Quantitative Precipitation Forecast
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295 FOUS30 KWBC 151552 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1152 AM EDT Sat Jun 15 2024 Day 1 Valid 16Z Sat Jun 15 2024 - 12Z Sun Jun 16 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE UPPER MIDWEST... ...16Z Update... The previous forecast was generally maintained across the Central Plains and Upper Midwest in conjunction with the SLGT risk. A minor adjustment was made to the southern periphery of the SLGT across northeast KS where a strong convective cluster this morning put out a local maximum as convection became anchored to the warm front and produced some totals between 1.5-2" within a few hours. This area has been primed for the next wave of convection expected later as indicated in the discussion below... Across KS, mid-level WV satellite shows our next shortwave making progress to the northeast which will assist in the primary ascent pattern that develop during peak diurnal heating through the overnight time frame. A generally modest environment currently will ramp up with the advection of richer theta-E`s and relevant instability as it pushes north with the aid of the nocturnal LLJ. Guidance is consistent on the heaviest precip focus within eastern NE up through northern IA and southern MN with the QPF maximum confined within that corridor in question. 12z HREF neighborhood probabilities are still fairly robust for at least 3" across the aforementioned corridor with percentages around 35-50% for the 3" threshold with even some low-end probabilities for >5" settling between 10-20%. Scattered flash flood signals are most likely the case with this kind of setup, but any organized clusters could promote a more significant areal extent for flooding prospects, especially if they make their way into the suburban and urban corridor centered over Minneapolis-St Paul. Given the above, the SLGT risk was more than valid for the period. The MRGL risks over North Dakota, the Southern Plains, Florida, and Washington state were all justified within the latest forecasted QPF fields on all 12z deterministic. There were no changes to the outlined areas considering the general continuity from guidance. South FL will see their chances dwindle after 21z with north- central FL seeing their best chance after 19z with the introduction of the seabreeze driven convection. Washington state will remain in a low-end flash flood concern through the period across portions of the Cascades where remnant burn scars within the terrain will allow for a localized threat with the elevated convective pattern pivoting through the area. The Southern Plains and North Dakota will have their MRGL risk generally after 23z with isolated flash flood threats bordering around 5-10% given the environment in place for the respective locations. General max of 1-3" is forecast in these locations with much of it occurring in short intervals. Kleebauer ...Previous Discussion... ...Upper Midwest to the Texas Panhandle... A potent upper level shortwave currently over western Nebraska and northeastern Colorado is spurring along a strong squall line across the Missouri River Valley between Nebraska and Iowa and eastern Kansas this morning. An impressive low level jet (LLJ) is increasing moisture ahead of the squall line with PWATs locally exceeding 1.75 inches and 850 flow of up to 45 kts. On the northern end of the line, a warm front will remain nearly stationary across southern/central Minnesota. This will allow for training of mostly light rain, but with embedded convection for much of the day. The heaviest rain is likely to develop well after sunset as the nocturnal LLJ strengthens further and advects instability to 3,000 J/kg into the Slight Risk area. This will allow for backbuilding convection into western Iowa and extreme eastern Nebraska, which will then track ENE across northern Iowa, southern Minnesota, and into western Wisconsin. The embedded convection with heavier rainfall rates will increase the flash flooding potential across the Slight risk area as the cold front continues pushing east. Much of the area has about average to slightly above average soil moisture content, which will support the development of widely scattered instances of flash flooding through tonight. The trailing cold front and associated moisture plume will allow some limited convection to develop across the Texas Panhandle north into Kansas today. Since coverage will be more widely scattered, any flash flooding will be isolated, and the Marginal Risk for these areas remains in place with no changes. A second cold front associated with a low over Saskatchewan and Manitoba tonight will spur its own convection across western North Dakota. This convection too will have plenty of instability to work with with values between 2,000 and 3,000 J/kg. Isolated flash flooding is possible as a result. ...Florida Peninsula... Typical afternoon convection is expected to redevelop again today across the Florida Peninsula. It will continue the downward trend as far as coverage