Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Sun Jun 22 2025 Day 1 Valid 16Z Sun Jun 22 2025 - 12Z Mon Jun 23 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHEAST NEW MEXICO/SOUTHWEST TEXAS AND PARTS OF PENNSYLVANIA/SOUTHERN NEW YORK... 16Z Excessive Rainfall Discussion... New Mexico and Texas... 12Z upper air showed precipitable water values in excess of 2 inches continued to be drawn northward into West Texas...on par with the 2 to 3 standardized anomaly precipitable water forecast from the overnight numerical guidance. With the large scale forcing largely in place...there were few changes needed here. Northeast... Introduced a Slight Risk in the area of instability ahead of on-going convection building west/southwestward towards northeast Pennsylvania and west-central New York. Based on cooling cloud top temperatures and resumption of 1 inch per hour rainfall being reported in Susquehanna county...combined with low 1-hour flash flood guidance in the area...felt a Slight Risk area was warranted despite some lingering questions as to how far west the activity will develop, Further details available in Mesoscale Precipitation Discussion 0492 from the WPC Metwatch desk. The Dakotas/Nebraska to Minnesota... Made a few adjustments to the northern portion of the Marginal Risk area given convective behavior earlier this morning...while a southward expansion into northern Nebraska was based largely on a consistent signal for convective development during the afternoon farther south and west than indicated by the 00Z/06Z numerical guidance. This area of convection looks to be progressive...but locally intense rates are possible given the precipitable water values which exceed 1.75 inches by early evening. Bann ...New Mexico and Texas... Today is the beginning of what will be a multi-day period of excessive rainfall potential over portions of west TX into NM. The mid/upper level pattern features a near record to record ridge over the eastern U.S. and a well-defined longwave trough over the west. Southerly flow in between these features will supply ample moisture to NM, with a connection all the way to the Caribbean Sea. The better forcing from the western trough will generally hold of until Monday and Tuesday (see the day 2 and 3 EROs), but today will still have enough moisture and instability around for scattered convective development. In fact PWs should already be getting towards climatological max values for late June over portions of southwest TX and immediate adjacent areas of NM. And while the better forcing arrives Monday, do note broadly divergent flow at 250mb Sunday which could help sustain convection. The greatest convective coverage is expected to be across portions of southwest TX, where this is considered a higher end Slight risk. Deep layer mean flow is weak, suggesting slow cell motions today. Convection forming on/near terrain will slowly move off the terrain, with outflows likely allowing for some cell merger activity. Given this will be occurring within a very high PW airmass, scattered areas of flash flooding appear likely. Both the 00z HREF and REFS indicate a high likelihood of rainfall locally exceeding 3", with at least some potential of isolated amounts over 5". A Marginal risk extends across much of eastern NM into adjacent area of west TX, where storm coverage should be less...but slow moving cells will still be capable of producing localized flash flood concerns. ...Eastern Dakotas into Minnesota... The setup looks favorable for another round of organized convection this afternoon and evening across the region. Strong shortwave energy ejecting into the northern Plains, a strengthening axis of low level moisture transport, and upwards of 4000 j/kg of MUCAPE will drive the convective threat. The combination of extreme instability, PWs well over the 90th percentile for late June, and a warm front focusing convection, all point to a heavy rainfall threat. Main uncertainty is the longevity of higher rainfall rates and the exact axis of convection. Cells will generally be moving along quick enough to preclude much of a flash flood threat, however some training along the warm front over far northern MN is possible. Activity may actually get going this morning, with some additional development by this afternoon. Also seeing a growing signal for convective development by this evening over southeast SD as convergence increases along the cold front. This activity should move northeast along the axis of the front across MN tonight. While this activity should be quick moving, heavy rainfall rates could still pose an isolated flash flood risk. ...Northeast... An area of organized convection is diving southeast across portions of NY as of 08z this morning. The evolution of this activity after 12z this morning remains a bit unclear, however some backbuilding/training potential may persist on the west/southwest flank of convection. While the motion of the complex should continue off to the southeast with the deep layer mean flow, upwind propagation vectors are off the southwest, a favorable sign for backbuilding. There will be plenty of upstream instability advecting in to sustain convection...however low level convergence should be on the decrease after 12z, and continued warm advection aloft will increase capping. Thus the degree of maintenance beyond 12z remains a question...but seeing enough of a threat for at least some backbuilding/training to justify a Marginal risk in the new day 1 ERO. PWs are certainly high enough to support at least a localized flash flood risk with any convection that is able to train/backbuild this morning. Chenard Day 2 Valid 12Z Mon Jun 23 2025 - 12Z Tue Jun 24 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NEW MEXICO AND WEST TEXAS, AND FROM KANSAS INTO IOWA... ...Southwest... The overall pattern over the Southwest described in the day 1 discussion will persist into the day 2 period. By Monday forcing, moisture, and instability should all be on the increase across NM, and thus expect convective coverage and flash flood risk to be on the rise as well. The gradual approach of the west coast trough will bring the right entrance region of the upper jet closer, and continued moisture advection in between the trough and the east coast ridge will push PWs well above the