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Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
583 FOUS30 KWBC 141600 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Sun Jul 14 2024 Day 1 Valid 16Z Sun Jul 14 2024 - 12Z Mon Jul 15 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL MIDWEST... ...16Z Update Summary... The previous SLGT risk was expanded to include a portion of Eastern IA and points east including; Southern WI, Northern IL, and the Southwest corner of MI around the Lake Michigan shore. MRGL risk across the Southwest was adjusted on the northern and western flank of the risk area to align with current QPF footprint and neighborhood probability fields. ...Midwest... Current MCS from this morning is pressing southeast with scattered moderate to heavy rain signatures progressing through northwest IN. This will continue to move east-southeast with a low threat of flash flooding as it makes headway through the rest of IN into OH before decaying. Upstream, a pretty robust vort max is quickly pushing eastward out of the SD/NE border with sights downstream on the Central Midwest corridor between Eastern IA, Southern WI, and Northern IL. CAMs this morning were struggling mightily with the handling of the previous complex which would have some implications on the environment downstream of where the vorticity maxima will move overhead. The HRRR is doing the best at handling the evolving environment with a relatively good handle on the current MCS propagation, and even its interpretation for later is fairly aggressive with potential heavy rain footprint intersecting where heavy rain has previously fallen. 12z HREF probability fields are suggestive of flash flood prospects focused within the aforementioned areas with the neighborhood signal for at least 2" across Southern WI and Northern IL between 40-60%, a significant percentage considering the antecedent conditions from previous complexes, and the urban factors running from Milwaukee down through Chicago. 3hr FFG exceedance probabilities are also running between 20-40% with upwards of 50% within a small portion of Northern IL, just west of the Chicago metro. That area was significantly impacted from last evening, so the prospects of flash flooding are reasonably higher within that corridor. This all culminates in a sufficient SLGT risk threshold for impact with locally significant flash flooding possible over Southeast WI down through much of Northern IL. The previous SLGT risk was expanded to include those zones, as well as the Southwest corner of MI where the forecast motion of the complex should take the northern apex of the disturbance across Lake Michigan into the bordering lake shore areas. ...North Dakota... Shortwave trough over the Southern Plains of Canada will continue pivoting to the southeast with large scale ascent pattern improving across the northern tier of ND by nightfall. Scattered thunderstorms will develop east of Minot and grow in coverage with forward propagating speeds generally quick in the overall pattern. This area has had some rain recently, but the conditions are still on the drier side as depicted within the latest NASA SPoRT soil moisture percentiles. The area of interest for potential flash flood impacts will be localized to the river basin areas around Grand Forks where the urbanization factors lend credence to the potential for higher risks. Rates between 1-2"/hr will be plausible in any stronger cell cores, but the relevant storm motions should be sufficient to limit widespread impacts. With coordination from the local Bismarck and Grand Forks WFOs, have maintained a MRGL risk with potential for a targeted SLGT if necessary this evening. Kleebauer ...Southwest... An influx of drier air at the lower levels around the 4 Corners region prompted a reduction in the Marginal Risk area for today. Guidance suggests the greatest moisture has pushed south and west and will be focused into Arizona for today. Otherwise, expect typical monsoonal coverage of thunderstorms this afternoon and evening. There will be widely scattered thunderstorms over much of the Intermountain West, but the prevalence of low level dry air should keep most of the precipitation from those storms as virga except into Arizona where the lower level moisture will remain. Of course, particularly persistent storms especially those tied to terrain may produce enough rainfall. A series of shortwave disturbances will track roughly west to east across the northern Plains and Midwest today through tonight. The latest wave has sparked several MCS`s over the Upper Midwest this morning. The one currently moving across MI/IN is expected to continue weakening as it moves southeast...however there is a second one tracking southeast down the Mississippi River between MN and WI. This one following close behind the first will raise additional flooding concerns across northern IL and Chicagoland this morning before moving into MI/IN and dying out later this morning. These MCSs have a history of producing locally significant flash flooding, particularly in those areas where cell mergers and the back edge of the line of storms get hung up, resulting in a prolonged period of heavy rainfall. A few spots saw 3 inch per hour rainfall rates. On the large scale, expect these MCSs to continue moving quickly...but small portions of the storms are likely to get hung up and cause locally significant flash flooding. CAMs guidance has been very poor with the handling of these storms, almost across the board too light and small with coverage. Thus, there is lower than normal confidence on how the storms likely to develop once again tonight will behave. Given the new flooding sensitivities across much of northern Illinois, even comparatively disorganized and fast moving storms redeveloping tonight over these same areas will have a high enough potential to cause flash flooding as to justify a Slight risk upgrade. It should be noted that HREF guidance is much lower on the potential for FFG exceedance in this area...though it`s all but certain to be using pre-storms FFGs. Thus, it is assumed the FFGs now across northern Illinois are much lower and will therefore be much more easily exceeded with additional shower and thunderstorm activity expected tonight.ll for long enough to overcome the dry air, but those are expected to be isolated incidences. Wegman Day 2 Valid 12Z Mon Jul 15 2024 - 12Z Tue Jul 16 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN ILLINOIS INTO SOUTHWESTERN MICHIGAN... ...Midwest... On Monday evening, a surge of moisture associated with the typical strengthening of the low-level jet (LLJ) will run into a cold front supported by a strong and expansive area of Canadian high pressure moving south across Saskatchewan and Manitoba behind a strong upper level low over western Ontario. The clash of abnormally moist and warm air pushing northeastward on the LLJ from the