is concerned, but may impact more northern areas of central Florida, which remain in a drought. While coverage will be greater across northern Florida, even isolated convection is possible across southern Florida, where the Marginal risk was extended to account for even isolated convection resulting in additional flash flooding as many areas of the urban I-95 corridor from Ft. Lauderdale south through Miami remain flooded with soils and reservoirs still full of water and unable to handle even the brief heavy downpours typical of this time of year in the area and are possible today. Today should be the last day where even a modest degree of organization is possible with the convection. ...Washington Cascades... Upslope flow off the Pacific will continue to uplift showers and a few thunderstorms into western Washington. Additional lift from the Cascades may result in some widely scattered rainfall totals of 1-1.5 inches, which could result in localized flash flooding on burn scars and any canyons and fast-flowing creeks and streams. A Marginal Risk was introduced with this update for this possibility. Wegman Day 2 Valid 12Z Sun Jun 16 2024 - 12Z Mon Jun 17 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE NORTHERN PLAINS AS WELL AS THE CENTRAL GULF COAST... ...Northern Plains to the U.P. of Michigan... The cold front associated with a low over Canada will stall out as the parent low moves into northern Canada. Meanwhile a new leeside low will develop over Colorado. This will drag abundant Gulf moisture over the nation`s midsection back to the west into the low. After sunset Sunday night, the strengthening LLJ will run into the stalled out front, turning it into a warm front. The front will act as a forcing mechanism...lifting the abundant moisture and instability along it, resulting in widespread convective initiation across South Dakota and Nebraska. Due to the abundant moisture, the storms will be capable of heavy rainfall rates as they track northeastward across southern South Dakota and into southwestern Minnesota in the predawn hours Monday morning. The strong parent low over northern Canada will keep strong westerly flow into the front from the cold side. The result is despite impressive moisture influx from the south with the LLJ, the front will struggle to move. This will turn the front into a "train track" of sorts, allowing numerous rounds of convection to develop and move northeast along it, with the LLJ fostering new convective backbuilding to the west. The flow will turn northeastward upon running into the front, resulting in largely unidirectional flow, supporting training and backbuilding. This will set this stage for a more impressive flooding event possibly into Monday. More on that in the Day 3 discussion. Portions of the Slight Risk area across SD and MN will have been hit with multiple rounds of heavy rain from now through Sunday morning. Thus, the start of the potentially bigger flooding event across SD/MN may have some support of already wet soils as the storms move through, resulting in widely scattered instances of flash flooding. The Slight Risk area was largely unchanged from inherited. ...Central Gulf Coast... The other area of increasing concern will be along the Gulf Coast. A nothing short of impressive plume of very deep tropical moisture straight from the Caribbean will track northwestward across the Gulf and into the central Gulf Coast on Sunday. PWATs associated with this air mass will be between 2.25 and 2.5 inches, exceeding the 95th percentile compared with climatology and as high as 4 sigma above normal. This deep moisture plume will result in convection with daytime heating during the afternoon on Sunday, generally from New Orleans south...but the more substantive rainfall will come Sunday night as atmospheric moisture continues to build. Widely scattered convection all of which capable of very heavy rainfall will move across the Central Gulf...generally between Mobile and New Orleans, where the Slight Risk remains. The Slight was shifted eastward with this update, reflecting a slowing of the westward movement of the moisture plume. This slowing would further enhance the potential for flash flooding, but a lack of forcing should keep convection rather limited in coverage. One piece of good news is that this area has been rather dry in the recent past, with the last heavy rain episode a week or more ago. Thus, for the first part of this event Sunday night, instances of flash flooding should be isolated to widely scattered, with the greatest threat over flood sensitive urban areas from Mobile to New Orleans. Wegman Day 3 Valid 12Z Mon Jun 17 2024 - 12Z Tue Jun 18 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF MINNESOTA AND PORTIONS OF THE EASTERN DAKOTAS AND WESTERN WISCONSIN AS WELL AS FOR THE MISSISSIPPI DELTA REGION... ...Upper Midwest... A potentially high-impact flash flooding event may develop across portions of Minnesota on Monday and especially Monday night. A very impressive 130 kt jet for June will be nearly stationary across Manitoba and northern