastern U.S with locally heavy rainfall forecast in each area during the mid to late-afternoon hours heading into the evening. Across the Northeast, a cold front will traverse the region by midday with an increasingly favorable environment for deep, moist convection ahead of the front. Instability is modest overall, but the ascent pattern along and ahead of the cold front will be sufficient for an organized line of thunderstorms to initiate from New England to the Mid Atlantic, moving eastward through the second half of the forecast period. Latest HREF mean QPF is quite a bit more robust than the NBM, although the latter has started to come around from its usual low bias at range. Considering PWATs within the 1-2 deviations above normal range, the chances for more efficient rainfall during the event are considerably higher compared to other events in the past. A general 1-2" range is forecast from near PHI up through NYC and portions of Southern New England with the heaviest precip focused around the Lower Hudson into the NYC suburbs. A lot of this is due to the combination of forecasted surface frontogenesis being maximized over the Northern Mid Atlantic along with the favorable upper level ascent within the RER of a jet streak moving over New England. Given the latest forecasted precip and the dynamic factors becoming more favorable, have expanded the MRGL risk further south and east to include the Philadelphia and NYC metro areas along with the surrounding suburbs. There was only some minor adjustments necessary to the MRGL risk across the High Plains as the upper level evolution remained steadfast in its run-to-run continuity. Heaviest rain will fall across the Front Range northeast of Denver/Boulder into Southwest NE and Northwest KS before ejecting off into the northern Midwest during the front half of the D3 period. Scattered signals for totals exceeding 2" have been indicated within the latest probability fields with the highest neighborhood probs of 60-70% located within the Sand Hills of NE. This will be important as that area has a much higher FFG interval compared to the surrounding locales. This creates a better threat further south where the setup is both favorable with those lower FFG`s comparatively. The threat for significant flooding is low and with the coverage of heavier rainfall likely more sporadic, a MRGL risk was sufficient to cover the threat. ...South Florida... No significant changes to the previous SLGT risk across South FL. A lot on the potential is contingent on what transpires the prior period in regards to antecedent conditions. The area will be wet, but the prospects only increase if there is more significant flooding with standing water present in any areas south of the Fort Meyers to West Palm Beach line. The worst will be over compared to previous periods as the front slowly sags south and the higher PWATs begin degrading, albeit still pretty moist, but the ascent pattern becomes less prominent leading to less organized waves of convection. Recent trends with the ensemble QPF signal much less in terms of the upper threshold of potential with the 90th percentile QPF more in line with 3-5" compared to the 10+" we`ve had in successive days. This will curb the higher end impacts to a degree, but the receding waters will take some time before we are in the clear. The the areas south of the line mentioned above will be closer to the higher end of the SLGT risk with a chance for an upgrade pending hi-res forecasts for thunderstorms during the period. For now, will maintain wording of potentially significant flooding, but will also mention the threat becoming more localized as the ascent pattern loses some favorability compared to recent periods. Kleebauer ...Previous Discussion... ...Rockies and Plains... A strong upper level disturbance exits northeast out of the Four Corners with a strengthening low in the lee of the Rockies. Scattered heavy rain signatures with some isolated totals exceeding 2" are forecast. Local enhancement of QPF is expected within the higher elevations of northeast New Mexico otherwise the bulk of the high QPF is expected across the Central Plains, roughly from Boulder to Sioux Falls and could easily pick up 1 to 3+ inches of rainfall. A vast majority of this region has 3-hr FFG near 1.5/2 inches however there are very isolated locations as low as 0.25 inch. A Marginal Risk is in effect from northeast New Mexico to Iowa/Minnesota/South Dakota. There may be a need for a small Slight Risk area with future updates if QPF trends continue to increase. ...New England... A cold front will advance through Ontario/Quebec and reach the Northeastern U.S during peak diurnal destabilization. Convection is expected to develop along and ahead of the front and will be capable of isolated heavy rain instances from Pennsylvania to New England and into the Canadian maritimes. The highest QPF is focusing across southeast New England where guidance suggests areal average of 1 to 2 inches. The progressive nature of the convection will limit the threat to marginal territories, although the urbanization factors in-of the Lower Hudson up through southern New England could produce some locally enhanced risks if a storm overperforms. Campbell/Kleebauer Day 3 Valid 12Z Sat Jun 15 2024 - 12Z Sun Jun 16 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF IOWA, MINNESOTA AND WISCONSIN... ..20Z Update... The upper pattern across the Plains into the Northern Midwest is still sufficient for a period of enhanced rainfall on Saturday afternoon and evening. Latest ensemble bias correction and NBM both agree on a core of 1-2" amounts with the upper quartile of NBM output generally placing parts of southern MN and northern IA closer to 3" which is pretty significant considering the setup a full 48 hrs out. Recent ML output is a bit further west with the core of the heaviest QPF, so it will be interesting to monitor if the SLGT risk in place will get positioned a bit further west into parts of the northern plains. Until there is some significant deviation from the mean, have maintained continuity from previous forecast. Over FL, the threat for flash flooding will be waning as the pattern responsible for the successive days of heavy precip will finally be broken down with the deeper moisture field shifting back west over the Gulf. Isolated to scattered thunderstorms are still forecast over parts of South FL during Saturday afternoon which could trigger some localized flood concerns thanks to the very wet antecedent conditions. The threat is currently on the lower end of the MRGL threshold, but at leads wanted to remain consistent and will adjust if necessary as we monitor the pattern evolution. Kleebauer ...Previous Discussion... ...Upper Mississippi Valley to Southern Plains... During this period a cold front will be pushing through the Northern Plains while a warm front lifts from the Central Plains into the Upper Mississippi Valley. PW values of 1 to 1.5 inches will be drawn into this system, reaching a peak of +2 standard deviations above average over Iowa and Minnesota. There will be strong mid-level ascent embedded with this pooled moisture which will help enhance rainfall amounts and coverage from the Plains to the Upper Mississippi Valley. Guidance is showing 1 to 3 inches will be common although local maximums may reach 5 inches. A Slight Risk is in effect from northwest Iowa to west-central Wisconsin. A Marginal Risk area spans from North Dakota eastward to Lake Michigan and southwest to Oklahoma. ...South Florida... Convection is expected to linger nearing the decaying front boundary and may produce an additional 1 inch or so. The soils will be over-saturated and sensitive to any additional rainfall. A Marginal Risk is in effect given the ongoing flooding concerns. Campbell 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