central Plains into the Great Lakes and much cooler and drier air associated with the Canadian high is likely to occur from the Mississippi River east across northern Illinois and into southwestern Michigan. This area has been hard hit with flooding rains overnight tonight, and while there will be a good 36 hours between these events (barring any rain tonight), saturated soils across northern Illinois will certainly support the development of additional flash flooding across this region. As always with stalled out front, the exact placement of the front will determine who sees the most rain and subsequent flash flooding. However, both saturated soils towards Rockford and urbanization in and around Chicago are lowering FFGs, such that the potential for multiple inches of rain in just a couple hours could result in locally significant flash flooding. With portions of northern Illinois having been hard hit this morning, expecting some rain again tonight, and then following that up with potential for multiple separate rounds of training thunderstorms Monday night, that area has been highlighted for a higher-end Slight, which depending on the behavior of the storms tonight may potentially need to be upgraded further. As the night wears on Monday night, the LLJ is expected to follow its typical nighttime evolution turning more westerly with time. This would align the resultant storms parallel to the front which should move them more quickly, and with less resistance may allow the front to continue to sag southward, but the potential for multiple rounds of organized training thunderstorms will persist, gradually moving south with time. ...Marginal Risks... South-central Colorado: The guidance is marginally more optimistic for heavy rain producing thunderstorms to develop across the mountains of southern Colorado, particularly the San Juans on Monday afternoon as compared with Sunday. A bit more westerly flow in the upper levels may allow the storms to have a bit more upslope support on the west facing sides of the mountains, which are the areas most likely to see flash-flooding producing thunderstorms Monday afternoon. That said, the chances are still on the lower-end Marginal side, and could be downgraded should drying trends continue. For now, have divided up the inherited Marginal to only highlight the areas the guidance has better agreement for heavy rains. Southern Arizona: Pacific moisture streaming northward from the Gulf of California will shift a bit to the east on Monday as compared with Sunday. This should allow somewhat more organized convection to develop off the mountains of Mexico...impacting far southeastern Arizona the most Monday afternoon and evening. The southerly flow of moisture will support storms drifting north out of Mexico into the southern Arizona desert. Further north, upslope lifting along the Mogollon Rim will create a secondary maximum of rainfall across central Arizona, though flash flooding will be isolated. Wegman Day 3 Valid 12Z Tue Jul 16 2024 - 12Z Wed Jul 17 2024 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL MISSOURI INTO SOUTHERN ILLINOIS AS WELL AS FOR NORTHERN OHIO INTO NORTHWESTERN PENNSYLVANIA... ...Central Missouri into Southern Illinois... A strong cold front pushing south across the Midwest and down the Mississippi Valley is expected to stall out across central Missouri by Tuesday night. An expansive Canadian High will advance southward bringing welcome relief from both the heat and humidity to the northern Plains and upper Midwest. The cold front will lead the high on the southern side. The front should make steady southward progress during the day Tuesday. However, come Tuesday night, the typical diurnal strengthening of the LLJ to the south/ahead of the cold front will cause a substantial push of hot, humid and very unstable air towards the north into the front over Missouri. This will cause the front to stall out overnight. Where this front stalls out will be critical to which areas see the heaviest resultant rainfall, as the hot and humid air rides up and over the front, causing widespread convective development along much of the frontal interface. Multiple ingredients favorable and necessary for the development of flash flooding are likely to be present. In addition to a stalled out forcing, PWATs with the hot and humid air mass are likely to exceed 2 inches Tuesday night. The LLJ gradually turning more southwestward with time will support multiple repeating rounds of training thunderstorms along and south of the front. Much of Missouri has also seen heavy rain in previous days and weeks, so soils are wetter than normal across much of the state, and will be saturated more quickly once steady rain starts. Finally, topography (the north facing slopes of the Ozarks), may also help to organize the storms. This is because the Ozarks will help to slow the southward movement of the front simply due to the rugged terrain, and they will also help to locally uplift the hot and humid air, supporting storms that are tied to the terrain and at least during formation will be slow-moving and more susceptible to training. In coordination with LSX/St. Louis, MO; SGF/Springfield, MO; EAX/Pleasant Hill, MO; and PAH/Paducah, KY forecast offices, a Slight risk upgrade was introduced with this morning`s update. There is considerably more uncertainty on the east and west sides of the Ozarks, where the topography will not be a potentially supporting factor in the slowing of the front. For most areas, the topography won`t be necessary as the highly contrasting air masses alone should support strong frontogenesis. Nonetheless with the nose of the LLJ expected to be over Missouri Tuesday night, expect areas both west and east of this primary area will see somewhat less rain. Thus, the Slight was largely confined to the portions of the area with highest certainty. It`s likely the area of the Slight will need to be adjusted with future updates. ...Ohio/Pennsylvania... Along the same front impacting Missouri, storms moving northeast along the front will be highly subject to training in the unidirectional southwesterly flow. The upper level jet will be stronger north of NY across eastern Ontario, so the upper level support will be greater supporting the storms across OH/PA. However, the moisture will be tracking parallel to the front instead of into it, so expect the front to at least make slow progress southeastward through Tuesday night as strong and heavy- rain producing storms track along it. The inherited Slight was nudged northeastward with this update following guidance trends in the strongest forcing. Otherwise the plentiful Gulf moisture will be present all along the front, so the increasingly strong forcing across Ohio should help to invigorate the storms more than across Indiana. Hence, the 2 Slight risks are separated for that reason. 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