Ontario provinces. The jet exists because unseasonably cold air will be digging south across the Intermountain West. Meanwhile, an unseasonably hot and humid air mass associated with a strong upper level high over the East Coast will pump impressive amounts of Gulf moisture up the entire Mississippi River and into Minnesota. A impressively strong shortwave trough will eject out of the Intermountain West and into the northern Plains Monday night. The warm moist air mass will run into the front that will be over central and northern Minnesota Monday and Monday night. Additional lift from the right entrance region of the upper level jet streak, the strong shortwave, and the stationary front will result in nearly continuous convection across this region from Monday morning straight through Monday night. While convection typically wanes through the day due to competing heating sources, the front and ample additional lift will keep the rainfall going. Then as the LLJ intensifies to over 50 kt after sunset Monday night, the associated storms will also increase in coverage. Since the front, acting as the primary forcing mechanism will be either stationary or very slow moving, the convection will continuously train and redevelop over Minnesota into Monday night. The combination of all the above ingredients could result in a high-impact flash flooding event across Minnesota. Guidance is in above average agreement that this will occur over central to northern Minnesota, but as would be expected, there is still disagreement as to whether the front will slowly move north or remain stationary. This will ultimately be the difference between more widespread instances of somewhat less severe flash flooding over a larger area, or a more concentrated area of high-impact flash flooding over a smaller area. Due to this uncertainty and in coordination with DLH/Duluth, MN; MPX/Minneapolis, MN; and FGF/Fargo, ND forecast offices, the Slight Risk area was expanded a bit, and is a very high-end Slight over the aforementioned areas. A Moderate Risk is very likely with future updates, and should the guidance suggest the latter scenario, a High Risk is not out of the question. The potential is there for a flash flooding event unrivaled in the area for at least the last 10 years. Stay tuned. ...Mississippi Delta Region... Concurrent with the potential flooding in the Upper Midwest, the plume of moisture moving into the central Gulf Coast from the Gulf will continue to intensify through the day Monday. PWATs will be nothing short of impressive, as they eclipse 2.5 inches along the Louisiana coast. Storms are likely to be ongoing across the area from the overnight period as the Day 3 period starts at 12Z/7am CDT Monday morning. Continuous southerly flow of deep tropical moisture will support continuous development and redevelopment of storms capable of very heavy rainfall rates across Louisiana and Mississippi on Monday. Corfidi Vectors will be both weak (under 10 kts, and out of the northwest, against the flow. This will support persistent training convection and backbuilding when Corfidi vectors are against the prevailing southerly flow. A subtle and weak disturbance in the upper levels may further support convection, but forcing should largely be low levels driven from outflow boundaries and cold pools that are constantly recycled with deep tropical moisture out of the Caribbean. In coordination with LIX/Slidell, LA; LCH/Lake Charles, LA; and MOB/Mobile, AL forecast offices, the Slight was greatly expanded with this update to central Louisiana and Mississippi, with a higher-end Slight introduced for southern Louisiana and the Mississippi Gulf Coast. This area is the most likely to be upgraded to a Moderate Risk with future updates, especially if Day 2 rainfall over this area overperforms or increases in future model guidance. Given rivers in some of these areas of far eastern Texas and western Louisiana are still in flood from events over more than a month, soils and reservoirs have not had sufficient time to recover from those events for another rainfall event of this magnitude. Further east into New Orleans and the Mississippi Gulf Coast, FFGs from urbanization are already lower. With potential for 3 inch/hour rainfall rates from convection, perhaps repeatedly, even flood resistant bayous will be unlikely to handle a deluge of that magnitude, which will be possible. ...Montana... Much of Montana will be on the "cold side" of the system, with high-elevation snow expected. The rainfall across the state will be much more reminiscent of a cold-season event, with ample upper level forcing involved and some Gulf moisture. Essentially some of the Gulf moisture from further east into Minnesota will curl back to the west as another leeside low that forms over CO early in the period tracks north into the northern Plains. Rainfall amounts should remain generally under 1 inch, but will be widespread enough across Montana that the inherited Marginal remains in place with no changes. Wegman 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