climatological 90th percentile and towards late June record levels. Instability is a bit of a question as cloud cover debris from Sundays activity could play a role...however the model consensus is averaging 1000-2000 j/kg over much of eastern NM into far west TX. Not a ton of deep layer flow, so convection forming on/near terrain should slowly move off the terrain and allow for some cell merger activity. The expected convective coverage should eventually erode instability enough that activity trends down by overnight...but not before what should be a more widespread scattered flash flood risk over eastern NM and west TX where a Slight risk remains. Far southeast NM into southwest TX looks to be a focus for more organized slow moving convection, with the 00z RRFS, GEM Reg and ECMWF all focusing higher totals over this area. In fact the RRFS and Gem Reg indicate localized amounts over 3" are likely, with a few spots over 5" possible. Another area of focus will be the sensitive burn scar areas of the Sacramento Mountains. High res guidance indicates a likelihood of strong convective development over the terrain here Monday afternoon, with high rainfall rates leading to what could be a locally significant flash flood risk. ...Central Plains to MS Valley... A Slight risk remains from portions of central KS into southeast NE and much of central and northern IA where convection along a stationary front poses an isolated to scattered flash flood risk. Model guidance indicates convective development along/near this front Monday afternoon, with some persistence and/or upscale development possible Monday evening as low level moisture transport increases. Corfidi vectors over this area are divergent, indicating a favorable environment for slow moving/backbuilding convection near this front. PWs should be well over the 90th percentile for late June along this front, as the same plume of moisture over the Southwest will be in place here as well. Thus any training of convection will pose a flash flood risk. Maintenance of instability is probably the main question mark, as the pool of forecast instability is not overly impressive and neither is the magnitude of the low level jet...so the extent of convective persistence into the overnight is a bit unclear. However do note some upward trend in instability, so while this convection should eventually erode the instability and weaken, we should still be able to realize several hours of possible training convection. The highest probabilities of excessive rainfall are likely across portions of north central KS into far southeast NE and far southwest IA. It is here along the southwest flank of the convection where we should have the strongest uptick in convergence Monday evening, and also the better upstream instability pool. As mentioned above, upwind propagation vectors are favorable for backbuilding and training along this axis. Do consider this a higher end Slight risk over this corridor, as the high PWs combined with training convection supports a scattered flash flood risk. In fact FFG exceedance may support MDT risk probs (40%), but given dry antecedent soil conditions and the narrow nature of the expected rainfall axis..will keep the risk at a higher end Slight level for now. Chenard Day 3 Valid 12Z Tue Jun 24 2025 - 12Z Wed Jun 25 2025 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF CENTRAL NEW MEXICO... ...New Mexico... Very similar setup as was described in the day 2 discussion continues into Tuesday. The moisture axis will shift a bit farther west by Tuesday, and with the upper trough getting closer large scale forcing will be slightly stronger. So should be another day of widespread convection over NM, with deterministic and ensemble guidance suggesting this will be the day of highest areal averaged rainfall. Flash flooding over this part of the country is heavily driven by rainfall rates...and whenever you see a widespread heavy QPF signal such as this you do wonder about the degree of instability that will be present. We have seen before where these more widespread rainfall days end up under performing flash flood wise, as cloud cover limits instability and keeps rainfall rates lower. So while that is something to keep in mind...all other factors suggest Tuesday will be a day with scattered to numerous instances of flash flooding over NM. It seems more likely that the modest westward shift and increase in the moisture axis/forcing will support areas or organized heavy rainfall rates. Central NM seems most likely to see more numerous impacts and thus a MDT risk was maintained across this area. Some of this MDT risk area will also have likely seen heavy rainfall Monday, thus hydrologic conditions over some of this area will likely be more sensitive by Tuesday, adding to the flash flood risk. This MDT risk area includes the sensitive burn scar areas of the Sacramento Mountains. While much of NM will see a flash flood risk Tuesday, the MDT risk area has the greatest risk of seeing more numerous and higher end impacts. Some rainfall totals over 3" appear probable, and would again not be surprised to see totals locally exceed 5" where convection persists the longest. ...NE/SD/IA and vicinity... A broad Slight risk was maintained across portions of NE/SD east into IA, southern MN and southern WI. Some convection will likely be ongoing across this area Tuesday morning, although would generally expect this to be of a weakening nature. By afternoon we should see additional development both the near the stationary front and over the High Plains. Convective details by this lead time become murky, and thus confidence on the details remain low. The ingredients (high PWs, instability, stationary front) will remain favorable for an area of excessive rainfall...and rainfall on preceding days may make hydrologic conditions more sensitive over portions of this area by this time. Ensemble probabilities and recent runs of the AIFS highlight northeast NE into northern IA as having the greatest heavy rainfall risk, and thus this portion of the Slight risk is considered higher end. This is where the best chance of scattered flash flooding seems to exist, especially if this additional rain overlaps any areas of heavy rainfall from Monday. Chenard Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt